Can SLA prototypes be tapped, bonded, painted, or post-processed further?
Yes, SLA prototypes can be tapped, fitted with metal inserts, bonded, painted, polished, metallized, drilled, and lightly machined after the named resin has been washed, dried, and cured. Tapped bosses may split, bonded seams may peel, paint may reveal residue, and finishing may change dimensions. Because SLA photopolymer is a cured thermoset, plan inserts, pilot-hole stock, joints, coating, and supports before prototyping. The RFQ should mark loads, assembly cycles, cosmetic zones, masking, post-finish dimensions, torque or bond requirements, and inspection stage.
Tapping and Mechanical Fastening
Yes, SLA parts can be tapped, but direct resin threads are most credible for light loads, limited assembly cycles, and prototype fit checks. Boss diameter, edge distance, thread depth, resin ductility, print orientation, post-cure, screw torque, and side load determine whether the thread survives. A sharp-looking printed hole is not proof of usable thread engagement.
Technique: Cure the part, support the boss close to the cutting load, and drill or ream the pilot hole when printed size is uncertain. Use a sharp cutting tap, low torque, alignment control, and chip clearing. Thread-forming taps and undersized holes can split brittle resin. Inspect the boss after tapping and after the specified screw torque.
Best Practice: Use threaded inserts or a captured metal nut for repeated assembly, preload, or a functional load test. Screw-to-expand, press-fit, bonded, and self-tapping inserts require different boss dimensions and installation loads. An insert sold as heat-set hardware must be glued into thermoset SLA resin rather than melted in. Qualify pull-out, torque-out, edge distance, adhesive, and installation sequence on a representative boss.
Material Choice: Tough or impact-modified resin can reduce chipping around threads and inserts, but a family label does not establish torque capacity. Compare the exact resin's post-cured elongation, notch response, creep, and temperature limit. Use machined plastic or metal hardware when the test must represent production preload, wear, fatigue, or service life.
Bonding and Assembly
SLA parts bond effectively when the adhesive matches the resin and mating material, the bondline is designed for shear rather than peel, and both surfaces are fully processed and dry. A strong adhesive cannot compensate for uncured residue, weak surface material, a large uncontrolled gap, or a joint placed at the highest bending stress.
Adhesives: Cyanoacrylate (Super Glue) suits small, close-fitting cosmetic joints and rapid fixtures, but it can bloom on visible surfaces and remains sensitive to peel and impact. Two-part epoxy is usually better for controlled gaps, larger areas, mixed materials, inserts, or moderate structural load. Resin, paint, primer, and adhesive compatibility still need a coupon test.
Surface Preparation: Light sandblasting or sanding can remove gloss and create controlled roughness, but aggressive blasting may erode edges. Remove dust with a resin-compatible method and let the joint dry. Do not assume a final IPA soak is harmless; excessive solvent exposure can craze or swell some photopolymers.
Process: Define bondline thickness, overlap, fixture datum, clamp pressure, cure temperature, and cure time. Keep adhesive away from optical, sealing, painted, or measured faces. Test the joint in tension, shear, or peel as used, then inspect for cohesive resin failure, adhesive failure, voids, and dimensional shift.
Painting and Finishing
SLA parts provide a good substrate for painting when liquid resin, wash solvent, support scars, and sanding debris are removed before primer. Paint can hide color variation but will not repair a soft, contaminated, cracked, or unstable substrate.
Post-Cure: Follow the named resin's wash, dry, and post-cure schedule unless a qualified coating system specifies another sequence. Drain blind features and check for tacky surfaces. Cure conditions affect dimensions, color, adhesion, and thin details.
Sanding: Start with the least aggressive grit that removes the measured defect, then progress until support marks and scratches are uniform. Wet sanding may reduce clogging and heat on broad faces when the resin and water exposure are compatible. Protect holes, seals, datums, text, and edges from stock loss.
Priming: Use a resin-compatible filler primer only where texture requires it. Record primer build and sanded build because small holes, engraved marks, snap gaps, and mating edges can close or soften. Mask bond faces, threads, inserts, contacts, and final inspection datums.
Painting: Apply compatible spray paint in light coats and observe flash and full-cure times before assembly. Conventional powder coating is usually unsuitable because electrostatic charging, powder flow, and oven cure can exceed the resin's process window. Qualified low-temperature systems are separate trials; controlled spray painting is the normal prototype route.
Clear Coating (Optional): A compatible clear topcoat can adjust gloss, protect color, and reduce handling marks. It also adds film thickness and may alter transparency, fit, friction, solvent resistance, and UV aging. Approve coating on a representative part and measure critical features after cure.
Other Post-Processing Options
Sanding and Polishing: Sanding and polishing suit reachable cosmetic faces, clear covers, and appearance models. Deep channels, fine text, and internal surfaces may remain unfinished. Define gloss, scratch visibility, lighting, viewing distance, and stock allowance rather than requesting a vague "smooth" finish.
Vapor Smoothing: Vapor smoothing is not a standard SLA process because photopolymer resins do not share one controllable smoothing solvent. An unqualified solvent can craze, swell, discolor, or weaken the part. Test mass, dimensions, surface, and strength on a coupon before any chemical smoothing trial.
Metallization: Vacuum metallization normally uses a smooth basecoat before the decorative metal layer. Electroplating requires a conductive seed and may alter small features. Separate appearance plating from shielding or structural requirements, then define adhesion, coverage, thickness, and masking.
Machining: Fully processed SLA parts can be drilled, reamed, trimmed, faced, or lightly milled. Support the feature near the cut, use sharp tools, control heat and feed force, and remove dust safely. Inspect for edge chipping, cracks, hole size, flatness, and datum shift after unclamping.
Engineering Guidelines
Design for Post-Processing: Add local finishing allowance, boss and insert edge distance, adhesive overlap, drain access, and masking notes. Keep supports away from cosmetic, sealing, bonding, and reference surfaces where possible. State whether drawing dimensions apply before or after coating and assembly.
Select Resins for Function: Use tough resin for handling, clips, and low-cycle fastening trials; use high-detail resin for appearance surfaces; use clear resin with a dedicated optical finishing plan. The exact grade, cure, temperature, load, and environment still control acceptance.
Process Order is Critical: No sequence fits every SLA assembly. Define wash, dry, cure, machining, thread or insert preparation, finishing, masking, primer, paint, bonding, and final inspection. Move an operation only for adhesive access, coating, heat, or datum control, then validate the route on a first article.
SLA prototypes support substantial post-processing when each operation has an allowance, failure criterion, and inspection point. Release the part after representative threads, inserts, joints, coatings, and machined features meet the specified torque, load, appearance, adhesion, and dimensional checks.
