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Trouble Shooting Latent problems IN INJECTION MOULDING OF PLASTICS

- Prabodh C. Bolur

Introduction:

During my tenor as Regional Manager of WINDSOR in Madras between 1978 and 1990, I had come across numerous problems of machines, moulds, materials, processing, support equipment. Each of these affected my customers’ performance and also profitability. I took up their problems seriously and tried to apply my mind to understand the root causes of these problems. I used to discover and learn new things every time I tried to solve the problems. I used to document these learning and later on I tried to fit all jigsaw pieces to form a whole picture. The end result is this book " A Guide to Injection Moulding of Plastics". Truly I should be grateful to my customers who gave me this opportunity to understand the subject. Now I want to distribute this knowledge to all – particularly to the students’ community. Please visit my website http://pcbolur.balasainet.com

QUALITY PROBLEM

Quality problems are

The root causes of quality and consistency problems may or may not lie in the present step - moulding process set-up.

The mistakes of the earlier steps may not be possible to correct at the later steps.

Some time it may be possible to correct partly. Such corrections are expensive.

The problems originating from the present step can only be solved in the present step

Quality problems in injection moulding can be traced to

ANALYSIS OF PLASTIC PART FAILURES by RAPRA

High failure cause to low failure cause- in descending order

The predominantly material causes involved in failure were:

The predominating process faults were:

Environmental Stress Cracking,

Excessive moulded-in stresses,

Cyclic Fatigue,

Voiding,

Notch Sensitivity,

Poor weld lines,

Chemical Attack and; Stress Cracking,

Adverse Orientation,

UV Degradation,

Material contamination,

Thermal Degradation,

Longer Residence time in the machine,

Creep,

Wrong material selection or poor part design,

Buckling,

Wrong material selection or poor part design

Moulding inconsistency and failure due to moulded-in stress are the major problem observed in moulding shop. Moulding inconsistency means shot to shot variation in quality parameters.

ROOT CAUSE for moulding inconsistency.

Root Cause can be due to

Physical cause is easily traced and trouble shooting is normally done at shop-floor level. All text books, polymer manufacturers literature provide these information in the form of trouble shooting table or chart.

Human errors are like wrong water line connection, usage of contaminated material, etc.

LATENT cause means existence of inherent problem at the part design or mould design stage. Basics of plastics part design lies in ensuring perfect mouldability for the part designed.

I have observed that if the following three balances are taken care of during design stage, then moulding consistency is generally ensured.





These THREE balances also prevent or reduce the moulded-in stress, thereby avoiding major cause of part failure in service.

Any unbalancing on the above three balances will result in moulded-in stress. This is the biggest culprit for over 50% part failure

CAE software can predict the part design related problems and gives vital inputs for design of mould. It can also provide the processing parameters with a range.

It is observed that moulded-in stress is behind most of the problems. It could be due to following reasons:

  • Unbalanced flow of melt in mould results in some overpacked region and some just packed regions.
  • Inadequate injection speed results in faster freezing, thereby demanding more filling pressure which causes overpacked moulding.
  • Incorrect switching over from injection speed to follow-up pressure resulting in overpaking. This is caused because the balance unfilled volume available (at switchover point, in the mould) to accommodate the relaxation of compressed melt less than required. This can be damaging the mould if the mould has slender parts.
  • High follow-up pressure and time resulting in overpacking. Melt transfer during pressure phase can reach only unfrozen regions, which is normally near the gate. Therefore, the pressure set should gradually reduce with time to prevent overpacking.
  • Poor cooling system giving rise to non uniform mould surface temperature which in tern resulting in overpacking in some regions of moulding.

It is observed that over packing increases the density of the moulding. If we measure the density at different region of the moulding, you may find the variation in density if the mould filling is unbalanced. The unbalanced mould filling can be due to part geometry or non uniform cooling. Over packing during pressure phase results in over packed region near the gate, which can be verified by checking the density of the regions near the gate.

MOULDING PROBLEMS- Mould, Machine and Material related

MOULDING PROBLEMS

MOULD related

MACHINE related

SINK MARKS

Part design related- non uniformity of wall thickness.

  • Injection speed profile and hold on pressure profile can minimise to some extent.
  • Gas injection Moulding technique can be considered.

WARP

  • Use technique to improve part stiffness with part design.
  • Random distribution of mould surface temperature. Cooling circuit design related.

 

WEAK WELD LINES

Part design related.

Injection speed can influence.

IN-CONSISTENT CRITICAL DIMENSIONS

  • Unequal shrinkage due to variation in mould surface temperature.

Cooling circuit design related.

  • Core and cavity dimension related. Correct shrinkage is not considered for core / cavity dimension.

Melt temperature, Injection speed and hold-on pressure can have little influence.

DEGRADATION

Check dimensions of runner system for excessive shearing.

Excessive Residence time due to use of oversized injection unit.

Lowering barrel temperature and injection speed can have some influence

EJECTION DIFFICULTY

Part design related -Unbalanced melt flow. Use flow leader to reduce unbalance in flow.

 

POOR IMPACT strength

Radius at projections and sharp corners as it act like notch from where crack propagates.

 

IN-CONSISTENT WEIGHT.

 

% age utilisation of shot capacity is closer to 90% resulting in inconsistent melt quality- if metering stroke is more than 3 times screw diameter. Borderline case for shot weight.

IN-CONSISTENT FILLING

Increase gate size to reduce pressure drop across the gate.

Available maximum injection rate is not adequate for flow ratio of part being moulded. Borderline case for injection rate.

STRESS CRACKING

Melt flow is unbalanced. Part design related.

Over packed part. Set correct pressure profile during pressure phase.

 

MATERIAL RELATED PROBLEMS

SINK MARKS

Filled polymer can reduce sink mark.

POOR MECHANICAL STRENGTH

Consider high molecular weight polymers.

Consider filled polymers. Pre heat polymer in dehumidifier if required for polymer as recommended for polymer.

DIMENSIONAL VARIATION

Check shrinkage characteristics of material.

DEGRADATION OF MELT

Check thermal stability of polymer as well as pigments and other additives if any used with material.

DIFFICULTY IN FILLING

Consider polymer of higher MFI

Steps in Injection Moulding

Injection Moulding of plastics involves quite a few steps before actual moulding operation.

The first step has to be

 

The root causes of quality and consistency problems may or may not lie in the present step - moulding process set-up. The mistakes of the earlier steps may not be possible to correct at the later steps. Some time it may be possible to correct partly. Such corrections are expensive. The problems originating from the present step can only be solved in the present step.

Mouldability

In mechanical engineering Part design has to be manufacturable. Similarly plastic part has to be mouldable to high degree. If moldability is poor than, you have all types of quality and production problems. Therefore part design has to be modified to make it more than 90% mouldable. General assumption that any geometry of the part is mouldable perfectly is not correct. General understanding- that once the mould is filled with the melt every thing has to be perfect- is not correct.

Let us understand the factors influencing quality consistency in processing and quality in performance

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