injection molding and copolymers by processing styrene
TRANSCRIPT
Processing styrene polymers and copolymers by injection molding
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2–0,04 mm) Barrier ring Thrust
hA*
A*
30°H*15°
ow cross sections in these regions muapproximately the same
0°
0°
Table of contents
1. Introduction
2. Styrene polymer and copolymer product range2.1 Composition of styrene polymers and copolymers2.2 Product range2.2.1 Crystal polystyrene2.2.2 High-impact polystyrene2.2.3 Styrolux2.2.4 Luran (SAN)2.2.5 Terluran (ABS)2.2.6 Luran S (ASA and ASA/PC)2.2.7 Terlux (MABS)
3. The injection molding machine and mold3.1 The screw injection molding machine3.2 Injection molding screws3.2.1 Single-flighted, three-section screws3.2.2 Other screw geometries3.2.3 Barrier screws3.3 Screw tip, nonreturn valve3.4 Injection nozzle3.4.1 Open nozzles3.4.2 Shut-off nozzles3.4.3 Directions for installing the injection nozzle3.5 Injection mold / Molded part design3.5.1 Sprue design3.5.2 Design of molds and wall thicknesses3.5.3 Venting3.5.4 Demolding3.5.5 Temperature control3.5.6 Materials of construction
4. Processing advice for styrene polymers and copolymers
4.1 Moisture content of pellets and drying of the material4.2 Temperature control on the cylinder4.3 Melt temperature4.3.1 Special product-specific characteristics4.3.2 Effect of processing on mechanical strength4.4 Mold surface temperature4.5 Peripheral screw speed4.6 Back pressure4.7 Injection pressure / Injection speed4.8 Holding pressure / Holding pressure time4.9 Cooling time4.10 Shrinkage and warpage4.11 Optimization of the injection molding process
5. Special processes5.1 Multi-component injection molding5.2 Internal gas pressure process
4
5
67
8
910
11
12
1314
15
16
1718
1920
22
23
25
4
2.2 Product range
2.2.1 General-purpose polystyreneGeneral-purpose PS is a transparent,colorless material. (Fig. 3) It is hard,stiff and has very good dielectricproperties (ie, it is an electrical insula-tor). The general-purpose grades aredifferentated by the unique combina-tion of stiffness, heat resistance andflowability that characterizes eachgrade.
Polystyrene, due to its amorphousstructure, exhibits high dimensionalstability and low shrinkage. Itabsorbs very little moisture. Poly-styrene is easy to mold and subse-quently print, weld, paint andemboss. Its neutral taste and odorare particularly important when itcomes to food packaging applications.
2.2.2 Impact-modified polystyreneImpact-modified polystyrene is madeby polymerizing styrene in the pres-ence of polybutadiene rubber, anelastomer that gives the polymer itsimpact strength (Fig. 4). The stiffnessand impact resistance of impactgrades is determined by the amountof polybutadiene present. HIPS isnaturally opaque, its surface appear-ance can range from matt to highgloss.
2.2.3 StyroluxStyrolux is a styrene-butadiene blockcopolymer that combines excellent
clarity and impact strengh. (Fig. 5)Styrolux is universally applicable, iseasy to thermoform and can be printed, welded and embossed. Also,it absorbs very little water, is physio-logically harmless and gamma-raysterilizable.
In many applications, Styrolux ismixed with general-purpose poly-styrene to custom modify physicalproperties. A high degree of clarity isrealized with high and low styroluxlevels when blended with GPPS. (Fig. 6)
0 230 240 250 260 270 280
Impa
ct-f
ailu
re e
nerg
y (J
) 50
40
30
20
10
0
Processing temperature (°C)
SHIPS
HIPS
Fig. 4: Impact resistance of a high impactpolystyrene compared to a super high impactpolystyrene
possess high dimensional stabilitywhile shrinkage and water absorptionare low.
By adding rubbers to SAN either ABS(acrylonitrile-butadiene-styrene, tradename Terluran) or ASA (acrylate ester-styrene-acrylonitrile, trade nameLuran S) polymers are obtaineddepending on the type of rubbercomponent employed. Luran S/C ismanufactured from Luran S and polycarbonate.
The styrene copolymer range is fur-ther supplemented by SBS (styrenebutadiene and block copolymers,trade name Styrolux) and MABS polymers (methyl methacrylate-acrylonitrile-butadiene-styrene, tradename Terlux). The chemical names,abbreviations and trade names areshown in Figure 2.
2.1 Composition of styrene polymers and copolymers
Styrene polymers and copolymers arematerials capable of being processedby thermoplastic transformation.Figure 1 shows the BASF styrenepolymer and copolymer productrange.
Crystal polystyrene (PS) and polybu-tadiene – modified polystyrene (high-impact polystyrene or HIPS) are themain focus of the styrene polymerfamily.
Styrene copolymers contain, in addi-tion to styrene (S), other monomers inthe principal polymer chain. Styrene-acrylonitrile copolymers (SAN, tradename Luran®) form most importantrepresentative and basic buildingblock of the entire copolymer productclass.
Luran® SAN is a rigid, hard and heat-resistant thermoplastic which is asclear as glass due to its amorphousstructure. In contrast with partiallycrystalline thermoplastics moldedparts produced from SAN or Luran
1. Introduction 2. Styrene polymer and copolymer product range
The following is a brief survey of thestyrene polymer and copolymer productrange. The aim of this publication isto provide the processors of ourproducts with information of both ageneral nature and specific to thevarious products in relation to injec-tion molding machines, molds andprocessing.
+ Acrylonitrile
+ Butadiene+ MMA
+ Polybutadiene + Polybutadiene+ Polyacrylate ester
blend with PC
Blend with functionaladditives
Styrolux(SBS)
+ Polybutadiene
Terlux(MABS)
Luran S(ASA)
Luran(SAN)
Styrene
Polystyrene(PS)
impact-modifiedPolystyrene
(PS)
Styroblend
Terluran(ABS)
Luran S/C(ASA + PC)
Fig. 2: Styrene copolymer product rangeFig. 5: Impact properties of Styrolux 684Dand Polystyrene compared
0 10 20 30
Forc
e (N
)
1500
1250
1000
750
50
25
0
Depth of penetration (mm)
PS 684 D
5
Fig. 1: Product range
Crystal polystyrene (PS)High-impact Polystyrene (HIPS)Styrolux (SBS)Luran (SAN)Terluran (ABS)Luran S (ASA and ASA/PC)Terlux (MABS)
0 10 20 30 40 50 60 70 80 90 100
Light transmission (%)
Clear SBS
Acrylic, SMMA
PC
PVC
PET
Clear ABS
SAN
PP
GPPS
88-91
88-92
86-89
76-82
84-90
72-88
85-89
35-82
87-92
Fig. 3: Transparency of clear thermoplastics in %
2.2.6 Luran S (ASA and ASA/PC)In contrast with Terluran, the ductilephase in the case of Luran S consistsexclusively of an acrylate ester rubber.As a result of this, excellent resistanceto UV and heat aging (Fig. 8) isobtained resulting in very good resis-tance to yellowing. Accordingly,Luran S is primarily suitable for out-door applications. In addition, it hashigh resistance to chemicals.
