What is High-Consistency Rubber (HCR)?

Silicone elastomers have long been a popular material for medical parts and devices due to their highly desirable mechanical and physical properties. One of the most common elastomers for manufacturing is High-Consistency Rubber, or HCR. It should be noted that the terms HCR and HTV, which stands for High Temperature Vulcanization, are often used interchangeably and refer to the same silicone material; for the purpose of this article, we will use the acronym HCR.

HCR is a type of silicone elastomer comprised of long polymer chains with a very high molecular weight. It is cured at high temperatures with a platinum catalyst or peroxides. HCR is known for its gummy consistency that is similar to peanut butter. Due to its higher viscosity compared to other elastomers, HCR is typically processed using compression and transfer molding methods, but can also be utilized for injection molding projects. HCR has many desirable properties such as excellent aging resistance, thermal stability, electrical properties, mechanical strength, elongation, and hardness. For these reasons, HCR is a good material for a broad range of applications and HCR products are found across many industries including medical, automotive, consumer goods, aerospace, and electrical uses.

Best Practices for HCR Injection Molding

Injection molding offers many benefits over other production methods, making it a very attractive manufacturing option. At a high level, the injection molding process for HCR is the same as with other elastomers: the silicone rubber is fed into a heated barrel and injected under pressure into a mold where the material completely vulcanizes into the desired part shape. However, due to its properties HCR processing presents some challenges when compared to other elastomers. But do not be alarmed – high quality parts are achievable with HCR when attention to detail is given during the part design phase as well as during manufacturing! Below are some best practices to consider when using HCR for injection molding projects.

  • High-consistency rubber takes longer to cure than many other molding materials. A longer cure time results in a longer injection molding cycle time. In order to make the project economics attractive, HCR molds often have a large number of cavities in order to accommodate the longer cycles and still achieve the desired production volume for each cycle – resulting in more attractive project economics!
  • One best practice that is especially important for HCR is preheating the material prior to injection into the mold. Preheating has a couple benefits. It reduces the viscosity and allows the HCR to more quickly fill the mold as well as more uniformly – reducing the cycle time and improving the quality of the final product.
  • HCR injection molded parts are susceptible to tearing when removed from the mold; this is referred to as hot tear. Hot tear is directly related to the mold temperature – the higher the temperature the more vulnerable the part is to hot tear; however, lower operating temperatures result in longer cure times – so manufacturers must adjust the temperature to optimize cure times while minimizing the potential for hot tear.
  • When compared to other silicone elastomers, HCR has higher part shrinkage rates. For this reason, it is crucial that OEMs work with their molding partner to design for manufacturing by selecting the right size and type of mold for their specific HCR project.
  • It is crucial to ensure the right size of shot for HCR projects to prevent over packing of the mold. When a shot size is too large for a given mold, HCR will seep from the mold resulting in flash. To avoid this situation, manufacturers must gradually increase the shot size until the mold completely fills without any mold leakage or flash on the final product.
  • To avoid air entrapment, the mold needs to have sufficient air flow and channeling when HCR is injected into the mold. Air that remains in the mold creates air bubble imperfections and inconsistencies in the final product. OEMs must ensure air flow is accounted for in the design and take steps to improve flow properties during production such as preheating and possibly a higher operating temperature.
  • Similar to other injection molding projects, selection of the right design for the mold, runners and gate, press, and other equipment are critical to the success of the project – and HCR injection molding is no exception. It is important to work with your design team to ensure the right equipment set-up during the design phase to set production up for success! Additionally, it is important to ensure the molding equipment are properly maintained over time.

The decision on which material is best for your injection molding project should be determined on a case-by-case basis and depend on a variety of factors including the requirements of the part and the OEM’s preferences. This is why it is important to team up with an experienced partner, such as ProMed, who will guide you through the selection process to ensure the right material is chosen for your project!

ProMed’s HCR Processing Capabilities

At ProMed, we combine industry-leading medical-grade expertise with the latest developments in silicone materials and technology – including HCR. We have garnered a reputation as the world benchmark of implantable silicone components and assemblies. From helping OEMs incorporate the latest medical-grade silicone formulations into their designs to delivering rapid silicone prototypes, we serve as a premier silicone molding contract manufacturer for medical device OEMs.

ProMed has expertise in working with the full spectrum of silicones covering a wide range of properties and characteristics. Our wide range of materials include: High-consistency Rubber (HCR): 20 to 80 Durometer, Liquid Silicone Rubber (LSR) 5 to 80 Durometer, Room Temperature Vulcanizing silicone (RTV). We will assist in your material selection to help ensure all design requirements are met.

Our manufacturing facilities and equipment are designed for a single purpose—to mold medical and implantable silicone, combination components, and bio-material grade plastics with uncompromising quality and service. We currently have four divisions that are located within two manufacturing sites. All are certified class 10,000 / ISO Class 7 cleanrooms.

Contact ProMed today at 763-331-3800 to discuss your next medical device project.

Summary
Best Practices for HCR Injection Molding
Article Name
Best Practices for HCR Injection Molding
Description
Overview of High Consistency Rubber (HCR) injection molding and some of the challenges manufacturers may encounter such as hot tear or over packing.
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ProMed Molding
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