processing data for the injection molder - mito polimeri processing data for the injection...

Processing data for the injection molder

2

Contents Page

1 Product overview 3

2 Preparations for production 4 – 72.1 Drying 4 – 52.2 Cleaning; material changeover 6 – 7

3 Machine selection and component options 8 – 193.1 Determination of screw diameter,

shot weight and metering stroke8 – 11

3.2 Suitable and feasible metering strokes 123.3 Determination of clamping force 13 – 143.4 Screw geometry 153.5 Nozzles 163.6 Protection against wear 17 – 19

4 Processing conditions 20 – 354.1 Mold and melt temperature 20 – 214.2 Temperature control of the mold 22 – 244.3 Melt temperature sensor 254.4 Injection and holding pressure;

injection speed26 – 28

4.5 Screw speed; back pressure 294.6 Cooling time 30 – 334.7 Optimization of machine settings;

production monitoring34 – 35

5 Processing reclaim; recycling 36

6 Measures for the elimination ofmolding faults

37 – 48

7 Product range 49

Processing data for the injection molder

3

1 Product overview

The purpose of this booklet is to give the injection molder a quick rundown on the processing of the complete range of en-gineering thermoplastics produced by Covestro.

Apec® PC-HT

Bayblend® PC+ABS; PC+SAN-I; PC+ASA

Desmopan® 1) TPU

Makrolon® PC

Makroblend® PC+PBT; PC+PET

For more detailed information regarding specific applications, please contact the experts at the Business Unit.

1) See brochure entitled „Processing Desmopan“

4

2 Preparations for production

2.1 Drying1)

If the polymer melt has an excessively high moisture content, this can result in surface defects (streaks) and impaired molded part properties (hydrolytic degradation). Since the majority of plastics absorb excessive water through non-moisture-proof packaging, storage and transport, these materials must be dried prior to processing.

Engineeringthermoplastic

Permissible residual moisture contentin % by weight (injection molding)

Apec® 0,02

Bayblend® 0,02

Desmopan® 0,02 to 0,05

Makrolon® 0,02

Makroblend® 0,01

Table 1: Permitted moisture content of granules

The moisture content of the granules should be measured by the Karl Fischer test method, or another appropriate method. If the permitted moisture content is exceeded, the finished part may still have suffered material degradation, even if it looks perfect on the surface. This applies in the case of Apec®, Bayblend®, Desmopan®, Makrolon® and Makroblend®.

1) See Technical Information Sheets entitled: „Determining theDryness of Makrolon® by the TVI Test“; „Injection Molding ofHigh-Quality Molded Parts – Drying“

5

Engineeringthermoplastic

Dryingtemperature

in °C

Drying time (h)Circulating

dryer(50 % fresh

air)

Fresh airdryer (high

speeddryer)

Dry airdryer

Apec® 130 4 to 12 2 to 4 2 to 3

Bayblend® 100 to 110 4 to 8 2 to 4 2 to 4

Bayblend® FR1) 75 to 100

Desmopan®2) 60 to 110 1 to 3 1 to 3 1 to 3

Makrolon® 120 4 to 12 2 to 4 2 to 3

Makroblend®PC+PBTPC+PET

100 to 105110

4 to 124 to 12

2 to 42 t0 4

2 to 42 to 4

1) Drying temperature 10 °C < Vicat2) Depending on the hardness

The above data apply to containers that have been stored at room tempe-rature. In the event of stoppages lasting four hours or more, we recom-mend reducing the temperature of the dryer by 40 °C.

See also Technical Information Sheet: Injection Molding of High-Quality Molded Parts – Drying

Table 2: Recommended drying conditions

The times specified above apply to containers that have been stored at room temperature. It is also assumed that the drying equipment is in perfect working order and that the recommend-ed drying temperature is observed.

6

2.2 Cleaning; material changeover

1) See also Technical Information Sheet: „Purging compounds for use when molding thermoplastics“

2) • When changing from a high-viscosity to a very low-viscosity material • When changing from a material that forms a boundary layer to one

that does not • When the production of transparent parts is planned3 ) For certain FR grades use a non-flame-retardant grade of the same

material

Material changeover

Apec®Bayblend®Desmopan®Makrolon®Makroblend®

• Empty the plasticating cylinder• Purge the cylinder with the new material or

with a mixture of the new material and a spe-cial purging compound1)

• When changing colors always change from light colors to darker ones where possible

• In special cases2), clean the plasticating unit (see under “Cleaning”)

• Start the cleaning at a high cylinder tempe-rature. After the material change, reduce the temperature to the level specified by the manufacturer

Production stoppages(prolonged interruptions and over the weekend)

Bayblend®Desmopan®Makroblend®

• Empty the plasticating cylinder3)• Move screw as far forward aspossible• Switch off the machine and heating system

Apec®Makrolon®

• Empty the plasticating cylinder• Set cylinder heaters to between 160 and

180 °C and ensure continuous heat soak• Empty hopper

7

End of production


• Purge plasticating cylinder with an appropriate high-viscosity molding compound, e.g. Bayblend® natural

• With low-viscosity resins such as PP, there is a risk that PP residue will remain in the cylin-der on re-starting and cause streaks in the molded part.

• Where appropriate, clean plasticating unit (see under “Cleaning”)

• For TPU purge screw with Desmopan® 385

Cleaning


• Cleaning/purging prior to a change of materi-al without severe encrustation of the plasti-cating unit (see under “Material changeover”)

• When transparent Makrolon® is used, clean-ing is best carried out with a higherviscosity, transparent Makrolon®

• Cleaning in the case of severe encrustation (e.g. boundary layers adhering to the wall)

• Pre-clean unit with cylinder cleaning agent1)• Additionally purge unit with special purging

granules where necessary• Dismantle unit and clean components with

steel brush while still hot, then polish with a cloth and polishing paste. Do not use emery paper!

