Determining Shut off Land Relief and Contact Stress in Multi Cavity Tooling
Balancing shut-off land relief area against clamp force prevents steel hobbing while holding plastic melt seal across multi-cavity injection tooling.
Hobbing Deformation
Meaning
Cold metal forming processes generate the highly detailed cavities required for polymer injection molds. Excessive force during the pressing of a hardened master hub into a soft steel block can result in hobbing deformation, which distorts the mold cavity profile. This distortion alters the final dimensions of molded plastic parts.
Material Flow
Plastic displacement of the tool steel during cavity creation must follow a uniform path. Uncontrolled metal flow induces hobbing deformation because the steel work-hardens unevenly during the process. Correct lubricants reduce the friction that leads to this non-uniform resistance.
Cavity Accuracy
Mold inserts must replicate the master hub with micron-scale precision to prevent flash and parting line errors in molded parts. When hobbing deformation occurs, the dimensional errors transfer directly to the molded parts. This leads to rejects during injection molding of precision gears.
Tooling Correction
Post-hobbing heat treatment restores the metallurgical stability of the mold insert before it is placed in the mold base. Relief holes drilled in the steel blank accommodate the displaced metal to minimize hobbing deformation during the pressing phase. Stress relieving at high temperatures removes the residual stress created by the cold working process.
This ensures that the cavity remains stable during millions of high-pressure molding cycles.