BLOW MOULDING

Blow moulding is a process for manufacturing hollow articles. The articles commonly made arebottles, edible and lube oil containers, jerry cans, drums, toys and automotive components.

TYPES OF BLOW MOULDING :

(i) Extrusion blow moulding - continuous or intermittent type(ii) Injection blow moulding(iii) Extrusion stretch blow moulding(iv) Injection stretch blow moulding

EXTRUSION BLOW MOULDING :

In extrusion blow moulding (Fig 1), a parison is formed by an extruder. Basically the plastic ismelted by heat which is transferred through the barrel by the shearing action of the extruderscrew as it passes through the extruder.Turning continuously, the screw feeds the melt through an annular orifice as an endless parisonor into an accumulator. Size of the part and the amount of material necessary to produce thepart (shot size) dictate whether an accumulator is required. The non- accumulator machineoffers an uninterrupted flow of plastic melt. With the accumulator, flow of parison through thedie is cyclic.When the parison or tube exits the die and develops a preset length, a split cavity mould closesaround the parison and pinches one end. Compressed air inflates the parison against the hollowblow mould surfaces which cool the inflated parison to the blow mould configuration. Uponcontact with the mould wall, the plastic cools and sets the part shape. The mould opens, ejectsthe blown part, and closes around the parison to repeat the cycle.Fig 2 shows a typical blow mould.

INJECTION BLOW MOULDING :

The basic process as shown in Fig. 3 consists of three stages :-(a) Injection moulding of a parison / pre-form onto a steel core pin.(b) Transfer of this core pin, with its pre-form to a blow mould and blowing of the pre-form

to the cavity shape.(c) Ejection of the blown container.

Fig 2. Blow Mould Construction and Nomenclature

ADVANTAGES OF INJECTION BLOW MOULDING :

(1) Superior gloss(2) Higher dimensional accuracy(3) No welded seams / weak points(4) Better weight control, both cavity-to-cavity and shot-to-shot.(5) Superior mechanical properties due to the orientation effect.(6) Product is flash free

DISADVANTAGES :

(1) Higher mould cost(2) The process produces centre neck containers only(3) No handled or asymmetric containers can be produced(4) Economical for high volume production(5) The maximum container size is 1 litre, normally containers less than 300 cm3 are

produced

STRETCH BLOW MOULDING :

Biaxially stretching the extrudate before it is chilled in the mould can impart improved propertiesto the finished bottles.The stretch-blow process can give many resins improved physical and barrier properties. Inbiaxial orientation, the bottles are stretched lengthwise by an external gripper, or by an internalstretch rod, and then stretched radially by compressed air to form the finished container. Thisprocess aligns the molecules along two planes providing additional strength and better barrierproperties than is possible without biaxial orientation. Other advantages include better clarity,increased impact strength, or toughness, and reduced creep. The actual increase is dependenton the ratio of blow up in each direction.Stretch blow moulding is possible for thermoplastic materials such as PET, PVC, PS, PAN &PP. The amorphous materials with a wide range of thermoplasticity are easier to stretch thanthe partially crystalline types.Stretch-blow moulding whether extrusion (Fig 6) or injection (Fig 7) initially involves theproduction of a pre-form, followed by tempering of the pre-form to the proper orientationtemperature. This is followed by axial stretching and blowing to achieve biaxial orientation.

Stretch-blow processing can be separated into two categories : In-line and two-stage. In-lineprocessing is done on a single machine, while two-stage processing requires either an extrusionor injection line to produce pre-forms and a reheat-blow machine to make the finished bottles.

RESIN CHARACTERISTICS FOR BLOW MOULDING

Blow moulding requires resins with higher melt strength for parison formation and goodprocessability during extrusion. Consistent melt swell is also important. Product requirementsusually include stiffness, impact strength and good environmental stress crack resistance(ESCR).The three important factors which determine the properties of polyethylene are

i. Molecular weight,ii. Molecular weight distributioniii. Density.

