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The Soft Component First

The various cable bushings with the TPE/PP material combination were injected in a family mold (© Günther)
In order to achieve a flat height on the "hot half", both manifold systems were housed in a manifold block (© Günther)

Günther Hot Runner System for Two-Component Application Offers Space even in Tight Conditions.

Low assembly effort, often shorter production times and lower-cost parts production overall are the reasons why 2-component injection molding is often used in hard-soft material combinations, including in the production of seals, cable bushings and handles. If the core-back process is to be used for production, however, mold structure and hot runner system design will pose a challenge.

Often, the best solution for a complex component is a clever combination of materials in order to make the production process more effective and increase the benefit of the product. In plastics processing, these are often metal-plastic composites or hard-soft combinations. Based in the town of Schlitz in East Hesse, Germany, the company S & S Werkzeugbau GmbH makes molds for the production of complex plastic parts, and has been renowned for the design of multi-component molds for over 30 years. Prestigious German suppliers to the car industry as well as companies in the medical and electrical industries are just some of the customers of the family-run company, which is now in its second generation. Harald Starch, Managing Director and Head of Design at S & S Werkzeugbau, states, "Many customers from the car industry come to us because of our reputation as a provider of 2-component technology."

One example is the current project with Woco Industrietechnik GmbH, a globally active group in the automotive industry, which Starch describes as a regular customer. "The task was to develop and construct a mold for a cable bushing." The parts were to be created via 2-component injection molding using a TPE/PP material combination – in other words a thermoplastic elastomer and polypropylene. Woco Industrietechnik is a well-known developer and manufacturer of complex components and complete assemblies for car manufacturing and industrial applications.

The company develops its functional solutions for improving comfort and safety in vehicles, reducing vibrations and sealing and separating gaseous and liquid media in the industrial sector with a focus on polymer materials. The Polymer Automotive business unit has a particular focus on sealing technology. Alongside the development of such components, Woco also produces elastomer materials with specific property profiles.

Advantage 2-Component – When It Is Done Properly

"That's probably also the reason why Woco put us in charge of the mold development for the cable bushing," explains Starch. "Traditionally, the customer comes from the rubber processing sector and produces many components in which rubber is combined and overmolded with another material. Since Woco places importance on an economical production process to create the molded part, it specifically looks for partners that are not only able to construct a mold according to specifications, but also apply their expertise in production efficiency. This is precisely the reputation we have built up over the years."

2-component injection molding is an efficient process that often brings out the properties of the different materials even better. The two components are injected into the mold one after the other, enabling them to bond in different ways. The success of the bonding depends on various factors such as the mold geometry and the compatibility of the different plastics and the processing parameters.

However, the most important aspects for a company like Woco Industrietechnik are the economic and logistical advantages. After all, 2-component injection molded parts are produced in a process so that no assembly or reworking is required. In addition to this, a huge variety of material combinations is possible, bonding between materials is more durable than with any other joining method and complex component requirements can be met. The only disadvantages of hard-soft composites are the limited restoring force at higher temperatures and the tendency to creep at higher strains.

Bottleneck between the Two Components

Especially the application of a soft layer made of a rubber-like material on a hard carrier is a commonly used method to achieve a certain seal on bushings or improve the grip of a component. Soft components can be found on a wide range of objects, from medical instruments through hand tools all the way to toothbrushes and seals in technical assemblies. One such product is also the grommet used to protect cables or wiring for car body bushings. In vehicles, such cable bushing has to fulfil a comprehensive property profile, which has an obvious effect on product design and therefore also on the mold design. At the project meeting, Woco handed over the article data to the engineers at S & S Werkzeugbau with the request to produce a 2-component family mold. Two cable bushings in two different sizes were to be produced in a single-stage process.

Due to the fact that the parts were to be injected in a core-back process, the distance between the injection positions of both components was extremely narrow. "It would not have been possible to position two hot runner systems in the limited gap of 35 mm between the hard and soft components," explains Harald Starch. "It was also not possible to build upwards given the low installation height that was required."

Due to the part design, the movable core that separates the soft from the hard components could only be constructed for the hard components. This determined the order to inject the TPE first, then the PP. Based on its extensive experience, S & S considered the challenge of maintaining this order to be a workable task. Had it been reversed, a more expensive 2-component process (rotary table or index plate) would have been necessary.

