SLA (stereolithography apparatus)
The term stereolithography was coined in the 1980s and is used by 3D Systems for its SLA equipment that uses a beam of UV laser light to cure a photopolymer resin. As the resin cures, it solidifies to create a thin ‘slice’ of the part on the surface of the resin. Once the layer is complete, the bed of the machine descends within the vat so the next layer can be cured. When each new layer cures, it fuses to the one beneath, thereby building a three-dimensional solid part.
If the part geometry includes overhanging features, these can be built by means of 3D printed support structures that prevent the overhangs from deflecting under the influence of gravity. Hollow parts can be built but drain holes need to be incorporated so uncured resin can be removed afterwards. Also, if there is no need for a part to be completely solid, the interior can be filled with a three-dimensional lattice to save weight and material costs.
After the part has been built, it is removed from the vat, any excess resin rinsed off, then it is placed in a UV oven for final curing. Secondary finishes can be applied afterwards if required.
SLA is quick, it builds parts with good accuracy and surface finish, and can operate with a choice of materials. However, the material properties of finished parts are not perfectly isotropic, being slightly weaker in the Z axis, so care needs to be taken when choosing the build orientation. Also, while the surface finish is often good enough for functional parts, light bead blasting or hand finishing will improve aesthetics.
DLP (digital light projection)
In common with SLA, DLP builds parts layer-by-layer from photopolymers. The major difference, however, is that an entire layer is cured using a single flash of light, which is much quicker than using a spot of laser light to trace all over the area to be cured. Exposure masking is achieved by means of an LCD through which the light is projected onto the surface of the photopolymer.
Like SLA, DLP can build overhanging features by means of 3D printed support structures, and hollow parts or lattice-filled ‘solid’ parts are possible. DLP parts also need to be cleaned and UV cured, as with SLA parts.
Thanks to each slice being cured with a single flash of light, DLP is quicker than SLA despite producing parts with similar resolution, accuracy and surface finish. In addition, whereas we offer a choice of three materials for SLA, we stock six materials for DLP and can order a further 11 specialist grades if required. Consequently, the range of material properties for DLP parts is much greater than for SLA parts, so DLP is used more often for end-use parts, while SLA is usually (but not always) chosen for prototype parts.
PµSL (projection micro stereolithography)
This 3D printing technology is similar to DLP because it uses masking to cure areas of resin with a single flash of light. However, it is different because the optics provide far better resolution, so parts are extremely accurate. The layer thickness is also smaller which means that, in conjunction with the higher resolution, the surface of the part is far smoother.
Each exposure cures a relatively small area, but larger parts can be 3D printed by using a step-and-repeat motion on the machine bed. The high precision of the movements ensures each masked area is aligned with its neighbours. Alternatively, the bed movement allows multiple parts to be built concurrently.
We operate our PµSL with a choice of three materials. These are all relatively stiff, strong materials with good dimensional stability, enabling fine details to be reproduced accurately. Furthermore, finished parts are not porous and have material properties that are essentially isotropic thanks to good interlayer fusing.
If identical parts were to be printed with DLP and PµSL 3D printers, the DLP part would be built more quickly because of the larger exposed area and the thicker layers. However, the capabilities of the PµSL technology mean it is used for smaller parts and different applications than DLP, so the speed differential is irrelevant. PµSL is often used to build parts that cannot be manufactured any other way, including alternative 3D printing technologies and CNC machining.
PµSL parts, as with SLA and DLP parts, have to be cleaned and UV hardened after they have been removed from the 3D printer. However, PµSL parts seldom need support structures for overhanging features due to the scale of the features and the buoyancy of the resin.
Resolution
SLA 3D printers can operate in UHD or XHD mode but we operate ours in UHD (ultra high definition) as standard, with a maximum resolution of 4000dpi. Layer thickness is 0.1mm as standard, though the 3D printers are capable of building 0.05mm layers if necessary. We quote a general tolerance of ±0.5mm yet we often achieve tighter tolerances, depending on the operating mode, build orientation, material and part geometry.
Our Figure 4 DLP 3D printer has a horizontal resolution of 65 microns and the layer thickness is from 10 to 100 microns, depending on the part requirements and material. As with SLA, we quote a general tolerance of ±0.5mm but several factors impact on the accuracy achieved and we often achieve tighter tolerances.
The question of 3D printing resolution can be a misleading one. We strongly recommend that designers talk to our 3D printing specialists about their part’s requirements so we can help them decide on the optimum 3D printing technology, material, build orientation and so on, as that will be the best way to ensure the part is fit for its intended purpose.