Tables 1, 2 & 3 show the effect of molecular weight, molecular weight distribution and densityon physical properties of polyethylene respectively.Higher the density better is the chemical resistance, gas barrier, stiffness and hardness howevertoughness and ESCR decrease.A lower melt flow index (MFI) gives better mechanical properties, ESCR and melt strength.A broader molecular weight distribution gives better swell, processability & ESCR but poorersurface gloss.HDPE blow moulding grades have broad MWD’s with optimum combination of stiffness andstress crack resistance. This can be achieved with a low MFI and appropriate density. Ahigher density grade would give lower ESCR but higher stiffness.

TABLE 1

EFFECT OF MOLECULAR WEIGHT (M.I) ON PHYSICAL PROPERTIES

PROPERTY MI INCREASINGMELT VISCOSITY ↓MELT STRENGTH ↓SOLID STRESS CRACK RESISTANCE ↓TOUGHNESS ↓ABRASION RESISTANCE ↓RESISTANCE TO CREEP ↓

TABLE 2

EFFECT OF MWD ON PHYSICAL PROPERTIES

PROPERTY MWD BROADENINGMELT VISCOSITY (HIGH SHEAR) ↓MELT STRENGTH ↑SURFACE GLOSS ↓TOUGHNESS ↓STRESS CRACK RESISTANCE ↑

TABLE 3

EFFECT OF DENSITY ON PHYSICAL PROPERTIES

PROPERTY INCREASING DENSITYIMPACT STRENGTH ↓STIFFNESS ↑HARDNESS ↑TOUGHNESS ↓STRESS CRACK RESISTANCE ↓CHEMICAL RESISTANCE ↑PERMEABILITY ↓

RELENE BLOW MOULDING GRADES

Table 4 lists the RIL blow moulding grades. The corresponding competitor grades are shown inTables 5 and 6

RIL blow moulding grades have the following characteristics:-

◊ Broad molecular weight distribution◊ High melt strength◊ Grades suitable for food contact & pharmaceuticals applications◊ Products exhibit

- Excellent ESCR characteristics- Superior drop impact strength and stiffness- Exceptional toughness even at low temperature- Good resistance to weathering

TABLE 4

RELIANCE HDPE GRADES FOR BLOW MOULDING

GRADE DENSITY(gm/cm3)

MFI(dg/min)

MWD TYPICALAPPLICATIONS

B 46003 0.946 0.30 Broad Detergent bottles / pesticidecontainers

B 56003 0.956 0.30 Broad Oil bottles / jerry cancontainers upto 100 litrecapacity

EB16 0.952 0.28 Broad Lube oil containers

PROCESSING GUIDELINES

EXTRUSION TEMPERATURES

◊ Melt temperatures in the range 175 - 205°C are recommended.◊ Normally, temperatures of 190 - 205°C will result in optimum ESCR properties◊ No premium in stress crack resistance can be expected by increasing melt

temperature above 215°C.

MOULD TEMPERATURE :

◊ Mould temperatures should be preferably 25-28°C to minimize cycle time. Thecycle time also depends upon mould design and atmospheric conditions.

◊ Mould should not be too cold, as this would lead to mould sweating

SHRINKAGE :

◊ Shrinkage limits will be between 0.010 & 0.050 cm/cm. Shrinkage will depend on:a) Mould designb) Melt temperaturesc) Mould temperatured) Blowing pressuree) Bottle sections

SWELLING CHARACTERISTICS :

1. HDPE resins comprise of long and medium chain molecules, which in the molten state havecomplex motions, with themselves and with each other, forming a coiled mass of molecules.

2. When extruded through the die, they are forced to uncoil and stretch in the flow direction onemergence these relax to their random orientation.

3. HDPE resins manufactured by different processes, differ in their swelling characteristics.Swells are of 2 types as shown in the fig.

- Diameter Swell- Thickness Swell ( Weight Swell)

Swell can be adjusted by adjustments in temperatures , screw speed & type of tooling used.

TYPES OF TOOLING

There are basically two types of tooling:-

DIVERGENT TOOLING (FIG. 9)

A divergent tooling (Fig. 9) is generally preferred for larger articles. It gives greater thicknessswell as compared to diameter swell.

