Lightweight construction, functionality, downsizing, cost savings are the major challenges facing the automotive industry today. Increasingly complex components, frequently manufactured by bonding a wide variety of materials with varying properties, are required. A combination of a flexible soft component such as rubber, LSR, or TPE, with a hard, mechanically load-bearing rigid component is often required. By using special thermoplastic molding compounds for the rigid compounds and suitable rubber recipes, it has been possible to manufacture a durable, reliable bond between the elastomeric material and the thermoplastics in one- or two-component injection molding by the plastic-rubber composite process (K&K process) described below. In this process, the rubber forms a reliable, substance-to-substance, hard-soft bond with a preform in the appropriately tempered injection molding tool. The preform can consist of PA 612 (VESTAMID D series), mPPO (VESTORAN series), or PPA (VESTAMID HTplus series). Wherever functionality is required in the form of seals, damping components, membranes, covers, or for haptic reasons, a variety of elastomers is used.
Where rubber components need to be secured or fixed durably, composite parts combining this type of elastomer with a rigid component have proven tried and tested. Traditionally, the rigid components of rubber composite parts were made of metal. In order to achieve lightweight design and considerably reduce weight in particular, plastic components are increasingly being used today. Plastics are light, they do not corrode, and they can be processed efficiently to create extremely complex molded parts. However, they must remain dimensionally stable at the usual vulcanization temperatures of 160 °C to 200 °C for rubber components. To ensure the durable functioning of composite parts, generally under static, and, in the case of newer applications, dynamic loadings, a bond that will remain inseparable is a major quality criterion. To achieve this adhesion promoters are generally used in the case of rubber composites.
Machine and mold technology for composite parts
In addition to further, somewhat complex operating steps for the application of the adhesion promoter, however, the process also requires protective measures against emissions of the solvents generally used, as well as their environmentally sound disposal. In contrast, the K&K process patented by Evonik dispenses with adhesion promoters and requires no special pretreatment. This has less impact on the environment and reduces the manufacturing costs. Basically, two processing options are available for manufacturing K&K adhesion promoter free composite parts: The two-stage process – an insertion process – and a single-stage process – multi-component injection molding in a single tool. The two-stage insertion process is particularly useful when the vulcanization time for the rubber is much longer than the cooling time for the rigid thermoplastic components. It is comparable with the conventional production of rubber-metal and rubber-plastic parts: The thermoplastic molded part is manufactured and delivered separately, and is inserted into the elastomer pad where the rubber mixtures are injection molded and in some cases fully cured. The intermediate adhesion promoter application step is not required for the Evonik K&K system. The system does not require any investment in new machinery. If there is little or no difference between the cooling time and the vulcanization time, the single-stage process can be adopted. Since no adhesion promoter is required, the composite parts can be manufactured in a combined mold with no intermediate steps: The cavities for thermoplastic and rubber components are each located in different parts of a combined injection mold. The thermoplastic preforms are manufactured in the „cold“ area of the mold. For example, these are generally demolded from the ejector side using an index platen, a turntable, or, in the implementation process, handling systems, and they are transferred to the hot area of the mold. Here, the rubber mixture is injected and vulcanized when the mold is closed. At the same time, new thermoplastic preforms are being produced in the cold area. For the high temperature resistant PPA molding compounds presented above, it is even possible to dispense with thermal separation in the mold. In this case, the cold area of the thermoplastic cavity may have the same temperature as the hot area. This is made possible by the high glass transition temperature of VESTAMID HTplus where mold temperatures of 160 °C to 180 °C are common. As regards molding technology, it is important to ensure that the elastomer side is well sealed with reference to shrinkage of the preforms so that there is no overmolding on account of the low rubber viscosity. For VESTAMID D and VESTORAN molding compounds, however, thermal separation of the hot area of the mold (temperatures approx. 160 to 200 °C for full, cost-effective, cross-linking) from the cold area (appr. 80 °C up to 100°C) is necessary and it is a criterion for good manufacturing quality. The manufacturing process can be automated to a large extent so that multiple handling of the parts is not required. This eliminates many potential causes of defects and reduces the scrap rate.
Material combinations and requirements
To cross-link rubber on the preform, the plastic must have the highest possible short-term thermoforming resistance, ranging from >150 °C, or ideally 180 °C. In some cases, even 200 °C is required in order to withstand downstream cross-linking processes without deformation, e.g. in the oven. The thermoplastics used must allow modification during polymerization or compounding. Overall, high-temperature polymers are particularly suitable for the K&K process. They achieve high laminate strength with rubbers, which cross-link radically using peroxide, amine, or sulfur. Polar rubbers with reactive groups are used for preference. Rubber recipes with short vulcanization times are best for the single-stage process. The K&K lamination systems listed in the table are available. With the combinations listed, it is possible to obtain durable composites, which withstand the usual temperature and media influences typical in the automotive industry for example. Rubber recipes may need to be modified in some cases. When developing the molded parts, appropriate preliminary testing of the desired material combination should always be carried out. In the course of development, the material combinations are subjected to tensile and peel tests, among others. Depending on the K&K variant, separating forces of between 4 N/mm and 20 N/mm were achieved in the peel test. When designing the composites, a fracture in the rubber matrix and, where possible, no fracture in the parting plane between the plastic and the rubber should be strived for (100% cohesive fracture). Up till now, it has been difficult to realize composites using ACM, CR, NR, or EOS without special modifications on the elastomer side, or if realized, the laminate strength was poor.
Polyamide meets EPDM
Where before it was only possible to obtain composite parts from polyamide molding compounds using X-NBR, HNBR, FKM, and AEM rubbers, adhesion-modified polyamide 612 molding compounds such as VESTAMID DX9325 are now also available for lamination with EPDM. It has also been found that it is possible to obtain combinations with excellent bonding with VMQ and these are of particular interest to the automotive industry. For example, EPDM seals can be used just as well for the brake or cooling water circuit as for the areas of door opening, cable glands, or steering. New additions are composites with a highly temperature-resistant polyphthalamide. The advantages of VESTAMID HTplus are its short-term high temperature resistance and dimensional stability as well as the favorable price/benefit ratio, making it possible to achieve further cost benefits, as a result of reduced vulcanization times at high cavity temperatures of up to 180 °C in particular. The adhesion-modified PPA molding compounds are particularly targeted at combinations with high temperature and chemical resistant rubbers such as FKM, HNBR, or AEM. But it is also quite possible to create a bond with EPDM without adhesion promoters. This means that the combination options for K&K lamination will be considerably expanded in the future, so facilitating new areas of application, which were inconceivable in the past, or which could only be realized by means of complex manufacturing technology with the application of adhesion promoters. The Evonik technicians will provide support for the selection of concrete molding compounds and with running preliminary testing. Initial applications have already been realized using PPA, e.g., components in the cooling circuit of well-known car manufacturers. A recently developed HNBR from Kaco, Heilbronn is being used for this. The direct bonding prevents undefined changes in the dimensions inside the component as a result of the application of adhesion promoters. Where tolerances are tight, changes of this type are unacceptable. Naturally, this means that any dimensional changes in the plastic component, which is in contact with the cooling media of various vehicle manufacturers, generally glycol-water mixtures, must also be kept to an absolute minimum. When in contact with media and subjected to high temperatures, the bond with the HNBR must also remain stable. This is ensured by the high chemical resistance of PPA for one and by the special equipment for the other. Depending on the travel path, smooth functioning is thereby facilitated for over a million switching cycles.
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