Polyurethane is used for its excellent durability, chemical resistance, and mechanical properties. Typically, polyurethane parts are made by casting or reaction injection molding. Formlabs PU Rigid Resins are developed with a hybrid chemistry which allows them to be compatible with Formlabs SLA printers. Parts printed with Formlabs PU Rigid Resins have extremely high impact strength and the physical properties of polyurethane, making them ideal for end-use applications that are intended to be exposed to harsh conditions.

Sometimes prints don't come out as expected. Compare symptoms of common issues with parts printed in PU Rigid Resins to narrow in on the underlying source of the problem and find a solution.

Immediately after printing, in their green state, polyurethane parts are much weaker than parts printed in Formlabs general purpose and engineering resins. Thin parts may break when removed from the build platform.

To prevent printed parts from breaking during removal from the build platform:

• Consider printing geometries with a wall thickness greater than 1 mm.
• Use one of the recommended removal tools for part removal from the build platform.
• When printing with a standard Build Platform: Print parts on rafts instead of printing directly on the build platform. Sliding a removal tool underneath a part printed directly on the build platform stresses the part and can lead to breakage. This is more pronounced for thin-walled parts and for parts printed with PU Rigid 650 Resin.
• When printing with a Build Platform 2: Printed parts can be separated from the build platform by flexing the part removal handles. If they do not come off, use the removal tool to carefully pry the parts from the build platform.
• Parts printed on Build Platform 2 with PU Rigid 1000 Resin come off more easily than parts printed with PU Rigid 650 Resin due to the higher stiffness of PU Rigid 1000 Resin. Print parts in the center of the build platform for best part release.
• Leave the raft adhered to the build platform—separate the part from the raft by cutting the support structure with a flush cutter instead.
• If you do print parts directly on the build platform, orient the parts to increase print success and to decrease stress on the parts during part removal.
• Consider heating the build platform with a hair dryer or another similar heat source to help loosen the parts before using the removal tool.

Any liquid resin left on the surface of a part printed in a PU Rigid Resin may cure and lead to visible surface artifacts, especially if relative humidity levels are high.

To fully clean parts and prevent surface artifacts:

• Consider changing the orientation of tall parts to reduce print times.
• Use a dry cabinet to maintain the lowest possible relative humidity levels during printing.
• Manually dip and agitate parts in a wash bucket of clean solvent to remove resin stuck to the surface of the parts.
• Use compressed air to blow away any remaining solvent and resin after washing your parts.
• Ensure that the parts are completely dry of PGDA solvent before humidity curing.

Removing supports can lead to divots, burrs, or other damage on your printed parts. Set up your prints to make supports easier to remove, and change when you remove supports depending on part geometry.

To avoid damaging your printed parts during support removal:

• For parts with wall thicknesses less than or equal to 2–3 mm, leave the parts on supports during humidity post-curing to prevent warping.
• For large and thick parts with wall thicknesses greater than 2–3 mm, consider removing supports before humidity post-curing.
  • Use a flush cutter to remove support structures from the green part instead of peeling away the support structures by hand.
  • If needed, use a fine sand grit paper (240–320 grit size CAMI) to sand the parts in the green state to obtain a fine surface finish. Use care while doing so to avoid damaging your parts.
• If you post-cure your parts on their supports, remove support structures with a flush cutter. Cut close to the surface of the part to avoid marring the surface through excess sanding.
• For parts printed in PU Rigid 650 Resin: If you are experienced with printing with PU Rigid 650 Resin, try reducing the support density from a default of 1.0 to 0.8 or less and reducing the touch tip size from a default of 0.6 mm to 0.5 mm depending on the model size, shape, and thickness. Less dense support structures with smaller touchpoints are easier to remove.

When printing parts directly on the build platform, the early layers may flare outwards beyond the designed bounds of the part. Formlabs LFS printers (Form 3 and Form 3L product lines) overexpose the early layers of a print to ensure good adhesion to the build platform. Print parts on rafts (or mini-rafts, if applicable) to ensure that the part’s geometry and tolerances remain intact.

The dimensional accuracy of 3D printed parts depends on the part geometry and part orientation during printing:

• Orient your parts according to Formlabs guidelines to help ensure better dimensional accuracy and print success.
  • Printed parts can shrink slightly during post-curing. The post-curing shrinkage is dependent on both geometry and part design. Holes and thin walls are particularly susceptible to shrinkage compared to thicker part sections. PreForm and Formlabs print settings account for material shrinkage, but you may need to adjust your model to achieve appropriate dimensional accuracy.
• Do not wash printed parts for longer than the recommended amount of time.
• Ensure that printed parts are completely dry before post-curing.

Support placement, density, and removal techniques all affect the cosmetic finish of part surfaces. Depending on your part geometry, you can address cosmetic issues during print preparation, after printing, or after post-curing.

Print preparation

• Orient the part in PreForm so that the support touch tips are located on non-cosmetic surfaces of the part. Consider editing the touch tips and deleting touch tips as needed on other surfaces.
• For parts printed in PU Rigid 650 Resin: If you are experienced with printing with PU Rigid 650 Resin, try reducing the support density from a default of 1.0 to 0.8 or less and reducing the touch tip size from a default of 0.6 mm to 0.5 mm depending on the model size, shape, and thickness.

After printing

For large, thick parts, remove supports after washing and sand with a fine grit sandpaper (240–320 grit CAMI) before post-curing. Formlabs does not recommend this technique for thin or fragile parts.

After post-curing

After post-curing, parts are stronger and can be sanded and polished with less risk of damage.

When sanding parts:

• Sand with a 320–650 grit sandpaper to remove blemishes and any leftover support marks.
• For more persistent support marks, use a rotary tool with sanding discs to achieve a more consistent surface finish. Use the finest abrasive available for your tool.

Most remaining blemishes can be removed by polishing your printed parts:

• Apply a plastic polishing compound such as NOVUS Plastic Polish to temporarily restore the surface finish. Note that scrubbing treated parts with IPA or doing additional ultrasonic cleaning can remove the polish and render parts to their original finish.
• For a more permanent polish, try different polishing compounds such as Tripoli and Jeweler’s Rouge. Apply these compounds with a rotary tool and a cotton polishing disc. Scrubbing treated parts with IPA or doing additional ultrasonic cleaning should not remove these polishing compounds.
• Optionally, recoat parts with one or two drops of liquid resin to restore the surface finish.
  • Liquid resin can be spread over the entire part with the help of smooth, clean nitrile gloves to achieve a uniform finish of an extremely thin layer of resin.
  • Formlabs recommends UV post-curing for at least 10 minutes in a Form Cure or Form Cure L at room temperature and moisture curing for at least 24 hours again to fully cure the resin layer.