Mold surface design for releasability of semicrystalline thermoplastics

Overview

Sticking of semicrystalline thermoplastics to mold surfaces is a persistent challenge in injection molding, hot press molding, and consumer packaging production. Researchers at the University of Twente have developed a mold surface design strategy that addresses this problem by engineering the polymer interphase, the thin layer of polymer that forms in direct contact with the mold surface. By applying coatings such as graphene, the interphase can be tuned to form a transcrystalline structure with reduced adhesion to the mold, enabling easier part release without relying solely on traditional release agents.

The approach is based on the insight that sticking is governed by the crystalline morphology of the polymer close to the mold surface rather than by bulk polymer properties. When a lamella-like interphase forms against a bare steel mold, the bonding between the lamellae and the steel is stronger than the lamella-to-bulk interface, causing cracks to propagate from the metal interface into the polymer. A 2D material coating alters lattice matching between polymer chains and the surface, producing a transcrystalline interphase whose internal strength exceeds that of the polymer-coating interface, so the part releases cleanly.

Technical specifications

Core mechanism:

• The polymer interphase crystalline structure controls mold sticking, not bulk polymer behavior
• 2D material coatings such as graphene promote transcrystalline interphase formation
• Fracture shifts from the polymer-coating interface rather than propagating through the polymer bulk

Validated materials and coatings:

• Semicrystalline thermoplastics tested include PA6, PAEK, and PPS
• Graphene coatings applied to hot press mold surfaces
• Steel used as the baseline mold substrate

Analytical techniques employed:

• Atomic force microscopy (AFM) for surface and fracture analysis
• X-ray diffraction (XRD) for crystalline structure characterization
• Hot stage polarized light microscopy for interphase microstructure observation

Technology readiness level

The technology has been validated through a two-year postdoctoral research project demonstrating the effectiveness of graphene coatings on hot press molds for PA6, PAEK, and PPS. Fracture surfaces and interphase microstructure have been characterized, and a working hypothesis describing the non-sticking mechanism has been proposed. The next planned steps include assessing the microstructure of consumer packaging plastics near the mold surface, evaluating whether the interphase structure can be tuned with alternative 2D materials, conducting a feasibility study on a real industrial mold with an industrial partner, and performing a lifetime performance study of the coated mold surfaces.