Like thermosets, thermoplasts are plasticised by means of heat application and friction. With the application of heat, however, there also comes a chemical cross-linking reaction, which works against plastification. High temperatures and long dwell times in the plasticising cylinder increase the risk of cross-linking, which means that the moulding material cures. This impairs the flow properties of the moulding material. Additionally, particles which are already cross-linked can be injected into the moulded part as well, which can create weak spots there.
In order to prevent the premature cross-linking of the moulding material in the plasticising cylinder, thermosets are plasticised at relatively low temperatures. The plasticising cylinder is temperature-controlled using liquid. To prevent excessive heat by friction, compression-free screws without non-return valves are mostly used. The L/D ratio of thermoset screws ranges from 12:1 to 15:1, and therefore up to 40 percent below that of thermoplast screws.
In addition to the level of the temperature, the duration of the thermal strain must also be observed. The longer the thermosets are exposed to the application of heat, the more cross-linkage they will exhibit.
Short, open nozzles as well as the low L/D ratio of the screw guarantee a short dwell time for the moulded material in the plasticising cylinder.
In addition, material cushion are not used during the injection of thermosets. Here, the screw is advanced into the drive cylinder up to the mechanical stop. Depending on the size of the injection unit, the drive cylinder is to be adjusted in such a way that a space of at least 0.5 to 1 millimetre remains between the nozzle and the tip of the screw. The screw must not impact the nozzle during injection.
When a plasticising cylinder is installed for the first time, or when replacing the screw or nozzle, the position of the screw in the front most position must be inspected and adjusted as outlined in the operating manual.