SLS · MJF · FDM · SLA

3D Printing Design Guidelines

Q: What tolerances can you hold?

Our standard tolerance is +/-0.3 mm for parts up to 100 mm in size. For larger dimensions it is generally calculated as +/-0.3 mm for the first 100 mm, plus an additional +/-0.1% for every 100 mm thereafter. Tighter fits, bores and sealing faces are achieved by printing near-net and machining the critical features - flag those dimensions at quote and we will plan and inspect around them.

Parts that print first time are designed for the process, not adapted to it. This is what our engineers check before a job goes on a machine — the handful of decisions that decide whether a part comes out right, and the ones that quietly cost you a reprint.

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Design for additive manufacturing · DfAM guidelines · escape holes and wall thickness · print orientation

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The process decides the rules

Most design mistakes in additive come from applying one process’s habits to another. The single biggest split is whether your part is built in powder or built with supports, because everything downstream follows from it.

Powder-bed — SLS and MJF

No supports · near-isotropic · nests densely

● The part is held up by the powder around it, so there are no supports to design in and no witness marks where they were cut off.
● Overhangs, undercuts, internal channels and lattices are effectively free — the geometry that saves weight costs nothing extra.
● Properties are far more consistent in every direction, so parts loaded on several axes behave predictably.
● The catch: enclosed cavities fill with powder, and it has to get back out. See rule 1.

Supported — FDM and SLA

Supports needed · direction matters · larger and finer

● Overhangs need support, and support has to be removed, which leaves a witness on the surface it touched.
● FDM is directional: layers bond to each other and that bond is the weak axis. See rule 4.
● In exchange you get the extremes — 500 × 500 × 900 mm in one piece on large-format FDM, and the finest surface we produce on SLA.
● Tell us which faces matter cosmetically and we will orient supports away from them.

The six that matter

What we check before your part goes on a machine

None of these are exotic. They are the things that come up again and again on real jobs, and catching them at CAD stage is free where catching them after a build is not.

1 Applies to: SLS & MJF

Give trapped powder a way out

Any sealed cavity in a powder-bed part finishes the build packed solid with unsintered powder. If there is no route out, it stays there — adding weight, rattling, and in a fluid part, contaminating whatever runs through it.

Two escape holes is the working minimum: one for powder to leave through, one to let air in behind it. More is better, and bigger is better than smaller. Put them where a cleaning tool can physically reach, not just where they look tidy on the model — a hole into a deep blind pocket is not much use if nothing can get down there to agitate the powder.

If a cavity has to stay sealed for the part to work, say so at quote and we will talk through whether it is printable as designed or better split and bonded.

2 Applies to: all four processes, most critical on SLS & MJF

Keep wall thickness even

Sintering and fusing are heat processes, and heat leaves a thick section more slowly than a thin one. Where a wall jumps abruptly from heavy to light, the two sides cool at different rates and pull against each other. That shows up as warp, as internal stress, or as a part that measures fine on the bench and moves later.

The fix is usually to shell a solid part down to a consistent wall rather than printing it solid, and to blend transitions with a fillet or a taper instead of stepping straight from one thickness to another. You get a straighter part, and because there is less material in it, a cheaper and faster one.

Ribs are the honest way to put stiffness back into a shelled part — they add strength without adding a thick section.

3 Applies to: SLS & MJF

Print the assembly, not the parts

With no supports to dig out from between components, a powder-bed build can produce a working mechanism in one go. Hinges, chains, captive nuts, ball joints, enclosed mechanisms — all of it can come off the machine already assembled and moving, with no fasteners and no assembly labour.

The whole thing turns on the clearance between the faces that move against each other. Too tight and the surfaces fuse into a solid lump that no amount of cleaning will free. Too loose and the joint is sloppy. It also has to be a gap powder can actually be cleared from, which ties it back to rule 1.

Tell us what has to move, how freely, and in which grade, and we will confirm the clearance before it goes on the machine rather than after.

4 Applies to: FDM above all

Orientation is a load-path decision

An FDM part is built from layers bonded to one another, and that bond between layers is weaker than the material within a layer. Load a part along the layers and it is strong. Load it across them, and you are testing the weld between layers instead of the plastic itself.

