Technological Solutions for Quality in Injection Moulding of Plastics by Prabodh C. Bolur-


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There are SMALL factories in Europe where virtually number of machines work with robot - without operators. Manpower is used for movement of finished parts for packing or further operations.

Since injection moulding is an automatic mass production technique, it is meaningless to use cheaper, inferior and imperfect moulds on expensive machine. Once the mould is fixed and process parameters are set on the machine for desired quality, the production should go on and on till the production schedule is over . This can be possible only if all concerned with operations understand the strength and significance of four 'M's.

MACHINE which gives excellent repeatability of set parameters - for which machine control is responsible. Machine should be able to reproduce machine parameters repeatedly with out any error. The machine should have microprocessor controls which is capable of giving

    • 5 - 10 steps of injection speeds
    • 5 - 10 steps of injection follow-up pressure.
    • 3 - 5 steps of back pressure.
    • 3 - 5 steps of screw RPM.

These features of multistep control of machine parameters can - if suitably set - can have influence on quality of moulding.

The machine is not responsible for

  • Deviations of melt flow with different velocities in the complex shaped space between core and cavity. The part design is responsible for this.
  • Uniform distribution of mould surface temperature and its consistency with respect to time. The design of cooling circuit in the mould and coolant temperature and circulation pump capacity are responsible for this.

There should be periodical machine preventive maintenance practice so that there should be no unscheduled interruption of production.

MOULD which has to mould part.

  • Part has to be perfectly mouldable. All complex shapes need not be perfectly mouldable. Therefore, there is a need for understanding the perfect mouldability of complex shapes. The constrains of moulding process can be avoided by suitable part design.

  • Mould has to be designed to achieve quick stability of the moulding process. Therefore, through understanding of injection moulding process is necessary. The good knowledge of Wander-wall's equation, PVT diagram, Cavity pressure profile, Characteristics of plastic melt, Heat exchange in mould etc. are essential.

  • Using right steel, the mould parts are machined precisely.

When above mentioned factors are considered before manufacturing mould , then only it can produce parts without interruption in production. There should be scheduled for lubrication, inspection, servicing of cooling system of mould.

MATERIAL including additives of excellent quality standards and consistency are used. Knowledge of material characteristics and it's properties essential.

MAN who is able to visualise and understand what is happening inside the mould and plasticising screw assembly. He should be able to visualise forces acting and its influence on various component of mould. He should have clear understanding of fundamentals of the subject - strength of materials of metal and peculiarities of plastics.

Mr. William J. Patton- a moulder and inventor, graduate of University of Manitoba in Mechanical Engineering ,Canada, Author of PLASTICS TECHNOLOGY : Theory, design & manufacturing says in the PREFACE of his book:

" Plastics are more complex material than metals and therefore, are less understood (even by technical personnel) and not always successfully applied...

Normally we are familiar with metals and we tend to view plastics as a type of metals. Metals are hard, stiff and elastic, they corrode where as plastics have none of these characteristics. All metals have similar behaviour but one plastic does not necessarily behave like any other plastic...

Different types of same plastic may be totally unlike. Solid Urea-formaldehyde is a hard material while urea-formaldehyde foam is the softest material. Polycarbonate is very tough material, but it is not tough if a notch is cut in it or wipe it with a solvent...

Plastics are exciting materials to use, but they are deceptive in ways that no metals are. Therefore, skilled practicing engineers should have imagination and caution in working with plastics... "


It should be realised that every intricate shape need not be perfectly mouldable. Therefore, there exists problems which can be recurring processing problem or quality related problems. To solve these problems we should have in-depth knowledge of plastics technology. It can be realised that the root cause of all processing and quality related problems lies in part design and mould design.

The problems may be due to

  1. defective part design from manufacturing point of view.
  2. defective mould design from plastic melt characteristics and processing point of view.
  3. poor machine specification or poor machine controls.
  4. inconsistent material quality.

The problems belonging to (a) can not be solved by searching for solution in (b), (c), (d).

The problems belonging to (b) can not be solved by searching for solution in (a), (c), (d).

The problems belonging to (c) can not be solved by searching for solution in (a), (b), (d).

The problems belonging to (d) can not be solved by searching for solution in (a), (b), (c).

The solution for problems have to be in it's own domain.

All theses quality problems - listed below- are not new. They existed right from the beginning and they continue to exists. Obviously this shows men concerned have not applied their mind to the problems and hence they are repeated. CAE analysis software like MOLDFLOW can help in detecting these problems at part design stage itself and it enables to correct the defect in computer even before mould is fabricated. The useful data obtained from the analysis can be used to design perfect mould.

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Technical Papers by Prabodh C. Bolur

Understanding Energy Consumption in Injection Moulding Machine(1994)
Understanding Heat Exchange in Injection Moulds (1994)
Ideal Moulding Shop (2001)
Understanding Selection of Injection Moulding MachineThis paper was part of authors lectures at CIPET since 1980. It has been regularly updated.
Technological Solution to Injection Moulding of Plastics (1999)
Technological Tools for Part Design, Mould Design and Mould Fabrication (1999)
Extrusion of Thermoplastics(1998)

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