When not to use resin 3D printing

1. The part is large and mechanically simple

A 300 mm electronics enclosure with 2.5 mm walls does not usually benefit from micron-scale XY pitch. FDM can make the same envelope with less liquid-material cost and without peeling a large cross-section from a release film thousands of times. For very large parts, segmentation and joining are also easier with thermoplastic processes.

2. Sustained load and creep dominate

Clamps, brackets, fixtures and preloaded joints may spend months under stress. Short-term tensile strength is not enough to predict dimensional retention. If the resin supplier does not publish creep or long-duration behaviour and the part is load-bearing, a known engineering thermoplastic or machined polymer can be a safer route.

3. Impact and abuse are expected

Production jigs get dropped. Guards are knocked. Latches are cycled. Many high-stiffness photopolymers fail abruptly at stress concentrators. Tough resins improve this substantially, but if impact tolerance is central, compare a real tough-resin test to nylon, TPU, PETG, PP or a moulded material instead of assuming the resin is adequate because the static strength looks high.

4. The service environment is hot, wet, outdoors or chemically aggressive

Specialist resins exist for heat and chemicals, but they must be selected and validated specifically. If a part will live next to an engine, under UV, in coolant, in cleaning chemicals or in high humidity, demand data at those conditions. A standard modelling resin is not a neutral engineering plastic.

5. You need threaded serviceability

Large threaded holes, repeated fastener cycling and field repair often suit thermoplastics or machined parts better. Metal inserts can be bonded or press-fitted into resin, but the joint design must manage local stress and brittleness. If every prototype requires inserts, adhesive, cure and rework, the “fast” resin workflow may not be fast.

6. Chemical handling is incompatible with the workspace

Uncured resin and wash solvents require PPE, clean/dirty zoning, spill control, ventilation and waste management. That can be entirely reasonable in a workshop or lab; it may be inappropriate in an office, classroom or shared domestic area. Do not solve a geometric problem by creating an unmanaged exposure problem.

7. The tolerance is tight but the workflow is uncontrolled

Resin can be very accurate, but only after exposure, temperature, orientation, post-cure and compensation are stabilised. If the organisation needs an occasional ±0.05 mm gauge but has no appetite for process qualification, machining or a qualified bureau may be more reliable than owning a hobby-class printer.

8. You need production economics at hundreds or thousands of identical parts

Resin printing scales well for small batches and complex geometries, but consumables, screen/film wear, labour, washing and post-cure remain. At higher volumes, moulding, casting or dedicated additive production processes may dominate. The break-even depends on tooling cost and change frequency; calculate it rather than treating additive as automatically economical.

Alternative-process map

Resin is excellent when used deliberately

This is not an argument against resin. It is an argument against buying it for a headline resolution number. Resin is exceptionally capable for small precise components, complex surface detail, patterns, intricate prototypes and geometries where FDM support mechanics become intrusive. The best process is the one whose weaknesses do not collide with the actual requirement.