This is not a slicer setting to be fixed later — it is a design input. A bracket that will see load in one direction should be oriented so the layers run with that load, and if the geometry will not allow it, that is worth knowing before the part is made rather than after it fails.

Send the load path with the part — even a sketch with an arrow on it — and we will orient to it. On SLS and MJF this matters far less, which is exactly why parts loaded in several directions at once tend to end up on those processes.

5 Applies to: all four processes

Print a coupon before you print the part

The cheapest habit in design for additive. When a part has one critical fit — a snap-fit, a dovetail, a bearing seat, a threaded boss — do not discover it is 0.2 mm out by running the whole part first.

Isolate that feature onto a small test coupon, print the coupon, check the fit in your hand, then commit the full part with the clearance corrected. A coupon is a fraction of the cost and turns round fast, and it removes the one variable most likely to force a reprint.

This matters most where the fit is against something we did not print — a bought-in bearing, a standard fastener, an existing housing. Send both and we will check the mating dimension with you.

6 Applies to: all four processes

Design for the build, not just the part

On powder processes you pay for the volume of the build your parts occupy, so parts that nest well are parts that price well. A batch that packs tightly into a bed costs meaningfully less per part than the same batch ordered piecemeal across separate builds — which is worth knowing before you split an order.

If the part is larger than the envelope, that is not the end of it. We split it, print it in sections, then bond and finish in-house. Send the overall dimensions with the enquiry and we will plan the split so joints land somewhere sensible rather than through a critical feature or a show face.

And if you are ordering repeats over time, tell us — consistency across builds is easier to hold when we know from the start that it matters.

Hard constraints

Build envelopes and lead times

The numbers that will not bend. Anything larger than these is printed in sections, bonded and finished in-house.

Process	Max build size	Lead time
SLS	165 × 165 × 300 mm, up to 490 × 490 × 740 mm for PA12-CF	1–3 days express, 5–7 days standard
FDM	500 × 500 × 900 mm (340 × 320 × 340 mm for PPA-CF)	1–3 days express, 5–7 days standard
SLA	353 × 196 × 350 mm	1–3 days express, 5–7 days standard
MJF	380 × 284 × 380 mm	5–7 working days, standard only — no express tier
Tolerance	±0.3 mm up to 100 mm, then ±0.3 mm for the first 100 mm plus ±0.1% for every 100 mm thereafter. Full detail on tolerances & accuracy.

Tighter fits are achieved by printing near-net and machining the critical features — flag those dimensions at quote. Send the CAD and we will confirm the envelope against your part.

FDM specifics

Three things that only apply to filament

FDM behaves differently enough from the powder processes that it is worth calling these out separately.

Before the print

● Orientation — covered in rule 4, and the single most consequential decision on an FDM part.
● Moisture — nylons absorb water from the air, and wet filament prints badly. We dry material before it runs; on your side, it is worth knowing that a nylon part will also take up moisture in service, which shifts dimensions slightly.

After the print

● Annealing — some grades gain heat resistance and stiffness from a post-print heat cycle, at the cost of a small dimensional change. If your part runs hot, ask us whether annealing is worth it for that grade.
● Layer lines — visible by nature on FDM. If the finish matters, paint or vapour smoothing is the answer, and it is a process with a cost, not a print setting.

Send us the CAD → Not sure on material? Use the selector

Minimum feature sizes

Process-specific design guides

Minimum wall thickness, hole diameter, embossed text height, pin diameter and clearance all move with the process and with the specific grade. One set of numbers across twenty-four production materials would be tidy, and it would also be wrong for most of them — a figure that holds for SLS PA12 does not hold for FDM PPA-CF or SLA Ceramic-Filled.

So we publish them by process. The SLS design guide gives the full set of reference dimensions for powder-bed nylon — walls, pins, holes, drain holes, clearances, lattice gaps and embossed and engraved text minimums — plus grade-specific notes for PA12, PA11, PA12-GF, PA12-CF and TPU 90A. Guides for the other three processes are in preparation.

