In this post we're going to take a look at three common trouble makers for injection moulding designers: Living hinges, bosses and overmoulding. These techniques allow you to incorporate more engaging features into your moulded part design, which is why it's really important to master them.
1) Living hinges
Living hinges are a great way to keep the two halves of a moulded container together. Take a look at a vitamin dispenser or mint box—chances are good there’s a clip of some sort on one side and a living hinge on the other. The biggest consideration here is material. Where polycarbonate might make a good clip, it definitely won’t survive the thousands or millions of cycles expected of a living hinge. Shoot for polypropylene instead.
2) Bosses & stand offs
A bad boss can be a major problem. But if you need somewhere to stick a threaded insert, a boss will certainly be necessary. Yet bosses, like tall ribs and thick standoffs, are potential problem areas. Additional draft up to 3 degrees or more might be needed to avoid ejection problems. Make any of these part features too thick and sink becomes an issue. The taller the feature, the deeper the mould must be, which means longer end mills and slower feedrates are needed to cut it. This also raises concerns for venting that may result in shorts, burning, or simply incomplete parts.
Some ways to avoid this include using vertical ribs or gussets around the periphery of the boss to support it, thus allowing thinner walls to be used. Be aware that Protolabs may need to place vent holes in deep (tall) ribs, standoffs, and bosses. And when angled, features such as this are a real pain in the neck because their axes diverge from both the direction of mould pull and the parting line, pretty much guaranteeing a hand-loaded insert will be needed.
3) Overmoulding
While overmoulding requires more complex design, processing, and materials choice than single-shot injection moulding, it offers significant benefits:
- It allows materials to be combined to provide characteristics that no single resin can deliver.
- It can eliminate assembly steps, saving both time and money.
- It can meld materials in a way that assembly processes cannot match.
- Inserts add strength and durability to parts.
When adding an overmoulded feature to your part design there are two things you need to consider. 1) Make sure the two materials used are compatible. 2) The two materials should form a secure bond (this is something a Protolabs Applications Engineer can help you with).
If you want to find out anymore about injection moulding at Protolabs, check out our service pages.