Properties such as flowability, heatresistance or impact resistance areadjusted differentially according toLuran S grade. Processing of Luran S at high temperatures is alsopossible without difficulty. The Luran S range also includesblends of ASA and polycarbonate. Inthese cases the ASA componentcontributes its excellent resistance toweathering and aging. By compari-son with ASA the ASA/PC blendsexhibit increased impact resistanceand thermostability at comparablerigidity and dimensional stability.
2.2.5 Terluran ABSAll types of ABS are styrene-acryloni-trile copolymers modified with finelydivided butadiene rubber which havehigh impact strengths and notchedimpact strengths (even at relativelylow temperatures). They are distin-guished by high surface gloss, goodscratch resistance and good ther-mostability.
Terluran “World Grades”Terluran “World Grades” were spe-cially developed for the needs ofglobal customers. They are standard,injection-molding and extrusiongrades supplied in light, extremelyconstant self-colors, in a limitednumber of standard colors.
2.2.4 Luran SANAs a styrene-acrylonitrile copolymer,Luran is both an individual plastic inits own right as well as the basis forsome other styrene copolymers(Terluran, Luran S).
The most prominent properties ofLuran are:
outstanding transparency, good chemical resistance thermal resistance high strengthrigidity (see Fig. 7), dimensional stabilityresistance to fluctuating temperatures.
For years Luran has proved to bevery effective in numerous applica-tions in the field of household articlesand tableware, cosmetics packaging,hygiene and toilet articles as well asfor writing implements and officesupplies. The Luran color rangeextends from a large number oftransparent and smoky topaz shadesthrough bright solid colors having ahigh surface gloss.
70
60
50
40
30
20
10
00 20 40 60 80 100
Duration of storage (weeks)Pe
netra
tion
ener
gy (j
oule
s)
ABSPC+ABS
Luran S
Fig. 9: Light transmittance of Terlux 2802 TRand Terlux 2812 TR as a function of wave-length (measured on injection-molded test
specimens 2 mm thick)
100
90
80
70
60
50400 450 550 550 600 650 700
Wavelength (nm)
Tran
smitt
ance
(%)
Terlux 2802 TR
Terlux 2812 TR
Fig. 8: : Impact resistance of ABS, Luran S and blends after heataging at 90 °C (Penetration energy for circular disks 2 mm thick)
Fig. 6: : Light transmittance as a function of % GPPS in blendswith Styrolux 684D
100
95
90
85
80
75
70
65
600 20 40 60 80 100% GPPS in Styrolux 684D
Ligh
t tra
nsm
ittan
ce (%
) Resin (MFR) 168M (1.5)158K (3.0)166H (3.5)148G (6.0)147F (9.0)145D (14)
76
Luran
Generalpurpose (PS)
PA 6
ABS
3 300 MPa
3 000 MPa (dry)
2 400 MPa
1,000 MPa(moist)
3 800 MPa
HIPS2 000 MPa
Fig. 7: Comparison of modulus of elasticity of different thermoplastics
In contrast with polycarbonate chemical resistance is better andprocessability is distinctly enhanced.A fire-protection grade free of chlorine,bromine and antimony rounds off theproduct range.
2.2.7 Terlux (MABS)Terlux is transparent, amorphousthermoplastic based on a MABSpolymer. Thus Terlux combines inideal manner typical ABS properties,such as a balanced relationshipbetween toughness and rigidity, withthe transparency familiar from PMMAmolding compounds (Fig. 9).
This combination of properties sin-gles out Terlux as a specialty in thefield of transparent thermoplastics.
98
In the flight depths illustrated in Figure 11, a distinction is madebetween standard screws and shal-low-flighted screws. Shallow-flightedscrews pick up less material, and asa result of which the residence time inthe plasticizing unit is shortened. Thiscan be advantageous when handlingthermally sensitive materials.
3.2.2 Other screw geometriesStandard or universal screws, suchas the three-section screw, are stillencountered most frequently in prac-tice and even today still cover a largeportion of the spectrum of require-ments and materials. In recent years,however, applications have increas-ingly appeared in which higherdemands are imposed on the qualityof the melt, than can be provided bya three-section screw. This may beencountered, for example, in criticalproduct formulations and also in in-plant coloring using master
batches or pigment concen-trates. In order to achievethe required homogeneity,additional shearing and/ormixing parts as well as bar-rier zones can be and areemployed.
Common to all the shearingand mixing parts are thebasic principles of screwclearance and the divisionand recombination of themelt stream. The Maddockand spiral shear sections togetherwith the toothed disk or diamondpattern mixing section are primarilyemployed (Fig. 12). Furthermore, in apilot plant trial good results wereobtained with the Twente mixing ring(TMR), a combination of nonreturnvalve and mixing part, developed bythe University of Twente in theNetherlands (Fig 13) and availablecommercially in North America.
comings in visual appearance, thishas the consequence of diminishingthe mechanical properties of themolded part.
What are known as three-sectionscrews have proved to be highlyeffective in practice for the process-ing of styrene polymers and copoly-mers. As the name already indicatesthis type of screw is divided intothree different zones (feed, compres-sion and metering section) each ofwhich has to execute different tasks.Located at the tip of the screw thereis additionally a nonreturn valvewhich during the injection and hold-ing phase prevents back flow of theplasticized melt.
Standard modern screws generallyhave an effective length of 20–23 D,the length of the feed section beingapproximately half the length of thescrew. (Fig. 10) The compressionand metering sections are of approxi-mately the same length. The pitch isusually 0.8–1 D. The flight depth ratioof the feed and metering sectionsranges from 2 to 3. A compressionratio of 2 to 2.5 has proved to beeffective for the processing of styrenepolymers and copolymers in injectionmolding applications. The flightdepths recommended by BASF forthree-section screws are shown inFigure 11 as a function of the screwdiameter.
3.2 Injection molding screws
3.2.1 Single-flighted three-sectionscrewsThe screw employed in the plasticiz-ing unit affects the properties of themolded part. In certain circumstancesthe choice of a screw which is unsuit-able for the material results in inho-mogeneities in the melt (temperature,color distribution, non-uniform melt-ing). As a result of an unfavorablescrew geometry, the melt can beexposed to high shear stresses andthus be damaged both thermally andmechanically. In addition to short-
3.1 The screw injection moldingmachine
It is customary nowadays to employscrew injection molding machines forthe processing of styrene polymersand copolymers. In these machines,the screw assumes the roles of con-veying, plasticizing and injection.Other types of machines (e.g. plungerinjection molding machines) are nowused only for special applications (forthe production of marbled effects, forexample).
