Gate position/Mould filling
Suitable gate away from hinge Avoid"Stop and GO" flow through hinge by locating gate so that major cavity completely fills before flow starts through the hinge.
Correct gate location for shallow boxes
Provide ample cooling over hinge area. High melt temperatures due to shear heating through gate can lead to delamination
Mould conditions Flow of polymer melt through hinge should be rapid to promote orientation. Flow should be in a parallel stream through the hinge. "Stop and Go" flow will produce a layered hinge structure of limited flex life. For optimum flex life, the hinge should be flexed through its full arc while still retaining moulding heat.
If properly moulded and flexed, virtually any injection moulding grade of PP will give acceptable flex life and strength.
Hinge failures can occur due to improper dispersion and particle size of colourants/pigments.
Moulding with nucleated grades
Injection pressure and hold on pressure should be minimum to avoid overpacking and achieving better clarity.