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.
Heavy Wall Injection Molding
Meaning
Thermoplastic manufacturing of parts with nominal wall thicknesses exceeding six millimeters requires specialized processing strategies to manage the slow rate of heat extraction. Through heavy wall injection molding, processors adjust cycle parameters and gate designs to prevent structural defects like internal voids and severe surface sink. This technique demands precise control over the cooling and packing phases to ensure the part solidifies uniformly throughout its bulk.
Defect Prevention
Internal vacuum voids occur when the outer skin of a thick part solidifies first, leaving the molten core to shrink away from itself as it cools. In heavy wall injection molding, this issue is addressed by using low melt temperatures and exceptionally long hold times on the press. Maintaining pressure on the melt prevents the center from pulling apart, which is critical for the mechanical strength of structural plastic parts.
Cooling Penalty
Heat dissipation in polymers is governed by their low thermal conductivity, which causes the required cooling time to increase exponentially with wall thickness. During heavy wall injection molding, cooling times can extend to several minutes, which significantly raises the cost of machine run time. To offset this, molders utilize thermal-conductivity alloys in the tool or design conformal cooling channels to accelerate heat removal.
Resin Selection
High-molecular-weight resins with low shrinkage characteristics are preferred for thick-walled applications to reduce dimensional distortion. Under heavy wall injection molding conditions, amorphous materials like polycarbonate are chosen because they shrink less than semi-crystalline polymers like polyethylene.