Shear Heating and Melt Degradation Analysis in Thin Wall Tooling
Thin wall injection velocity must balance viscous dissipation heat spikes against gate clearance geometry to prevent local polymer backbone degradation.
Injection Speed
Meaning
Velocity parameters that govern the movement of a molding machine piston or screw during the filling stage of a cycle determine how quickly material enters a cavity. Setting the injection speed correctly is necessary to control the shear rate and ensure the plastic fills the mold before freezing. This setting directly affects both the strength and the surface finish of the molded parts.
Flow Control
Varying the rate of cavity filling allows the molding process to adapt to different part geometries. High injection speed generates frictional heat that lowers material viscosity, aiding the flow into thin-walled sections. If the rate is too high, it can cause jetting or burning of the plastic.
Mold Venting
Displacing air from the mold cavity must occur at the same rate as the polymer enters.
Production Quality
Optimizing the flow profile is an important step in transitioning a part from a prototype mold to a high-volume production line. When a higher injection speed is used to reduce cycle times, the increase in shear stress can lead to cosmetic defects or warpage. If these parameters are set incorrectly before a continuous run begins, the resulting high reject rate can destroy the economic viability of the production campaign.