What You Can Expect — and What to Design Around

Tolerances are one of the most misunderstood aspects of thermoforming. Engineers often apply injection molding tolerances to formed parts, leading to rejected parts, redesigns, and project delays.

This guide explains how tolerances work in thermoforming and what to specify on your drawings.

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Why Thermoforming Tolerances Are Different

In injection molding, the part takes the exact shape of the mold cavity. Tolerances are tight and repeatable because every cycle produces the same dimensions.

In thermoforming, the sheet is heated, stretched, and pulled against the mold. The process involves:

• Material stretching (which varies with geometry)
• Cooling shrinkage (which varies with wall thickness)
• Vacuum/pressure dynamics (which vary with part geometry)
• Trimming after forming (which adds its own variability)

This means formed parts have looser tolerances and more variation than injection molded parts.

Tolerance Categories

Thermoformed part dimensions fall into three categories:

Typical Tolerance Table

Tolerances are guidelines for ABS and similar materials. Other materials may vary.

What Affects Tolerance Capability

Tolerance Design Rules

1. Separate Critical from Non-Critical Dimensions

Don't apply tight tolerances to every dimension. Identify which dimensions are critical for:

• Assembly fit — mounting points, mating surfaces, alignment features
• Functional performance — clearances, sealing surfaces, electrical gaps
• Appearance — visible gaps, surface alignment, edge alignment

Apply tight tolerances only to these critical dimensions. Use reference tolerances for everything else.

2. Use Datums

Design the part so it can be consistently located for inspection and assembly. Use two or three formed features as datums:

• Datum A: A flat formed surface
• Datum B: A formed edge or flange
• Datum C: A hole or slot

All critical dimensions should be referenced from these datums, not from an arbitrary edge.

3. Specify the Inspection Method

Different inspection methods have different capabilities:

• CMM (Coordinate Measuring Machine): ±0.05mm — use for critical dimensions
• Handheld gauge: ±0.2mm — use for production inspection
• Go/No-Go gauges: pass/fail — use for repetitive checks
• Visual inspection: subjective — use for appearance only

Include the inspection method in your drawing notes.

4. Consider the Part in Service

A dimension that's "off" by 0.5mm might be fine if it's a hidden internal surface, but unacceptable if it's a visible gap between panels.

Think about:

• What gap is acceptable at the visible seam?
• What variation is acceptable at a mounting point?
• What happens if the part is 1mm too small or too large?

5. Design for Assembly, Not for the Drawing

The most important dimension is how the part fits with its mating components. If the frame, brackets, and adjacent parts have their own tolerances, the formed panel's tolerance stack-up matters more than its absolute dimension.

Best practice: Provide the mating component drawing to your thermoforming partner so they can design the part to fit the actual assembly, not just the nominal dimensions.

Common Tolerance Mistakes

1. Applying ±0.1mm tolerances — thermoforming cannot hold this consistently. Even injection molding struggles at this level.
2. Tight tolerances on free surfaces — free surfaces (non-tool-side) are the most variable. Don't specify tight tolerances here unless necessary.
3. No datums — without clear datums, inspectors measure from different references, leading to inconsistent results.
4. Ignoring trim tolerance — trimmed features have different tolerances than formed features. Don't assume all dimensions have the same tolerance.
5. Not considering thermal effects — a part that fits at 20°C may not fit at 60°C or -20°C. Thermal expansion affects assembly fit.

Tolerance vs. Quality

Tight tolerances don't equal high quality. A part with ±2mm tolerances that fits perfectly in its assembly is better than a part with ±0.2mm tolerances that doesn't fit.

The goal is not minimum tolerance — it's consistent assembly fit.