• By way of an alternative, the dismantled components can also be cleaned in alu-minum oxide vortex baths, oil baths or the appropriate solvent baths (with the assis-tance of ultra-sound, if required)

• Warning: subsequent “blasting” with glass or steel spheres will damage the surface of the steel parts

1) See also Technical Information Sheet: “Purging compounds for use when molding thermoplastics“

8

3 Machine selection and component options

3.1 Determination of screw diameter, shot weight and metering stroke1)

When molding parts with a defined shot weight, it is best to useonly screws from a specific size range (diameter range), whichwill ensure that the metering stroke works out at between 1 x Dand 3 x D (D = diameter). Conversely, a screw of defined diame-ter should only be used to produce articles from within the cor-responding weight or shot volume range.If the machine size falls outside of this range, the quality of themoldings can be impaired through molecular weight reductionsor through surface defects caused by air trapped in the screw(see Fig. 3).The nomogram below shows the correlation between shotweights and efficient screw diameters.This can be used to establish the screw diameter (machine size)and the planned part weight when processing thermoplastics on injection molding machines. The nomogram is based on find-ings as to the optimum metering stroke (for a metering range of 1 to 3D) for three-zone screws with an L/D ratio of between 18:1 and 22:1 (see also Fig. 2).

1) See also Technical Information Sheet: Relationship between screw di-ameter, metered volume, density and shot weight. This contains an en-larged version of the nomogram explained below.

9

Explanation of the nomogram (Fig. 1)

Reference line *1 relates to a metering stroke of 1D and refer-ence line *2 to a metering stroke of 3D.Taking as an example a part with a weight of 2,500 g including the sprue, the nomogram shows that this part can be injection molded with a minimum screw diameter of 100 mm (using themaximum metering stroke of 3D) and a maximum screw diame-ter of 150 mm (using the minimum metering stroke of 1D). A melt density of 0.85 g/cm3 is taken as the basis here. If the density increases, the requisite screw diameter will be reduced.The two density reference lines that are drawn in cover the range of melt densities from unfilled thermoplastics through to thermoplastics with a high filler content and hence take in the en tire current range of Covestro thermoplastics. It is, of course, always possible to draw in the density line of a special product. If the melt density of a molding compound is not known, it can be established on an approximate basis from its density at room temperature. To do this, the density at room temperature should be multiplied by 0.85 for unfilled melts and by 0.95 for highly filled melts (filler content approximately 60 %).

10 11Bild 1: Bestimmung von Schneckendurchmesser, Schussgewicht und

Dosierweg

*2 Thermoplast hohe Dichte*1 Thermoplast niedrige Dichte

Erläuterungen zum Nomogramm siehe Seite 9.

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3.2 Suitable and feasible metering strokes

1D to 3D: optimum range3D to 4D: possible in exceptional cases< 1D and > 4D: not recommended

Fig. 2: Suitable and feasible metering strokes for injection molding screws

1D

< 1D > 4D

2D 3D 4D

Screw

Fig. 3: An excessively long metering stroke leads to the intake of air

After metering

MeltAir

GranulesAir

1D 2D 3D 4D

MeltAir

Residual granules

MeltMelt

After injection

Melt

After the start of the next metering stroke

Melt

GranulesAir

MeltAir

Residual granules

MeltAir

Residual granules

MeltAir

MeltResidual granules

Granules Air

13

3.3 Determination of clamping force

General formula:

The actual clamping force required is determined first and fore-mostby the two variables included in the formula. Over and above this, the clamping force is influenced by other factors, such as the rigidity of the machine and the mold, the design of the molded part, the permitted breathing, the processing parameters and the molding compound itself.

The empirical values set out below in Table a) are thus onlyintended as a guide. Projected surface = sum of all the surfaces subject to pres-

sure projected on to the plane of the clamping platen Exam-ple: truncated cone-shaped disc

Clamping force ≥ mold opening force in kN =projected surface in cm2 · mean cavity pressure in bar

100

d D

Projected surface A Molded part

Projected surface: A =( D2 – d )24

.

Fig. 4: Projected surface of a molded part (schematic diagram)

14

Mean cavity pressure

a) Values from actual experience

Apec® PC-HT 300 bis 500 bar

Bayblend® PC+ABS; PC+SAN-I; PC+ASA

250 bis 500 bar

Desmopan® 1) TPU 300 bis 700 bar1)

Makrolon® PC 300 bis 500 bar

Makroblend® PC+PBT; PC+PET 250 bis 500 bar1)

1) For material grades with very good flow behavior, it may be necessary to use the higher pressures (greater clamping force) in order to pre-vent flash formation.

b) Values from rheological calculations If a filling pressure of 700 bar is taken as a basis for the cavity when the design of the molded part is worked out, then, in the case of amorphous thermoplastics, a mean opening pressure of some 500 bar can be em-ployed for the calculation, as is shown in the diagram that follows.

Example:

800700

Switchover pressure2)

Filling pressure from rheological calculation

500 bar450 bar

100 bar

Cavity pressure over flow path [p]

Fill

ing

pres

sure

at

gat

e [f

]

Pressure at gatePressure at end of flow path

= 800 bar= 100 bar

= 900 bar : 2 = 450 bar~ 500 bar

Fig. 5: Determination of the mean cavity pressure (mold opening pressure) for wall thicknesses of up to approx. 3 mm

Pressure at gate = 800 barPressure at end of flow path = 100 bar = 900 bar : 2 = 450 bar ~ 500 bar

2) Safety allowance for melt compression prior to switchover to holding pressure

15

3.4 Screw geometry1)

Three-zone screws with an L/D ratio of between 18:1 and 22:1and a flight depth ratio of 2:1 to 2.5:1 are suitable for use withCovestro thermoplastics.

Metering stroke 4D max./1D min.

