Practical 3D Printing: DFM - Design For Manufacturing

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DFM - Design For Manufacturing

10.42

00:00	DFM considerations need to be kept in mind whether we're designing a part we're going to print ourselves or we're using a manufacturing service.
00:07	The worst case looks something like this.
00:10	We send a part to the manufacturer that's not designed with the limitations of the process in mind.
00:16	Then they make the results and the parts are poor and we still have to pay for it.
00:21	With that said a good manufacturer should review the design first and communicate any potential issues and recommended revisions.
00:28	If we're ever unsure of anything, then the manufacturer is the first port of call.
00:34	Essentially we should be asking, how can I design this part so you can make it as accurately and efficiently as possible? It's also not just about the limitations, we want to make sure we use the manufacturing method to its fullest potential.
00:47	This is critical to SLS because as we know, the parts can be printed without supports and so we don't need to be as wary of overhang issues or witness marks.
00:57	This also allows us to print multiple parts stacked on top of each other without any issues if we can fit more parts into the build volume.
01:05	Of course the material usage and process time will go up but this is still generally cheaper than printing them all individually.
01:13	Services like Xometry, Craftcloud and Prototech to name a few use a somewhat of an automated booking system.
01:20	Essentially connecting us to one of their long list of manufacturers from all around the world.
01:26	While these services are aware of DFM issues and have processes in place to help avoid them, the risk isn't zero and we should always do our best to cover all the bases.
01:36	Each of these services have a design guide or design tips on their website for each manufacturing service.
01:42	We've reviewed these, compiled the information and added some of our own experience to help create the HPA 3D printing design guide which you'll find linked under this module.
01:54	This will help you design motorsport specific parts with DFM considerations in mind for whatever form of 3D printing that you intend to use.
02:02	So let's get into it, what are the DFM considerations for SLS? First and this is relevant to all forms of 3D printing, are the size limitations.
02:11	The most basic of which is the overall part size limitation.
02:15	The service provider should state a maximum here for the largest printers that they have available.
02:21	This will usually be the build volume of the printer.
02:24	Naturally every machine will be different but for most services this tends to be around 300x300x300mm.
02:32	With that said, bigger machines can allow one of these dimensions, usually the Z axis, to be closer to 600 or 700.
02:40	This requirement of a bigger machine often means a slightly higher rate for the process time though, so there's no free lunch.
02:47	Keeping in mind that depending on the geometry of the part, it can be possible to print a larger part than what we might initially expect by positioning it on an angle.
02:56	But also keep in mind that this will likely come with some compromises to print orientation.
03:02	On that note we won't delve too deep into the print orientation considerations for SLS as this is mostly the service provider's job and they should have the expertise to get it right.
03:13	In short though, the print orientation can be used to control the heat distribution in the part to prevent warpage and the geometry of the part has an impact here as well.
03:22	Unfortunately large flat surfaces are subject to warpage during the fusing process.
03:28	For example having large flat faces of the part on the build platform which is the area printed first and that cools the fastest is likely to cause warpage.
03:39	Centralising the part in the build volume and targeting a consistent cross sectional area is ideal for reducing thermal stresses and if we're printing multiple parts at once then they should be distributed evenly.
03:50	Where this can't be avoided it's possible to add ribs to provide some support as the part cools.
03:57	Another thing to note here is that well printed SLS parts typically don't have layer lines due to thermal diffusion between layers when sintering the powder.
04:06	This also means the structure is more isotropic in its mechanical properties.
04:11	Put simply, orientation doesn't usually have a significant impact on the strength or surface finish of the part.
04:18	The other size limitations we're about to discuss depend on the material being used to some degree, however there's still some typical values we can apply as good design rules.
04:29	Recalling an unsupported wall is one that connects to the rest of the part by one side only.
04:34	For SLS these should generally be at least 0.7mm thick for nylon 12 but 2mm thick for carbon reinforced nylon.
04:43	For supported walls we can usually get away with about 0.6mm or so, however thin walls are likely to over thicken from the heat of the laser so generally targeting over 1.3mm is ideal.
04:57	The minimum hole size should be about 1.5mm, any smaller than this can cause powder to get stuck inside.
05:04	For accurate small holes though it can be worth undersizing slightly and then using a precision drill bit to open them up to the desired size.
