The Mechanical Nature of Flash: Why Clamping Force Alone Won't Fix It

Author: Precisioner Engineering Team | High-Pressure Die Casting Mold Specialists

Introduction

Flash occurs when local contact pressure drops below melt pressure. But why does simply increasing clamping force often fail?

Body:

Flash is directly caused by local contact pressure on the parting line falling below the local cavity pressure. Simply increasing clamping force does not linearly reduce flash, because elastic deformation of the mold frame, plates, and tie bars absorbs much of the added force.

where Kcontact is the local contact stiffness coefficient, which depends on plate thickness, tie bar preload, mold height, and guide clearance. Engineering measurements show: Kcontact≈0.9–1.0 in the central region, ≈0.4–0.6 at the edges, and as low as 0.3 at the corners.

Measurement Method:

Pressure-sensitive film (e.g., Fuji Prescale, range 40–200 MPa) directly provides a pressure map of the parting line. Research at Precisioner has shown that when tie bar preload deviation exceeds 10%, the edge pressure can drop by an additional 15–20%.

· For molds with clamping force >1000 tons, a multi-point independent locking system is recommended

Case data:

An 800-ton mold had a flash rate of 4.2%. Pressure-sensitive film showed that corner pressure was only 42% of the center value. After increasing the tie bar pre-torque from 600 N·m to 750 N·m and redistributing the tightening sequence, corner pressure rose to 78% of the center value, and flash rate dropped to 0.8%.