Photopolymerisation involves the use of photopolymers - a polymer that changes properties when exposed to light. For the case of photopolymerisation, exposure to visible or UV light initiates polymerisation or cross linking reactions, “curing” the polymer. . This can be achieved by using specific monomers and oligomers, or by the addition of photoinitiators that become reactive species that initiate polymerisation when exposed to certain frequencies of EM radiation. Currently, available photopolymers are very limited.
Photopolymerisation reactions are exploited for a variety of additive manufacturing methods, the first of which is Stereolithography. SLA uses a vat of liquid photopolymer (shown as yellow in the animation below) with a laser, allowing selective and local solidification the liquid. A platform in the vat is slowly lowered as the laser moves across the surface of the liquid, curing the polymer a layer at a time. In some cases, a blade passes over the surface of each layer to produce a smooth surface for the next layer to build on. Once the object is completed, excess polymer is drained away.
Digital Light Processing (DLP)
Another process is Digital Light Processing (DLP) which is very similar to SLA, but instead of curing with a laser, the photopolymer is cured with a projector that projects the image of the desired layer onto the liquid surface.
Generally, DLP is faster than SLA but with lower resolution in the x-y plane and worse surface finish. This is due to the DLP having a certain minimum pixel size, so any edge will therefore have its accuracy and resolution limited by the size of these pixels. In SLA, however, the laser can be pointed at a given x-y coordinate. The laser can then be moved to produce accurate and precise edges and shapes due to the fine control a printer usually has over laser positioning. This is true even if the laser point is larger than the pixel size.
Post-processing is needed for both DLP and SLA. This involves removal of support structures, smoothing and, potentially, further curing for increased hardness. It is also worth noting that some SLA and DLP machines have platforms that rise upwards rather than downwards, with the projector or laser under the liquid.
Benefits and Limitations
Unlike most other additive manufacturing processes, SLA and DLP are able to produce isotropic properties. This is because, when initially curing a layer, the plastic is only partially reacted, known as a green state, where there are still monomers and oligomers available for reaction on the layer surface. . As the next layer is cured, the polymerisation reaction involves the previous layer, chemically bonding the two layers.
A further advantage is DLP and SLA printers generally have much higher resolution and smaller minimum feature sizes than other AM methods.
However, the two methods are both limited by the fact that you can't print fully closed hollow sections as they will be filled with liquid. The design would therefore need drainage holes to avoid this.
Print times can take a long time, especially for SLA. For comparison, a print which may take 2-3 hours for other methods (FDM or SLS ), may take upwards of 11 hours for SLA. Despite each layer being made quickly, layers are often very thin in SLA meaning many more layers are needed for SLA and DLP than FDM (see material extrusion section for more on FDM printers).
Cost for SLA and DLP is also high, with the printers themselves being far more expensive at desktop and industrial scales than other methods. The plastic used is also more expensive with 1 kg of photopolymer resin costing around between £50-120 compared to that of 1 kg of PLA plastic filament used for FDM being roughly £20-50.
Finally, both methods are limited by the material properties of the plastic used. The nature of the plastic means prints are brittle and are susceptible to creep, meaning they are not suitable to be used as functional parts nor used over longer periods of time. There is some control over properties, however, by further curing the print to harden and strengthen the object if needs be.