Molded Pulp Tolerances: What ±1.5 mm Really Means for Your Design

Typical molded pulp tolerances run ±1.5 mm on formed features, improving to ±0.5 mm on trimmed edges. Parts that must nest with other components need datums, not hope.

The Scenario: A lid that did not seat

A brand designed a molded pulp box with a lid that needed to close over a rim. Prototype lids rocked because every dimension floated; the rim varied ±2 mm run-to-run.

Pain Points

• No datums meant every feature referenced a different edge;
• Shrinkage differed between parts of the mold, compounding error;
• The fit check was visual, so issues appeared at assembly.

The Solution: Datum-based tolerancing

The redesign defined the rim as the primary datum and controlled the lid from it, with trimming for the closure edge:

1. Datum-first design All critical features reference one rim datum, cutting stack-up error.

2. Trim for closure edges Die-trimmed edges hold ±0.5 mm where the lid meets the rim.

3. GD&T-style notes Tolerance callouts follow ISO 2768 general tolerances.

The Result: Lids seat every time

The lid seated consistently across 1,000 prototype parts. The brand adopted datum-first design for all pulp packaging going forward.