Have you ever heard a designer say, “form follows function”. What does that mean anyway?

I had someone ask the question “should my part be injection molded or vacuum formed?”. Great question and it really comes down to the form of the design and the desired function. Let me explain …

Form follows function. If an object has to perform a certain function, its design must support that function to the fullest extent possible. Part of design is taking into account all of the constraints that push and pull the design.

Often times, cost is a major design constraint. As a rule of thumb, a vacuum formed part will be less expensive in both tooling and piece part.

Great, just vacuum form it then! Not so fast. Not all designs, i.e. functions, are possible with a vacuum formed part. So how do you know which it is?

Visualize this. Imagine the part you designed needs be completely covered by a thin sheet, imaging a bed sheet floating down perfectly flat and when it hits your part, it conforms to the shape you want. Like the sheet was soaked with water and the sheet is a bit stretchy in parts.

If it can do this, it most likely can be vacuum formed. The part also needs to a uniform thickness. Look at the plastic covers on a cup of coffee; those are most likely vacuum formed.

The picture to the left shows the parts before they are cut from the sheet of plastic they are made from.

If your part has a very detailed inside and outside then probably it needs to be injection molded. Have you ever taken apart a TV remote control or a power drill? The parts that you hold, the housings, have very complex, precise and intricate shapes that allow them to be assembled to other parts. The walls are of different thicknesses and there are areas where it has shapes that hang over, these are called undercuts.

Here’s a picture in lieu of 1,000 more words.

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