CONVERGENT TOOLING (FIG. 10)

A convergent tooling (Fig. 10) is generally preferred for smaller articles. It gives greaterdiameter swell as compared to thickness swell.

TROUBLE SHOOTING IN BLOW MOULDING

SR.NO PROBLEM CAUSE SUGGESTED REMEDY1. Streaking a) Contamination in die head

b) Degraded material ontooling

c) Cold spot in tooling or diehead

a) Clean die head

b) Clean tooling

c) Raise temperature & checkheating bands

2. Rough surfacefinish

a) Insufficient venting

b) Material temperature toolow

c) Air pressure too low or rateof air flow too slow

d) Air leakage around blowpin

a) Add venting by eithersandblasting the mouldsurface adding channels onthe mould faces or ventingthe interior of the mould

b) Check fuses, heating bandsand thermocouples. Raiseheat inputs

c) Check air pressure and airlines for foreign material

d) Increase blowpin diameter

3. Parison notbeing blown

a) Blow timer defectiveb) Clogged blow linesc) Pinch-offs too sharp

d) Clamp not cushioninge) Cut parison

a) Replace blow timerb) Clean blow linesc) Stone pinch-offs to create

more pinch aread) Reset cushione) Clean tooling

4. Air bubbles inparts

a) Insufficient back pressure

b) Air leaking into head

c) Nozzle valve not seatedproperly

a) Clean back pressure valveand reset

b) Check fit of mandrel informing pin

c) Check seat of spool toprevent air from beingdrawn in by movement ofspool

SR.NO PROBLEM CAUSE SUGGESTED REMEDY5. Varying parison

wall thicknessa) Parison not dropping

straightb) Loose mandrel

c) Parison not centred withmould

d) Loose forming pin

a) Adjust centre bushing

b) Tighten mandrel on mandrelstud; and check fit ofmandrel in forming pin

c) Check alignment of mouldwith parison

d) Tighten forming pin andcheck O.D

6. Poor colourdispersion

a) Insufficient number ofstraining rings on screw

b) Poor blending of material

c) Back pressure too low

d) Clogged screen pack

a) Remove screw and cleanstraining

b) Check ratio of colouredresin to natural resin

c) Raise back pressure toimprove mixing by screw

d) Change screens7. Inadequate

stripping ofmoulded parts

a) Stripper stroke not longenough

b) Insufficient air pressure

c) Parts sticking in mould

a) Adjust knockout stroke forlonger movement

b) Check air pressure and linesc) Lengthen exhaust time,

check for large undercut8. Parison curling a) Mandrel and bushing not

flushb) Bushing too cold

a) Remachine tooling

b) Raise bushing temperature9. Flashing a) Melt too hot

b) Blowing air pressure toohigh

c) Clamping mechanism out ofadjustment

d) Excessive material beingforced into mould

a) Reduce melt temperatureb) Reduce air pressure

c) Reset clamp or increaseclamp pressure

d) Increase recess at pinch-offareas to accommodatemore material

10. Warped ordeformed parts

a) Slow parison extrusionb) Melt temperature too lowc) Parts ejected too hotd) Design of ribs, undercuts,

threads, etc. inappropriate

a) Check extruder rateb) Increase melt temperaturec) Increase cooling cycled) Review and alter mould

design if necessary

SR.NO PROBLEM CAUSE SUGGESTED REMEDY11. Excessive

shrinkagea) Mould temperature too highb) Melt temperature too high

c) Blowing air conditionsinadequate

a) Reduce mould temperatureb) Decrease melt temperaturec) Increase blowing air cycle

and / or blowing airpressure

12. Parts blow-out a) Blow-up ratio too largeb) Mould separation

c) Pinch-off too sharp

d) Pinch-off too hot; no welde) Parts blow too fast

a) Use large dieb) Increase clamp pressure or decrease blow pressurec) Provide wider land in pinch-

offd) Cool Mould pinch offe) Use low-pressure blow

followed by high-pressureblow

13. Parison tailsticking to parts

a) Parison too long a) Shorten parison length orincrease pinch-off land areato cool compressed tail.