Multi Component Injection Moulding: 2-Shot LSR Expertise

Multi Component Injection Moulding

Two Shot LSR Expertise and Experience

Two shot (2K) injection thermoplastic molding dates back several decades, from the multicolor molding of typewriter keys (to produce permanent characters on the keys), to the integration of the multiple parts that make up a car’s headlights. This process has since advanced to allow for the consistent and cost-effective manufacture of multicolor/multifunction thermoplastic/thermoset products for a variety of innovative applications. SIMTEC Silicone Parts prides itself in being at the forefront of two shot LSR/Thermoplastic technology.

Two-shot LSR-Thermoplastics Molding

The two shot or multi component injection moulding process is sometimes also referred to as over-molding, in-mold, or assembly injection molding. In general, this process consists of injecting an LSR – in the same injection unit as the substrate – immediately after the thermoplastic substrate has been molded. With the multi component injection moulding process, it is possible to obtain the best features of both materials, while at the same time reducing or eliminating post-molding assembly operations. The biggest benefits of the multi component injection moulding process are the reduction or elimination of post processing steps, as well as the integration of several functions, parts, and materials into one component. Two shot molding (or multi component injection moulding) in one sequential cycle requires the following: an injection molding machine equipped with multiple injection screws; either a rotating tool (as can be seen in the figure), a transfer tool and/or a moving core tool; and all necessary ancillary equipment.

In a transfer injection tool, the substrate is first molded employing hot runners, it is then transferred to the Liquid Silicone Rubber (LSR) side where the LSR is injected using cold runners. A ready two shot part is then de-molded with the assistance of a robot. The bond between the two different materials can be mechanical, thermal, chemical, or a combination of a chemical and mechanical.

Another method of the two shot multi component injection moulding employs a rotating tool with two molds integrated into one: one for the thermoplastic substrate and one for the LSR. Once the substrate is molded, a hydraulic or electric servo drive rotates the core and the part by 180 degrees (or 120 degrees for a three-shot part), allowing alternating materials to be injected.

In another approach, the substrate is first molded and then transferred to a second injection molding machine, where it is then over-molded with either a second polymer or an elastomer. The loading and pick up of the already molded substrate is carried out by robots. This production method is also called insert molding, and it is also suitable for metal/LSR parts integration; in the case of a metal substrate, the metal part (after cleaning and pre-treatment) is fed directly into the LSR injection molding unit.

Another variation involves automatically expanding the original cavity geometry using retractable (movable) cores or slides while the substrate is still in the mold. This process is called core-pull or core-back. To be specific, the core retracts after the insert has solidified to create open volume to be filled by the second material within the same mold. Material selection is vital for multi component injection moulding. A thorough analysis needs to be conducted to determine material compatibility, chemical and wear-resistance, environmental performance, and other program-specific requirements.

Two shot components can also be designed to combine a soft elastomeric material like Liquid Silicone Rubber (LSR) onto a rigid substrate to create the soft-touch feel, which improves the handling, user friendliness, and the appearance of a finished product. In addition, this process allows for the placement of the LSR exactly where its function is needed, allowing for the consistent production of high precision parts.

When deciding on the most suitable molding technique, the following should be considered: component shape and geometry, production volume, and quality requirements.

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As a trailblazer for this emerging technology, SIMTEC has secured its position as the leader in the two shot LSR/Thermoplastic multi component injection moulding industry. In 2011, SIMTEC invested in three additional two shot injection units; one of which (with a 350 T clamping force) is the largest for LSR/Thermoplastic injection outside of Europe.

[1] Osswald, T. A.; Baur, E.; Brinkmann, S.; Oberbach, K.; Schmachtenberg, E.: International Plastics Handbook; 4th edition, Hanser Gardner Pubilcations, Cincinatti, 2006

[2] Schmachtenberg, E.; Johannaber, F.: Montagespritzgießen – Verfahrensprinzipien und Definition; Technical Conference Montagespritzgießen, Institute of Polymer Technology, Erlangen, Germany, Nov. 2007, p. 1-18

[3] Schmachtenberg, E.; Schuck, M.; Kuehnert, I.; Forming and Assembly in One Process; Kunststoffe International 4/2007, p. 8-13

At SIMTEC Silicone Parts, a leading company in manufacturing high precision parts and components, we are exclusively focused and specialized in the production of LSR and LSR/Thermoplastic (Two-Shot) components.

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