Cavity Surface Finish in Molded Pulp Molds: How Mold Polish Becomes Part Texture

Published: 2026-09-01 | Category: Mold Design | Author: Yisen Pulp Editorial

In molded pulp, the cavity surface is the part surface: the screen and mold face texture transfer to every part. A polished cavity yields a smooth part; a textured cavity yields a grain. This direct transfer makes cavity finish a quality lever — and a common source of unexplained surface defects when it drifts.

The Scenario: The Surface That Changed Overnight

A plant produced smooth premium trays for months, then parts began arriving with a rough, pebbled surface. The cause was a re-polished cavity — maintenance had changed the finish from RA 0.8 to RA 3.2 to "save time." The surface defect cost a week of rejected production before the finish was matched.

Pain Points

Cavity polish drift changes part surface silently — RA values move without anyone noticing.
Inconsistent screen mesh and cavity finish produce patchy surfaces that fail premium specs.
Texture intended for grip or aesthetics is applied without knowing how it transfers to fiber.
Surface defects are blamed on fiber when the cavity finish is the real variable.

The Solution: Controlling Cavity Finish

Cavity finish control is a specification, not a hope — the working rules:

1. Specify RA by surface zone Functional interiors: RA 1.6-3.2. Visible/premium surfaces: RA 0.4-0.8. Record the RA in the mold spec and verify with a surface profilometer at mold acceptance and after every maintenance.

2. Know the transfer ratio The part surface roughness is typically 60-80% of the cavity RA for wet-press parts. A RA 1.0 cavity gives ~RA 0.7 parts; matching the finished-part spec starts with the cavity spec.

3. Texture for function, verify on fiber Molded-in texture (grain, logo, grip) is machined or etched into the cavity. Always validate on real fiber parts — texture reads differently in pulp than in plastic.

4. Screen side vs cavity side The screen side carries screen marks; the cavity side carries polish. Design the visible face against the cavity side and specify both finishes.

The Result: Specifying the Surface Back to Life

After the polish-drift incident, the plant locked cavity RA values in the mold spec, added profilometer checks at acceptance and maintenance, and eliminated surface-finish rejects entirely across 14 consecutive production months. The premium tray line recovered its smooth finish and its customer s trust.