DLLL
D
0.5-0.550.25-0.3
0.2
0.8-1.0
Outer diameter of screwEffective length of screwLength of feed sectionLength of compression sectionLength of metering sectionFlight depth in the metering sectionFlight depth in the feed sectionPitchNonreturn valve
DLLELKLAhAhESR
20– 23
hEhAR S
L
LA LK LE
D
Fig. 10:Plasticizing unitwith three-section screw
Fig. 12: Three-section screw with shear and mixing sections
Screw diameter D (mm)
Flig
ht d
epth
h (m
m)
30 40 60 80 90 130
2
4
6
12
50 70 100
10
8
110 120
hE
hA
hE = flight depth in the feed sectionhA = flight depth in the metering section
Standard screwShallow screw
Fig. 11: Flight depths for three-section screws
Maddock-shear section Spiral shear section
Diamond pattern mixing sectionToothed disk mixing section
Fig. 13: Twente Mixing Ring
3. The injection molding machine and mold
Using this system in contrast withtwo other shear and mixing sectionsstudied in parallel, molded parts freefrom streaks were producible espe-cially in the case of critical colors andlow-concentration pigment pastes.
These mixing and shear sectionsshould be designed as far as possibleto be pressure-neutral. This designkeeps the material throughput, mini-mizes wear and avoids detrimentaleffects on the melt temperature.
A shut-off nozzle and/or subsequentretraction of the screw can be usedto prevent escape of melt duringplasticization operations.
When using open hot runnersystems, retraction of the screw isadvisable so that the pressure in thehot runner is released and escape ofthe melt from the hot runner is pre-vented.
There is, however, the danger that airis drawn in between the hot runnersystem and the injection nozzle. Thiscan be prevented, for example, byusing a long-reach nozzle.
The screw tip has to be designed in amanner to promote flow (Fig. 15 ).The angle C at the screw tip and noz-zle inlet should be the same so thatas little melt as possible can settle inthe cylinder head or in the nozzle. Ina poorly designed screw tip, the meltcan be thermally damaged by a rela-tively long residence time in the plas-ticizing cylinder. In this case mechani-cal and optical impairment of thematerial cannot be precluded.
3.4 Injection nozzle
3.4.1 Open nozzlesDue to flow-related advantages, opennozzles (Fig. 16 ) allow simple clean-ing and rinsing as well as rapidchange of materials and colors. Inorder to prevent thermal damage, theopening of the nozzle bore should beat least 3 mm wide. The angle at thenozzle inlet should be the same asthat of the screw tip.
The more air drawn in due to enlargedmetering or injection strokes, the moredifficult it is for this air to escape againto the hopper and not to move intothe space in front of the screw.Indrawn air which is included in themelt and gets into the mold causesstreaking in the molding, an effectwhich must be prevented. The mini-mum precondition for qualitativelyflawless parts is that the maximumpossible metering stroke and theeffective length of the screw have alimited ratio to one another (i.e., forexample, that the maximum meteringstroke for a screw 20 D long shouldbe restricted to 3 D).
3.3 Screw tip, nonreturn valve
The designs of the tip of the screwand nonreturn valve are important forthe troublefree flow of the melt in theplasticizing unit.
A constant melt cushion and longholding pressure times can only bemaintained by means of nonreturnvalves. The clearance between thecylinder and nonreturn valve shouldlie between 0.02 and 0.04 mm (Fig. 15) at the operating tempera-ture. In order to avoid any back pres-sure from the melt, the flow crosssections in the different regions (A, hA
and H) must be of the same size.
The length of screw or which barrierapproach should be employed mustbe determined according to applica-tion. The requirements, relating in par-ticular to homogeneity, can varygreatly from case to case.
Very large plasticizing units that makeuse of only a small part of the availableswept volume result in long residencetimes of the melt in the cylinder. Thiscan result in thermal damage to theproduct. Furthermore, even small fluc-tuations in the stroke of the screw cangive rise to considerable variations inthe shot weight.
Larger shot volumes are achieved atlow cost and by structurally simplemeans like enlarging the meteringlength. Enlargement of the meteringlength has the consequence that theeffective screw length shortens. Thiscan result in unmelted material andinhomogeneities in temperature. Afurther risk with larger metering lengthis that even more air can be drawn in,especially during injection.
3.2.3 Barrier screwsThe mode of operation of all barrierscrews is in principle the same. Thecharacteristic feature is the division ofthe screw channel into a solids chan-nel and a melt channel (Fig. 14). Thesolids channel is separated from themelt channel by the barrier partition.The barrier partition has a greater gapwidth than the main or barrier flight sothat only fused material or particleswhich are smaller than the gap widthin at least one direction can get intothe melt channel. In flowing over thebarrier partition, these particles areexposed to an additional definedshear stress resulting in further fusionof the remaining particles of solid. Thebarrier partition contributes moreoverto the homogenization of the melt.
As the channel length of the barriersection increases, the cross section ofthe solids channel decreases while atthe same time the channel cross sec-tion of the melt channel increases. Thischange in the cross section of bothchannels is achieved in the differenttypes of barrier sections by a changein the dimensions of the flight depthsand/or flight widths.
* 30° to 60°
C*
Temperature sensor
Fig. 16: Open nozzle with temperature sensor
Shut-off needle Shut-off spring
Temperature sensor
Fig. 17: Spring-loaded needle valve nozzle
Cylinderinternal
Clearance (approx. 0,02–0,04 mm) Barrier ring Thrust ring
hA*
A*
30°H*15°Screw tip
* Flow cross sections in these regions must be approximately the same
30°to60°
Fig. 15: Nonreturn valve
Barrier partition
Solids channel
Melt channel
Fig. 14: Illustration of the principle of a barrier screw
10 11
3.4.2 Shut-off nozzlesThe use of shut-off nozzles allows theinjection nozzle to be retracted duringplasticization. As a result of this, thetransfer of heat between the temper-ature-controlled injection nozzle andthe cooler mold is cut off.
Shut-off nozzles are also advanta-geous when operations must be con-ducted with elevated back pressureand unwanted stringing is to beavoided. Mechanically or hydraulicallyoperated needle shut off nozzles arehighly suitable for styrene polymersand copolymers (Fig. 17). By com-parison with open nozzle systemsshut-off nozzles suffer from the dis-advantage of higher-pressure losses.
1312
3.5.2 Design of molds and wall thick-nessesThe wall thickness needed is deter-mined by the requirements forstrength and rigidity as well as for aneconomical cycle time for the compo-nent. In Figure 19, the effects of wallthickness, melt and mold surfacetemperatures are shown for an amorphous and a partially crystallineproduct. The maximum and minimumcooling times of a product areobtained by using the maximum andminimum melt and mold surface tem-peratures found on pages 16 and 18,respectively. It is evident that the wallthickness of the molded part playsthe dominant role in determining thesetting time.
