Pitfalls in Injection Moulding of Plastics
Click to see Myth in Injection Moulding PITFALLS
Injection Moulding
of Plastics


Things to Know in Injection Moulding of Plastics

Pitfalls in Injection Moulding

Myth in Injection Moulding

A Guide to Injection Moulding of Plastics

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During my tenor as Regional Manager of WINDSOR in Madras between 1978 ∓mp; 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.

Steps in Injection Moulding

Injection Moulding of plastics involves quite a few steps before actual moulding operation. The first step has to be plastic part design. There after mould design, mould fabrication, selection of correct machine and process setting, post moulding operations, assembly etc.

Quality in Injection Moulding

The root causes of quality problem may or may not lie in the present step. 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.


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.

APPLICATION of BASICS- Physics and Engineering

Similarly trainability of the people depend on their understanding of basics of school physics. Those who can recall meaning or definition of each technical terms -he uses every day- is trainable.

It is necessary to understand HEAT, TEMPERATURE. THERMAL CONDUCTIVITY and HEAT TRANSFER. In fact, with out understanding heat exchange that is taking place in the mould one can not understand the causes for inconsistent- shot to shot variation of - dimension, dimensional distortion or warping. Other important basic technical terms are Viscosity, Shear, Pressure/stress, Speed, Power, Energy. All these terms are learnt in secondary school. Therefore, knowledge of school physics is adequate to understand the processing of plastics.

Let us understand that the learning of any subject. The subject can be broken-down to number of elements / parts. These elements/parts can be grouped in to two groups. This grouping has to be based on the ease of grasping the each parts/elements.

  • Very Obvious and hence simple to grasp for any one and
  • Not very obvious with out understanding basics of physics hence difficult to grasp.


I can summarize the successful moulding to three balancing tricks;

  • Flow balancing – part geometry dependent.
  • Heat balancing in one cycle time- dependent on design of cooling in the mould and cooling water supply system.
  • Uniform freezing of melt in the mould. – Dependent on cooling design and supply system.

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

PITFALLS- not Obvious

My book " A Guide to Injection Moulding of Plastics" discusses the following not obvious topics;

  • Shear thinning nature of polymer melt and resulting complications while mould filling phase of moulding cycle.
  • Limited thermal stability of engineering polymer causing generation of gas bubbles / degradation of polymer resulting in poor quality of moulding.
  • Understanding the moulding process from the point of view of understanding the settings of process controls in the microprocessor controlled machine. Earlier machines did not have the capability to control the injection speed/rate in multiple steps (of stroke) during the filling phase of the moulding cycle and to control follow-up pressure in multiple steps (of time) with constant injection rate.
  • Significance of injection rate while selecting the machine is discussed. The melt has to move at an injection rate, which enables it to reach farthest regions in the mould before no-flow point -freezing- is reached.
  • Clamp unit appears big but it requires lesser power to operate. Whereas injection unit appears smaller but it consumed more power. How different types of hydraulic controls influence power consumption is discussed in the book.
  • Testing of Injection Moulding machine before purchase should be of interest to all the users of the machines.
  • Screw plastcating and screw design parameters are discussed very thoroughly.
  • PvT characteristics and its importance.
  • How to determine shot weight of the machine?
  • Principles behind setting up of barrel temperature profile.
  • Shrinkage influence on dimensions of mouldings.
  • Root causes of quality problems like dimensional stability, sink mark, warpage, shrinkage, stress cracking, weld lines, orientation are discussed.
  • Root causes of mechanical failure of mould
  • Advancement in molding technology, use of CAE to produce flawless mouldings multicomponent, gas injection, thermoset moulding,

Glossary of technical terms includes definitions of basics of physics, engineering and computer and software terms.

To see this paper "Pitfalls in Injection Moulding" with hyperlinks to sketches and notes, please visit following websites:




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Prabodh C. Bolur

Landon Towers Flat 27, 47 Landons Road, Kilpauk, Chennai 600010.-INDIA

Phone: 91-44-641 20 32 e-mail: bolurpc@hotmail.com


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