Moulds: designing a 3D printed insert

Design a 3D printed insert mould in Fusion 360.

Eleven steps, two printing routes, and a finished example file to work from.

A 3D printed insert is a mould cavity you print yourself and drop into an aluminium housing. It gets you injection moulding a new part without commissioning an aluminium mould first. That matters when you are still testing a design, or when you only need a short run. Print it, inject, change something, print again the same day. An insert will not last like machined aluminium, so treat it as a tool for a run of parts rather than a production mould.

This guide uses Fusion 360 because it is free for personal use and it has the four commands the job needs: Split Body, Shell, Extrude and Combine. If you already work in another parametric CAD package, the same steps translate.

It does assume you can find your way around Fusion 360. If you have not used it before, run through Autodesk's introductory tutorials first, then come back.

What you need Jump to the steps

The finished insert mould open in the Fusion 360 workspace, ready to export for printing. — A finished insert mould modelled in Fusion 360, ready to print and drop into the aluminium housing.

Before you start

What you need

The shared kit is the same whichever way you print. From there, pick one of two routes: an FDM printer running a high-temperature filament, or an SLA printer running a high-temperature resin.

Shared essentials

Sustainable Design Studio Injection Mini machine
A 3D printed resin holder mould (the aluminium housing)
A computer running Autodesk Fusion 360

FDM route

An FDM printer with high-temperature capability and a heated chamber, such as the Bambu Lab X1C
Polycarbonate filament (higher melt temperature), such as Prusament PC Blend

SLA route

An SLA printer, such as the Elegoo Saturn 3
A high-temperature resin, such as Phrozen TR300 or Siraya Tech Sculpt Ultra White (+250°C, not as good as Phrozen TR300)
A washing and curing station, such as the Elegoo Mercury X

Optional extras

Fine-grit sandpaper (800-1200 grit)
Silicone mould release spray (260°C)

Sustainable Design Studio can design your mould for 3D printing or aluminium.

Learn more

FAQ

Questions about 3D printed inserts

Still unsure? The team replies within one to two working days. Get in touch

Why do we need an aluminium housing?

Two reasons. Injection puts a lot of force behind the plastic, and cured resin cracks under it without something taking the load. The housing takes it instead. It also sits between the hot nozzle and the resin, so the insert stays cooler and survives more shots before it fails.

Are there design constraints compared to a full aluminium mould?

Yes. Because the mould is more delicate than a full aluminium mould, avoid using it for objects with fine detail such as small text or intricate logos. Stick to larger, thicker items, or accept that you may only get a few injections before the detail is lost. Draft angles also need to be much bigger than normal for the product to release.

What machines can I use?

Only our Injection Mini works with the 3D printed insert moulds. This is because we need to accurately control the injection pressure to prevent flashing, overflow, or breaking the mould.

Starter files

Download an insert block

Pick a block size, import it into Fusion 360, and use it as the starting solid for your mould. Or open the worked example and see a finished one.

Small insert

STEP · 50 × 50 × 15mm

The smallest starter block. Best for compact parts with a small footprint.

Download

Medium insert

STEP · 75 × 75 × 25mm

The middle size. A good default for most everyday parts.

Download

Large insert

STEP · 100 × 100 × 25mm

The largest starter block. Use it for bigger, thicker parts.

Download

Worked example, medal mould

F3Z · Fusion 360 archive

A finished medal mould you can open in Fusion 360 and pick apart. Useful if you would rather see a finished one before working through the steps.

Download

The build

Step by step in Fusion 360

Eleven steps from starter block to a finished mould ready for printing. Steps 6 to 10 apply only to SLA moulds. If you are making an FDM insert, follow steps 1 to 5 then skip straight to Step 11.

Step 1

Download and import the block

Choose and download the block size you want from the files above (small, medium, or large). Open Fusion 360 and import the block into your workspace.
Step 2

Create two copies of the block

Duplicate the block and stack one on top of the other, keeping them aligned.
Step 3

Import your object

Import the 3D model of the object you want to inject. Position it 50/50 between the two blocks and centred on the X and Y axis.
Step 4

Split the blocks

Use the Split Body command. Select the two blocks as the bodies to split and the object as the splitting tool. Confirm to create the initial mould cavity.
Step 5

Add the injection point

On one of the four external side faces of the mould, sketch a 6mm circle aligned with the injection point on the aluminium housing. Use Extrude to cut into the mould until it intersects the product cavity, keeping the intersection diameter at 2-3mm.
Steps 6 to 10 are for SLA moulds only

If you are making an FDM insert, skip straight to Step 11.
Step 6

Hollow out the moulds (SLA only)

Use the Shell command to hollow out each mould half to a uniform 3mm thickness.
Step 7

Add a hexagonal texture (SLA only)

On the mould face, sketch a hexagonal pattern and extrude it to create a textured surface.
Step 8

Split the texture (SLA only)

Use the Split Body command to split the hexagonal texture with the hollowed-out mould halves.
Step 9

Combine the parts (SLA only)

Use the Combine tool to merge the hexagonal pattern with the corresponding mould half.
Step 10

Repeat for the other half (SLA only)

Follow steps 6 to 9 to finish the second half of the mould.
Step 11

Finalise and export

Your mould is complete and ready for printing and assembly. Export it as an .STL file to the slicer of your choice.

Get the most from your insert

Tips

Two habits that get more shots out of an insert before it fails.

Temperature

Always inject at the lowest temperature that works

Start low and only raise the barrel temperature if the part will not fill. Excess heat is hard on a 3D printed insert and shortens how many times you can use it before it breaks down.

Pressure

Always inject at the lowest injection pressure that works

Use just enough pressure to fill the cavity cleanly. Lower pressure reduces the force on the delicate insert and helps prevent flashing, overflow, and breakages.