In order to prevent or reduce sinkmarks, gating should be placed asclose as possible to the region of thegreatest wall thickness because herethe holding pressure can act for thelongest time.
If gating is placed in regions of thinwall thickness, volume contractions inthe thicker regions can no longer becompensated for since the thin wallsfreeze earlier. Gating in this way gen-erally results in more distinct sinkmarks.
As a matter of principle, reinforcingribs should be constructed with alower wall thickness than the basicwall thickness and be tied by a cer-tain radius to the base wall. The tieradius prevents flow problems andnotch effects. A tie radius which istoo large can, however, have aneffect on the cycle time (Fig. 20)and/or the quality of the molded part(sink marks due to accumulation ofmelt).
3.5.3 VentingMolds must also be vented. Whenthe two mold halves close, a volumeof air is trapped in the mold cavity.During injection of the polymer, thisair, if not removed, will compress andrise in temperature. The temperaturecan rise enough to cause autoignitionof the air/polymer volatiles mix. Thisis called dieseling and results in aburn mark through the plastic. Inextreme cases, the mold cannot befilled. The removal of the trapped airis accomplished using vents. Figure 21 shows an example of amold venting scheme.
Vents are typically placed around thecircumference of the parting line,spaced as close as 3-4 inches apart.The vent consists of a “land” areaapproximately 0.0004” deep by 1/4”to 1/2” wide by 1/8” to 1/4” long. The0.0004” is a “steel safe” startingthickness that allows air to escapebut not viscous plastic. Beyond the1/8” to 1/4” dimension, the tool steelis removed to a greater depth to facil-itate gas flow and is channeled to theperimeter of the mold. On occasion,
3.5 Injection mold / Molded partdesign
3.5.1 Sprue designIn the design of injection molds forstyrene copolymers, care should betaken that runners and gates are ofan adequate size. Flow resistanceduring injection and holding has to bekept as low as possible. Undersizedgates can cause streaking, charringdue to shear and delamination.Premature solidification of the melt inthe sprue frequently results in sinkmarks and voids in the molded partsince the contraction in the volume ofthe melt during the holding pressurephase cannot be compensated for.
3.4.3 Directions for installing theinjection nozzleIn order to ensure troublefreedemolding of the molded part, carehas to be taken during installation ofthe nozzle that the radius of the noz-zle head is always smaller than that ofthe sprue bushing the mold. Thediameter of the nozzle bore mustalways be smaller than the diameterof the runner of the sprue bushing(Fig. 18).
A thermocouple for measuring themelt temperature should be installedin all nozzle structures. This providesthe processor with evidence of themelt temperature actually prevailing inthe space in front of the screw (Figs. 16 and 17).
Fig. 20: Effect of rib and wall thicknesses andradii of curvature on cooling times of a rib-reinforced injection-molded part
Wall thickness (mm)
Cool
ing
time
(s)
1.0 2.0 4.0
5
10
15
30
3.0
25
20
hE
hA
00.0
Terluran® 877M
Ultramid® A3K
PS Wall thickness s = 2.0Rib thickness a = 1.0Radius R = 0.3Control circle d = 2.24=1.12·sSetting time 11.1sec
Ra
sd
Ra
sd
Ra
sd
s=2.0a=2.0R=0.5d=2.7=1.35·sSetting time 15.9 sec
s=2.0a=2.0R=2.0d=3.5=1.75·sSetting time 26.1
Fig. 21: Structural scheme for venting a mold
Moldpartingline
Moldcavity
2
3
1
Bore2 to 5 mm
0,015 + 0,005
Fig. 19: Theoretically determined settingtimes for plaques of differing thickness untilthe maximum wall temperature reaches thedemolding temperature
Fig. 18: Conical gate and sprue
R15
R16
ZConical gate
Slope> 1°
Needle valve nozzle
Conical nozzle boreSlope > 1°
DK DD
Detail Z
Sprue
due to the gate placement, gasentrapment can occur away from theoutside edge of the cavity space. Inthose cases a vent pin, which is similar to an ejector pin but with a0.0005” (to start with) flat cut on oneside,can be used to vent gas out theback of the core into the ejector box.Sintered metal plugs have also beenused successfully when ventedthrough the mold body.
During construction, care has to betaken that the venting channels runinto an adequately sized exhaustchannel so that cleaning of thechannels is rendered practicallyunnecessary. Nevertheless, the venting channels should be checkedregularly and cleaned if necessary inorder to prevent any decrease in theefficiency of venting with progressiveusage of the mold caused by deposits.
For prototype work where a smallnumber of parts will be produced,lead/zinc alloys are often used.These materials are inexpensive, canbe cast into near net shape to reducemachining cost, and can be remeltedand recast again. This family materi-als are not durable because thelead/zinc structure does not possesshigh tensile strength, toughness, ordurability against erosive effects.These materials also exhibit muchhigher rates of heat conductivity ver-sus steel, which could affect finalpart properties. This effect would beespecially critical in blends or alloysof thermoplastic materials where thecooling rate affects domain size ofthe blend constituent. It is also veryimportant to remember that this fam-ily of mold materials loses physicalproperties, primary tensile strengh,and surface toughness at tempera-tures above 80°C (175°F).
While there are many materials thatproduction molds can and have beenmade from including aluminiumalloys, bronze, and stainless steel,the most common material choice formolding thermoplastic resins is P-20steel. P-20 is chosen for its uniquecombination of toughness combinedwith surface hardness that can befurther enhanced by heat treatmentmethods. P-20 can be machinedusing standard methods; and the lowsulfur grades, through harder tomachine, are typically used becauseof improved surface finish when pol-ishing and photo etching is required.
The cooling rate of the part iseffected by the material of construc-tion. Aluminium and beryllium coppercan conduct heat twice as fast astool steel and four times as fast asstainless steel mold.
4.1 Moisture content of pelletsand drying of the material
In order to produce moldings havinga satisfactory surface and goodproperties, the moisture content ofthe pellets must not exceed a certainvalue. Figure 22 shows the permis-sible moisture level in the pellets forprocessing.
The moisture content in the containersrises as a function of the ambientstorage conditions and time.Especially in the winter months whenthe material is transferred from a coldstorage area into a heated process-ing shed, condensation from thecooling ambient air on the pellets canoccur when the container is openedimmediately. The occurrence of con-densing moisture is prevented bystoring the closed container in theprocessing shed for a certain time orby bringing it to room temperature inan intermediate store.
In order to ensure the production ofcomponents free of streaks and havea flawless surface with a good levelof gloss and properties, the pelletsshould be dried using the times anddrying temperatures listed in Figure22. Circulating air driers are oftensufficient for drying the pellets. Sincedry air can absorb a relatively largeamount of moisture, dry air driers aregenerally more effective.
