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.
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.
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.
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.
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.
What Is Multi-shot Injection Molding?
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.
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 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.
Other terms commonly used to describe the 2-Shot injection-molding process includes:
- Hard/soft over-molding
- Multicomponent injection molding
- In-mold assembly
- Double-shot injection molding
- Twin-shot molding
- 2K injection molding
The “K” in 2K injection molding comes from the German word “kunststoff,” which means “plastics.”
Multi-shot Injection Molding Manufacturing Techniques
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.
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.
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.
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.
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.
Here are some other techniques used under the umbrella of 2-Shot injection molding:
- Molding a base substrate in one mold, then transferring the first-shot component to a second mold located in a different machine to over-mold the second shot.
- Using a multi-shot mold and reducing the cycle time to form a very thick component — part by part. The procedure only uses one material in all of the shots and builds the part layer by layer.
- Injecting two types of material, consecutively, at the same injection point on the mold. This process, which is called “co-injection”, creates a two-part component that consists of a core and a surface layer.
- The use of a machine with multiple barrels to facilitate color changes on a part.
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.
The Benefits of Two-Shot Injection Molding
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 benefits of the process include:
- Intricate parts: 2-Shot molding enhances your design options, making it possible to create detailed parts designs that are durable, functional and aesthetically appealing. The process combines multiple materials, product features and/ or colors to create parts or products that are difficult to achieve with other manufacturing techniques.
- Improved quality: Two-shot injection molding enhances the quality because each part is manufactured from compatible materials, which ensures the formation of a strong bond during the fabrication process, thus improving the reliability and useful life of the part. The repeatability of the process eliminates the risk of contamination because it removes the typical two-tool process.
- Improved bonding: Double-shot injection molding utilizes a single molding cycle to join two compatible materials to create a powerful molecular bond without the use of adhesives between the two parts. The technique produces a far more robust bonding process compared to an over-mold process that involves two separate tools and molding operations.
- Realize cost reductions: The ability to mold multiple parts with the two-shot molding process decreases molding and tooling costs, eliminates (or enables) assembly that would not otherwise be possible and reduces material usage and component counts.
LSR two-shot injection molding produces high-performance lightweight parts and reduces production time — all of which add to your bottom line.
2-Shot Injection Molding Design Tips
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.
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.
A correctly functioning part should be able to:
- Absorb or transmit energy
- Seal or transmit a fluid
- Provide structural support
When designing a part, it is crucial to consider the environmental factors that the part will be exposed to, such as:
Parts should also have the correct properties to ensure they work as intended, such as:
- High elongation, which is the ability to stretch without breaking.
- High modulus or resistance to deformation.
- High Compression set, which is the resistance to set under extensive load.
- Resistance to dimensional changes in the presence of heat or fluids.
The part designer and engineers must also determine how long the part must meet these performance specifications to perform correctly.
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.
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:
- Heat resistance
- Extreme low-temperature flexibility
- Chemical resistance
- Inherent lubricity
- Odorless and tasteless
- Chemical inertness
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.
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.
The TPEs most commonly employed in the multi-shot molding process includes Polypropylene (PP) and Polybutylene Terephthalate (PBT). Other material options include:
- Styrene Block Copolymers (TPE-S)
- Thermoplastic Polyurethane Block Copolymers (TPEU)
- Copolyesters (TPE-E)
- Copolyamides (TPE-A)
- Olefinic Thermoplastic rubbers (TPE-O)
- PVC/NBR blends
- Vulcanized Phase blends (TPE-V)
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.
Industries and Applications
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.
The industries and applications that use multicomponent injection molding continue to grow across a variety of end-user markets, including:
- Medical devices and equipment. LSR has become the first choice for many medical device manufacturers because of its odorless and tasteless qualities, inertness to bodily fluids and ability to meet USP Class VI regulations, making it a very feasible medical-grade material. Medical applications include catheters, prosthetics, drug delivery, surgical tools and instruments.
- Automotive. The automobile industry and OEMs depend on the 2-Shot injection molding techniques because of the greater latitude offered for part design and the feasibility of designing bearings, airtight/watertight seals, complex micro components and other parts. The process also reduces the risk and cost associated with secondary assembly. Common applications include thermal management, electronic control units, electrical systems, drive train and transmission parts, as well as braking and safety components.
- Consumer products. Many consumer products require impact-resistant surfaces to increase durability, non-slip handle grips to improve tactile properties, shock absorption attributes and other features to improve aesthetics or differentiate the final product. Thousands of consumer products used on a daily basis were created using the 2-Shot injection molding process, including tooth brushes, bakeware, power drills, cell phones, shower heads, tub fixtures and sports equipment.
Other industries that utilize LSR two-shot injection molding include telecommunications, construction, electronics, household appliances, food and beverage, as well as other sectors.
Key Considerations for Successful Two-Shot Molding
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:
- Cleanliness. When you mold the substrate separately and transfer it from one machine to another, it is important to ensure the substrate is clean and dry. If you mold the substrate in a multi-shot mold along with the LSR, it will not be as much of a concern; however, you must still err on the side of caution.
- Optimal material bonding and mechanical bonding of different materials. Obtaining the best fit and functionality possible of the final product depends on the adhesion of the substrate on the over-molded component. To create the optimal bond, it may occasionally be necessary to preheat the substrate to bring the surface temperature closer to the melt temperature of the over mold. You can also use mechanical techniques such as installing a runner system on the underside of the substrate or creating a through-hole with a counter bore to enhance bonding of the materials.
- Facilitate the flow. Some over-molded parts will have isolated or multiple portions of the substrate that are difficult to fill. Utilize runners that originate from the thicker sections of the second shot to feed material into hard-to-reach areas. Create through holes in the substrate to deliver the over-molded plastic to the isolated portion of the part.
- Gate locations and wall thickness. It is important to visualize the gate location on the thermoplastic component. To eliminate voids and sink marks, design the gates of thermoplastic components in the thicker portion of the part. Keep in mind that the gates must be positioned in the thermoplastic part in a manner that allows the LSR to flow. The variation of wall thickness on the thermoplastic part must be kept consistent or you risk warping, sink marks, dimensional control problems and other integrity issues. LSR allows for wall thickness variations, but the changes need to be gradual.
- Optimize the material bond. To produce the best covalent-bond strength, you must design the part by using interlocks such as through-holes on the rear side of the component and other techniques to facilitate the flow of the melt and to provide large spaces where the LSR and thermoplastic can join and maximize the bond between the two materials.
- Maintain proper substrate temperature. Over-molding LSR on cold substrate slows the curing stage. To ensure the proper chemical reaction and the bonding of the substrate to the thermoplastic, the substrate components must be hot. On average, the mold temperature for liquid silicone rubber processes are between 300 to 400-degrees F, with a minimum temperature of 250-degrees F. Use a hot plate or conveyor oven to preheat cold substrates.
- Additives and mold releases. Additives that contain amines or sulfur can inhibit the curing of LSR. You should avoid using thermoplastic substrates with external mold releases. Occasionally, internal mold releases can also cause a problem.
- De-molding. When de-molding, be aware that the adhesion and cure may not be in its final state, so avoid stretching or pulling the LSR. The application of a PTFE mold coating can also help ease the release of the LSR.
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.
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.
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.
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.
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.