Plastic injection molding machine that can be classified as reciprocating extruder  are among the most widely used machines in industry. However, most studies in the injector design were based on steady state models developed for reciprocating extruder  that involved no reciprocation. This over-simplified model leaves out the most important aspect of reciprocation. The authors of this paper have derived a transient melting model that takes care of the change from the conventional steady extrusion to that of a discontinuous transient process. This paper describes simulations conducted on the derived model to explain observation that cannot be explained by the steady extrusion model in practical experiments. The simulation was conducted by using parameters given in Donovan's experiment [Polym Engng, 11 (1971) 353]. The simulation results are found to qualitatively match with the experimental results. It proves the validity of the model. Simulation has also been conducted with the model on materials that their viscosities are temperature and shear rate dependent. The result has shown that screw rotation speed, screw axial movement speed, barrel thickness, barrel heat capacity, temperature of heater and polymer are factors affecting the melting speed and the transient effects.

Precise feeding of a rubber compound into a twin-screw compounding extruder is achieved by using a short single-screw rubber processing extruder with downstream gear pump.

Hannover, Germany-based KraussMaffei Berstorff has developed an innovative system for TPE-V production by merging its engineering capabilities in rubber processing and compounding.

A compounding extruder with downstream gear pump ensures precise feeding of the rubber compound into the twin-screw compounding extruder.

The single screw extruder and gear pump combination provides outstanding high precision feeding performance, said KraussMaffei.

Bale rubber cut into strips can be processed directly instead of having to use pellets.

Process efficiency and cost-effectiveness are substantially improved since time-consuming and expensive pre-pelletising of the rubber is no longer necessary.

The benefits offered by the new KraussMaffei Berstorff system are especially significant in the production of soft TPE-V types.

In many cases, soft TPE-Vs are produced using oil-extended EPDM rubber, which is rather difficult to pelletise owing to its sticky surface and because separating agents must be used.

* Precise rubber feeding - the single-screw extruder ensures reliable feeding and pre-compounding of the rubber.

A pressure-based control system adjusts the extruder speed in such a way that the downstream gear pump is always completely filled.

The gear pump is designed to achieve precise volumetric feeding of the rubber into the twin-screw compounding extruder.

* Compounding process - inside the twin-screw compounding extruder, the rubber is mixed with the thermoplastics material, and the cross-linking reaction of the rubber phase takes place to form the TPE-V required.

The phase morphology and the physical properties of the finished TPE-V are determined by the configuration of the twin-screw extruder.

The first extruder section is designed for the compounding and mixing of the main recipe components, usually EPDM and PP, and for the incorporation of additives and processing oils.

Dynamic cross-linking takes place in the downstream mixing zones of the extruder.

The last extruder section is used for vacuum degassing.

* Combination with internal mixer - the new system can also be used with an internal mixer.

In this case, rubber, fillers, plasticiser oils and additives are pre-mixed in the internal mixer and fed into the extruder in the form of strips.

With this solution, the twin-screw compounding extruder is merely required to mix in the thermoplastics material and, if necessary, the cross-linking agent and to perform the dynamic vulcanisation process.

* More about TPE - owing to their superior material properties, thermoplastics elastomers (TPEs) are becoming increasingly important, said KraussMaffei.