Design for manufacture using the injection moulding process requires a certain approach. Working with us saves time, money and results in a suitable plastic moulding and tool design from the outset. All through the process we share our expertise with customers and together we optimise the choice of plastic material, physical shape, ease of production, cost and performance. The aim for the most cost-effective and sustainable solution is to design parts in their finished form, i.e. without the requirement for secondary operations and with no waste of materials. There are various design for manufacture considerations and more information can be found in our knowledge bank on our website.
A wall section that is too thin can lead to structural failure or poor insulation characteristics. However, a wall section that is too thick can result in appearance defects and an overweight or over-engineered part. With the latter point it is also worth remembering that wall thickness governs the moulding cycle time – the thicker the section the longer the cycle time and therefore the more expensive the part becomes. Furthermore, plastics shrink during cooling which in thick sections can result either in the surface of the part forming a sink mark or an internal void. In most applications, a thin, uniform wall with ribs is preferable to a thick wall. There are general guidelines for how thick a part should be, however, this does depend on the design and function of the part concerned. It is possible to mould much thicker sections if required, Rutland Plastics has successfully moulded void-free parts up to 120mm thick.
Another point to consider is to avoid sharp corners because:
- Sharp internal corners tend to stick in the mould as the part shrinks onto the core and so impacts the ejection of the parts.
- Resin flows more easily around smooth radiused corners rather than sharp ones.
- Shear around sharp corners can degrade the material and cause aesthetic problems.
Bosses can be incorporated into the design and these are usually incorporated to facilitate mechanical assembly. They can be designed to accommodate self-tapping screws, push-in or moulded-in inserts or used for ultrasonic welding. Therefore, the boss may have to withstand a variety of forces – tension, torsion, compression, shear and flexing.
Undercuts should be avoided if at all possible. In certain circumstances, mouldings with slight undercuts can be ‘bumped’ off the core. The same results as an undercut can be achieved using certain design techniques.
With a normal cavity and core, the cavity is stationary and the core is attached to the moving platen. In the majority of cases the part ‘sticks’ to the core due to shrinkage of the material moving it away from the cavity and tightening it onto the core. In order to facilitate part removal from the mould, it is necessary to incorporate draft angles.
Textures and lettering can often be moulded onto the surface of the part. Texturing can help change the aesthetics of the part and lettering can add a brand logo or product information at the point of manufacture so reducing the need for a secondary operation. We frequently modify tools using laser engraving to add lettering to an existing tool so that the lettering is incorporated at the point of manufacture. The video shows lettering being added to an existing tool.
These are just some aspects to consider with design for manufacture and we are happy to offer advice and support at this critical stage, just contact us for more information or take a look at our design guidelines.