Meteringzone20%

Compres-sion zone

20%* Feed zone 50–60 %

Screw length (according to EUROMAP)Residual length halved

* Longer compression zones are preferred for Desmopan® (see Desmopan processing brochure)

Fig. 6: Three-zone screw

1) See also Technical Information Sheet: „Injection Molding of Quality Parts – Production Equipment/Machinery“

Fig. 7: Correlation between screw diameters, flight depths and flight depth ratios

Feed zones

For amorphous thermoplastics

For semi-crystalline thermoplastics

**

**

Flig

ht d

epth

H (m

m)

Screw diameter D (mm)

16

12

10

100 120 160

8

6

4

2

00 20 40 60 80 140

14

Constant flight depths

Flight depth ratios

2.5:12.5:12.4:12.3:12.3:12.2:12.1:12.0:1

*

*

1.9:1 2.0:1 2.1:1 2.1:1

Compression zone

H~D0,7

Metering zone

16

3.5 Nozzles

Open nozzles should be used wherever possible. Shut-off noz-zles can be used for easy-flow materials, although these can more easily lead to problems such as material degradation, specks and malfunctions, depending on the design involved (see also key points set out below). Care should be taken to ensure that the nozzle aperture and gate diameter are correctly matched on all nozzles.

Guide values:Nozzle aperture = gate diameter minus 0.5 to minus 1.0 mm

Of the shut-off systems available on the market: sliding shut-off nozzles, needle valve nozzles, bolt-type shut-off nozzles,

the hydraulically operated bolt-type shut-off nozzles seldomcause problems. Particular attention should be paid here toerrors in the channel hole alignment in the bolt (in the openposition).Spring-loaded needle systems lead to higher injection pres-sure requirements and to higher short-term material shear.Mechanically controlled sliding shut-off nozzles and systemsthat are hydraulically or pneumatically driven on both sides donot suffer from these disadvantages.The success of all needle and sliding shut-off nozzle systemsdepends to a large extent on the melt channel being suitablydesigned in flow engineering terms (no dead spots or flowdivisions).On all shut-off systems, movable actuating elements should beprovided with a certain amount of “play” so as to permit “melt lu-brication” and an intentional, slight leakage flow to the outside.

17

3.6 Protection against wear

As with all mechanical equipment, the plasticating unit is subject to wear when thermoplastics are processed (partic-ularly those containing fillers or pigments). A basic distinc-tion is drawn here between abrasion and corrosion. These can occur in isolation and also together.Wear on components is frequently only detected at a late stage, once malfunctions occur. In many cases, however, this wear will have started to affect the molded parts much earlier on, through discoloration of the surface or similar defects. Sometimes these defects are located inside the molding and are not visible on the surface.High costs are incurred not only through replacing worn, unserviceable machine components, such as screws, cylinders and non-return valves, but also through reject production parts and the reduced availability of the ma-chines due to stoppages and repair time.Screw and cylinder components made of “standard nitrid-ed steels” frequently no longer meet the increasingly strin-gent requirements here. The geometrical design of the components is also a crucial factor.“Wear-protected” plasticating units are now available, which fulfill the current specifications much more effec-tively. Experience has shown that the throughput-based cost of wear on such plasticating unit components can be reduced by a factor of 3 to 6 or more. This is without taking into account the additional improvements that are achieved in terms of more costefficient production (fewer rejects), reduced machine stoppage time and more consistent quality. When selecting the grade of steel and the surface treatment method to be used for wear and corrosion-pro-tected plasticating units, knowing which of the two wear mechanisms predominates can be decisive. As a rule, it is best to select a “universal” type of protection that will cope with both sorts of wear. The following table contains advice on the type of material that should be selected.1)

1) See also Technical Information Sheet: „Injection Molding of Quality Parts – Wear Protection for the Plasticating Unit“

18

Material selection for wear-protected injection molding units (universal protection against corrosion and abrasion)

Cylinders1. Centrifugal deposition of a suitable wear-resistant coating, gene-

rally Fe-Cr-Ni-B-based; unalloyed C steels and Cr-V alloyed special steels for the supporting tubes

2. Insertion of centrifugally coated bushes; supporting tube in nitri-ded steels, e.g.

34 Cr Al Ni 7 (1.8550) 31 Cr Mo V 9 (1.8519)3.Boride diffusion layers, small diameter4.PM-HIP materialsScrews1. High Cr-alloyed depth-hardened (up to about 60 mm diameter and

1500 mm in length), additionally ionitrided in some cases, e.g. X 155 Cr V Mo 12 1 (1.2379) X 165 Cr Mo V 12 (1.2601) X 210 Cr 12 (1.2080) X 220 Cr Mo 12 2 (1.2378) X 210 Cr W 12 (1.2436)2. Stellite hard facing for screw flights with ionitrided Cr steels for all

diameters, e.g. X 35 Cr Mo 17 (1.4122) tempered X 22 Cr Ni 17 (1.4057) tempered3. Stellite hard facing for flights and chromium-plated root surface

and flanks, e.g. 31 Cr Mo V 9 (1.8519)4. Boride diffusion layers, small diameter5. All-hard-metal coated screw contour and PM-HIP materialsCylinder head1. High-alloy Cr steels, ionitrided (see “2” under “Screws”)2.Standard nitrided steels, hard chromed, e.g. 31 Cr Mo V 9 (1.8519)Non-return valve1. Tip and back-up ring – Flights of wing tip 1.1 High-alloy Cr steels, ionitrided if necessary (see “2” under

“Screws”) 1.2 High Cr-alloyed depth-hardened steels

(see “1” under “Screws”)2. Locking ring – High Cr-alloyed steels with good toughness,

through-hardened or tempered/ionitrided, e.g. X 155 Cr V Mo 12 1 (1.2379) X 40 Cr Mo V 5 1 (1.2344) X 35 Cr Mo 17 (1.4122)3. All structural elements made of – Hard materials or – Boride or – CVD1)/PVD2)-coated.

1) Chemical Vapour Deposition 2) Physical Vapour Deposition

19

Fig. 8:Non-intact sealing face on the front end of the screw with degraded melt right up to the threaded hole

Fig. 9:Molding displaying pronounced discol-oration on account of degraded melt

Sealing faces: Nozzle, nozzle head and non-return valve

One frequent cause of wear problems is non-intact sealing fa-ces inside the plasticating unit. Melt that penetrates the gaps inthese non-intact sealing faces becomes damaged (dead spots,residence time and temperature) and is picked up again by thenew melt flowing past it. The damaged melt can then lead todark streaks, cloudiness or specks in the molded parts. When assembling the plasticating unit, bedding-in paste (ap-

plied as thinly as possible) should be used to ensure that the sealing faces are fully in contact with each other.

