PP-TD20 Injection Temperature and Shrinkage: Four Variables That Move Dimensions

Order of magnitude

Homopolymer PP shrinks about 1.5-2.0%, copolymer PP 1.2-1.8%, PP-TD20 roughly 0.9-1.2%, and PP-GF20 can reach as low as 0.3-0.5%. Differences of that magnitude mean tolerances must be fixed together with the material at design stage, not drawn first and specified later.

Filled compounds also retain a flow versus cross-flow shrinkage difference, though much smaller than glass-reinforced systems.

The four variables

One, mould temperature. Higher mould temperature means fuller crystallisation and more shrinkage, but better surface and heat resistance. 30-60 °C is typical.

Two, hold pressure and time — the most direct lever. Insufficient holding raises shrinkage markedly, causing sink marks especially in thick sections.

Three, injection speed. High speed creates more orientation and can widen the directional shrinkage gap.

Four, wall thickness. Thicker walls cool slowly and crystallise longer, shrinking more — the reason uniform wall thickness matters so much.

Measurement and tolerance advice

After sampling, condition parts at 23 °C and 50% RH for 24 hours before measuring; post-crystallisation continues to shift PP dimensions slightly for several days.

For assembly-critical holes, run mould-flow prediction first, then correct against first-article measurement.