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.
Core Pin Thinning
Meaning
Mechanical stress during the high-pressure filling phase can cause slender steel inserts to deflect or bend, altering the internal dimensions of the molded part. In core pin thinning, the cross-section of these projections is intentionally reduced to facilitate specific polymer flow paths or to accommodate intricate part features, at the cost of structural rigidity. This design choice increases the risk of pin deflection and unbalanced wall thickness in the finished article.
Deflection Risk
Mold pressure distribution during injection exerts asymmetric lateral forces on any cantilevered cores within the cavity. When core pin thinning is applied, the mold designer must analyze the flow front to ensure that the plastic does not push the pin to one side, which produces uneven walls and scrap parts. Symmetrical gating helps balance these forces, but high-viscosity resins can still cause substantial deflection of the thinned metal.
Tooling Wear
Repetitive thermal cycling and hydraulic loads shorten the fatigue life of slender steel inserts. Under the conditions of core pin thinning, the stress concentrations at the base of the pin are magnified, leading to premature fatigue failure and cracks. Selecting high-strength tool steel and using generous transition radii are necessary to extend the service life of these delicate features.
Process Limit
Injection velocity must be carefully controlled to prevent high shear stress on fragile core elements. With core pin thinning, the process window narrows, demanding precise control over the filling speed to avoid mold damage.