Cutting 3D-printed sprues and runners.

Note: as of July 2024 the Shapeways 3D printing service has closed down. Most of the information on this page is related to plastic processes used by Shapeways, and thus this page is mostly obsolete.

I've left it here in case it's of interest to anyone.

Cutting 3D-printed sprues and runners

Shapeways now charges significant extra fees for each separate part in a product. That means a designer has to make a product more affordable by joining all the parts together with little plastic rods or sticks. These are often called sprues or runners, by analogy to the similar-looking sticks found in injection moulded model kits. You then have to cut the pieces apart.

Unfortunately, it’s easy to damage small 3D-printed parts when cutting it off a sprue. Here are some tips for making it easier, based on my experience working with printed detail parts.

Never twist or snap off the sprues

Unlike model kits made of styrene, where you can sometimes get away with a little brute force, the acrylic resin used to print 3D parts tends to be very brittle and will easily snap. And it may not break off at precisely the location you want.

Use a proper cutter or nipper designed for cutting sprues. These are marketed under many names - sprue cutters, sprue nippers, runner cutters, flush cutters, etc.

Use a flush-cut sprue cutter only

There are two basic types of sprue cutters. Flush-cut ones have two blades which come together and touch along the sharp edges. These blades are aligned with each other.

Shearing cutters, designed for cutting styrene or wire, have two blades that side-swipe each other like scissors. Never use these on acrylic. I’ve found that they tend to crack and destroy brittle 3D resin! Xuron is one popular maker of side shear cutters, which they grandly proclaim possess "Micro-Shear blade by-pass cutting technology." Sure sounds like a fancy-ass way of saying "just like scissors!"

Don’t use knives

Knives are fine for removing small amounts of material via whittling or scraping actions. But don’t use them for chopping off sprues. 3D printing acrylic resin isn’t soft like styrene, so the pressure and sudden release from the knife tends to cause shattering.

Always use a Perfect sprue cutter

The blades must be sharp. Blunt blades, or chipped and damaged old wire cutters, do you no favours, since the pressure from the blades is spread over a wider area.

Press the flat side against the part

This is important. Cutting tools generally have a flat side and a curved or open side. Make sure the flat side is pressed firmly against the part, so that the sliced-through sprue can fly off away. Cutting further along a sprue can cause uncontrolled or unexpected cracks to occur.

Score the area to cut

It doesn’t hurt to lightly score the areas you need to cut first, especially if the part is thin and delicate. Use a sharp knife, and lightly and repeatedly drag the back of the knife tip over the join between part and sprue. Basically score the area where the blade of the cutter will touch.

The basic idea is that you want to encourage certain areas of the plastic to crack, thus keeping other areas intact. Scoring the plastic helps do so. It's no surefire guarantee that the crack will go where you want, but it ups the chances.

Remove the nub with a knife and file

You often end up with a subtle bump or nub left over where the sprue was. Use a sharp fine-tipped craft knife and gently chisel or chip it away. You can’t slice through brittle but hard acrylic the way you can cut through paper or wood, say. Another approach is to use extremely tiny flat needle files. Filing take time, but offers a lot of control.

Don’t use a sprue cutter

Unlike soft bendy styrene, brittle 3D-print acrylic will typically shatter or crack if a sprue cutter is used for trimming purposes. Don’t do it! Use a really fine razor saw instead.

This may sound like contradictory advice, since I suggest using a sprue cutter for cutting 3D-printed sprues. But there’s a reason for this. A sprue cutter places a lot of pressure on the plastic at a specific point. If that pressure goes through to a thin sprue, then the sprue will take the force and crack, leaving the main part intact.

But it can be difficult controlling where a shockwave cracks through the plastic, in the case of a part where you’re not trimming off something small. There’s a risk that areas of the part you want to keep will get damaged. Experiment with some scrap acrylic material if you don’t get what I mean.