Design Essentials for Injection Moulding

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Protolabs’ Insight video series

Our Insight video series will help you master digital manufacturing.

Every Friday we’ll post a new video – each one giving you a deeper Insight into how to design better parts. We’ll cover specific topics such as choosing the right 3D printing material, optimising your design for CNC machining, surface finishes for moulded parts, and much more besides.

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Insight: Design Essentials for Injection Moulding


Good afternoon everybody, and welcome to another Insight video.

Now, the world of part design can be a pretty complicated one, and if you’ve been following us for a while, you’ll know that many of the videos in the series deal with particular techniques, materials or even industries that might want to make use of the tech.

However, while it’s great to know about all these esoteric methods, it’s important not to lose sight of the fundamentals. That’s why, today, we’re going to simplify things a little and focus on the essentials - the core elements of designing parts for injection moulding.

First up, let’s talk a little bit about draft.

If you’re a little unsure about what draft is, it’s essentially the angle you put on your part, when compared to the mould. If your part is sticking right out from the mould in a straight line – like, say, you were making a perfect cube – there would be zero draft. Angle those walls in just a little and you might have something like 3 degrees draft.

Make sense?

Draft is important because it’s what you need to ensure that a part ejects nicely. Material has a habit of shrinking onto the mould core, and if you’re trying to pull a part with minimal or no draft out, it can require a whole lot of pressure. This can damage parts or even the mould.

By adding a decent draft – 1 degree per 25mm of depth is a good rule of thumb – ejection goes a lot smoother.

Another seemingly minor thing that makes everything go a lot smoother is the use of radii. Which basically means avoiding sharp corners and adding curves instead. There are a couple of reasons for this, but the biggest is that it helps material flow. After all, you’re relying on the flow of liquid plastic to fill your mould, and liquids like curves. I don’t know about you, but I’ve never seen a river with a 90-degree bend in it.

Don’t get me wrong, you don’t NEED to make sure every corner has a radius, but it does make things a lot easier.

Right. Next on my list of essentials is… wall thickness.

The thickness of part walls is in somewhat of a Goldilocks situation. If you make them too thin, they can warp and bend, too thick and you run the risk of sink and having internal voids caused by air pockets. What counts as too thin and too thick depends on the material you’re using to make them, so something made of nylon can be a bit skinnier than an ABS part.

If you know what material you’re planning to use, make sure you stick up the recommended thickness somewhere near your workstation, and check that your design doesn’t exceed those limits if you can possibly avoid it.

On the subject of things to avoid if you can… if you’ve been watching these videos, you’ll know that we talk a lot about undercuts in designs. And while, yes, we do advise that you avoid them if possible, it’s worth baring in mind that they aren’t completely impossible to work with or anything of that sort. There are a handful of techniques we can use to make undercuts workable, from bumpoffs to hand-loaded inserts, and sometimes people forget this.

The final item on our list of essentials is to remember about gating and ejection.

Gates are the places where you inject the liquid plastic into the mould, while ejector pins are used to push your completed part out of the mould when everything’s cooled.

There are a handful of different types of gate, each of which have their own strengths and weaknesses. We could do a whole video on the minutiae of gate choices, but for now just remember that they exist, and make sure you think of them while putting your part together.  Perhaps have a chat with your supplier, after all they work with injection moulding day in day out so they could probably bore you silly about gates and loads of other stuff.

Similarly, remember that you need to get your part out of the mould, so keep potential pin placement in mind. And again if in doubt, talk to your manufacturer – you’ll probably be making their life easier down the road, so they should be happy to advise you.

Right. That’s everything done for this video. I hope it’s left you with a solid grasp of some of the really core things you need to keep in mind when dreaming up parts and designing moulds.

It just remains for me to wish you a great weekend and I look forward to seeing you again next week.




With special thanks to Natalie Constable.

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