Thermal Stability

The first purpose of the injection unit for molding a crystalline material is to deliver to the mold the necessary amount of a homogeneous melt ( with no unmelt and no degraded material ). The rules of construction of the injection unit are then dependent on the molding material requirements in term of thermal behavior and heat needed. The first point to take into account for a crystalline material is the thermal stability at melt temperature, to avoid degradation. Then, screw, nozzle, backflow valve, adaptor should be designed to provide efficient melting of crystalline material and delivery of molten polymer to the mold.

Melting behavior is different between plastics are different. The amorphous polymer starts softening just after Tg and presents a continuous change in viscosity. This gives a very large temperature range to operate ( but a large variation of viscosity with temperature). In contrast, the crystalline polymer stays solid up to the melting point and suddenly melts to the liquid phase at a high temperature. This limits the processing range of temperature between unmelt and thermal degradation.

The second factor is the time the material stays at that temperature. For all polymers, the molecules can withstand a certain temperature before degradation can start. Obviously, the acceptable time limit becomes shorter when the temperature is higher. Degradation will result in the generation of gases which cause bubbles in the melt, splays on parts, mold deposit, yellow and brown marks on the parts.

The average residence time (or Hold-Up Time, HUT) in the injection unit is linked to the amount of polymer in the cylinder, the shot weight and the cycle time and can be calculated with the following:

HUT= (Weight of Resin in Cylinder) x Cycle Time / Shot Weight

or

HUT= (Maximum Screw Stroke x 2) x Cycle Time / Current Screw Stroke

Note: Effective screw stroke = distance the screw travels during rotation only

Leave a Reply