How to Choose a Blow Moulding Machine

Buying a blow moulding machine can be a nerve racking experience for the fresh entrepreneurs. There countless specification, capability and price factors to be considered. This blog sets out to help anyone who wants to buy a blow moulding machine but does not know where to start.
Mould Size:
The most important spec is the mould size of the machine. The range of product you want to produce should be matching the model. For this refer the maximum mould size mentioned in the machine specification chart.
Production Capacity:
Estimate your production capacity required like containers per month and cross check with the machine supplier whether this can be met. It is advisable to keep a 25% extra capacity. This relates to the extruder size, plasticizing capacity and possible cycle time of the machine.
Quality of the machine:
Quality of the machine is a relative subject. Normally the cost of the machine will vary proportionally with the quality of the machine. Quality itself is in two sections – machine built quality & product (container) quality. The product quality depends on your end user. The machine built quality affects the maintenance cost and breakdown cost of the plant.
Other Machine Specifications:
The Die-Core (Cup-pin) size of the machine should be enough for the proposed containers intended to manufacture. The mould clamping force also critical with respect to your products. Then for the accumulator head type machines. The accumulator shot capacity is important.
Automation:
The machine automation can be done by a PLC control or by a wired relay logic (this become outdated because of the limitations and bulkiness of control panel). The PLC control has very wide range of options available. This can be from ordinary custom built micro processor controller to industrial grade established control systems like MOOG, B&R etc.
Additional Options:
Automatic Deflashing:
Here the product is automatically deflashed or trimmed in the attachment of the machine. Fully ready container will come out of the machine. If deflashing option is not present, every product (container, bottle etc) have to be trimmed by an operator manually.
Ancillaries:
Selection of ancillaries like air compressor, scrap grinder, cooling system are also required to be selected in accordance with the machine manufacturers recommendations.
Parison control:
A parison control system helps to improve the product quality and reduce the flash waste. This will be useful for large containers.

Trouble Shooting of Blow Moulding Process

Pitted surface appearance

Possible Reasons:

• Air entrapment inside the mould
• Condensation
• Low Blow Rate
• Low Blow Pressure

Possible Solutions:

• Vent the mould to reduce the air entrapment.
• Check mould finishing, polished mould causes air entrapment. Very coarse mould surface results in rough part surface. Sand blasted mould surfaces are usually desirable.
• Check mould for condensation coolant temperature
• Increase the size of air orifice to allow faster blow rate
• Increase blow pressure.

Vertical groves and lines in the part

Possible Reasons:

• Dirty and / or damaged die

Possible Solutions:

• Clean die surface.
• Scratches in die or core, to be removed
• Check for contamination.
• Set the parison straight to eliminate hold up in die.

Irregular markings on the part

Possible Reasons:

• Water condensate or water leakage in the mould

Possible Solutions:

• Run faster cycle.
• Run warmer mould.
• Stop leakage in the mould.

Rough part surfaces and / or poor gloss.

Possible Reasons:

• Rough parison

Possible Solutions:

• Increase die temperature
• Increase mould temperature
• Increase melt temperature
• Increase extruder speed
• Use low molecular weight resin
• Clean die core

Rippling on parison / melt fracture

Possible Reasons:

• Unsuitable die shape
• Extrusion rate too high

Possible Solutions:

• Increase die land
• Increase die diameter
• Reduce extruder speed

Thin areas or holes on the part surface

Possible Reasons:

• Airflow in head is faster
• Parison is not straight
• Moisture in resin
• Contaminated resin in head or die

Possible Solutions:

• Reduce airflow through head
• Set parison
• Pre heat the resin
• Clean the head and die core

Poor wall thickness distribution – longitudinal

Possible Reasons:

• Parison sagging
• Parison control not working

Possible Solutions:

• Change extrusion rate
• Decrease melt temperature.
• Use low M.F.I. resin.
• Check parison control timings.
• Check parison control pressure.
• Check flow control valve setting.

Poor wall thickness distribution- circumferential

Possible Reasons:

• Non symmetrical parison
• Non symmetrical article

Possible Solutions:

• Set parison straight
• Use larger die
• Optimise article weight
• Use larger die

Excessive dimensional shrinkage of article

Possible Reasons:

• Low blow pressure
• Insufficient cooling
• Melt temperature is too high

Possible Solutions:

• Increase blow pressure
• Decrease mould temperature
• Increase cooling time
• Decrease melt temperature.

Warpage

Possible Reasons:

• Insufficient cooling
• Non uniform cooling

Possible Solutions:

• Reduce mould temperature
• Check for efficiency of water flow in mould channels
• Increase cooling time.
• Check efficiency of mould venting.
• Provide more cooling, especially in pinch of area of the mould.

Parison doughnut formation

Possible Reasons:

• Core temperature is low
• Core gone inside the die

Possible Solutions:

• Allow core to reach temperature equilibrium with rest of the system
• Lower the core slightly (to get the weight of the article, reduce the core diameter)

Thinning or stretching at parting line

Possible Reasons:

• Low blow pressure
• Pinch off is too sharp

Possible Solutions:

• Increase blow pressure
• Check efficiency of airline
• Widen pinch of land
• Reduce head and die temperature

Parison tail sticking to the article

Possible Reasons:

• Parison too long

Possible Solutions:

• Shorten Parison length.
• Increase pinch-of land to compression cool the tail

Black specks in parison

Possible Reasons:

• Degraded resin
• Hold ups in the die head

Possible Solutions:

• Thoroughly clean and purge extruder. Avoid prolonged shutdowns with resin held at high temperature
• Improve streamlining of the melt inside

Contamination

Possible Reasons:

• Poor house-keeping

Possible Solutions:

• Keep production area clean