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Cost considerations in design

2016/11/2 11:08:38

There are two areas of consideration in designing for injection moulding:

Design of the part to minimise production costs

Design of the part to minimise tooling costs

Production Considerations

Injection moulding is a heat process and plastics are poor conductors of heat,  that is to say it takes a relatively long time for plastic parts to cool down.  With this in mind the aim should be to keep sections of parts as thin as  possible, this will not only mean shorter moulding cycle times but also less  material content.

Strength - If the part needs some strength this can be achieved by the addition  of ribs rather than thickening of the section. With plastics, thicker sections  do not necessarily mean stronger parts, in some instances beyond a certain  thickness the part can become more brittle due to the resulting lack of flex.

Material- Another consideration is choice of material type and grade.  Thermoplastics can range in price from just over £1 per kg for basic polyolefins  up to £25 per kg and beyond for PEEK and other specialised polymers. By  designing strength into a part it may be possible to use a cheaper material but  sometimes the reverse can be true and advice should be sought from the moulder.

The price of polymers is also governed to a certain extent by the amount  purchased so some benefit may be gained by using a grade already used by the  moulder. Be guided by the moulder rather than sticking rigidly to a specified  grade.

Eliminate Assembly- If a number of parts are required, and especially when  converting from metal or a different production method, look at the  possibilities of combining two or more parts into a single moulding so  eliminating assembly. It is often possible to produce complex single mouldings  that would not be possible with a different method of production. In a similar  vein, where assembly is required every effort should be made to make this as  simple as possible with the use of snap-fits, for example.

Tooling Considerations

Combining Parts - Injection mould tooling is relatively expensive but it is not  always correct to assume that it is only suited to long production runs. If it  is possible to combine a number of parts into a single moulding, for example,  then the cost of assembly and possible other ancillary parts can be saved making  shorter production runs economical.

Keep it Simple- The simpler the mould tool the lower the cost. If possible avoid  holes in side walls of parts, undercuts and other complex features. The mould  tool can then be what is referred to as straight open and close. If the part  does need side holes, for example, then these can either be moulded in which  would require side movements in the mould tool, or a secondary operation will be  necessary. Which approach is adopted will depend on anticipated quantities of  the part – the higher the number of parts required the more economical it is to  have the features produced by the mould tool rather than as a secondary  operation.

The same rule applies to threads. Internal threads can of course be tapped  afterwards but the usual method is to have them moulded in. This can be achieved  in two ways, either with hand loaded cores or with auto unscrewing in the mould  tool. The former requires operator intervention during the moulding process so  adding to the part cost whereas the latter means the mould tool can be run fully  automatically but will mean a higher tooling cost at the outset.
Multiples- To reduce both total tooling and part costs where a number of similar  parts are required in equal quantities and in the same material, it may be  possible to produce a family tool, i.e. all the parts moulded in a single cycle  from a single tool. Also, where large quantities of a part are required, a  multiple cavity tool could be used, i.e. two or more of the same part produced  in a single cycle. Although the tooling cost would be higher than for a single  impression tool, the part price will be lower so making it cost effective. The  moulder can advise on the optimum configuration.

Conclusion

There are a number of factors that impact upon part and tooling cost: material  selection, section of part, complexity of design, etc. What is certain is that  the further down the design/production cycle you are the more costly any  alterations become. Therefore, it is advisable to get the initial design right  for injection moulding and to this end you should involve your injection moulder  at the very early stages of the project to ensure correct design and material  selection for the given application.

Tags: plastic mould, injection mould,