Attention should be paid to the detailed instructions supplied by machine manufacturers on correct assembly of the indivi-dual components, such as the cylinder head and the nozzle.

20

4 Processing conditions

4.1 Mold and melt temperature

The figures given for mold and melt temperatures in the table below apply to all the injection molding grades of that particular-thermoplastic (except specialty products) and can therefore-serve only as a guide. Generally speaking, the melt temperature for easy-flow grades will be taken from the bottom of the range or, for more viscous grades, from the top of the range. Long re-sidence times in the plasticating cylinder, which are due to long cycle times or to under-utilization of the shot volume, require a reduction of the melt temperature in order to prevent thermal degradation.

Thermoplastic Moldtemperature

in °C

Melttemperature

in °C

Apec® 100 to 150 310 to 340

Bayblend® 1) 70 to 100 240 to 280

Desmopan® 20 to 50 190 to 245

Makrolon® 80 to 100 280 to 320

Makrolon® GF 80 to130 310 to 330

Makroblend® PC+PBT 60 to 80 250 to 270

Makroblend® PC+PET 60 to 80 260 to 280

1) The higher values should be used with high PC contents

Table 3: Recommended mold and melt temperatures

It should be noted that the melt temperature often differs con-siderably from the setpoint cylinder temperature, as a function of the screw geometry and operating conditions (speed, back pressure, injection time etc.). In the event of temperature- dependentdifficulties being encountered, the melt temperatureshould be measured (see Section 4.3).

21

Under the recommended processing conditions, small quantities of decomposition product may be given off during processing.

To preclude any risk to the health and well-being of the machine operatives, tolerance limits for the work environment must be ensured by the provision of efficient exhaust ventilation and fresh air at the workplace in accordance with the Safety Data Sheet.

In order to prevent the partial decomposition of the polymer and the generation of volatile decomposition products, the pre-scribed processing temperatures should not be substantially exceeded.

Since excessively high temperatures are generally the result of operator error or defects in the heating system, special care and controls are essential in these areas.

22

4.2 Temperature control of the mold

The mold temperature has a decisive influence on molded partquality. This applies especially to such properties as inherentstresses, warpage, dimensional tolerances, weight and surfacefinish. The cooling time is also determined to a large extent bythe mold surface temperature. It is only possible to comply with production specifications, and particularly with dimensional tolerances, if a defined mold tem-perature is maintained. As a rule, the heating/cooling equipment employed to this end can only ensure a constant mold tempera-ture, at a specific level, with certain limitations. First of all, the ca-vity surface is heated up by 5 to 15 °C during the injection phase when it comes into contact with the melt. By the time the next in-jection cycle commences, this temperature increase will have been offset once again through the removal of heat. With a steady -state cycle, therefore, a periodic temperature fluctuation will re-sult (a “sawtooth” profile). During production start-up, however, the mold temperature will increase for a certain period of time, until a state of equilibrium has been a chieved between the sup-ply and the removal of heat. This temperature can be 10 to 30 °C higher than the setpoint value on the temperature control unit. It also has the control fluctuation of the temperature control unit superimposed upon it, which can be quite considerable at times.

emp

erat

ure

[°C

]

Cavity wall temperature

Medium input temperature

T

Time t [min]

120

100

90

80

70

606 8 10 12 14 16 18 20 22 24

110

Fig. 10: Profile of the equilibrium temperature at the cavity wall following production start-up

23

6-

4-

2-

6-

4-

2-

Channel: 12 mm Ø

Heating/coolingchannels [m]

Pre

ssur

e lo

ss ∆

p [b

ar]

Flow rate[l/min]

18 110

8

2825

20

15

10

2 4 6

The equilibrium temperature and the time taken for thermalequilibrium to be attained are a function of the heating/coolingmedium throughput and the flow resistance. The flow resistanceis determined by the number of heating/cooling channels andcorners in the mold (more than one heating/cooling circuit con-nected up in a series arrangement). In many cases, the pump on the temperature control unit does not supply sufficient pressure for the requisite throughput of heating/cooling medium to be achieved (10 to 15 l/min). In other cases, the maximum pressure level may be kept very low by a pressure-limiting valve. This re-sults in a “creeping flow” and hence in an insufficient exchange of heat in the mold. The temperature differential between the in-flow into the heating/cooling unit and the outflow from it provides an indication as to whether the throughput is too low. This diffe-rential should be less than 4 °C.

Fig. 11: Pressure losses in heating/cooling channels of different diameters

24

One essential requirement for the rapid attainment and reliablecontrol of the required mold temperature is a sufficient heatingand cooling capacity in the temperature control units em-ployed. The following diagram provides guide values for the heat-ing capacity, as a function of mold size and mold temperature.

{

101 102 103 104kg

Mold weight

100

101

102

kW

Hea

ting

cap

acity

Insu

late

d

pla

ten

area

ϑW (°C)

160

120

80

40

Fig. 12: Requisite heating capacity as a function of mold size for different temperatures

25

4.3 Melt temperature sensor (schematic diagram)

Probes which are suitable for connection to any injection mold-ing machine are available for measuring mold and melt tempe-ratures (e.g. melt temperature sensors).

