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Injection Moulding Cosmetics – Cosmetic defects and how to avoid them

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Insight: Injection Moulding Cosmetics – cosmetic defects and how to avoid them

20.12.2019

Transcript

Hello and welcome to Proto labs' Insight video series. 

Today we’re going to take a close look at cosmetics and what you need to consider for a good-looking injection moulded part.

So let’s start with sink. As the name implies this is when you get a dimple or depression on the surface of a moulded part. It can be caused by thicker than normal cross sections, a non-uniform part design or by placing the gate in the wrong place.

It’s worth noting that some plastics such as polypropylene and acetal are more susceptible to sink than others; while it’s less of a problem for fibre and glass filled materials. Your supplier should be able to recommend ideal wall thicknesses for each material.

To avoid sink, we recommend that a workpiece’s minimum thickness should be no less than 40 to 60 percent of its thickest section and that material flow should travel from thick to thin whenever possible. This means that you need to think about the mould cavity orientation and where the gate is.

What is the gate? – I hear you ask. Well, it’s where the hot resin enters the mould cavity and it will leave a small ugly spot that is unavoidable, so position it on a non-cosmetic surface if you can.

Okay, let’s move on to warping. Now this is caused by walls that are too thin. Just like sink, the best way to avoid this is to stay within the wall thickness guidelines for a material. You’ve guessed it, your part’s walls should be neither too thick nor too thin.

If your walls are a bit on the thin side, then design parts with gussets to support them. Or for large flat surfaces, think about ribbing.

Again, different materials are more susceptible to warp, but this time it’s the glass filled materials (which are less prone to sink) that are more susceptible to warp.

Next up is flash.

If you look closely at a rubber O ring you will notice a thin line.  It’s a parting line, or seam, where two halves of the mould come together.

With free-flowing materials such as unfilled nylon, a small amount of flash can sometimes ooze into the seam and may need trimming after your part has cooled. For a donut shape, you have no choice about where the parting line is, but other parts have sharp corners which allow a clean junction for the mould to separate.

Talking about cosmetics, what if you want a part in cornflower blue or perhaps beige?  This should not be a problem, ask your supplier what colourants they have in stock. These are mixed with the resin pellets before moulding and, hey presto! You have your coloured part.

But – and there’s always a ‘but’ isn’t there – this is not an exact science and the final product colour may vary depending on the polymer used, the texture and polish of the tool, and swirling during the mixing process.

If you want an identical colour match on your part, then the best thing to do is to purchase colour matched, pre-compounded resin.

Next is something that you might notice, but is actually nothing to worry about. If you look very closely at a part you may see some fine lines. These are not hairline cracks; they are knit lines which are formed when two opposing flows of material join together in the cavity mould. 

You will generally see it at the edge of a hole and it’s not a problem unless they are present in an area of the part that will receive a lot of stress, such as at the head of a screw.  If that is the case, then you can design a strengthening boss feature around the hole or just skip the hole completely and drill it afterwards.

Next up is something that I have already covered in a previous video: draft. You should avoid vertical walls in your part, they are hard to eject and may leave you with drag or scrape lines. The answer is to design walls with a slight angle. We recommend a draft angle of at least ½ a degree, but 2 degrees is better.

Exactly what surface finish you want will depend on the part and its function. Not everything needs to have a cosmetic finish, but if it does, it’s generally quite simple to manually polish the tool. Most suppliers will offer a range of finishes from non-cosmetic through to something very smooth. You can also have a textured part to leave a uniform matte finish.

So, I’ve briefly outlined some common issues that could affect the cosmetics of your part and also what to think about. There are sometimes others issues, but in general there is nothing that you can’t solve by modifying your part design or selecting a different material.

Above all, work with your supplier from the design right through the process and you will end up with an injection moulded part that is both fit for purpose and good looking.

Right that’s it for this week. I look forward to seeing you again next Friday.

 

With special thanks to Natalie Constable.

 

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