Partly filled containers have to becarefully closed immediately afterwithdrawal of pellets. The pellet hop-per on the machine has to be cov-ered by the lid.
The pellets should be predried toreduce moisture to an acceptablelevel to assure satisfactory part qual-ity. Directions for predrying can befound in the processing section ofthe relevant product brochure.
3.5.4 DemoldingFor styrene polymers and copolymersdemolding drafts of 1° are generallysufficient. Under no circumstances,however, should they be less than0.5°. The knock-out pins or stripperplates should be designed with aslarge an area as possible so that thefinished part is not deformed ondemolding. This may also allow ear-lier demolding and hence a shortercycle time.
3.5.5 Temperature controlAs early as determining the positionof the molded part in the mold, con-sideration should also be given to theuniform temperature control of themolded part. The effective tempera-ture of the mold at the surface of themold cavity has a substantial effecton the cycle time, surface quality(gloss, brilliance and prominence ofthe weld line), shrinkage and warpageand thus on the tolerance of themolded parts.
Mold cooling is achieved by installingcooling channels in the cavity andcore plates in order to circulate wateror an ethylene glycol solution throughthe mold. The channel diameter andplacement, along with the circulatingcapacity of the cooling system, willdetermine overall effectiveness of thedesign. The bore diameter for coolingchannels are typically drilled toaccept 0.25-0.375 inch pipe. Inter-connecting hoses should have thesame internal diameter to assure theflow rate through the system remainsin the turbulent regime. Turbulent flowachieves three to five times as muchheat transfer as does non-turbulentflow.
The size of the coolant channel, itsposition relative to the surface of themolding, the flow rate and tempera-ture of the coolant are important foruniform cooling of the molded part.Often, segments on the mold surfacemust have their temperature con-trolled by different cooling circuits.Uniform cooling, providing a tempera-ture gradient of no more than 10°Facross the mold, will allow for uniformparts.
3.5.6 Materials of constructionThe choice of material to construct amold is based on several factors.These include the number of parts tobe produced, the size and shape ofthe part, surface durability, cost andmolding force required. Depending onthe gating system used and the wallthickness of the part, thermoplsticmolding can generate pressures ofbetween 2 and 5 tons/in2 in the moldcavity. These pressures can distort orcrack a mold if improperly designed.
Fig. 23: Temperature control profile on the cylinder
240 240
Heater 6 5 4 3 2 1
Temperaturecontrol (°C) 240
255 250 245 240
Fixed
230 220Rising
240 250 240 230 220 210Rising
240 240 240
Decreasing
Drying time Drying Max. permissibletemperature moisture content
of granules(h) (°C) (%)
Circulating air drierPolystyrene*LuranTerluranLuran S 2-4 70-80 <0.1Styrolux 2-4 60 <0.1Luran S/C 2-4 100-110 <0.1Terlux 2-4 70 <0.1
4. Processing advice for styrene polymers and copolymers
Fig. 22: Processing of styrene copolymers
* Normally not required
1514
4.2 Temperature control on thecylinder
The quantity of heat needed for melt-ing the pellets is introduced fromoutside by heating the plasticizingcylinder and also by friction from therotation of the screw. The possibilitiesfor temperature control given inFigure 23 are for setting the bandheater temperatures.
A fixed temperature control profile isused when, as a result of a shortresidence time or utilization of the fullplasticizing capacity, a lot of heat hasto be supplied as quickly as possible.
1716
4.3.1. Special product-specificcharacteristics
BASF polystyrene products should beprocessed between 180 and 270 ºC(356 - 518 ºF). Temperatures above280 ºC (536 ºF) and long residencetimes can cause thermal degradation.A number of molded article propertiescan be controlled by adjusting themelt temperature (Fig. 25).
Styrolux® SBSThe maximum processing tempera-ture for Styrolux is 250 ºC. Above thistemperature, the rubber componentof Styrolux starts to crosslink, caus-ing poorer melt flow and haze in themolded article.
LuranEspecially when processing lightlycolored, transparent Luran formula-tions, changes in the shade of thefinished part can occur when the meltis subjected to excessive thermalstress. This effect, peculiar to SANmaterials, can only be prevented byprocessing under gentle conditions.
Terluran® ABSAt melt temperatures above 260°C(500°F), particularly in the case oflong residence times of the materialin the cylinder or high heat produc-tion through shear, products contain-ing polybutadiene can suffer adecrease in impact strength due tothermal damage.
Luran SBy comparison with Terluran, theLuran S and Luran S/C gradesexhibit higher stability to tempera-ture and processing. Neverthelesswhen processing is carried out inthe upper temperature range, careshould be taken to keep residencetimes as short as possible in orderto prevent any thermal damage tothe material. This may be recog-nized in the case of bright colors bya change in color, usually towards asomewhat lighter shade.
StyroluxStyrolux feeds easily in the barrel.The temperature profile shouldincrease gradually in the direction ofthe screw tip. The feed zone shouldbe cooled. The back pressureshould be only as high as neces-sary. Melt temperatures must notexceed 250 ºC (482 ºF). Excessivetemperatures damage the materialby causing crosslinking, which pro-duces poorer flow and turbidity.Residence times in the barrel shouldbe kept short to prevent sucheffects. In addition to a good moldsurface, transparency and gloss arealso favored by a high injection rate.Note, however, that a high injectionrate may lead to a slight loss inimpact strength.
Terlux® MABSTerlux is slightly cloudy immediatelyafter demolding (while the tempera-ture of the molding is still high). Onlyon cooling does the transparency ofthe product become apparent.Optimum transparency is achievedat melt temperatures of 230–260°C(446-500°F) and with a mold sur-face polished to a high gloss. Longresidence times can cause slightyellowing. Mixtures with any otherthermoplastics result in permanentclouding of the product.
4.3.2. Effect of processing tempera-tures on mechanical strengthDepending on which product isbeing processed, the processingtemperature has a greater or lessereffect on mechanical strength(notched impact strength, see Fig. 26). Particularly noteworthy isthe impact strength maximum ofTerluran GP 22 at 260°C (500 ºF). In
A rising temperature control profile isemployed when residence times arerelatively long. This permits relativelygentle fusion of the material.
The initially rising temperature profilewhich then falls on approaching thenozzle is used primarily for open noz-zles in order to prevent the melt run-ning out or to prevent stringing.The aim of all temperature profileshas to be the setting of that melt tem-perature in the space in front of thescrew which is required for process-ing. By means of the thermocouplefitted in the nozzle head, the actualmelt temperature can be measuredand the temperature profile of theband heaters together with the rotaryspeed of the screw can be corrected.
