Draft is one of the most important, yet rarely discussed, parts of injection moulding. Draft means angling the walls of your product so that the part doesn’t get stuck up on your tool. Most people say that 3 degrees of draft is sufficient to allow the part to fall off the tool when it’s ejecting the part. And 90% the time, most people would be right, but I’ve seen tools with 7 degrees of draft on them, stick to the tool like white on rice. And how did that happen? Well the 3-degree rule breaks down as you get smaller and smaller.
A 10mm high wall with 3 degrees of draft, will angle out about 0.52mm, which will be plenty to pop that part off the mould, with sufficient ejector pins. Take that same 3 degrees and put it onto a 1mm high wall, it will angle out 0.05mm – to put that number in perspective, take a human hair and then cut it in half, and you’ll get about 0.05mm. Now, most of you will have picked up that it only has to travel 1mm to pop of the tool, and you must be saying “Surely it wouldn’t stick to it, Mark.” But believe me, I’ve spent hours melting and scraping plastic off a multi cavity tool that had 7 degrees of draft and plenty of ejector pins.
I learnt two lessons that day when it comes to draft. First, it doesn’t matter how much draft angle you’ve got, if the distance is less than 0.5, your part won’t survive. And second, I don’t care if you’ve got 3 degrees or 4, I will always ask for more. And guess what, I always ask my design team for more draft, because it never hurts to have more than enough.
A little caveat to that second rule. The main reason we have draft is so that it can eject cleanly from the mould, but you never want people to see the draft in your product, and most never will. Customer’s eyes will look at it and say “It’s very close to straight” and all their brains will hear is “It’s Straight” and then they will forget about how it looks. When I say more draft is always better, that’s completely true but just be aware that it will affect how your part looks and how people will subconsciously process your product. We understand that it’s important to match your clients expectations, just like we want to exceed our clients’ expectations.
If you have a part that needs draft, or just want some expert advice to analyse the part you want to make, call in for a free, no-obligation 1 hour meeting with our design team and we’ll be able to point you in the right direction. Or if you’re in a rush contact us directly.
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