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Selective Laser Sintering

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Our Insight video series will help you master digital manufacturing.

Every Friday we’ll post a new video – each one giving you a deeper Insight into how to design better parts. We’ll cover specific topics such as choosing the right 3D printing material, optimising your design for CNC machining, surface finishes for moulded parts, and much more besides.

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Insight: Selective Laser Sintering - Fully Functional Parts with SLS

21.02.2020

Transcript

Hello and welcome to this week’s insight. 

This week I’m going talk to you about one of the most useful technologies for rapid prototyping and low volume production of plastic parts. 

It’s a type of 3D printing called Selective Laser Sintering, or SLS for short.

Sintering applies heat and/or pressure to fuse bits of metal, ceramic and polymers into a solid mass.

The process is nothing new, nature has been fusing sedimentary materials into slate and quartzite for eons. But while nature can take its time over millions of years, I’m guessing that you need your prototypes or parts on a bit of a tighter deadline, right?

Well the good news is that using SLS, you should expect prototypes or parts in less than 7 days.

Unlike nature, SLS uses a computer-controlled CO2 laser to “draw” slices of a CAD model in a bed of material to build the part. It fuses micron sized particles of the material one layer at a time.

The great thing is, it does not need support structures and it creates fully functional parts. It is the only 3D printing technology that can create durable living hinges and snap fit assemblies.

This makes it an excellent choice for prototyping products that might get produced by injection moulding, or even as a low volume production alternative to it.

However, it’s not the answer to everything and there are limitations. For one thing you can only have SLS parts in Nylon, but nylon is a pretty versatile polymer with a wide range of applications.

Exactly what nylon material you select will depend on what you want the part to do. 

Let’s take a look at one as an example. PA 850 Black, which is similar to an unfilled Nylon 11, is a tough bioplastic that is a great choice for parts needing a living hinge, like a lid on a pill container. It offers one of the highest elongation break thresholds in the nylon family.

You’ve probably already guessed that it is black, but is also produces a smooth surface finish and good part detail and it’s very resistant to chemicals and water absorption. This makes it ideal for products as diverse as fuel lines, catheters, tennis shoes and electrical connectors.

Of course, there are plenty of other nylon materials and I could bore you to death with their properties, but if you talk to your supplier, I am sure that they will help you select the best material for the job.

There’s also far more to effective part design than selecting the right material. For a good quality part, we also need to control in-build curl and post build warping.

A lot of this control is down to your supplier, so have a chat with them about how they will do this.  One way is for the technician to tip parts slightly in the build chamber. This can result in some stair stepping and a slightly rough feel, but we’ll deal with that issue in a minute.

Many of the design rules that we use for injection moulding also apply to SLS. This makes it a great choice for prototypes that will eventually be moulded. 

So, what do you need to think about when you are sitting in front of your CAD? Well things like using hole bosses and support struts and avoiding thick cross sections are all good. Here’s my list of other points:

Add corner radii where walls meet to reduce stress.

Aim for a uniform wall thickness.  We recommend between 1.5 and 3.8 mm to reduce inbuild curl and warping

And talking about warping, consider using integrated ribs for support, especially on large flat surfaces.

If you are prototyping for a part that will contain over-moulded parts or threaded inserts, then don’t despair because SLS parts can do something similar using heat stake inserts as a secondary process.

Now do you remember that I mentioned that SLS can feel a bit rougher than other additive manufacturing technologies?

To be honest, it’s not something that you should worry too much about. It will still work well for most functional prototypes and your supplier should be able to bead blast most parts to create a smooth matte finish.

What else? Well very fine text can be an issue with the minimum feature size of SLS being 1mm. It means that very small fonts can get jammed with powder, making typography less clear. Inset text is better, but is still limited to features no smaller than 0.5mm.

And finally, SLS is slightly less accurate than other laser sintering processes, such as direct metal laser sintering. But on the plus side SLS has a build frame of 700mm by 380mm by 580mm, which dwarfs that of its metal counterpart.

So just like other technologies it has its place and for the right product or prototype it might well be your best option. It’s always a good idea to find a supplier who offers a range of different 3D printing and other technologies so that you can find the one that best meets your needs.

Right that’s it for this week. I look forward to seeing you again next Friday.

 

With special thanks to Natalie Constable.

 

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