Compare shrinkage allowance: A Practical Thermoforming Guide – Thick Gauge Work

This guide covers how to compare shrinkage allowance on a vacuum formed part: what the designer has to decide, what the mould maker needs to know, and the checks that catch problems before tooling is cut.

What it affects

The main effects are on wall thickness, cycle time and how much hand work the part needs after forming. On a thick gauge part these three decide the piece price, so the decision taken here shows up in every release afterwards.

Typical values

How it is controlled

1. Trimming is done on 5-axis routers or with a dedicated trim fixture to meet the drawing.
2. Drawings are reviewed and a manufacturability report is issued with wall thickness, draft and radii marked up.
3. Sheet is dried when the material needs it, then heated to forming temperature on the machine.
4. Tooling is cut: aluminium for production volumes, composite or printed moulds for samples and first articles.
5. The part cools on a jig so the shape is held while the material stabilises.

Common mistakes

• call out the finish on the drawing: mould texture, paint, or as-formed gloss
• keep a draft angle of at least 3 degrees on vertical walls so the part releases cleanly
• hold the draw ratio near 1:1 for deep parts, or specify plug assist
• avoid sharp steps in the mould, which show as chill marks on the finished surface

How to specify it on the drawing

Put the decision on the drawing rather than in an email: state the material and grade, the gauge, the finish, the surfaces that must not carry marks, and the tolerance that applies to the trimmed edges. Where the part has to mate with another component, mark the datum faces so the trim fixture and the inspection report use the same reference. A drawing that states these points removes most of the questions that otherwise surface during sampling.

Checklist before tooling is cut

• Material and grade fixed, including flame retardancy, UV or food contact requirements
• Gauge selected from the load case, not from habit
• Draft angle and corner radii checked against the sheet thickness
• Trim line placed on a face the router can reach
• Finish and colour standard agreed, with a reference if the colour is critical
• Inspection points agreed and noted for the dimensional report

Related terms

• Wall thickness distribution
• Regrind content
• Flame treatment
• Uneven heating
• Tolerance stack up
• Sheet clamping

Frequently asked questions

Does this change the tooling cost?

Yes. Decisions taken at drawing stage move the tooling cost more than any later change, which is why the manufacturability review is done before the tool is cut.

What information do you need to advise on it?

A 3D file, the material if it is fixed, the annual quantity and the finish. With those the answer comes back with a recommended route and a quotation.