Your Insight into Cooling Rates
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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: Fill and Cool Rates in Moulding
Hi everyone, and welcome to another insight video.
Today we’re going to be tackling one of the most finickity aspects of injection moulding – how to work around your part’s filling and cooling rate.
And I cannot tell you how hard it was to resist making a joke about how cool the process is, or how this is going to be a really cool video.
Anyway. When we fill a mould with our resin, it’s easy to think of the plastic as being totally liquid – like we’re pouring water into an ice-cube tray. However, in reality, the molten plastic is… well, it’s plastic. It’s pretty viscous stuff and it sets into a solid shape relatively quickly – which is the whole reason we’re using it in the first place.
The fill rate, then, is a measure of how quickly the resin fills up your mould. The cool rate is a measure of how quickly it cools down. Just remember that in this business “cool” really means “solid” – you probably don’t want your bare hands to go anywhere near these parts when they’re coming off the machine.
Now, in many ways upping the cool rate is good, because a lot of the injection moulding cycle – usually more than half – is spent waiting for the resin to cool down. However, this needs to be applied carefully and balanced against the need to get the entire mould filled up.
If the resin begins to set the moment it enters the mould it can force you to squeeze the rest of your liquid plastic through increasingly narrow gaps, upping the pressure around your injection gates. The higher the pressure, the more difficult the fill, which can translate to weak knit lines as well as unwanted flash and sink.
In order to avoid this kind of uneven setting, it’s vital that you keep the mould temperature as uniform as possible. There are a handful of factors that play into this, but the most important of them is the thermal conductivity of your mould – the higher it is, the more easily that heat can flow about and even itself out. This makes the choice of moulding material incredibly important.
If you have relatively high tolerances on your part, you can likely get away with something like tool steel. However, if you really need to keep the design of your part as precise as possible, you may need to look at something like beryllium copper, which has a thermal conductivity three to five times higher than the tool steel.
Of course, sometimes there’s only so much that some fancy materials can do for you. If you’re finding that a part is still struggling to get reliably filled, you may want to look at techniques like adding in multiple gates to fill it from several angles at once or increased wall thickness. Sometimes even a change in your plastic material might be the answer you’re looking for.
If your part has many narrow channels that could easily be blocked by setting resin, you may even want to head back to the drawing board and see if there’s a way to eliminate, or at least diminish, the problem.
Once you have all the kinks worked out, you’ll probably want to make sure things are running as efficiently as possible. In practical terms, this means cracking open your moulds as soon as the resin has cooled.
Working out this ideal cooling time can be tricky, as it depends on things ranging from your thermal conductivity to the material’s density and specific heat capacity, with things getting even more complex if you have a complex geometry.
In the past, you sometimes had to rely on plain old trial and error to find the perfect answer, but thankfully some modern tech can help to smooth things out a little. We now have access to some pretty great simulations that can take in the shape of your design and the materials, and do a lot of the heavy lifting for you. Which is a relief, I’m sure.
Right. That’s about all we have time for this week. Make sure you check back in next Friday when we’ll have something brand new to chat about. Have a good weekend everyone.
With special thanks to Natalie Constable.