This two-shot injection molding guide can assist you in your understanding of the process and how your company can use its capabilities to help you produce high-performance parts efficiently.

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Although manufacturers have some experience with thermoplastic or liquid silicone rubber applications, many do not grasp the significant advantages of combining the two materials. This is especially true in an era of intense global competition, tight budgets, and mandates for leaner operations and improved customer satisfaction. LSR two-shot injection molding offers companies the best features of using thermoplastics and liquid silicone rubber to make their parts.

The TPEs most commonly employed in the multi-shot molding process includes Polypropylene (PP) and Polybutylene Terephthalate (PBT). Other material options include:

The multi-molding injection process plays a significant role in changing the design, aesthetics and functionality of parts and products across a broad range of industries. The technique is becoming increasingly appealing because it allows companies to develop complex parts, create textured or soft-touch grips for products, produce two-color aesthetics, give products ergonomic attributes and provide designers and engineers with so much more flexibility to create a diverse set of products.

Once the thermoplastic and LSR materials are selected, the 2-Shot molding of the component is achieved through the combination of the most efficient product design, mold design, material chemistry and manufacturing process.

For instance, some manufacturers used the term to describe the insert molding process, which entails the placement of an insert into the injection mold prior to molding. Then, melted plastic material is injected into the mold to fill the cavity and mold around the inserted part. The insertion technique is also used for metal/LSR parts integration. The metal undergoes cleaning and pre-treatment before feeding the metal substrate directly into the LSR injection-molding unit.

The overmolding injection molding process involves adding material (overmold) to a rigid part (substrate) such as metal or plastic to form around it.

Today, blended TP and LSR components make up products ranging from consumer electronics and automobiles to medical devices and industrial/defense assemblies. Companies, across a variety of industries, can produce complex, precision LSR components in high volumes and parts that meet the most demanding applications.

– In terms of product durability, overmolding is recommended because its process creates a chemical bond between the substrate and the material called « bonding ». This effect allows to create a better link between a substrate and a material. Bonding replaces gluing or fixing and guarantees optimal durability.

Early on in the design phase, it is critical to work with an experienced two-shot, LSR-thermoplastic manufacturer. Collaborating with the right designer and/or manufacturer assures you have the critical understanding needed to address the relationship between material, parts and the final end-use performance of the part. The manufacturer should also be experienced in silicone molding and material compatibility.

In many applications, Fluorocarbons, EPDMs and other thermoset compounds are not suitable for 2-Shot molding. It is only natural that companies would want to benefit from combining LSR/thermoplastic materials. However, the thermoplastic material must have the elastomeric and thermoplastic properties that allow the material to withstand temperatures of 300-degrees F or more and are able to co-polymerize to the LSR material.

In order to determine the most suitable molding technique, you must consider a number of factors, including part shape and geometry, production volume and quality requirements.

Other industries that utilize LSR two-shot injection molding include telecommunications, construction, electronics, household appliances, food and beverage, as well as other sectors.

You can obtain consistent results with the use of self-adhesive liquid silicone rubber whether you use a separate mold in two machines or a multi-shot injection mold with a single machine. If you must use two machines in your two-shot molding process, automation will ensure you maintain a consistent temperature for the over-molding and eliminate the possibility of contamination from human handling. Prior to doing a full-production run, a preliminary test should be conducted to give a good understanding of how well your particular substrate will bond to the LSR material you choose.

The two-shot (2K) injection molding technique is the ability to create injection-molded parts by forcing two different materials into different locations in the same mold. Consequently, a minimum of two shots are injected into the mold as compared to the single injection common in traditional injection molding methods. Often, the term “Two-Shot Injection Molding” and the use of associated expressions have caused a bit of confusion about the process.

LSR two-shot injection molding produces high-performance lightweight parts and reduces production time — all of which add to your bottom line.

As the multicomponent injection-molding process relates to liquid silicone rubber, it describes the injecting of a LSR material in the same unit immediately after the molding of the rigid substrate to create a soft-touch feel. This process makes many consumer products more aesthetically appealing, allows for the application of the LSR in the precise position needed, and increases consistency.

In the past, long-term adhesion when using the multi-molding process for molding of TPEs onto rigid thermoplastic substrates or over-molding LSR on thermoplastics was a major challenge for manufacturers. However, advances in the development of self-adhesion LSR material make it possible to use certain grades of LSR that bond with thermoplastics. Self-adhesive LSR eliminates many of the processing steps of substrate preparations, such as plasma, UV light treatment or chemical primers.