For anything not yet covered, send the part and tell us the grade, or let us pick the grade, and we will confirm the real minimums for that combination before you design around a guess. It is free, it comes with the quote, and it takes an engineer rather than a table.

Read the SLS design guide → Get the figures with your quote

FAQ

Design for 3D printing FAQ

Do I need to design supports for SLS or MJF parts?

No. Both build in a bed of powder, so the surrounding unsintered powder holds the part up as it grows. That means no supports, no witness marks where supports were removed, and no orientation compromise to make supports reachable. Overhangs, undercuts and internal channels cost nothing extra. FDM and SLA do use supports, so on those two the orientation conversation matters.

Why do enclosed cavities need escape holes?

Because on SLS and MJF the cavity fills with powder as it builds, and if there is no way out the powder stays in the part. Two holes is the working minimum — one to let powder out and one to let air in — and bigger is better than smaller. Put them where a cleaning tool can actually reach, not just where they are tidy on the CAD. If you leave them off we will usually spot it at quote and ask.

Why does wall thickness need to be consistent?

Sintering is a heat process, so thick sections and thin sections cool at different rates. Where a wall steps abruptly from thick to thin, that difference in cooling pulls on the part and shows up as warp or internal stress. Shelling a solid part to an even wall and blending transitions rather than stepping them fixes most of it — and it prints faster and costs less, because there is less material in the part.

Can you print assemblies that come out already moving?

Yes, on SLS and MJF. With no supports to remove from between the parts, hinges, chains, captive fasteners and enclosed mechanisms can be printed as one build and arrive working. The clearance between the moving faces is the whole game: too tight and the parts fuse, too loose and the joint is sloppy. Tell us what has to move and how freely, and we will confirm the gap before it goes on the machine.

Should I print a test piece before committing to the full part?

For anything with a critical fit, yes — and it will save you money. Rather than running a large part to find out a clip or a dovetail is 0.2 mm out, isolate that feature onto a small coupon, print the coupon, check the fit, then commit the full part. A coupon costs a fraction of the part and turns around fast. It is the single cheapest habit in designing for additive.

Does print orientation change how strong an FDM part is?

Yes, significantly — and this is the main thing that separates FDM from the powder processes. FDM builds in layers that bond to each other, and that bond is the weak axis, so a part loaded across the layers is weaker than the same part loaded along them. Tell us the load path and we orient the part to it. SLS and MJF are far more isotropic, which is why they suit parts loaded in several directions at once.

What if my part is bigger than the build envelope?

We split it, print it in sections, then bond and finish it in-house. Our largest single-piece envelope is 500 × 500 × 900 mm on large-format FDM, and we have taken parts well beyond that in bonded sections — a full show-car body among them. Send the dimensions with the enquiry and we will plan the split so the joints land somewhere sensible rather than through a feature.

What tolerances can you hold?

Our standard tolerance is ±0.3 mm for parts up to 100 mm in size. For larger dimensions it is generally calculated as ±0.3 mm for the first 100 mm, plus an additional ±0.1% for every 100 mm thereafter. Tighter fits, bores and sealing faces are achieved by printing near-net and machining the critical features — flag those dimensions at quote and we will plan and inspect around them. Full detail on the tolerances and accuracy page.

How fast can you turn a part around?

Express lead time is 1–3 days on SLS, FDM and SLA, with 5–7 days standard. MJF runs 5–7 working days, standard only — there is no express tier on that process. Tell us the date you need and we will put the part on whichever process makes it.

How do I find the minimum feature size for my material?

For SLS, the full set of reference dimensions is published on our SLS design guide — walls, pins, holes, drain holes, clearances and text minimums, with grade-specific notes. Guides for MJF, FDM and SLA are in preparation. For anything not yet covered, send the CAD to our instant quote tool or talk to the engineers, and we will confirm the figures for the grade your part is actually going on — free, and before you redesign around a guess.

Ready when you are

Send it over and we’ll review the design with you.

Every quote comes with a free design-for-additive review — we tell you what will print, what will not, and what we would change. Printed, finished and inspected under one UK roof.

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Printed, finished and inspected at our own ISO 9001 facility in Shipston-on-Stour, Warwickshire. Call us on 024 7736 0144 or request a free sample pack.