Fig. 13: Diagram showing the Covestro melt temperature sensorinside the nozzle with the measuring point for the nozzle heater control

Fig. 14: Section through a nozzle with a melt temperature sensor

Sealing face

Measuring point 2 for nozzleheater control

Measuringpoint 1 for melttemperature

26

4.4 Injection and holding pressure; injection speed

The injection and holding pressures, and also the injectionspeeds required, depend on the type of material being moldedand the nature of the end product. The injection and holdingpressures are set as hydraulic pressures. The latter must be high enough to achieve sufficient cavity pressure to enable the mold to be filled completely, without any sink marks. They can differ considerably for a given mold, depending on factors such as injection speed, melt temperature and nozzle geometry.The injection speed is matched to the size of the molded partand to its shape and should generally be fast. The injection pressure should be high enough to ensure that the injection speed does not drop below the required setpoint value(s) during the entire injection process. If the injection speed drops towards the end of injection, this indicates that the injection pressure is too low or the set speed too high. In order to avoid surface defects close to the gate (dull spots, cold slug, delamination), it is a good idea to sharply reduce the speed at the start of the injection process (graduated injection). A constant flow front speed can be achieved by implementing a velocity profile over the entire screw stroke (optimization of the filling process). In many cases, empirically determined velocity profiles are of assistance in remedying flow engineering problems (entrapped air, weld lines, bubbles, tear drops, streaks, diesel effect). By reducing the speed directly prior to switchover, it is possible to level out the pressure profile and help prevent a backflow of melt. The cavity pressure required for full-scale mold filling, the “fil-ling pressure”, is a measure of the viscosity of the melt (provid-ing that the associated filling time is kept constant). This can be used for process control purposes.Another important factor is to switch over to holding pressure at the right moment in order to prevent overpacking of the mold.

27

The holding pressure serves to offset the volume shrinkage that takes place as the molded part cools in the mold. The level of holding pressure will be a function of the quality re-quirements on the molded part. These can include dimensional stability, low stresses and surface properties (sink marks, reproduction). The pressure level will generally be set as low as possible.The holding pressure should be maintained until the gatesystem has “frozen” (in order to avoid any backflow of meltwhen the pressure is removed). The minimum holding pressuretime (also known as the gate open time) can be establishedthrough weight checks on the molded part (Fig. 17) or from thecharacteristics of the cavity pressure curve (Fig. 18).

Unfavorable sequence

Favorable sequence

Cav

ity p

ress

ure

Holding pressure phaseInjection phase

Packing phaseTime

Fig. 15: Switchover to holding pressure

28

Fig. 16: Determining the holding pressure time from the increase in weight

Fig. 17: Determining the holding pressure time from the cavity pressure curve

Constant molded part weight

Mol

ded

part

wei

ght

Cavi

ty p

ress

ure

p W

No pressure drop≙ minimumholding pressuretime tND min(gate open time)

Holding pressure time tND

Holding pressure time tND

Minimum holding pressure time tND min

29

4.5 Screw speed; back pressure

The screw speed should be selected in such a way that the pe-ripheral screw speed (Vs) is between 50 and 200 mm/s. A speed of 300 mm/s should never be exceeded. Higher peripher al speeds can cause processing problems.

The back pressures that will ensure even melting are normallyof the order of 100 ± 50 bar (hydraulic pressure usually 5 to15 bar).

The following rules of thumb apply:

to improve melt homogeneity: increase back pressure to prevent uneven screw retraction (corkscrew effect): in-

crease back pressure occasional interruption of melt transport: reduce back pres-

sure metering time too long: reduce back pressure

Fig. 18: Correlation between screw speed and screw diameter

Scre

w s

peed

nS [

min

– 1 ]

Screw diameter D [mm]

30

4.6 Cooling time

The following diagrams (Figs. 19 to 22) show the calculated cool-ing time of injection moldings as a function of material type wall thickness mold temperature (ϑW) melt temperature (ϑM)

The essential factors that influence cooling are wall thicknessand mold temperature. The melt temperature has only a slightinfluence on cooling time.NB: Cooling time is understood here as being the time from theinitial application of holding pressure through to the point ofdemolding.

Fig. 19: Cooling time/wall thickness diagram for Apec®

See also Technical Information Sheet:„Optimized Mold Temperature Control“

Wall thickness s [mm]

Non-reinforced

Cool

ing

time

t K [s]

31

Fig. 20: Cooling time/wall thickness diagram for Bayblend®

°

Wall thickness [mm]

Coo

ling

time

t K [s

]

Non-reinforced

90

5

32

Reinforced 60

50

5

Non-reinforced


Coo

ling

time

Coo

ling

time

[s

]

Fig. 21: Cooling time/wall thickness diagrams for Makroblend®

33

Fig. 22: Cooling time/wall thickness diagrams for Makrolon®

70

60

50

40

30

20

10

0

Reinforced 120/340

80/300

60

50

40

30

20

10

00 1 2 3 4 5 6

Non-reinforced

Coo

ling

time

tk


100/320

60/280

100/320

80/300

ϑW /ϑ M (°C)

[s]

34

4.7 Optimization of machine settings; production monitoring

The properties of injection moldings are critically influenced byprocess control.

The following are influenced during the injection phase: mechanical properties surface finish visibility of weld lines warpage

The following are influenced during the packing phase: completeness of cavity filling flash formation

The following are influenced during the holding pressurephase: weight dimensional stability shrinkage voids sink marks ejection characteristics weld line strength dimensional accuracy (warpage)

35

Fig. 23: Cavity pressure as a function of time

The decisive process parameters here are: mold temperature melt temperature injection speed cavity pressure

Proper control of these parameters: simplifies setting enables immediate recognition of deviations during production This helps to improve quality assurance.

If accurate, detailed information is to be obtained on the process, then sensors need to be installed in the mold. Modern machines are capable of recording process data and further processing this for process optimization and alarm functions as well as for process control and process documentation. On older models of machine, these key functions can be performed by suitably adapted external process and operating data acquisition units.

t3

Influencing variablesInjection phase:– injection speed– oil, molding compound, molding temperature– polymer viscosity

Influencing of:a) material parameters:– viscosity– molecular degradation– crystallinity– orientation in surface layer

b) molded part properties:– surface quality

Influencing variablesPacking phase:– switchover to holding pressure– pressure limit setting

Influencing variablesHolding pressure phase:– level and duration of holding pressure– mold wall temperature– mold deformation– stability of clamping unit– level of clamping force

Influencing of:a) material parameters:– crystallinity– orientation inside molded part– shrinkage

b) molded part properties:– weight– dimensional stability– voids – sink marks – relaxation – ejection characteristics

Influencing of:a) material parameters:– crystallinity– anisotropies

b) molded part properties:– extent to which part is filled out– flash formation– weight

