Injection Moulding Product Design Guide
This is a short guide for designing optimum plastic injection moulded products. The aim is to give you a brief insight into the complexity of designing plastic components that satisfy Fit, Function and Manufacturability, as well as OPPD – Optimised Plastic Product Design.
The concept of heating plastic until it becomes fluid, then forcing it into a much colder metal mould to cool and take shape, seems fairly simple. But there are many pitfalls that a product designer must avoid, if components are to be reliably manufactured at the lowest cost.
Production cost is a combination of the weight of the plastic part (polymer cost) and the production rate or cycle time. Put simply, the faster the parts are made, the lower the manufacturing cost. Therefore, the product designer should minimise the polymer content and enable fast production when developing an optimum design. However, due to physics, the design process is often complex.
Thermoplastic polymers shrink in volume (increase in density), as they cool from being molten and fluid enough to be forced into the metal mould. Polymers solidify until they are rigid enough to be ejected from an opened mould. If this change was consistent throughout the geometry of the part, then all that would be needed would be to make the mould larger by the percentage that the polymer shrinks. Indeed, this is commonly done by the mould maker by using an average value for the polymer.
Different polymer types shrink more or less than others. But the biggest issue is that the polymer will shrink more or less in different regions of the component. This happens for a variety of reasons, but understanding these reasons has always been a major challenge for the product designer. Nowadays, computer based predictive technologies, such as Autodesk Moldflow, provide detailed information.
Using predictive technology, the total injection moulding process can be tested using the product design’s CAD geometry. Every aspect of dimensional deviation from the CAD and its cause can be identified. However, determining the causes of these deviations is not simple.
The differences below may combine to prevent the production of the perfect part:
- Wall thickness
- Lack of symmetry
- Feed position
- Pressure needed to fill the mould
- Orientation effects of the polymer flow
- Packing time and pressure
- Mould’s heat removal efficiency
One solution is to extend the moulding cycle time by increasing the time the mould stays closed, so that the polymer cools more. At lower ejection temperatures, the polymer has a higher modulus (stiffer), so it is able to freeze in internal stresses that would otherwise distort the product. Of course, this will increase manufacturing costs by decreasing output per unit time.
Another key point to consider is that this ‘solution’ may lead to product distortion or even failure, as the latent residual stresses work overtime to comply with the laws of physics.
So, here are some brief design guides:
- Before creating any geometry, learn the characteristics of the chosen polymer: Is it amorphous of semi-crystalline?Does it contain a filler, such as talc or glass fibres?
- Keep wall thicknesses as even as possible, so that the polymer cools at the same rate throughout the part.
- Feed the mould cavity on at least one axis of symmetry.
- If you want ‘A’ surface perfection, eliminate or minimise ‘B’ surface detail, such as ribs and bosses.
- Use predictive technologies to evaluate the mouldability of the geometry early in the process. Develop the optimum geometry through an iterative process of analysis and change.
- Accept the fact that physics determine all outcomes
Do you have a product design that you would like to discuss?