4.3 Melt temperature
Apart from a few exceptions, styrenepolymers and copolymers can beprocessed over a broad range of melttemperatures.
Figure 24 shows a summary of theprocessing temperature ranges forstyrene polymers and copolymers.
If possible, processing in the mediantemperature range is preferred inorder to protect the material. Tocheck on this, we recommend thatthe melt temperature be measured bymeans of a needle thermometer inthe purged-off melt.
While processing is done in the uppertemperature range, residence timesshould be short in order to preventyellowing of or damage to the mater-ial. Furthermore, when high process-ing temperatures and/or long resi-dence times are used the tempera-ture of the first band heater (close tothe hopper) should be set a littlelower. This will prevent prematuremelting of the pellets in the feed section.
0220
Processing temperature (ºC)230 240 250
ISO
179/
1eA
notc
hed
impa
ct s
treng
th (k
J/m
2 )
260 270 280 290 300
51015202530354045
Terluran GP 22Terluran 967 KLuran S 778 TTerlux 2802 TR
Fig. 26: Notched impact strength as a function ofprocessing temperature
Fig. 24: Processing temperature ranges for styrene copolymers
PS/HIPS
Polystyrene
Luran
Terluran
Styrolux
Luran S
Luran S/C
Terlux
180 190 200 210 220 230 240 250 260 270 280 290 300
Melt temperature (°C)
Fig. 25: Instrumented impact of high impactpolystyrene vs. melt temperature and injection speed
1800
1600
1400
1200
1000
800
600
400
200
200
300
400
500
600
700
800
900
1050
1200
above
4.03.5
3.02.5
2.01.5
1.00.5
0.0 400 420 440 460 480 500 520
Melt Temperature (°F)
Injection Speed (in/sec)
Peak Force (N)
the example of Luran S 778 T, a slightincrease in impact strength can bediscerned as the melt temperaturerises. The highest impact strength isreached at 250 ºC (482 ºF). By com-parison, with ABS, only a marginaldecline in impact strength occurseven at high processing temperaturesup to 280°C (536 ºF).
18
about 0.2 m/s and as far as possi-ble, even in intensive use, notexceed 0.3 m/s. When using ascrew having a diameter of 80 mmfor example, this corresponds to aspeed range of 48 to 72 rpm.
4.6 Back pressure
In order to prevent air intake and toproduce a sufficiently homogeneousmelt, it is necessary to work withback pressure. In doing so, the levelof back pressure should as much aspossible, but not exceed a hydraulicpressure of 10 bar (150 PSI). A backpressure which is too high results inexcessive heating by friction andthus in an uncontrolled and generallyexcessive processing temperature.This can result, for example, in ther-mal damage to the melt and conse-quently in considerable losses in thequality of the molded part.
4.7 Injection pressure / Injectionspeed
A good molding surface can beachieved by filling the molduniformly. To prevent the injectionspeed from falling below the desiredvalue throughout the injectionprocess, a high injection pressurehas to be set. A drop in the injectionrate at the end of the injectionphase indicates that the injectionpressure is too low or the targetspeed is too high.
Marked differences in wall thicknesswhile the injection speed remainsthe same result in nonuniform fillingof the mold and hence in flowmarks. If necessary, the injectionspeed has to be varied in steps orthe component geometry has to berevised.
Especially in the case of thin-walledmoldings, high injection speedsshould be chosen. As a general rule,high injection speeds result in ahigher melt temperature and ahigher surface gloss (Fig. 28). Theybring about improvements in flowa-bility and especially in the case ofglass-fiber reinforced productimprovements in surface quality.High injection speeds give rise to amore uniform solidification process.In many cases, weld lines are lessvisible and the impact strength ofthe products is enhanced.
At a higher injection speed, thepressure requirement rises and thedemands on mold venting becomemore stringent. In the case of a rela-tively cold melt, there is a danger ofdamage to the melt and of higherstresses in the molding.
4.4 Mold surface temperature
The surface temperature of the moldis among the most important para-meters in the injection moldingprocess. Figure 27 presents BASF’srecommendations for the mold sur-face temperatures. Apart from sur-face quality, (gloss and weld linemarks) the effective mold surfacetemperature also affects the mechan-ical properties, weld line strength andthe dimensional tolerances of themolded part (Fig. 29). Locally differ-ent mold temperatures result in differ-ent rates of cooling of the moldingand hence in differences in shrinkagecaused by cooling and in warpage.
In general a higher mold surface tem-perature brings about slower freezingof the melt. Shrinkage can be com-pensated for with longer coolingtimes resulting in lower residualstresses.
Figure 28 shows the effect of themold surface temperature (MST) onthe surface gloss of a molding madefrom Terluran 967 K. Increasing themold surface temperature from 30°Cto 60°C (86°–140°F) resulted in thecase of a test mold with an appropri-ately polished surface in an increasein gloss of 24% (melt temperature230°C).
However, in the case of Terluran, ahigh melt temperature in combinationwith a low injection rate and low moldsurface temperatures can result indistinct deterioration of the surfacegloss. Increasing hold pressure andmold temperature can have a signifi-cant effect on peak force (Fig. 29).
4.5 Peripheral screw speed
The rotary speed of the screw can beselected in such a way that the timeavailable in the cycle for plasticizationis utilized to the greatest possibleextent. As Figure 30 shows theperipheral screw speed VU should be
Screw diameter D (mm)
Scre
w s
peed
n (m
in-1
)
Vu=0.05 m/s
0 40 6020 80 100 120 140 160
40
200
80
0
120
160
Vu=0.2 m/s
Vu= n·D·π60 000
(m/s)
Fig. 30: Recommended screw speed as a func-tion of screw diameter and peripheral screwspeed (VU)
Melt temperature (°C)
Glo
ss (%
)
0.4s
0.4s2.0s
2.0s
MST 60°C
MST 30°C
230
50
70
255
60
28040
Fig. 28: Gloss forthe example ofTerluran 967 K
0 10 20 30 40 50 60 70 80 90 100Mold surface temperature (°C)
Polystyrene
Luran, Luran S
Terluran
Luran S/C
Terlux
Styrolux
Fig. 27: Recommended mold surface temperatures for styrene copolymers
Fig. 29: Instrumental impact of high impact polystyrenevs. hold pressure and mold temperature
100
80
60
40
20
30
40
50
60
65
70
75
80
85
90
above
13001200
11001000
900800
700600
500400 105 1
15 125 1
35 145 1
55
Mold Temperature (°F)
Hold Pressure (PSI)
Peak Force (N)
19
21
4.9 Cooling time
The criterion for an optimum coolingtime is the demolding of the injectionmolded part without any lasting orsubsequent deformation of the com-ponent. The cooling time can bedetermined for many geometries withthe aid of computer programs.