PµSL is in a league of its own in terms of resolution and part accuracy. We quote a general tolerance of ±25µm due to the 3D printer’s small layer thickness (5-40µm) and fine resolution of 10µm, subject to the part’s geometry and build orientation. Parts also benefit from exceptionally smooth surfaces, typically 0.4-0.8µm Ra on the top and 1.5-2.5µm Ra on the sides, so no secondary finishing operations are necessary. PµSL can therefore build parts with very fine details, thin walls and sharp edges, all at a scale that is unachievable with other 3D printing technologies.
Build envelope
We have a total of nine SLA 3D printers, with build envelopes (XYZ) from 250 x 250 x 250mm up to 508 x 508 x 534mm.
Our Figure 4 DLP 3D printer has a build envelope of 124 x 70 x 196mm.
Finally, the Boston Micro Fabrication (BMF) microArch S240 PµSL 3D printer has a build envelope of 100 x 100 x 75mm, though it is unlikely we would build a single part that fills that envelope.
Materials
The three materials we use in our SLA 3D printers have similar properties to polycarbonate, polypropylene and ABS, with the first of these benefitting from USP Class VI capability for patient contact applications.
We stock a wider choice of materials for DLP 3D printing, including the following: one with high strength, stiffness and temperature resistance (>300 °C); another that is UL94 V0 flame-retardant; a production-grade elastomer with a Shore A hardness of 65 and a high elongation at break; a white plastic for long-term use parts requiring impact strength, elongation, and tensile strength; and a clear, production-grade material that is stable in the presence of UV and moisture, compatible with a range of chemicals and has thermoplastic-like mechanical properties.
In addition to the stock materials, we can source a broad range of specialist grades for DLP 3D printing. These include materials with properties similar to polypropylene, ABS, elastomers, hard rubbers and investment casting wax. There are also materials suitable for applications requiring biocompatibility.
We offer three materials for PµSL 3D printing. One is a high-performance engineering material with excellent strength, rigidity and heat resistance (up to 114 °C). Another is biocompatible and sterilisable for non-implantable medical applications. The third is a durable engineering material suitable for functional testing and end-use parts. This material does not absorb moisture and is biocompatible.
To find out more, visit our dedicated page about materials for 3D printing.
Finishes
When it comes to finishing options for parts 3D printed using SLA, DLP and PµSL, the first thing to say is that PµSL parts seldom require secondary finishing. They are usually highly detailed, accurate, functional parts with extremely smooth surfaces. Consequently, finishes are not required and, if they were applied, they could have an adverse impact on the feature details and tolerances.
For SLA and DLP parts, the choice of finish depends on the application requirement and material. As there is a wider choice of materials for DLP, you might assume there are more finishing options. However, many of the DLP materials are targeted at functional prototype or end-use parts, for which secondary finishes are less often specified.
Typical finishes for clear SLA parts include polishing and lacquering to produce a part with high clarity, or tinted finishes can also be applied for parts such as prototype automotive light lenses. In addition, vacuum metallisation on a polished surface provides high reflectivity, such as for the inside of light units.
SLA and DLP parts can also be sanded, primed and painted if a good aesthetic is required, and a rubberised soft-feel coating can be applied.
A quicker, lower-cost finish for functional parts is light bead blasting, which improves the appearance compared with the as-built finish.
Another option popular with functional parts is a blackout/RFI/EMC coating on the internal ‘B’ surfaces.
Service levels
To meet the needs of customers whose requirements are dictated by time pressures or budgets, we offer a choice of service levels. SLA and DLP parts are available with Express Delivery (parts shipped the next working day), Standard Delivery (parts shipped in three working days) and Economy Delivery (eight working days).
PµSL is a more specialised 3D printing technology, so delivery requirements discussed with the customer when we prepare the quotation.
Prototyping and low-volume manufacturing
SLA, DLP and PµSL are all suitable for both prototyping and low-volume manufacturing (Additive Prototyping and Additive Manufacturing).
The suitability will, of course, depend on the material and the part’s intended use, including the operating environment. However, although SLA and DLP are often thought of as 3D printing technologies for prototyping, they can also be appropriate for low-volume manufacturing.
PµSL is suitable for prototyping and low-volume manufacturing. Indeed, the technology can be used to make end-use parts that simply cannot be made any other way – such as for microfluidics applications.
Which photopolymerization 3D printing technology is best?
For some parts, particularly those with very fine detail or requiring tight tolerances to be held, PµSL is the only feasible option.
Larger parts tend to be built with SLA compared with DLP simply because of the size of the build envelope. However, DLP parts can be built from a wider range of materials.
If you are unsure which 3D printing technology is best for your application, talk to our team who can give unbiased advice based on many years of experience with 3D printing.
Talk to us
If you need prototype or end-use parts 3D printed with SLA, DLP or PµSL, talk to our experts by calling 01763 249760.
