Injection moulding is essentially injecting molten plastic into a cavity between tightly clamped pieces of steel. The plastic cools and solidifies, the mould opens and the solid part is ejected; this is referred to as a ‘cycle’ which is repeated to mass produce parts. When it comes to good part design, the main factors that should be considered are the amount of material that is being used per impression, the time it takes to produce the part and of course the cost. In short, good part design will serve its purpose and satisfy its requirements using as little material as possible and being processed in as short a time as possible. Material costs money therefore the less you use the less it will cost. The time on a ‘press’ (the machine which puts the mould to work) also costs money so the quicker the cycle time the less money will be spent on labour and use of a moulding facility. Understanding the moulding process and optimising your design for production can go a long way to improve the efficiency and subsequently the overall competitiveness of the end product by keeping costs down. |
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Assemblies If assembly is required it’s important to note that minimising separate components by moulding sections as one piece where possible is often the best route, especially if cost is a main driver of the product. The more separate components you have as part of your design, the more moulds or cavities will be required and moulds can be expensive. Material Careful consideration of the materials used will help to reduce costs, many materials have similar properties and would work just as well as more expensive alternatives if the options are laid out clearly and criteria matched appropriately. |
Part function Mechanical characteristics needed for part integrity mean an understanding of the function/design requirements of the part are an important consideration. If this part is under constant load it may require a thicker wall section than a part that is not under any load for example. Also, the uneven rate of cooling of these thick and thin cross-sections is likely to result in distortion of the part, even after it has been ejected from the mould. This distortion is often severe enough to prevent the part from meeting specifications. A thick cross-section is also likely to result in a depression on the surface called a sink mark, particularly if the cross-section is of varying widths. |
![]() Wall thickness is dictated by multiple inputs to the part. It is helpful to design all part cross-sections as thinly and uniformly as possible. |
Draft and tapers Amount of taper or “draft” refers to the amount of taper of molded parts perpendicular to the parting line. The draft taper should be determined early in the plastic part design process. The need for part surface tapering facilitates ejection of the part from the mold, especially in high speed, high volume production applications. |
![]() For rougher surface finishes, larger draft angles should be used. |
These are just a few of the principles which can be applied to improve the performance, cost efficiency and structural integrity of plastic parts. A detailed analysis is often necessary to understand the function of your part to direct us down the best route to building an injection mould tool.
If you have any questions just give us a call and we’ll be glad to look at your design in more detail.