Meaning
The vertical boundaries of injection mold cavities guide the flow of molten resin and form the outer surfaces of molded plastic parts. In mold construction, tool steel sidewalls must withstand high injection pressures and thermal cycling while resisting the abrasive action of the flowing polymer. These surfaces require careful design to ensure the mechanical integrity of the mold.
Mechanical Stress
During the injection and packing phases of the molding cycle, the tool steel sidewalls experience massive outward pressures from the highly pressurized polymer melt. This intense stress can cause significant elastic deflection or even permanent deformation of the cavity if the steel thickness is insufficient. Mold design rules dictate specific thickness-to-depth ratios to ensure that structural deflection remains well below the allowable tolerance limit of the molded component, thereby preventing flash and dimensional variation across production runs.
Finishing Requirement
Achieving the specified finish on the tool steel sidewalls is challenging because of their vertical orientation and limited accessibility. Mold makers employ a combination of manual stoning, rotary polishing, and selective texturing to achieve the required surface roughness. Any residual machining marks or surface damage from electrical discharge machining can lead to stress concentration and eventual mold failure.
Release Efficiency
Ejection efficiency of the cooled polymer parts depends heavily on the surface quality of these sidewalls. Draft angles must be incorporated into the sidewall design to facilitate easy ejection and prevent part scratching or sticking. Highly polished tool steel sidewalls can actually increase sticking with certain soft polymers, requiring a slightly textured finish to reduce the contact area and improve release efficiency.
