FDM vs SLA: which 3D printing technology suits your part

Choose SLA (resin printing) when you need fine detail, a smooth surface or a watertight wall, typically for display models, figurines, jewellery, dental applications and casting patterns. Choose FDM (filament printing) for anything that has to hold, carry or survive heat. Put simply: SLA is more accurate and better looking, FDM is tougher, cheaper and more resistant to the environment.

How each works

FDM (Fused Deposition Modeling) melts a plastic filament and lays it down through a nozzle in layers. The layers bond by melting, mechanically.

SLA (stereolithography) cures liquid photopolymer resin with light, layer by layer. According to Formlabs this creates a chemical bond across layers, so the part is fully dense, watertight and isotropic, strength does not change with orientation (Formlabs).

That single difference explains most of the others.

The comparison in numbers

Figures for layer height, tolerances, material prices and build volumes come from the Formlabs comparison; they are an SLA printer manufacturer, which is worth keeping in mind when reading them.

Accuracy and detail

The accuracy gap is not marketing, it is an order of magnitude. Formlabs states for the Form 4 and Form 4L a tolerance of ±0.15 % on features from 1 to 30 mm with a lower bound of ±0.02 mm, and ±0.3 % on features from 81 to 150 mm with a lower bound of ±0.15 mm. The figures apply to features in the XY plane and were measured on Grey V5 resin at 100 µm layer height (Formlabs). Ordinary FDM printing sits an order higher.

In practice: text 1.5 mm tall, gearing below module 0.5, mesh, filigree or a thin antenna on a figurine are SLA jobs. FDM will either fail to print them or turn them into a blob.

One detail that applies to both technologies: accuracy in Z is worse than in the XY plane, because layer height limits it. So where a dimension is critical, how the part sits on the bed matters.

Strength: isotropy is not the same as durability

This is the most common misunderstanding. "SLA is isotropic" sounds like "SLA is stronger". It is not.

Isotropy means the part has no weak direction. It does not mean overall strength is high. Ordinary SLA resins are brittle: under impact or bending they crack rather than deform. Tough and engineering resins exist, but at a substantially higher price, and they still do not match the toughness of PETG or nylon.

An FDM part, by contrast, has an obvious weakness (tension perpendicular to the layers), but in the plane of the layer it is tough and forgives an impact. We go into this further in the article on materials for end-use parts.

Rule of thumb: if the part might slip out of someone's hand, you want FDM.

Surface finish and the hidden finishing time

With FDM, finishing is simple: remove the supports and you are done. If you want a smooth surface, sand and fill.

An SLA part looks finished as it comes off the machine, but it is not. According to Formlabs the process has three steps:

1. Washing in isopropyl alcohol or an alternative solvent to remove uncured resin.
2. Support removal. SLA needs more supports than FDM, so it leaves marks that have to be sanded.
3. Post-curing to reach the highest strength and stability.

These three steps are not optional, and they are why SLA costs more in labour as well as material.

Cost and running requirements

Material: filament costs around 50 USD per kilogram according to Formlabs (engineering variants 100 to 150 USD), resin 149 to 200 USD per litre. The gap is a multiple.

Running SLA brings in things FDM does not have: gloves and ventilation when handling uncured resin, a vessel of isopropyl alcohol, a curing chamber, and waste disposal per the manufacturer's instructions. Uncured resin is an irritant and belongs neither in general waste nor down a drain.

With FDM you open the foil, load the spool and print. That is why FDM established itself in workshops and SLA more in specialist facilities.

Part size

Per Formlabs, FDM on desktop and semi-professional machines handles up to 300 × 300 × 600 mm, SLA typically up to 300 × 335 × 200 mm. Larger SLA formats exist, but at a substantially higher price.

If you need a tall part, FDM is practically the only accessible route, and if it does not fit there either, the part gets split and bonded.

When to choose which

Choose SLA when:

• the part has fine detail that must survive (text, texture, filigree),
• you need a smooth surface without sanding,
• the part must be watertight or gas-tight in a thin wall,
• it is a casting pattern, a dental or a jewellery application,
• dimensional accuracy matters more than durability.

Choose FDM when:

• the part holds, carries or takes impact,
• it works at elevated temperature or outdoors in the sun,
• it is larger than your palm,
• you need several pieces at a sensible price,
• it is a prototype, a spare part or a small batch.

What we print

We focus on FDM printing and 3D modelling. Not because SLA is worse, but because the vast majority of the work that comes to us (enclosures, brackets, jigs, spare parts, small batches), falls into the first category and is made better and cheaper on FDM.

When your brief is clearly an SLA job, we will say so straight away. That is cheaper for both sides than printing a part that will not meet expectations.

If you are not sure which category your part falls into, send us the model or a description. The basic check is free and you will usually have an answer within 24 hours.