4.10 Shrinkage and warpage
The shrinkage of a component isdetermined both by the nature of thematerial as well as by the processingconditions and the geometry of themolding (shape of the molding, wallthickness and position of sprue).Interactions among these factorsmake it difficult to predict the exactshrinkage.
Accordingly, the shrinkage values setout in the material data sheets applyonly to the specified processing andmold surface temperatures and thegeometry used for the component. Atthe same time, a distinction has to bemade between the values for free andimpeded shrinkage parallel and atright angles to the direction of flow ofthe melt.
At 0.3–0.7%, the free shrinkage ofamorphous materials is smaller by afactor of three or more than that ofpartially crystalline plastics which is1–3%. Figure 32 shows the valuesof free shrinkage for the processingtemperatures in question. These weremeasured on a 110 x 110 x 2 mm(4.3” x 4.3” x 0.08”) plaque.
Shrinkage also affects molded partsin two ways other than part dimen-sions, namely sink marks and voids.Both of these molding problems arecaused by the shrinkage of the hotmelt as it cools on the inside of themolded part. In the case of sinkmarks, as the polymer cools andshrinks, material is pulled away fromthe wall of the part, creating adepression in the surface of the part.Voids occcur from the same rootcause, shrinkage of material on theinside of the part; but in this case, thewalls have frozen solid and cannot bepulled away from the walls. As thematerial shrinks and volumedecreases, a void is created on theinside of the part. Correction of bothof these problems requires increasedpacking of the molded part to com-pensate for the shrinkage. Often,mold design changes are required toavoid thick sections in the part andminimize shrinkage to acceptablelevels.
The processor can control shrinkagesubstantially by selective changes inthe holding pressure and in the maxi-mum effective holding pressure timeup to the gate sealing point. The aimshould be to compensate for thecontraction in volume occurring dur-ing the freezing process for as longas possible by feeding melt throughthe holding pressure and holdingpressure time.
A high mold surface temperatureaffects the cooling rate of the moldingand thus prolongs the possible dura-tion of action of the holding pressure.On the other hand, the injectionspeed and melt temperature haveonly a minor influence on the courseof shrinkage.
4.8 Holding pressure / Holdingpressure time
The holding pressure is intended tocompensate for the shrinkage of thesolidifying plastic. For this purpose itis important that the changeover tothe holding pressure occurs exactlyat the volumetric filling of the cavity.
Depending on the geometry of thecomponent, voids and sink markscan be very largely balanced out bychoosing an appropriate holdingpressure value and a holding pres-sure time. On account of their amor-phous structure, the holding pressuretimes for styrene polymers andcopolymers are distinctly lower thanfor partially crystalline polyamide forexample.
If the geometric design or choice ofgate are unsuitable, an extremelyhigh holding pressure or an extremelylong holding pressure time can resultin overfeeding of the moldings andhence in a lower impact strength ofthe components (Fig. 31). As a rule,high stresses can be demonstrated,especially in the sprue region andthese are frequently the cause ofpremature failure of the molded part.This can give rise to demolding prob-lems. The problem of overfeeding canpossibly be minimized by means of astaged holding pressure profile.
0.3–0.7%
0.4–0.7%
0.4–0.7%
0.3–0.7%
0.4–0.7%
Luran
Terluran
Luran S
Luran S/C
Terlux
Shrinkage values measured at average processing and mold temperatures*Free processing shrinkage in sheet measuring 110 x 110 x 2 mm
Fig. 32: Processing shrinkage* of styrene copolymers
Fig. 31: Relationship between fracture energyand holding pressure
Frac
ture
ene
rgy
(Nm
)
A B C
10
20
0
30
Terluran 967 KMelt temperature = 250°CMold temperature = 60°CWall thickness = 1.5 mm
600 bar800 bar
A
CB
20
Warpage in the molding is generallycaused by differences in shrinkage inthe direction of flow transversely toflow and also by different wall thick-nesses in the component. Since theholding pressure acts for very muchlonger close to the sprue than farfrom it, the different shrinkage alongthe component can also give rise towarpage.
The processor has the possibility ofcontrolling warpage of the compo-nent by means of differential temper-ature control on the two halves of themold as well as by means of partialtemperature control of individualparts of the mold.
5.1 Multicomponent injectionmolding with styrene and styrenecopolymers
The multicomponent injection mold-ing technique makes it possible toselectively combine different productshaving their own specific properties ina molding. Styrene copolymers incombination with different thermo-plastics and thermoplastic elastomersare also highly suitable for this specialprocess in injection molding technol-ogy.
The processes combined under thegeneric heading of multicomponentinjection molding include injectionmolding. Here, two plastic melts areprepared in different units andinjected one after the other through agate into the mold cavity. The result isa skin, or outer component (firstinjected component) and a core com-ponent (material which flows after thefirst component). A molding with asandwich-like structure is produced(Fig. 35).
This three-layer structure has theadvantage that the core componentis usually completely enclosed by theskin component. In this way it is pos-sible to produce moldings havingspecific properties matched to theapplication in question. For example,these can exhibit on the one hand avery good surface quality and on theother hand possess a reinforced orexpanded core. Typical applicationsof the process are thick-walled arti-cles, such as handles, and housings.
The distribution of the core compo-nent in the molded part usually has adirect effect on the quality of the fin-ished part. It is controlled by theflowability of the products, the pro-cessing parameters, the positioningand design of the sprue and by theshape of the molding. In order toensure adequate utility of the mold-ings, the components must be com-patible with one another.
If, for example, they do not adhereadequately to one another, delamina-tion of the layers can result in areduction of functionality or service-ability. On the basis of experience,styrene copolymers undergo compa-rably good bonding to one another.On the other hand, a materials com-bination involving styrene copolymerswith polypropylene for example is notrecommended.
A increasing number of new applica-tions are produced by a further multi-component injection moldingprocess, sandwich injection moldingor over-molding. In this process, pre-moldings produced out of the firstcomponent are first manufactured.On completion of the setting time,these are covered in part with at leastone further product (Figs. 36 and37).
22
4.11 Optimization of the injectionmolding process
Properties of a molding are frequentlyaffected by changes in individualmachine and mold as well as theprocess parameter adjustments dueto the machine/mold changes. Byway of example Figure 33 shows inwhich phases of the injection moldingprocess the key properties of a mold-ing can be changed. Figure 34 illus-trates the magnitude of these interac-tions on part properties and processresponses.
Here the key process and mold para-meters are the melt temperature, themold surface temperature, the injec-tion speed and the pressure in themold cavity. Measurement of thesevariables during the injection processsimplifies not only the adjustmentprocess but also shows potentialproblems in production at an earlystage.
