RUBBER INJECTION MOLDING – PRODUCTION WITH CONCEPT
ECONOMICAL, DURABLE AND RESISTANT
The rubber or elastomer injection molding process is very well suited for the economical production of large quantities of elastic components such as seals or dampers. Customer-specific injection molding tools are designed by SPT and equipped for the appropriate injection molding machine. The process is run in by experienced technicians and the parameters for series production are defined. In this way, our customers receive quality molded parts over the entire duration of the project – shot by shot – including the appropriate care and maintenance of the tools.
THE PROCESS IN ELSTOMER INJECTION MOLDING:
Elastomers such as natural or synthetic rubber vulcanize by the influence of heat. The molds or injection molds are hotter tempered than the rubber compound during injection.
- The compound is fed by a special screw, which introduces little shear into the plasticized mass. The temperature of the cylinder around the screw is approx. 70-80°C so that the mass is not overheated and premature vulcanization is prevented.
- The mixture is kneaded in the screw and processed homogeneously.
- At the injection molding machine, the dosage for filling the cavities in the mold, including the sprue channels and, if necessary, flood grooves, is determined. During the injection process or flow of the mixture in the nozzle and the runners, frictional heat is generated, which shortens the vulcanization time compared to conventional presses (compression molding).
- When the tool is full and the closing pressure is built up, the heating time begins. This always depends on the polymer base or rubber compound, cross sections of the runners, injection parameters and ultimately on the wall thickness of the molded parts. Vulcanization takes place at mold temperatures between 170°C and 195°C. While the parts are vulcanizing in the mold, the injection molding machine starts dosing for the next shot in parallel.
- The press operator or robot removes the parts after the tool opens automatically after the heating time has expired. After demolding, the cavities are cleaned and the next cycle is started. If the molded parts are not yet 100% vulcanized on the injection molding machine, they can be post-cured in an oven.
THE USE OF COLD RUNNERS IN RUBBER INJECTION MOLDING
The use of cold runners can save a lot of material in rubber injection molding, depending on the mold concept. The cold runners are tempered separately and sit on the upper mold sections. The mixture remains flowable in the cold runner and does not vulcanize. Due to the nozzle distribution of the cold runner, the cavities in the mold can be filled directly (valve gate cold runner) or placed as close as possible to the cavity with a small residual gating.
When the rubber molded parts have been cross-linked after the elastomer injection molding, the condition can no longer be reversed. The rubber molded parts retain their properties even under the most difficult conditions due to the chemical, mechanical cross-linking process.