Harmonized System Subheading Verification Using Immersion Density Protocols
Immersion density testing verifies polymer tariff subheadings by resolving specific gravity at 23 °C on void-free annealed plaques.
Compression Molding
Meaning
Thermoset and thermoplastic shaping is achieved by placing raw polymer into a heated mold cavity and applying high force until the material flows to fill the volume. This process uses preheated material placed directly into an open, heated mold cavity before the press is closed. In polymer processing, compression molding yields high-strength components with minimal internal stress and very low fiber orientation.
The method is widely used for fiber-reinforced composites where retaining long fiber lengths is essential for structural performance. It differs from injection molding because the material does not travel through a gating system, reducing shear-induced degradation.
Processing Variable
Control of temperature and clamping force determines the final quality of the molded part. During compression molding, the viscosity of the charge must drop sufficiently to fill the intricate details of the tool before the curing reaction begins in thermosets. If the tool temperature is too high, the polymer cures prematurely, which leads to incomplete part filling or knit lines.
Precise speed control during the final closure phase prevents air entrapment and voids.
Molding Defect
Surface imperfections and dimensional inconsistencies arise when the charge weight or pressure distribution varies. A common problem is flash, which occurs when excess material squeezes out between the parting lines of the tool. If the mold receives insufficient material, the part exhibits short shots or surface sink marks.
Operators must optimize the preform shape and charge weight to minimize these defects.
Economic Choice
Tooling costs for this method are generally lower than those for injection molding due to the simpler cavity design. However, the cycle time is significantly longer because the polymer must either cure or cool within the closed press before ejection. This trade-off makes compression molding more suitable for medium volume runs of large, high-strength structural parts than for rapid production of small consumer goods.