Two Materials – One Manifold Block

In close collaboration through on-site discussions, the engineers at S & S Werkzeugbau and the application consultant at Günther Heisskanaltechnik GmbH, Walter Ehlert, came up with a practical solution that enabled a flat mold design whilst simultaneously reaching all of the injection points. The two different injection weights made cascade control of the hard component obligatory. "The application consultants at Günther are deeply involved in mold technology and have many years of experience in mold design and injection molding. Especially when faced with such problems, competent advice is indispensable," states Starch.

With the idea of incorporating the two manifold systems in one manifold block, it was possible to implement a very flat height on the "hot half". "This enabled us to keep the mold compact," notes Ehlert. "For the soft components, we now use the 5DTT3-80C-1.0 nozzle and for the hard components with valve gate, we use the 6NTT3-100LAZ-1.4 nozzle. The length of the nozzle for soft components is 80 mm and 100 mm for the hard components. The function of the nozzle head is the same in both cases." The "TT" design of the hot runner nozzles (with front mounting) made the flat height of the "hot half" possible for the engineers. This was also helped by the fact that no intermediate plate is required.

Walter Ehlert explains, "Our front-mounted nozzles have the additional advantage that they have a sealing area at the nozzle head and a barrier is created between the manifold and the nozzle cabling and thus offer the highest level of safety when it comes to the seal between nozzle head and manifold. With front-mounted nozzles, the manifold area is thermally separated from the cable channels in the nozzle holding plate. The design also offers the benefit that individual nozzles can be removed from the mold as required for servicing without having to dismantle the entire hot runner system. "Openings in the mold clamping plate also enable the immersion depth of the shut-off needle to be adjusted from outside.

Core-Back Combined with Ejector Function

The philosophy of Günther is that rather than having to adapt components individually to every application, users are supported by the existing product portfolio. Customers are then quickly provided with a replacement in case of service and do not need to wait for customized production. "Of course, the manifold layout is particular here and was custom-made for this special case," signals Ehlert. All other components such as hot runner nozzles, connecting nozzles and shut-off needles are standard Günther parts. The valve gate actuator lifting plates were in this case designed and built by S & S.

During the production process, the TPE is injected first via an intermediate gate, since the part design did not permit the direct gating of the soft components. Next, the PP is cascade-injected directly via the valve gate system, since the two different weights of the hard components have to be injected at the same time. "On the one hand, the position of the hot runner nozzles on both components caused us quite a headache because of their close proximity," outlines Harald Starch. "On the other hand, however, we also had the operation of the core-back together with the ejector system. After all, we don't only use the core-back in retraction mode, but also as an ejector. Nevertheless, we have ample experience with such tasks," continues Starch.

Hot Runner and Mold Expertise Required

In the future, 2-component or multi-component molded parts will be increasingly used in all sectors. The advantage of receiving ready-to-install serial parts from an injection mold is clear to see. Especially in car manufacturing, it can increase productivity. If, like in this example, seals need to be inseparably and precisely connected with the structural part, it is absolutely indispensable to have an expert mold maker on the one hand and a hot runner system supplier who can offer process and user know-how on the other. After all, increasingly short product development times leave no room for time-consuming iteration loops in the search for the perfect solution.

The author

Dipl.-Ing. Horst-Werner Bremmer is in charge of technical application consulting and sales at Günther Heisskanaltechnik GmbH, Frankenberg, Germany

Company Profile

S & S Werkzeugbau GmbH was founded in 1985 by Heinz Starch and Walter Susemichel as Susemichel und Starch Werkzeugbau. In 1998, certified engineer Harald Starch joined the company. With the design and production of two injection-molding tools with 2-component index plate technology in 2008, this technique grew to become the company's specialty. Today, 2-component injection molds with index plates, rotary tables or core-back technology count for over 60% of all molds made by the family-run company.

About GÜNTHER Hot Runner Technology

With more than 220 employees, Günther Heisskanaltechnik GmbH produces customized and user-friendly hot runner nozzles and systems for the plastics industry. Its international customers include leading companies from the car, electrical/electronic, medical technology, packaging and consumer goods industries.


Core-back, which is also known as composite injection molding, injects the different materials one by one. Once the first cavity has been filled, linear-moving blocking elements open the filled cavities towards the empty cavity area to fill the remaining component with the second material through another gate. A prerequisite for the core-back procedure is a mold geometry that allows the cavity for the second component to be made available through a linear core pull movement so that both materials can adhere to each other. Due to the temperature proximity of the two materials TPE and PP, very good adhesion is possible

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