Thermoformed Part Design Features: Draft, Ribs and Radii

It depends on depth, texture and detail; deeper and more detailed parts need more draft. Values are agreed per project and confirmed on the sample.

Adding structure later means reworking or re-cutting tooling, so ribs are settled during the design review before tooling.

Material stretches into corners during forming; small inside radii and deep draws concentrate the thinning.

Wrinkle-like folds that appear when large flat sheet areas have nowhere to go during forming—addressed through geometry and cavity layout, not just temperature.

Thin walls occur where the heated sheet stretches furthest or too early. Review cavity depth and footprint, sharp radii, near-vertical walls, spacing, flange restraint and uneven sheet heating. Map finished thickness at the base, walls, corners and rim, then compare controlled trials of geometry, heat zoning and available plug, bubble or vacuum assistance rather than changing several variables at once.

There is no universal minimum angle for PP. Required draft changes with depth, texture, radii, resin grade, shrinkage, tool surface and release direction. Start with the largest draft the product and nesting function permit, review it with the toolmaker, and confirm clean release, scuffing, dimensions and wall distribution on production-intent samples.

First create a thickness map and identify where material is being consumed. Useful trials may include softening sharp transitions, increasing workable draft, changing cavity spacing or flange restraint, balancing heat zones and evaluating plug, bubble or vacuum assistance supported by the equipment. Hold other inputs constant, measure the same locations after each trial, and confirm that better distribution does not create webbing, poor definition or release problems.

No percentage is typical enough to serve as an acceptance limit: variation depends on sheet, grade, geometry, heating and forming route. Define functional measurement points at the base, sidewall, corners and rim; sample production-intent parts; then set location-specific limits from fit, stiffness, stacking and transport tests. Nominal sheet gauge alone is not a finished-wall specification.

Tool temperature changes how quickly PP cools and freezes against the surface, which can affect definition, release, shrinkage and the point at which material stops redistributing. Treat it together with sheet temperature and forming sequence. Instrument a controlled trial, keep material and geometry constant, map thickness and dimensions, and choose a stable process window from actual parts rather than copying a temperature from another tool.

Compare the specific PET and PETG grades, not only the family names. Define clarity, depth, detail, impact, heat exposure, downstream sealing or printing, contact requirements and recovery route. Form both candidate sheets on representative geometry and compare haze, whitening, wall distribution, release and functional handling; supplier data guides the shortlist but does not replace finished-part validation.

Calculate the draw demand by region, flag narrow gaps, sharp transitions, small radii and low-draft walls, and identify critical thickness and fit locations. Review the drawing with material, tool and forming constraints together. Then use a prototype or production-intent sample plan that records sheet, heat zones, forming assistance and measurements, with pass criteria for release, definition, thickness, fit, stacking and use loads.

Thin corners point to excessive local stretch; webbing to trapped excess sheet or layout; poor definition to heat or pressure/vacuum delivery; sticking or scuffing to draft, surface or cooling; distortion to uneven cooling or restraint. Identify the defect location and timing, inspect vents and tooling, map heat and thickness, change one likely cause at a time, and recheck dimensions and function on representative parts.