Cav

ity p

ress

ure

Packing phase

Holding pressure phase

Time

Injection phase

36

5 Processing reclaim; recycling

Recycling production wasteScrap suitable for recycling: short moldings sprues mechanically damaged parts

Special points to note: all parts being reground must be of the same sort of plastic all reject moldings must have been made of correctly

processed material no reject moldings with signs of thermal degradation (due to

overheating) should be used if possible the recycling of parts with streaks caused by moisture should

be avoided if possible no dirty or contaminated moldings should be used the pellet size of the reclaim should be roughly the same as

that of the virgin compound drying instructions must be observed

Adding reclaim to virgin material: 10 to 20 % always possible, depending on the application up to 100 %, following tests, for injection moldings where

properties are of secondary importance

The amount of reclaim that can safely be added should always be established through individual tests (e.g. tests for molecular weight reduction, mechanical properties). The different product departments at Covestro will gladly provide assistance. You should contact your Covestro field work er for information on the recycling of post-consumer waste, or of production rejects that have undergone final treatment.

37

6 Measures for the elimination of molding faults

Contents Page

Impurities in compound 38

Contaminated regrind 38

Moisture streaks 39

Silver streaks 39

Streaks 39

Burn streaks 40

Delamination 41

Gray streaks 41

Cloudy appearance 42

Blackish specks 42

Dull spots 43

Record grooves or rings 43

Cold slug 43

Voids and sink marks 44

Blisters 44

Jetting 44

Short moldings 45

Weld strength insufficient 46

Warped moldings 46

Part sticks to mold 47

Part is not ejected or is deformed 47

Flash formation 48

Rough, matt part surfaces 48

38 Fault Possibleappearance

Possible causes Suggested remedy

Impurities incompound

Gray foreign particleswhich appear shiny,depending on angleof light

Dark specks,discolored streaks

Colored streaks,surface layer nearsprue comes adrift

Abrasion from feed pipes,containers and feed hoppers

Dust or dirt particles

Presence of other plastics

Pipes, containers and feed hoppers should not be made of aluminum or tinplate but of steel or stainless steel (cleaned on the inside); pipes should be as straight as possible

Keep dryer clean and regularly clean air filter, carefully close opened sacks and containers

Separate different plastics, never dry different plastics together, clean plasticating unit, check subsequent batches for purity

Contaminatedregrind

As for fresh compound(see above)

Abraded material from pelletizer

Dust or dirt particles

Other plastics regrind

Check pelletizers regularly for abrasion and damage, and repair when necessary

Store scrap away from dust, clean parts before pelletizing, discard parts containing moisture (PC, PBT) and thermally de-graded parts

Always keep different types of regrind separate

39

Fault Possibleappearance


Moisture streaks

U-shaped, elongatedstreaks open towardsflow direction; or, in aless pronouncedversion, in the form ofsmall lines

Residual moisture content ofpellets too high

Check dryer or drying process, measure pellet temperature, observe prescribed drying time

Silver streaks Elongated silverystreaks

Overheating of melt due totoo high a melt temperature,too long a residence time

Too high a screw speed,nozzle and runners too narrow

Check melt temperature, use a more suitable screw diameter, reduce screw speed, widen nozzle and runner diameter

Streaks(entrapped airin compoundor mold)

Elongated streaks overa wide area, generallyrestricted to individuallocations

injection speed too high,entrapped air due to incorrectmetering, back pressure too low

Reduce injection speed, increase back pressure within permit-ted limits, use optimum metering stroke (> 1D to 3D), possibly increase the melt cushion to 1D



With transparentmaterials, bubblesmay also be apparentas striations, blackdiscoloration (dieseleffect) at points whereflows merge

Entrapped air inside moldcavity

Improve mold venting, especially near flow lines and near de-pressions (flanges, studs, lettering), correct flow front (wall thicknesses, gate position, flow leaders)

Burn streaks Brownish discolorationwith streaking

Melt temperature too high

Residence time too long

Unsuitable temperatureprofile in hot runner

Check and reduce melt temperature, check temperature controls

Reduce cycle time, use a smaller plasticating unit

Check hot runner, temperature controls and thermocouples

41



Occasional brownishdiscoloration withstreaking

Worn plasticating unit ordead spots near sealing faces

Parts of the plasticating unitand hot runners impede flow

Injection speed too high

Check cylinder, screw, non-return valve and sealing faces for wear and dead spots

Eliminate flow restrictions

Reduce injection speed

Delamination Surface near sprueflakes off (especiallywith blends)

Contamination through other,incompatible resins

Clean plasticating unit, check subsequent material for purity

Gray streaks Gray or dark stripes,unevenly distributed

Worn plasticating unit Exchange whole unit or worn parts, use a plasticating unit with an abrasion and corrosion-resistant coating

Dirty plasticating unit Clean plasticating unit



Cloudyappearance

Extremely fine specksor metal particles incloud formation

Cloud-like,dark discoloration

Worn plasticating unit

Dirty plasticating unit

Screw speed too high

See above

Clean plasticating unit

Reduce screw speed

Blackish specks

Less than 1 mm2 tomicroscopic

Bigger than 1 mm2

Worn plasticating unit

Screw and cylinder surfacedamaged and flaking off

See above

Clean plasticating unit, use unit with an abrasion and corrosion-resistant coating. For Makrolon®: run cylinder heater at 160 to 180 °C during breaks in production (for Apec® HT 180 to 220 °C)

43



Dull spots Velvety spots nearsprue, sharp edgesand changes in wallthickness

Disturbed melt flow ingating system, at transitionsfrom large to small-diameterrunner and at bends (shear,tearing of already solidifiedouter skin)

Optimize gate, avoid sharp edges, especially where gate joins mold cavity. Round off transitions near runners and sudden wall thickness changes and polish them, inject in stages: slow – fast

Record grooves or rings

Extremely fine grooveson part surface(e.g. with PC)

Too high a flow resistance inmold, so that meltstagnates; melttemperature, moldtemperature, injectionspeed too low

