The strong twist of the screw during the plasticizing of thermosetting materials or the special characteristics of some polymers, requires special screw geometries to prevent breakage.
The thermosetting materials for injection molding are using temperatures that vary from 70°C to 100°C in the cylinder. In the mold a temperature of 140°C to 190°C is reached to complete the conversion of the material. At temperatures up to 100°C the viscosity of the thermosetting material is still high and in combination with a high amount of fillers or reinforcements, which can reach up to 70%, the screw undergoes a strong torsion and abrasive wear especially in the part towards the side of ‘nozzle. In this area the flights faces a strong mechanical stress, especially if, due to a non-sealing of the flights, material flows back over the flight top and hardens (transform) inside the cylinder. This difference of molding process of thermosetting materials compared to thermoplastic materials therefore requires a special screw design.
A screw for plasticizing a thermosetting material has the function of transporting the material towards the injection nozzle. This screw is without tip assembly and transfers the plasticized mass directly into the mold.
The major thermosetting reaction is exothermic (heat develops during the hardening reaction). It is therefore necessary to avoid stagnation, abrasion and localized overheating in the process. For large size screws it may be necessary to make an cooling hole to control the temperature and to avoid overheating of the melt.
The screw design parameters:
- Compression ratio between 1 and 1.2
- Length of the screw must not exceed 15 ÷ 19 L / D.
- The height of the flight must be greater than 3.0 mm.
- The angle of the screwnose can vary from 75 to 90°.
- The width of the flight vary from 0.1 up to 0.16 D.
Screws for thermosetting materials have similar geometries. The main difference is the length that can change in relation to the quantity, material type to be injected and to its reactivity. For example with epoxy resins, due to the great reactivity, the screw must be short and the material must be transferred in the shortest possible time (up to a maximum of 3 cycles); with phenolic resins you can use longer screws to inject higher quantities. It will be clear that the definition of the infeed zone lengths, and plasticization, injection and the relative height of the flights should be adjusted according to the process.