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Insight: Complex Undercuts
Hi it’s almost the weekend, which means it’s time for another Insight video.
This week I’m going to talk about knit lines. Don’t turn off, I’m not talking about knit one pearl one or whatever you do when making a cardigan, but about a problem that you may face with injection moulding.
We all know that injection moulding is great for producing both custom prototypes and end-use production parts, but if you are not careful it can cause some problems such as stress factors, warp and of course knit lines.
In this episode I’m taking a closer look at knit lines. I’ll tell you what they are, how they are caused, problems they can cause and how to avoid them, or at least minimise their effects.
So, what is a knit line? It’s a line that may or may not be visible where two resin flows meet.
To understand how to avoid them, let’s take a step back and look at the injection moulding process itself.
It involves heating resin in a barrel to a molten state and then injecting it into a mould to form a thermoplastic resin. As you inject the resin into the mould it cools, which is why the leading edge of the resin flow is the coolest and therefore the closest to solidifying.
This can cause a problem if an obstacle divides the resin flow so that it meets again on the other side of the obstruction.
Ideally these two flows will meld together to form a solid joint, but if they have cooled down too much they will not meld properly and will form a knit line.
A good example might be the core that creates a rectangular hole in a cover plate. Think about your TV remote control for instance. The cover has multiple holes for the buttons, which could potentially create several knit lines. There are ways to avoid this of course, but more on that later.
Another situation where you might experience knit lines is if you use more than one gate. A gate is where we inject the resin into your part. Your supplier will not normally use multiple gates, but if they need to then check the gate and ejector layout to see if you have any critical cosmetic or strength requirements about halfway between them.
Whether this is an issue really depends on where a knit line is on your part. It might not be a problem at all, it could just be a cosmetic issue or in the worst-case scenario it could cause a potentially serious structural problem.
One place where a knit line is an issue is behind a boss. Just to remind you, a boss is a feature with a hole designed to accommodate a thread fastener. It is created by a core pin inside the mould around which the resin flows, so when they meet on the other side, they can form a knit line.
If the boss is near the edge of a part, then the knit line will be very short leaving very little surface holding the two faces together. When you come to drive a screw into the boss, that knit line could become a crack and then you have a real problem.
There is another factor that can cause problems when it comes to knit lines and that is using filled resins. Let’s say you are using a glass fibre filled resin for your part. As the resin front moves through the part, the fill material, in this case the glass fibre, will be behind the front edge because it does not flow as smoothly as the resin itself.
This means that when the two fronts meet and solidify there is little or no fibre crossing this line. It does not mean that the knit line will be weak, but it will not be reinforced by the fibre.
Understanding what can cause a knit line and where they could be a problem will help you design a solution that overcomes this issue.
Admittedly you can’t eliminate features like bosses, but you could thicken the part walls to slow resin cooling but remember not to thicken them too much or you could cause sink. Where possible you can also place knit line causing features further from the edges of parts.
You can also choose to avoid using filled resins in parts that will have features like through holes.
Two final thoughts for you. The first is general good practice – talk to your supplier on injection moulding, they know what to do to help avoid or minimise the effects of knit lines in your mould.
Second think about prototyping before moving to production. If your supplier uses aluminium moulds then you can test your part design before you commit to production. Better still you don’t have to wait the 10 -12 weeks for a steel mould and then find out that you have an issue. At Protolabs for example we would typically take about 15 days to produce the mould.
Right that’s it from me this week. Have a great weekend and I’ll see you again next Friday.
With special thanks to Natalie Constable.