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Plastic Injection Mould Tool Design

Good Practice

There is an extremely wide variety of mouldings that have been produced, ranging in complexity from straightforward to highly technical, since the introduction of injection moulding.

Despite the wide range of geometric forms of mouldings that are produced, it is possible to classify the mould designs into two broad groups when a new component has to be moulded:

  1. Those that may be based on existing proven mould designs
  2. Those that require a totally new design

It is clearly advantageous to use a previously proven design and adapt it to suit a mould design for the new part when it is possible, since with all existing designs the bugs will have been ironed out and their behaviour in production is fully known

If a sample moulding has been supplied when a new mould tool is required, this will often provide useful information such as gate position and size, position of ejector pins etc. If a part has been successfully moulded before, the new mould design should be based on this unless, of course, the reason for laying down a new tool is the fact the first was not successful.

Simpler designs always beat the complex, highly intricate ones for reliability and product quality. It is also good practice to use tried and tested  designs and mould components as opposed to totally new untried ones.

Gaining experience in mould design is largely a question of learning over a period of time and by achieving the following:

  • Become involved in the Design of as many mould tools as is viable
  • To study and understand other existing tool designs
  • To become familiar with toolmaking techniques
  • To observe and understand how mould tools work while in production
  • To have an appreciation of the different types of plastic material, particularly the difference between amorphous and semicrystalline materials.
Mould Tool Design

Predesign Analysis

New designers should place the emphasis on solving problems. Look at all the mouldings you can find and try to detect how they were moulded, were the split lines and gates are, where they were ejected and how many special features were formed.

Try to come up with at least three different approached for split line positions, gate positions and type of ejection. At this stage the cavity construction must also be taken into account as must water cooling.

Observing Mould Tools

Many mould designers may never see the mould they have designed in production!! This is a serious shortcoming. All designers should attend initial tool trials for two reasons:

  • Moulds are often complex and need to be explained to the sampling technician. Many breakages have occurred when complex unscrewing mechanisms or phased latch controls have not been fully explained by the tool designer:

Example of threaded mould:

  • The mould designer can obtain valuable feedback on the design and can implement improvements or “tweaks” that will help the mould run more efficiently

It is also very instructive to look at and observe other mould tools while they are running to study other people’s designs.

Summary of good design practice

  • Use the simplest possible design
  • Make full use of standard components
  • Use tried and tested design in preference to new, unknown designs
  • Critically examine any existing samples that have been provided for gate positions, ejector positions, sinks distortion etc.
  • Check with the toolmaker that they have understood your design
  • Attend sampling trials for essential feedback information and to advise the sampling technician on tool function.

Thread Tool Trim Video