Tolerances and fits for SLS printed parts

An SLS part does not come out at exactly the nominal dimension in the model. The deviation is small, but it matters when the part has to fit onto a bearing, a metal shaft or another printed part. This guide explains what tolerances you can expect, where the deviations come from and how to design clearances, holes and threads so that the part fits without rework. The values are the ones Formlabs publishes for the Fuse printers, measured on PA12.

What tolerance to expect

In X and Y, the threshold between the 2 values is 60 mm. Up to 60 mm, ±0.3 mm applies, and above 60 mm, ±0.5% applies, which is ±0.5 mm on a 100 mm dimension and ±1 mm on a 200 mm dimension. In Z, the axis along which the layers are added, the deviations are larger, so treat height as less accurate.

Other materials can deviate somewhat more. In the Formlabs table, PA12 GF has the same dimensional accuracy as PA12. Formlabs recommends PA12 Tough for uniform accuracy across the whole chamber and for reduced warping on large cross-sections. In TPU 90A dimensions are less accurate, and PA11 reproduces fine detail less well. For tight dimensions we recommend PA12.

Where the deviations come from

During printing, the chamber and the powder are heated to just below the melting point, and the laser only adds the difference. As it cools, the molten polymer shrinks. The print preparation software scales the model up by a factor that compensates for the average shrinkage, but the actual shrinkage varies slightly with wall thickness, with the amount of material around the part and with its position in the chamber. The tolerance comes from these variations.

In Z, the way the part cools also matters. Formlabs found that the sintered material beneath a part affects how it cools, so the same part can come out slightly shorter at the bottom of the chamber and slightly taller at the top. This is why critical dimensions are oriented in the XY plane wherever possible.

At the surface, the surrounding powder partly sticks to the part. This is why the surface is matte and grainy, and why small holes tend to come out slightly tighter than in the model. Below 1 mm, a hole can close up completely.

Large, flat parts

The tolerance above applies to dimensions, not to flatness. Large flat plates and parts with very uneven wall thickness can curl as they cool, even if their length and width stay within tolerance. Formlabs recommends a few measures:

• Walls of thickness as uniform as possible, with no massive sections next to thin ones.
• Ribs or fillets on long, thin sections.
• Orienting wide, flat parts at about 20°, which we set when we prepare the build.

PA11, TPU 90A and polypropylene warp more easily than PA12. For flat plates and long frames we use PA12 Tough, which curls less, but it prints only on the Fuse 1+ 30W, so the part has to fit within 165 × 165 × 300 mm. If the flatness of a face matters, say so in your request.

Clearances for assembled and moving parts

The values below are the ones Formlabs recommends, depending on the area of the mating feature. The first 2 rows are for PA12, and for PA11 and TPU 90A the starting value is adjusted after a test print.

The clearances for parts assembled after printing are smaller than the ±0.3 mm tolerance. Mating parts are oriented the same way in the chamber, so they deviate from nominal in a similar way, and Formlabs reports good repeatability in X and Y. If the printed part fits onto a metal or injection-moulded component, the deviations no longer cancel out, and the clearance has to allow for the full tolerance.

In mechanisms printed already assembled, any area where the clearance is below the minimum fuses the parts together. The grainy surface, with a roughness Ra of about 8 to 16 µm after blasting, also matters for parts that slide against each other. For hinges, joints and guides we recommend a small test print, in the same orientation as the real part, before the batch.

Holes and pins

• Minimum diameter. Holes under 1 mm can close up, and pins under 0.8 mm can deform or break. Thin pins can also break during cleaning, so add a fillet radius at the base.
• Orientation. Holes and pins are most accurate with their axis vertical (Z). A hole with a horizontal axis is built up from 0.11 mm layers and comes out less round.
• Tight fits. For bearings, bushings or shafts with an interference fit, design the hole slightly undersize and bring it to size by reaming after printing. For the hole to stay undersize even in the worst case, the difference from the final diameter must be at least equal to the tolerance.
• Concentric holes. For holes that have to be coaxial, the same method, with a pilot hole and reaming, gives the most reliable result.
• Precise location. Use metal dowel pins pressed into reamed holes instead of printed pins.

Threads

Printed threads are a reasonable option from M6 upwards, for parts that are assembled rarely. The thread has to be modelled as geometry. Many CAD programs show the thread only as an annotation, and it is exported as a plain cylinder.

For screws smaller than M6, or for joints that are tightened often, it is better for the thread to be metal or made after printing.

• Heat-set threaded inserts. Brass inserts hold best under repeated assembly and under load. Model the hole for the insert as a plain hole, at the diameter given by the insert manufacturer.
• Tapping after printing. The hole is printed at the tap drill diameter and the thread is then cut with a tap. The method is simple, but a thread cut in plastic holds up less well than a metal one under repeated assembly and disassembly.
• Self-tapping screws for plastics. They work well in PA12 and PA11 and need no work on the hole.

In PA12 GF, small printed threads break easily because the material is brittle, so for this material we recommend inserts or tapping. In your request, say which holes are for inserts, for tapping or for self-tapping screws, so that we know their diameter is critical.

How to mark critical dimensions in your request

We choose the orientation of the parts in the chamber. Critical dimensions tell us what has to take priority, for example precise holes oriented along Z or an important dimension kept in the XY plane. If you mark only the dimensions that really matter, we do not treat the whole part as critical.

1. Send a PDF drawing with the critical dimensions marked and the tolerance required for each. All other dimensions stay at the general tolerance.
2. State what each critical surface fits onto, for example another printed part, a metal part or a bearing.
3. Mark the holes that will be reamed after printing, so that we know they are undersize on purpose.
4. Mention the holes for inserts, for tapping or for self-tapping screws.
5. If a face has to be flat, or if you have requirements on orientation, write this in your request.
6. If you need a tolerance tighter than ±0.3 mm or ±0.5%, tell us from the start, so that we can discuss the options before quoting, for example reaming, bushings or a test print.

How to prepare the file and what other information to include in your request is covered in the guide to preparing files.

Repeatability within a batch and between batches

In a batch, identical parts are placed in the same chamber, at different positions and heights. Formlabs reports stable X and Y dimensions between positions in the chamber and between different builds. In Z, as explained above, height can vary slightly with the position of the part in the chamber.

Between batches, results repeat as long as we use the same files, the same material and the same orientation. Mark the critical dimensions with your first order, so that we keep the same orientation for them in later batches. If you change the model between batches, change the version name too, for example “lid-v4.step”. For a new series, we recommend checking a few test parts in assembly first.

PA12 absorbs little water (0.66% per ASTM D570), so its dimensions do not change noticeably with humidity. PA11 absorbs even less (0.07%).

As a rule of thumb, use the clearances in the table, keep critical dimensions few and preferably in the XY plane, and where you need better accuracy than ±0.3 mm, allow for reaming, bushings or metal inserts.

All SLS minimum feature sizes are in the SLS design guide. When your model is ready, you can send it for a quote, together with the drawing of the critical dimensions.

Sources

• Formlabs, Measuring SLS Dimensional Accuracy and Print Repeatability (tolerances and repeatability)
• Formlabs, Fuse Series SLS Design Guide (minimum features, clearances, orientation and flat parts)
• Formlabs, 3D Printing Threads and Adding Threaded Inserts to 3D Printed Parts (threads, inserts and tapping)