Increase melt and mold temperature, increase injection speed

Cold slug Cold melt particlesentrapped in thesurface

Nozzle temperature too low,nozzle aperture too small

Use band heater with higher capacity. Fit nozzle with thermo-couple and controller. Increase nozzle aperture, reduce cooling of sprue bush, retract nozzle earlier from sprue bush



Voids and sinkmarks

Round or elongatedbubbles, visible only intransparent plastics,surface depressions

No compensation for volumecontraction during thecooling phase

Molded part does not havethe right design for a plastic(e.g. wall thicknessdifferences too great)

Increase holding pressure time, increase holding pressure, reduce melt temperature and alter mold temperature (in the event of voids this must be increased, and in the event of sink marks, reduced), check melt cushion, increase nozzle aperture

Redesign part avoiding sudden changes in wall thickness and accumulations of material, adapt runners and gate cross- sections to part

Blisters Similar to voids butsmaller diameter andmore of them

Moisture content of melt toohigh, also too high a residualmoisture content in granules

Optimize drying, if necessary use a normal screw instead of a vented screw and pre-dry material. Check dryer and drying process and use dry-air dryer if necessary

Jetting Melt which hasentered cavity first isvisible on part surface

Unfavorable gate locationand size

Prevent jetting by moving the gate elsewhere (inject against a wall), increase gate diameter

45



Injection speed too high

Melt temperature too low

Reduce injection speed or inject in stages: slow – fast

Increase melt temperature

Short moldings

Incomplete filling of ca-vity, especially at end of flow path or near thin-walled areas

Plastic does not have sufficiently good flow

Injection speed too low

Walls of part too thin

Insufficient contact between nozzle and mold

Diameter of gating system too small

Mold venting inadequate

Increase melt and mold temperature

Increase injection speed and/or injection pressure

Make walls thicker

Increase nozzle contact pressure, check radii of nozzle and sprue bush, check centering

Enlarge gate and runner

Improve mold venting



Weld strengthinsufficient

Clearly visible notches along weld line



Walls too thin


Increase melt and mold temperature, improve flow conditions by moving gate elsewhere if necessary

Increase injection speed

Increase wall thickness


Warped moldings

Parts are not flat, aredistorted, do not fit together

Wall thickness differences too great, different flow speeds inside mold, glass fiber orien-tation

Mold temperatures unsuitable

Unfavorable switchover point from injection to holding pressure

Redesign part, change position of gate

Heat mold halves to different temperatures

Alter switchover point

47



Weld strengthinsufficient

Clearly visible notches along weld line



Walls too thin


Increase melt and mold temperature, improve flow conditions by moving gate elsewhere if necessary


Increase wall thickness


Warped moldings

Parts are not flat, aredistorted, do not fit together

Wall thickness differences too great, different flow speeds inside mold, glass fiber orien-tation

Mold temperatures unsuitable

Unfavorable switchover point from injection to holding pressure

Redesign part, change position of gate

Heat mold halves to different temperatures

Alter switchover point



Part sticks tomold

Dull spots, fingerlikeor cloverleaf-shapedshiny hollowson surface (usuallynear sprue)

Cavity wall temperature toohigh in certain places

Part ejected too soon

Reduce mold temperature

Increase cycle time

Part is notejected or isdeformed

Part has jammed;ejector pins deformpart or penetrate it

Mold overloaded, undercuts too deep, cavity insufficiently polished near flanges, ribs and studs

Vacuum is formed between-part and mold during injection

Elastic deformation of mold and core displacement through injection pressure

Part ejected too soon

Reduce injection speed and holding pressure, eliminate undercuts, re-work cavity surfaces and polish in longitudinal direction


Increase rigidity of mold, support cores

Increase cycle time



Flash formation

Polymer meltpenetrates mold gaps(e.g. parting line)

Cavity pressure too high

Mold parting surfaces havebeen damaged by overpacking

Clamping or locking forceinadequate

Reduce injection speed and holding pressure, bring forward switch-over point from injection pressure to holding pressure

Re-work mold near parting surfaces or contours

Increase clamping force or use machine with a higher clamping force

Rough, matt part surfaces (with glass fi-ber reinforcedthermo-plastics)

Rough, matt surfaceswith flaky appearance;glass fibers visible

Melt temperature too low

Mold too cold


Increase melt temperature

Increase mold temperature, equip mold with thermal insulation, use a more efficient heater


49

7 Product range Covestro

PolycarbonateApec® (PC-HT)High-heat polycarbonate

Makrolon® (PC)Polycarbonat

Makrofol®/Bayfol® (PC/[PC+PBT] blend)Engineering films

Polycarbonate blendsBayblend® (PC+ABS; PC+SAN-I; PC+ASA)Blends of polycarbonate and ABS, SAN or ASAMakroblend® (PC+PBT; PC+PET) Blends of polycarbonate and PBT or PET

Thermoplastic polyurethanesDesmopan®/Texin®1) (TPU)Thermoplastic polyurethanes

1) Texin® is a product line of Covestro LLC, USA

50

51

52

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Streaks, stripes, specks, dotsStreaks open towards the flow direction 5.1 ▶Large-area silver streaks 5.2 ▼ 2 ▼ 3 ▲ 1 CushionStreaks due to overheating, needle-like streaks 5.3 ▼ 1 ▼ 3 ▼ 2 ▲ 4Craters 10 ▼ 1 Type, quantity of carbon blackWhite patches 28 ▼ 2 ▼ 3 ▼ 4 ▶ 1 (PA-GF)Black or brown dots 30 Clean plasticating unit, soiled granules ▶Gray dots 32 Worn plasticating unit ▶Jetting 37 ▼ 1 2 Deflector surface

ColorRings 7 ▲ 1 ▲ 2 ▲ 3Homogeneous discoloration 16.1 ▼ 1 ▼ 2 Residence timeBlack discoloration 16.2 Eliminate dead spotsDarker color at weld line 18 ▼ 2 ◀▶ 1 ▼ 3 ▲ 4Partial color change 24 ▼ 3 ▼ 1 ▲ 2 ▲ 4Overheating marks (black) 26 ▲ 1 ▼ 3 ▲ 2Cloudy appearance 35 ▼ 1 Plasticating unit