05:12	This brings us to printing threads which put short, isn't ideal as the surface finish is grainy.
05:18	We can chase the threads with a tap but it's usually better to just drill the hole to the correct size and then we can tap the thread manually.
05:25	This will mainly be in nylon parts, due to its flexibility it's unlikely that we'd ever want threads in TPU parts.
05:32	For stronger fittings on our nylon parts we'll still commonly use nut pockets and threaded inserts.
05:39	For other fine details, like pin features for example, a minimum diameter of 0.8mm is suitable for good definition.
05:47	Embossed features where the details are proud of the face or engraving where they're into the face should be at least 1mm in depth or height to be clear and visible.
05:56	Anything smaller risks wearing off during media tumbling if we use this as a post process.
06:02	This should be 2mm in height minimum though, with a 14 point font size at least.
06:08	And to get really specific, it's also best to use sans serif as the serif at the end of the strokes are fine details that aren't often captured with the smaller font sizes.
06:17	SLS will usually provide an accuracy of plus or minus 0.3%.
06:23	So this depends on the part size but for typical print sizes around 100mm or so, this ends up being about plus or minus 0.3mm for most parts.
06:34	This is relatively good for plastic 3D printing although it still falls short of typical injection moulded parts in CNC machining.
06:41	So we need to keep this in mind for any critical dimensions and allow for some tolerance to avoid some fit and function issues.
06:49	This might be as simple as opening up or slotting some holes but in extreme cases we may need to reconsider the design.
06:56	More specific to 3D printing, it's relatively easy to hollow out parts.
07:00	This is a great way to reduce weight and also material usage which as we know, lowers the part price.
07:07	It's hopefully obvious that this needs to be considered along with the strength of the part and if the inside forms a complete enclosure, the unfused powder will be trapped as the walls are fused, rendering any weight and material savings useless.
07:21	In this case, for powder bed fusion methods, we need to add an escape hole to allow the powder to be removed.
07:27	The recommended minimum hole size for this is around 3.5mm which depending on the design may require significant revisions.
07:36	Another reason why it's good to keep these factors in mind from the start of the design phase.
07:42	Next we need to consider interlocking parts.
07:44	This is where we can print multiple parts at once that fit with each other but still allow for relative movement so they can be disconnected.
07:52	A pin in a hole, a jigsaw style joint or even fabric like chainmail are examples of this.
07:58	The precision and strength of SLS and not having the need for support makes it well suited to interlocking parts.
08:04	In any case we can't have extremely tight fits as we won't be able to remove the powder from between the parts and they can often be fused together.
08:14	For SLS we should leave at least 0.2mm but more ideally about 0.5mm of clearance to avoid these issues.
08:22	We can use a similar idea to design things like moving axles into our parts and nylon is well suited to this as it's a natural bearing material with good wear and low friction.
08:33	Bearing surfaces can be as tight as 0.3mm but we should still target a gap of around 2mm in cases like an axle housing so we can easily remove any unfused powder.
08:45	Finally let's briefly discuss living hinges.
08:48	This is essentially a thin section of the part that provides some functional flexibility, like the cap on plastic containers with snap lock fits.
08:57	TPU and polypropylene provide good flexibility for these, as can nylon when it's properly annealed.
09:03	This is a process of heating something to remove internal stresses and allowing it to cool and become easier to work.
09:11	This is as simple as dipping a nylon hinge in boiling water and working it back and forth a few times.
09:16	We should be targeting about 0.3-0.8mm thickness to allow for this and at a minimum about 5mm of hinge length.
09:26	This covers all the main DFM considerations for SLS, some specific and some general to 3D printing so let's recap the main points.
09:34	Part size limitations involve the overall size which is mostly determined by the printer.
09:40	Print orientation can be used as a tool to help here as well and while there are some considerations around this to avoid warpage, for SLS it has a relatively low impact on the surface finish and strength of the part.
09:53	For large flat surfaces of the part we may need to add some extra structure in the form of ribs to support the part and prevent warpage.
10:01	The wall thickness, supported or unsupported as well as the holes, features and text size all have minimum values that we've included in our design guide resource.
10:11	While printing threads and chasing them with a tap is possible, it's recommended to drill the holes to nominal sizes and tap them or use nut pockets or threaded inserts.
10:21	If the part is hollow we need a relief hole to remove unfused powder and any interlocking parts we need some clearance for the same reason so they don't fuse together.
10:32	Other features like live hinges and bearings are possible with SLS printing with some careful consideration.