Meaning
Thermodynamic heat generation occurs when a gas is compressed rapidly without transferring heat to its surroundings. During the injection moulding process, adiabatic gas compression takes place when entrapped air in the mould cavity cannot escape through the vents as the front of molten plastic advances. The temperature of the trapped air rises to several hundred degrees Celsius within milliseconds.
Thermal Outcome
Melt temperature limits are easily exceeded when this localized pocket of high pressure and temperature forms. This rapid rise in temperature can burn the surrounding polymer, causing unsightly black or brown marks on the molded part. The polymer degrades thermally, reducing the structural integrity of the component at that specific location.
Defect Generation
Extreme heating of the entrapped gas causes the cosmetic defect commonly known as a burn mark. This localized thermal degradation from adiabatic gas compression can also damage the steel surfaces of the mould cavity over time. It can deposit corrosive residues that pit the tooling and lead to premature wear.
Prevention Method
Mould venting channels must be positioned at the last areas of the cavity to fill to allow the gas to escape before it is compressed. These vents must be clean and sized correctly to handle the displaced volume of air. Adjusting the injection speed profile can also reduce the rate of compression and lower the peak temperature.
When the fill rate is profiled to slow down right before the final fill point, the air has more time to escape, which avoids the temperature spike entirely and protects both the plastic and the steel from degradation.