5. Special processes
Fig. 35: Co-injection molding
A = skin component, B = core component
AB
A = skin component, B = core component
A B
Fig. 37: Lid for a mixing cup made from Luran 388 S(transparent) and Luran S 778 T (white ring)
Time (s)
Inte
rnal
mol
d pr
essu
re (b
ar)
0 1 2 3 4 5
100
700
300
0
400
600
200
500
Shaping of contours,flash formation, webbing
Voids, sink marks,weight, shrinkage,warpage, internalstresses, demoldingdifficulties
Stresseson melt,qualityof surface
Injectionphase
Compressionphase
Holding pressurephase
Fig. 33: Modification of the properties of themolding in the injection molding process
Fig. 34: Effects of Injection Molding Parameters on BASF HIPS and the molding process.
1100.0
584.72300.00
Melt Temperature Mold Temperature Injection Speed Hold Pressure
1100.0
507.53
100.00
12.500
10.926
9.0000
95.00072.430
30.000
410 500455
110 150130
0.5 3.52.0
500 1200850
Glo
ss 6
0°In
st. I
mpa
ctSc
rew
rec
over
y
Peak
Pre
ssur
e
Fig. 36: Composite injection molding
23
5.2 Internal gas pressure process
The internal gas pressure method(IGP), also known sa gas assist, wasdeveloped in the middle of the seven-ties. It has been used on a large scaleonly since the middle of the eighties.
The principle of the IGP method is theselective production of cavities atpoints in the molding having greatwall thicknesses by the injection ofinert gas – nitrogen as a rule – intothe cavity partially or completely filledwith plastic melt. The gas can beintroduced by injection needles bothvia the machine nozzle as well as viaa gas nozzle installed directly in themold. In principle, all thermoplasticsused in conventional injection molding,i.e. including styrene polymers andcopolymers, can be processed byIGP. Figure 38 shows an example ofthis.
Depending on the type of plasticmolding to be produced, a choicecan be made between the inflationmethod and the blowing out method.
The decision whether to use IGP orconventional injection molding is sub-stantially determined by the moldgeometry. In order to make optimumuse of the advantages of the IGPmethod, it is necessary to start think-ing differently as early as the designof the component and the mold. Thefactors affected are: position andlayout of the sprue, design of wallthicknesses, use of ribs and diversionpoints for the melt.
The following advantages relative toconventional injection molding cometo the forefront when the IGPmethod is used.
Greater design freedom in the development of the molding.
Simpler, more suitable injection molds.
A great level of integration, i.e. the production of one part instead of two or more individualparts which subsequently have to be assembled.
Prevention or minimization of sink marks.
More uniform shrinkage, lower internal stresses and less warpage on account of the uni-form distribution of gas pressure in the molding.
Optimization of weight or mater-ial consumption while the rigidity of the molding is relatively high.
Reduction of cycle times.
Use of machines having a lower clamping force.
On the other hand, the followingdisadvantages can occur.
Gas spreading and the hollow space formed is not predictable. Appropriate computer simula-tions, such as FEM programs for example, can provide indications.
The gas spreading, size of the hollow space and the resultant residual wall thickness can only be affected within certain limits by the process parameters such as melt temperature, mold sur-face temperature, gas pressure and gas delay time.
Additional resources are needed to carry out the process. These include gas pressure generator and pressure control modules, process gas and injection nozzles.
Patent situation (license fees).
Injection molds must be made with great precision and be perfectly balanced – this applies especially to multicavity molds –in order to ensure uniformspreading of the gas.
All injection molding machines com-monly available on the market aresuitable for the IGP method.
The manufacturers of gas systemssupply different systems, such asAirmould, Airpress, Cinpress, Gain,etc. Some systems or variants ofthe process are protected bypatents. For that reason, theprocessor should obtain compre-hensive information about thepatent position before starting technical utilization.
The multicomponent injection mold-ing machines usually used are fittedwith a number of units correspondingto the number of components. Thesespecial multicomponent injectionmolds, depending on the part to beproduced, can have a very complexstructural design because premold-ings and finished moldings are pro-duced in one mold. Tried and testedmold concepts for this are rotary andsliding split molds. In some cases,the premoldings are positioned inwhat is known as the final injectionstation with the aid of a handlingdevice. Composite injection moldingis also possible, however, using con-ventional molds and machines. In thiscase, the premolding is placed in asecond mold and co-molded. Thetype of mold and machine employeddepends on the part geometry, thepiece numbers and the planned com-bination of materials.
The good bonding of the participatingcomponents required in most casesis ensured when styrene copolymersare used both in composite injectionmolding as well as in sandwich injec-tion molding (usually as rigid/rigidcomponents). In rigid/flexible applica-tions, thermoplastic polyurethanes(TPE-U), for example, which undergogood bonding to Terluran and Luran,are suitable for the flexible compo-nent. Known applications are hous-ings having a direct mold-on seal orhandles which have injected overtheir surface a flexible component toimprove their feel.
Since the bonding strength isaffected by many factors and is difficult to predict, preliminary trialswith test molds and prototype moldsis frequently recommended to deter-mine suitable combinations of materials or geometries of the bondingsurfaces.
If the processed components do notadhere sufficiently well to oneanother, what are known as clampingmechanisms can also increase bonding strength. In this case, theundercuts or openings provided inthe premolding are permeated duringthe subsequent overmolding by themelt of the second component andmechanically coupled. The disadvan-tage is the higher effort in mold engineering by comparison withbonding achieved by adhesion.
Fig. 38: External automobile mirror made fromLuran S 778 T
Cavity
24 25
26
Plastics Line of the BASF Corporation
Products of BASF CorporationTerluran® Acrylonitrile/butadiene/styrene copolymer ABS Luran® S Acrylonitrile/styrene/acrylate copolymer ASA, (ASA + PC) Luran® Styrene/acrylonitrile copolymer SAN Terlux®* Methyl methacrylate/acrylonitrile/butadiene/ MABS
styrene copolymerCrystal PS Polystyrene PS Avantra®*, high-impact PS Styrene/butadiene polymer HIPS Styroblend® Blend based on styrene/butadiene polymer HIPS blend Styrolux® Styrene/butadiene block copolymer S/B/S Ultradur® Polybutylene terephthalate PBT, (PBT + ASA)Ultraform® Polyoxymethylene POM Ultramid® Polyamides PA 6, 66, 6/66, 6/6TUltrason® E Polyethersulfone PES Ultrason® S Polysulfone PSU Styropor® Expandable polystyrene EPS Neopor® P Expandable polystyrene EPP Neopor® E Extruded rigid polystyrene foam EPE ® = registered trademark of BASF Aktiengesellschaft®*= registered trademark of BASF Corporation
NoteThe information submitted in this publication isbased on our current knowledge and experience.In view of the many factors that may affect processing and application, these data do notrelieve processors of the responsibility of carryingout their own tests and experiments; neither dothey imply any legally binding assurance of certain properties or of suitability for a specificpurpose. It is the responsibility of those to whomwe supply our products to ensure that any proprietary rights and existing laws and legislation are observed.