The primary challenge of any 2-Shot molding application is the achievement of maximum adhesion between the TPE and LSR materials. Proper adhesion requires a thorough understanding of the materials and the related nuances of molding of these materials. When designing the over-molded component, product designers and engineers must evaluate different material characteristics and behaviors under processing to optimize cost-per-unit production and to assure end-use performance reliability.

Two-shot molding has inherent characteristics that make it a relatively easy operating process, but there are still factors that must be considered to ensure the most efficient operation:

Another technique uses a rotating tool that has two molds incorporated into one machine. One mold fabricates the rigid substrate and the other mold applies LSR over-mold material. Upon completion of the thermoplastic substrate, a hydraulic or electro-server drive rotates the core and the part at a 180-degree angle (or 120 degrees in a 3-shot method), which allows for the injection of the alternating materials.

However, product designers have the ever-present challenge of creating high-performance products with the fewest parts. In recent years, advances in material technology have given designers the luxury of combining silicone rubber with thermoplastics in a 2-Shot molding process. This method allows manufacturers to produce single-molded components comprising multiple materials with unique performance characteristics.

Some manufacturers utilize a method that fabricates the substrate, and then, using robots, transfers the part to a second injection molding machine, which over-molds the piece with an elastomer and a second polymer.

The part designer and engineers must also determine how long the part must meet these performance specifications to perform correctly.

You must collaborate with an expert who understands LSR two-shot injection molding and the different nuances of working with LSR and thermoplastics as well as the appropriate tooling and processes for each.

The industries and applications that use multicomponent injection molding continue to grow across a variety of end-user markets, including:

Material compatibility will be one of the primary keys to the success of your 2-Shot molding component and ensuring it meets the necessary criteria. LSR has grown into a popular choice as a material for liquid injection molding processes because it outperforms TPEs (and other options) in a number of areas, including:

Another over-mold technique utilizes a transfer injection tool that employs hot runners to mold the substrate, and then transfer it to the LSR side where the material is delivered via cold runners. A ready two-shot part is automatically de-molded using robotic technology. This process creates a mechanical, thermal, chemical or a combination of a chemical and mechanical bond between the two materials.

The advantages of this process are multiple. They can be seen both in the production process and in the quality of the final product.

Historically, Thermoplastics (TP) and Liquid Silicone Rubber (LSR) occupied distinctive universes. Although similar in many ways, each method solved an array of challenges by employing different manufacturing processes. Since its introduction into the world of materials science in the 1970s, LSR has become the “go-to” material for product designers and manufacturers across a broad range of industries. The unique properties of LSR — such as stability, non-reactivity and resistance to extreme temperatures — make it the ideal material choice for numerous applications.

– Overmolding considerably reduces the need for secondary operations by avoiding manual assembly between two molded parts. Operators then have more time to perform other tasks, which lowers production costs.

Another approach uses automation to expand the original cavity geometry by employing retractable (movable) slides or cores while the substrate remains in the mold. Referred to as core-pull or core-back, the core retracts after the solidification of the insert, which creates an opening in the part that is filled with a second resin contained within the same mold.

We offer greater development and production efficiencies to help our customers, which include Fortune 100 companies across a wide range of industries, to minimize both short- and long-term production costs. Contact a SIMTEC representative to learn how you can get the highest performance possible from your two-shot injection molding parts.

The design of a multi-shot component begins with providing the answers to some some basic questions about the use of the part, the environment in which the part must operate and the desired characteristic of the final product.

For example, air entrapment can become an issue. An expert would understand the possibility of air entrapment in a cavity and the importance of making allowances for the fact that LSR injects at a lower psi — 1,000 to 2,000 psi — and thermoplastic injects into the mold at 20,000 to 40,000 psi.

The uncertainty has to do with the wide range of multi-material applications that go beyond the using of soft materials on rigid substrates. The lack of clarity may also result from the properties of the molded component or base process.

For example, perhaps the part specifications require the rigidity of a thermoplastic and the soft flexibility inherent in LSR. With two-shot injection molding, thermoplastic material functions as the substrate to accommodate the liquid silicone rubber‘s inherent quality of compression and elongation.

SIMTEC has a global reputation for our expertise on the liquid injection molding process, including two-shot injection molding. Unlike other manufacturers, our single focus is the liquid silicone rubber injection industry. Our design and technical capabilities allow us to provide the assistance you need from initial concept to final production.