SLA

SLA – Stereolithography

Stereolithography – or SLA 3D printing – is used mainly where surface finish, a high level of detail and dimensional accuracy are the most important properties for you. You can customise your part by choosing several different materials to obtain the correct transparency, flexibility or rigidity.

Dimensional accuracy and surface finish – check!

Stereolithography is the most accurate method of additive manufacturing; the dimensional deviation is seldom more than 0.1 mm. The method is therefore the obvious choice if the model is to be used as the master pattern for a subsequent process (e.g. vacuum casting, plaster casting etc.). SLA models are more fragile compared to SLS and FDM technology, but they have considerably better definition and surface finish.
Stereolithography was the first commercially exploited AM (additive manufacturing) method on the market. We have been working with SLA technology for over 20 years.

SLA – Technical information

The machine has a tank which is filled with a liquid epoxy resin. A build platform descends layer-by-layer into the tank. A UV laser cures the material layer-by-layer, based on the information in the layered CAD file. Once the process is complete, the parts are removed from the build platform, washed and cured further in a UV oven. Finally, the agreed post-processing is carried out, e.g. polishing, blasting and lacquering.

Tolerances: min ± 0.1 mm and ± 0.15% of dimension. Finer tolerances by special arrangement.
Minimum wall thickness in x/y is 0.25 mm. Minimum wall thickness in z is 0.4 mm.
There is a risk of increased tolerances in the case of split and glued parts.

Guidelines

In the case of transparent models, the best results come from applying clear lacquer to class A models. Transparent models can also be obtained by applying clear lacquer to class B models, however all construction stages and defects will then be evident.
Material shrinkage is comparatively small. However, a phenomenon that can occur is what is known as “warping”. Large horizontal surfaces shrink more than vertical surfaces.
As with other technologies, a discussion about which dimensions and surfaces are important or desired is invariably beneficial.
Bear in mind that enclosed volumes or geometries that are difficult to access will be filled with uncured resin and support. To allow uncured material to be drained off, drain holes are inserted, which are subsequently stopped with plugs. Long narrow channels can also be a problem to wash clean.
Threads can be manufactured, but will not be stronger than the plastic itself. Threads of less than M10 are not recommended directly in the plastic. For strength, and in the case of smaller threads, HeliCoil thread repair inserts are a good choice. HeliCoil is available in dimensions down to M3. External threads directly in the plastic that are less than M10 are not recommended.
Normally, the models are made with a layer thickness of 0.10 mm. Minimum wall thickness in Z is 0.40 mm, and for vertical surfaces it is 0.10 mm. Vertical cross-sections less than 1 mm2 will be weak and brittle. Thinner walls and finer parts can be manufactured with SLA XHD. The layer thickness is then 0.05 mm, and the time taken in the machine is considerably longer.
For surfaces with an angles of less than 20° to the X/Y plane, distortion due to the layer effect (stepping) can be marked. The steeper the angle, the better the surfaces.
For press fitting, a gap of 0.05 mm in the 3D model is suggested. Vaseline is useful as a lubricant to obtain better slip; silicon oils can sometimes have the opposite effect.

Customer case - Spectacles

A product which can literally be scrutinised from all angles.
This is an example of a design model, and we opted for SLA technology on account of its ability to produce fine surfaces with high resolution. After manufacture, we lacquered them, added graphics and then assembled them.

Mission accomplished!