Process Window Compensation versus Tool Steel Alterations for Heavy Wall Sections
Altering tool steel to eliminate heavy wall sections cuts cycle time and unit cost far more reliably than forcing process windows past physical thermal limits.
Polymer Residence Time Degradation
Meaning
Thermal and chemical breakdown of plastic resins occurs when they are held at elevated processing temperatures for an extended duration within the heating barrel of an extruder or injection molding machine. Through polymer residence time degradation, the molecular weight of the polymer is reduced, which negatively impacts the mechanical properties and surface finish of the molded parts. This degradation occurs when the machine screw size and shot size are poorly matched, leaving material in the barrel for too many cycles.
Defect Mechanism
Discoloration, silver streaks, and splay marks on the surface of molded parts are typical visual indicators of thermal breakdown. During polymer residence time degradation, the polymer chains shear and generate volatile gases that are trapped in the melt as it enters the cavity. These gas bubbles produce cosmetic defects and reduce the impact strength of the finished product, causing early structural failure.
Process Sizing
Matching the shot weight of the part to the total barrel capacity of the molding press is the primary method to control material exposure to heat. Under polymer residence time degradation, the material should ideally pass through the barrel in fewer than three to five injection cycles. If the press is too large for the part, the resin resides in the heated barrel too long, which is a common cause of scrap in low-volume or small-part runs.
Cost Impact
Scrap material and purged resin represent a major loss in material efficiency during long production runs. When polymer residence time degradation is detected, the run must be stopped, the barrel purged, and the process parameters reset, which increases machine downtime and labor costs.