Designing with Confidence: 8 ways to leverage low volume injection moulding

By Laura Reeves
Aluminium mould tool

Steel tooling for injection moulding can take months to be ready.  Using aluminium tooling you can bridge that production gap and make sure that your design is spot on before committing to mass production.  It still pays to get the design right so here are our top tips.

Mass production may bring your organisation savings in the long term, but equally the cost of getting the design wrong is every designer’s nightmare.  You’ve got months of waiting until your steel tooling is ready and if you or your boss are not happy with the finished result, you’ve got several more months of waiting for a second iteration.

This could hold up product launches and cascade right through the business.

But you don’t need to lie awake at night worrying.  There is an interim solution that can cost effectively ensure you get the right design and also fills the production gap while you are waiting for your long-term tooling.

Finding a supplier that provides aluminium tooling and extra production capacity allows you to check your design through rapid prototyping of different iterations or to bridge a manufacturing gap to produce anything from 25 to 10,000 parts.

While aluminium tooling is more cost effective than steel and also has a faster turnaround, it still pays to get the design right.  Consider the following factors to get the most from your design so that, when you are ready to opt for steel tooling, you have real peace of mind in an already proven solution.

1. Design for Manufacturability Analysis.

Once you receive a quote from your chosen supplier, you can set primary manufacturing requirements such as your preferred material, surface finish and what production quantity you require.

The first thing to check is that your supplier offers automated design for manufacturability analysis (DFM).  This will include recommendations on how and where to modify part geometry to improve mechanical strength, reduce sink, adjust wall thicknesses and many other factors.

2. Surface finish

Ask yourself whether you really need the highest quality surface finish, is it really necessary for the part’s function, because it will add to your cost and the lead time for your tooling.

A very smooth SPI-A2 finish requires mould cavity surfaces to be hand-polished with diamond buff 10 1-2Ra.  The least expensive part finish is a non-cosmetic PM-F0, which is an as machined surface that will probably show some tool marks.  There are of course other options in between these two extremes.

As a general rule use the lowest-cost finish that is suitable for your application.

drafted vs. undrafted

3. Don’t dodge the draft

Imagine your frustration if you can’t get the ice cube out of the tray for your gin and tonic. Fortunately, ice cubes have tapered and rounded edges (or draft) so that you can get them out easily.

It’s exactly the same with injection moulded parts, designing in a draft allows it to be ejected.  Your part should carry at least 0.5 degrees of draft on all vertical surfaces and, if your design allows it then, 2 -3 degrees is even better.  And if you have a heavily textured or complex surface, then you may need to allow 5 degrees or more.

Most CAD programmes can take care of this for you during part design as long as you understand how the part will be orientated in the mould.  If you do miss the draft during your initial design, then your suppliers DFM analysis should pick it up.

4. Walls – don’t be thick

Surely thick walls are good I hear you cry.  But remember your material is plastic and if your walls are too thick for a particular thermoplastic family then they could warp and sink during cooling.  As a general rule you should design walls to a consistent thickness of between 1 to 3.5mm.

As always there are exceptions, acrylic can go to 12mm, polyurethane to 18mm and certain fibre reinforced plastics can have 25mm or more thick walls.  Check out how thick you can go with different plastics here.  

5. Watch the Radii

Sharp internal corners are a problem for a couple of reasons.  From a practical point of view aluminium moulds are machined using end mills so they are hard to achieve, but more importantly they tend to create stress and weaken the structural integrity of your part.  We recommend being generous with your radii in part design.

As always talk to your supplier for advice.

Protolabs IM machine centre

6. Multiple cavities can be good

To really save production time and cost, you may get the opportunity to mould several parts in a single injection cycle by using a multi cavity or family mould.

For prototyping, you could use a multi cavity mould to produce two or more part designs with minor iterations to test different product designs.  You can also use them to reduce your cost per part, but you need to bear in mind that their use is dependent on geometry.

7. Material choice

Different plastics have different properties in terms of durability, strength, chemical and thermal resistance.  Other options might include liquid silicone rubber, so think about what you want your product or part to do and then ask your supplier for advice.  You can find out more about different materials used in injection moulding here.  

8. Moving to mass production

Understanding what is and what is not possible when designing a product or part for injection moulding is vital if you want to get it right.  Aluminium tooling offers you the option of testing your design through quick turn-around injection moulding.

It means that when you are ready to commit to steel tooling, you have every confidence that the design will be right from the beginning.  You will not hold up production or a product launch and you can sleep at night.

Oh, and if you need to bridge that gap between waiting for your steel tooling to arrive and production then aluminium tooling could be the answer.