How tight can thermoforming tolerances be?
Thermoforming tolerances are looser than injection molding or machining and scale with part size, draw depth, and material shrink. Tolerances at trimmed and CNC-machined features are tightest; formed surfaces are looser. The right approach is to tighten only mating and mounting features and open everything else.
What drives thermoforming tolerance
Three things move a thermoformed dimension: material shrink as the part cools, draw depth and wall thinning, and whether the feature is formed or machined. Formed surfaces carry the most variation. Features cut afterward on CNC carry the least, because they are referenced to fixture datums, not to the formed surface.
Tighten the features that mate
Put your tight tolerances where the part actually has to fit — mounting holes, mating edges, bolt patterns, interfaces with other components. These are almost always trimmed or drilled on CNC, where tight, repeatable tolerance is achievable. Reference them to a clear datum scheme.
Open everything else
Cosmetic surfaces, overall part envelope, and non-mating features do not need tight tolerance. Specifying it there adds inspection cost and scrap with no functional return. A realistic, open tolerance on formed surfaces keeps the part affordable.
Use a datum strategy, not a blanket callout
A single blanket tolerance on a large formed part is a recipe for argument and scrap. Define datums, dimension critical features to them, and let the rest float within a general profile tolerance. This is standard GD&T practice and it maps cleanly onto how thermoformed parts are trimmed and inspected.
Settle it during DFM
The cheapest time to right-size tolerances is before tooling. ThermoFloe reviews tolerance callouts on every drawing during design-for-manufacture, flags anything unrealistic for the process, and recommends where to tighten and where to open — within five business days.