GlossDull spots 1 ▼ 1 ▲ 2Matt surface defects at hot runner elements 2 ▼ 2 ▲ 1Cold slug 3 ▲ Ejector clawDamage to grain on molded part 15 ▲ 4 ▲ 3 ▲ 2 ▼ 1 Optimize removalGloss differential on molded part surface 21 ▶ 2 ▲ 1Gloss level not achieved on polished surface 22 ▶ 3 ▲ 1 ▲ 4 ▲ 5 ▲ 2 ▶ 6Matt appearance not achieved on textured surface 23 ▲ 2 ▲ 1 ▼ 3 ▲ 4Frosting 38 ▲ 2 ▲ 1

Removal behaviorSprue remains attached 6 ▲ 1 ▼ 2 ▼ 3 ▼ 5 ▲ 4Scratches 8 ▲ 3 ▶ 4 ▲ 1 ▼ 2 Optimize removalNoises as the mold opens 9 ▲ 1 ▼ 2Mold fails to open 19 ▲ 3 ▲ 2 ▼ 1Stress-whitening 36 ▲ 2 ▼ 1 Optimize removalMolding remains caught in cavity 44 ◀▶ 6 ▼ 1 ▲ 3 ▼ 2 ▲ 4 ◀▶ 5Part deformed during removal 47.1 ▶ 4 ▶ 1 ▲ 3 ▼ 2 Optimize ejection, surfaceEjector marks 47.2 ▶ 4 ▶ 1 ▲ 3 ▼ 2 ▲ 5 ◀▶ 6 Optimize ejection, surfaceFracture of part during removal 47.3 ▶ 4 ▶ 1 ▲ 3 ▼ 2 ▲ 5 ◀▶ 6 ▶ 7 Optimize ejection, surfaceCracks, microscopic 50.1 ▲ 1 ▲ 2 Check media contactCracks, macroscopic 50.2 ▼ 1 ▲ 3 ▼ 2 Optimize removal

UnevennessSink marks, localized 11.1 ▼ 3 ▼ 4 ▲ 1 ▲ 2 ▲ 5 Wall thick./rib ratioLarge-area sink mark 11.2 ▼ 1 ▲ 2 ▲ 3Notch along weld line 12 ▲ 1 ▲ 2 ▲ 3 ▲ 5 ▶ 4Grooves 13 ▲ 2 ▲ 3 ▲ 1Local, glossy, finger-shaped depressions 14 ▼ 3 ▲ 1 ▲ 2 ▲ 4Flakes 29 ▲ 2 3 ▼ 1 Especially with mineralsTear drops 31 ◀▶ 1 ▲ 2Delamination 33 ▼ 2 ▲ 1 Foreign materialPockets 34 ▲ 3 ▼ 2 ▲ 1

DimensionsFlash 40 ▼ 5 ▼ 4 ▲ 2 ▼ 3 ▲ 1 Sealing facesVariations in size 42 ▶ ▶Variations in wall thickness 43 ▲ 1 ▼ 2 ▲ 3Short molding 45 ▲ 1 ▲ 2 ▲ 3 ▼ 4 ▲ 5Weight variation 52 ▲ 1 ▶ 2 Non-return valve

Mechanical propertiesMechanical problems with part, cracks 17 ◀▶ 1 ◀▶ 2 ▼ 3 ▶ 4Weld line strength insufficient 27 ▲ 2 ▲ 3 ▲ 1 ▲ 4 ▶ 5

Processing/plasticationVoids 20.1 ▲ 1 ▲ 2 ▲ 3Large bubbles 20.2 ▼ 2 ▲ 1 High melt cushionSmall bubbles 20.3 ▶Cycle too long 25 ▼ 4 ▲ 2 ▼ 1 ▼ 3Unusual odor 39 ▼ 1 ▼ 2 ▼ 3 Residence timeWarpage 46 3 ▼ 1 ▲ 2 ▶ 4 GF orientationStringing 48 ▶ 1 ▲ 2 ▼ 3Mold corrosion 49 ▼ 2 ▼ 3 ▼ 4 ▲ 1 Suitable steel types

Injection molding Faults, causes, remedies ▲ increase, earlier

▼ reduce, later

▶ optimize (e.g. position)

◀▶ vary

1–7 order for making changes

Covestro Deutschland AGBusiness Unit Polycarbonates D-51365 Leverkusen

www.plastics.covestro.com

Typical valueThese values are typical values only. Unless explicitly agreed in written form, they do not constitute a binding material specification or warranted values. Values may be affected by the design of the mold/die, the processing conditions and coloring/pigmentation of the product. Unless specified to the contrary, the property values given have been established on standardized test specimens at room temperature.

The manner in which you use and the purpose to which you put and utilize our products, technical ass-istance and information (whether verbal, written or by way of production evaluations), including any sug-gested formulations and recommendations, are beyond our control. Therefore, it is imperative that you test our products, technical assistance, information and recommendations to determine to your own satisfaction whether our products, technical assistance and information are suitable for your intended uses and applications. This application-specific analysis must at least include testing to determine sui-tability from a technical as well as health, safety, and environmental standpoint. Such testing has not necessarily been done by Covestro. Unless we otherwise agree in writing, all products are sold strictly pursuant to the terms of our standard conditions of sale which are available upon request. All information and technical assistance is given without warranty or guarantee and is subject to change without notice. It is expressly understood and agreed that you assume and hereby expressly release us from all liability, in tort, contract or otherwise, incurred in connection with the use of our products, technical assistance, and information. Any state-ment or recommendation not contained herein is unauthorized and shall not bind us. Nothing herein shall be construed as a recommendation to use any product in conflict with any claim of any patent re-lative to any material or its use. No license is implied or in fact granted under the claims of any patent.

With respect to health, safety and environment precautions, the relevant Material Safety Data Sheets (MSDS) and product labels must be observed prior to working with our products.

Edition: 2016-01Ordner-no.: COV00072231 Printed in Germany · GB

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