Should I use SLS or SLA for a functional prototype?
The immediate reaction to a question of this sort is to say Selective Laser Sintering (SLS) – after all, a large part of the growth in laser sintering applications is due to the functionality offered by nylon. However, if we look at the material properties of Duraform (standard nylon for laser sintering) in comparison to Stereolithography (SLA) resins (in this case Xtreme), we can see how far SLA resins have developed.
All might not be what it appears to be!
That said, all might not be what it appears to be! For example, as engineers we are really interested in the material yield data and these figures are frequently not quoted. The reason is that for most, SLA materials yield and ultimate values are virtually identical. In some cases SLS may offer a difference, but getting consistent repeatable data is another thing altogether.
Elongagtion to break
Taking elongation to break, the figures look better for SLA. That said, and particularly with SLA materials, what is not quoted is the effect of age. Data is difficult to come by, but whatever the specification says, these numbers will fall substantially with time. Further, if this is an issue, then Duraform is not the best SLS material. Alternatives such as EX will do a much better job (i.e. 30 – 45%).
Impact
Looking at impact, the figures quoted for Xtreme are surprisingly good. DMX is also pretty good (65 – 80 J/m), but choosing Duraform as the benchmark is not doing SLS any favours either (again EX is much better).
Temperature, humidity and moisture
Finally we have temperature/humidity/moisture to contend with. The heat deflection temperatures for SLA materials are all well down on the equivalent for SLS materials and thus load at temperature is a real issue. In this case Xtreme is one of the better SLA materials, but this should be watched even in our climate!
Both temperature and moisture can destroy the properties of an SLA part as well as leading to enough growth as to render the part unusable. Fortunately with wise material selection this can be controlled or at least reduced within the realistic operating life. Nylon also absorbs water. The standard materials are nylon 12’s with circa 0.2% absorption, this can be more of an issue with nylon 11, i.e. the EX based material.
A thermal postcure of SLA materials can significantly improve heat deflection (HDT) data. For example on ProtoGen 18420, a thermal postcure can raise the HDT from 53 to 93 degrees C, tensile and flexural strengths also improve, but elongation is adversely affected. Nanotool has one of the highest HDT’s of all SLA resins at 225 degrees C and even this can be raised to 260 degrees C.
It's all about the trade offs…
So, don’t loose sight of SLA. It has not been superseded by SLS. It still has a better surface finish, typically better accuracy. It just offers different advantages – but check which resin you are getting! Bottom line, it’s all trade offs.
|
|
ASTM |
Duraform |
Xtreme |
||
|
Tensile strength, yield |
D638 |
N/A |
|
|
|
|
Tensile strength, ultimate |
D638 |
43 |
Mpa |
38-45 |
Mpa |
|
Tensile modulus |
D638 |
1586 |
Mpa |
1790-1980 |
Mpa |
|
Elongation at yield |
D638 |
N/A |
|
|
|
|
Elongation at break |
D638 |
14% |
|
14-22% |
|
|
Flexural strength yield |
D790 |
N/A |
|
|
|
|
Flexural strength ultimate |
D790 |
48 |
Mpa |
52-72 |
Mpa |
|
Flexural Modulus |
D790 |
1387 |
Mpa |
1520-2070 |
Mpa |
|
Impact strength,Notched Izod |
D256 |
32 |
J/m |
35-52 |
J/m |
|
Heat deflection temperature |
D648 |
|
|
|
|
|
|
@0.45MPa |
180 |
deg C |
62 |
deg C |
|
|
@1.82MPa |
95 |
deg C |
54 |
deg C |
