Making Changes

Your masterclass in product design and development


Protolabs’ Insight video series

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.

So join us and don’t miss out.


Insight: Making Changes


Hi and welcome to another Insights video.

This time around we’re going to be tackling an incredibly important concept that’s really rather easy for us all to forget – making changes to your design.

Why is this so important? Well, as much as we’d like to think that our first ideas are always the best, that isn’t always the case. In fact, now I think about it, it almost never is. There’s a very good reason why people come to us to help develop their prototypes, as over the course of testing they find that this wall is a little too thin, or that particular type of catch doesn’t work quite the way they want it to.

The whole purpose of the prototyping process is to allow yourself the option of tweaking your model before locking in a final design for production, but while this is an incredibly important stage you can see costs creep up if you approach things in the wrong way. If you’re injection-moulding plastic prototypes, for example, making what might seems like a few small tweaks can entail the creation of a whole new mould. This doesn’t have to be incredibly expensive, in fact, with the right manufacturer it can be very affordable, but if you can make the change by modifying your existing mould, the costs will be even lower still.

With this in mind, there's one critical fact you need to remember if you want to be able to modify your part by making a few changes to the original mould – namely, that while it’s relatively easy to remove metal from an existing metal mould, adding it can be really tough. Or, for that matter, pretty much impossible if you’re using rapid injection moulding.

In short, remember that you can add plastic, but you can't take it away.

Designing with this principle in mind is called "steel safe" or "metal safe," depending on who you talk to, and doing so can save you both money and time when you have to modify your design.

In a practical sense, this has a few major impacts on the way you make things.

For example, you may be able to thicken a wall as you fiddle with the prototype, but you can’t make them thinner. Similarly, you can add features – bosses, raised text, ribs, pins, that kind of thing – but you can't remove them.

When it comes to holes, you can reduce the diameter of a hole by adding plastic around the perimeter, but you can't increase the hole size. And, likewise, you can eliminate holes, but you can't add them.

The main rule to remember when initially designing your part is this: maximise metal, minimise plastic.

If you aren't entirely sure whether a feature is needed or whether a feature is the right size, you might want to review things feature by feature, asking yourself which ones may need to be changed in later versions before committing your design for prototyping.

When there’s any doubt as to how thick a feature should be, start thin and then step things up if needed – we can modify the mould to make things thicker, but getting thinner requires a whole new mould.

Unsure whether you'll need a rib to strengthen your part or a brace to prevent warp? Leave it off and add it later if it's needed.

Have two mating parts that might - or might not – need alignment pins? Put the holes in and leave the pins off. If you need them, you can add the pins in the next iteration. If you don't, you can easily remove a little bit of metal and eliminate the holes later.

Now, sometimes this can all be a little counterintuitive, as you need to bear in mind that removing things from the mould has the opposite effect on the part, and vice versa. So, the fact that we can remove bits of the mould means we can’t remove bits from the part, if that makes sense. If it doesn’t, don’t worry too much. Once you get used to it, things start to make much more sense. Trust me.

Anyway, on that note I think we’re out of time for today, so that’s it from me until next week. Have a great weekend, and I’ll see you then.



With special thanks to Natalie Constable.

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