Plastic part or sand castle?

In a previous article, we explained how the shape of a part plays a crucial role in its ability to be removed off its mold.

Once we have decided the shape of the part and how the mold will open, we can finally design the technical bits, such as the grid that make the part stronger, or the screw holes that we use for assembly.

If we want to be able to take the part off the mold, we have to make sure that any bit we add has a correct shape (i.e. can be taken off the mold as well). And we need to make sure that no surface is vertical.

Screw holes shape

Let’s look at one of the screw hole shape on the back cover, as highlighted below.

Screw hole, back cover

As you can see, its outside is not perfectly upright. It has a slope of a few degrees.

We use this feature to allow the air to get into the mold easily when we remove the part. If the air can not get in - as it would if the surface were upright - a suction effect happens, which makes it almost impossible to remove the part off its mold.

Vertical mold, injection

The mold has vertical surfaces, the air cannot get in, there is a suction effect.

Slight slope, injection

As the surface has a slight slope, the air can get in. We can easily remove the part.

And if you remember the bucket you used at the beach to make sand castles, then you probably remember that they had a slight slope as well. That way you could actually unmold your sandy masterpiece without breaking it. But also, because most of these buckets are made using plastic injection themselves, they also had to be taken off their own mold at some point !

The clearance angle

The angle of that slope is called the clearance angle.

And it is not just for cylindrical shapes. Everything is designed with clearance angles: none of the surfaces you can see on an injected product can be perfectly upright. This applies for example to the lines you can see inside the back cover to make it more rigid, or even to the outside surfaces.

The actual value of the clearance angle depends on a few parameters, such as the depth of the mold cavity or the chosen aspect of the plastic material. For instance, the angle will be bigger for a rough surface than a smooth one.

So, once you have correctly designed the shape of your part part, found a parting line that works, and added a clearance angle to all the surfaces, you should be able to take your brand new part off its mold quite easily !