SLA resin printing on the Form 4: what the resin library unlocks

Most people choose a resin printer for the surface. Layer lines vanish, small features stay sharp, and the part looks like it came out of a mould. That is a good reason to use SLA, but it is not the reason engineering customers keep coming back. They come back for the materials.

We run a Formlabs Form 4, and we can print resins from its library rather than a single grey. That changes what SLA is for: not just a prototype that looks right, but a gasket, a mould insert, a light pipe or a casting pattern that does a job. Here is what each resin family is good for, what we hold, and where resin is the wrong answer.

What the Form 4 is, in plain terms

The Form 4 is a masked-LCD stereolithography printer: it cures liquid resin with light, a whole layer at a time, rather than tracing each layer with a laser. In practice that means layers from 25 to 100 microns, 50 micron resolution in the plane of the part, and a build volume of 200 × 125 × 210 mm. Every part is washed and UV-cured, and supports are removed before it ships. That is the baseline for our SLA resin printing service. What sits on top of it is the resin you choose.

Grey resin: the default, and what it is for

Our standard is a grey, ABS-like resin. SLA grey resin is the right choice for visual models, master patterns, fit checks and any small part where finish and fine detail matter more than toughness. It is dimensionally stable and prices from £8 for a small part. It is also brittle (elongation of 8–12%), softens at 50–65°C and yellows in sunlight, so it is not a snap-fit or outdoor material. For those jobs we will usually point you to FDM.

Where the specialist resins earn their place

Clear Cast is a water-clear resin that polishes to near-glass clarity: light pipes, lens prototypes, display cases and flow-visualisation parts where you need to see inside. It is also an investment-casting resin with under 0.2% ash, so a printed pattern burns out cleanly for casting in metal. Heat deflection is 73°C and, like every clear resin, it yellows with prolonged UV, so it is for indoor work.

High Temp is for parts that see heat. Its heat deflection temperature is 238°C at 0.45 MPa, the highest in our range, which makes it suitable for short-run mould inserts, hot-air and fluid channels and thermal-test fixtures. It holds fine detail, but it is stiff and brittle, and it needs a thermal post-cure to reach full temperature resistance.

Silicone 40A is the one filament cannot imitate. It is a true silicone at Shore 40A, with 230% elongation and stable performance from -25°C to 125°C, which makes it useful for seals, gaskets, grommets and soft-touch parts, one-off or small batch, with no mould. Above a few hundred units, conventional moulded silicone is cheaper, and we would tell you so.

Castable Wax is for lost-wax casting. We print the pattern, washed and left uncured so it burns out cleanly, and your caster or a trade casting house invests it and pours the metal. It suits fine jewellery and small detailed parts where Clear Cast is too coarse for the geometry. We print and post the pattern; we do not cast metal.

A representative example

A small instrumentation firm needs four transparent housings so engineers can watch a fluid path during testing, plus a soft gasket to seal each one. Clear Cast covers the housings: the channel geometry prints cleanly and the walls polish clear. Silicone 40A covers the gaskets, printed from the same CAD so the groove and the seal match. Neither needs tooling, and a design change is a new file rather than a new mould. This is a representative job, not a specific customer's; special-order resins are ordered in for the job, so we confirm the lead time when we confirm the price.

What we can and can't do

We can print any Form 4-compatible resin, but we do not stock all of them. The grey is on the shelf; specialist resins such as Clear Cast, High Temp and Silicone 40A are special orders, with any surcharge shown in the quote tool. If the resin you want is not offered there, email the file and we will price it by hand.

The limits are real. Parts top out at roughly 200 mm on the longest edge, and large resin parts get expensive quickly. Most resins are more brittle than engineering thermoplastics, so for load-bearing, impact or outdoor parts, FDM printing is usually the better answer. We do not sand or paint resin parts, and small nib marks where supports met the surface are normal; tell us which face is cosmetic and we will orient the supports away from it. There is no food-contact or medical certification, and a printed part is not a substitute for a certified safety-critical component.

How to start

Upload an STL or STEP file, choose SLA and pick a resin. If you are torn between two, or the part has to survive heat, load or sunlight, ask first. Choosing the right resin costs nothing; reprinting in the right one costs a week.