What is Rubber Injection Molding?

Rubber injection molding is similar to rigid plastic injection molding, except that the materials being used have flexibility and elasticity to make parts that require adaptability in their environments. These rubber-like materials are durable and can be produced in a variety of shore values, making them useful for many applications. Some applications of rubber injection molded parts may include electrical overmolding, seals, stoppers, soft-touch grips, baby bottles, and other pliable parts. 

Although multiple flexible materials are available for rubber molding, the two primary types used are TPE and LSR. For the sake of simplicity, this article will only discuss these two main competitors. 

Thermoplastic Elastomer (TPE)

TPE, or Thermoplastic Elastomer, is a rubber-like thermoplastic that uses the same equipment and production method as rigid plastic injection molding. This involves feeding TPE pellets through a hopper and barrel where they are melted and injected into the mold. Once cooled enough to retain its shape, the part is ejected from the mold, and the next cycle begins. 

Because it is a thermoplastic and, therefore, its particles are not chemically bonded, it can be remelted and remolded repeatedly without change to its mechanical properties. This makes it easily recyclable. TPE is also processed at a lower temperature, and the average mold for this material may be heated to only 70-120°F, meaning that it takes less energy to manufacture than LSR and other thermoplastics. This lower temperature also contributes to a shorter molding cycle time, which can be as little as 20 seconds depending on part size and design. 

While TPE has many advantages, including its remoldability, these can be also be areas of weakness. TPE will deform and melt at elevated temperatures, including those found in high heat applications. For this reason, it is best to use this material only in moderate temperatures to avoid part warp or failure. Likewise, TPE will deform under continuous stress. 

Liquid Silicone Rubber (LSR)

LSR, or Liquid Silicone Rubber, is a rubber thermoset that is most commonly injection molded, but it requires different equipment than rigid plastic injection molding due to the nature of its processing. LSR is made when two different liquids, a base and a catalyst, are mixed together, causing a chemical reaction. As soon as the two materials are combined, chemical bonds begin to form. For this reason, the two materials are kept in separate containers until right before the injection mold process begins. To successfully process LSR, the two materials are poured into a mixing chamber where they are evenly mixed at a 1:1 ratio. The mixture is then moved through a barrel to the nozzle of the machine so it can be injected into the mold. As opposed to typical injection molding, this material is kept cool in the barrel because heat causes LSR to cure, meaning that the material would likely get clogged in the barrel and make injection difficult or impossible. Instead, the mold is what is heated. Once the cool LSR is injected into the hot mold, the material is vulcanized by the heat so it will retain its shape. 

Being a thermoset, LSR is a material that will harden permanently under heat. This enables it to be used in high heat conditions, up to 400°F, and not melt. It can also retain its mechanical properties in temperatures as low as -100°F. This material is biocompatible, hydrophobic, and chemically inert, meaning that it has high resistance to water, oxidation, and some chemicals. Its high tear strength makes it a practical choice for applications where high amounts of abuse or stretching will be expected. Additionally, it offers greater design flexibility than other materials because it can fill thick to thin wall transitions in the mold easily. 

Despite its superb mechanical properties, LSR has some negative attributes than can make it inefficient. Once cured, its shape is permanent, which means it can neither be recycled nor reprocessed to correct faulty parts without wasting material. Its processing can also be expensive because LSR requires demanding tooling that can be heated to 320-365°F for curing. Heating up LSR to cure will also take more time than typical injection molding materials, and cure time may take 1 minute or more depending on the part size and design. Additionally, LSR shrinks more than TPE, which must be accounted for in part design. Finally, though flame retardant, LSR can burn if exposed to extreme heat. 

Choosing Your Material

Both TPE and LSR are great rubber-like materials, but it can be difficult to know which one may be better suited for your part. The most important consideration when selecting any material is your part’s intended use. You will want to focus on what mechanical properties are required or most desired to get the job done. This can include heat resistance, strength, and more. 

Ultimately, LSR will be a more durable and versatile option because it can perform in a wider temperature range, but TPE excels with manufacturability because it uses the same equipment and molds as rigid plastic injection molding. This may be an additional area of concern if your budget is small. 

Injection Molding at 3 Space

Here at 3 Space, we offer injection molding and other manufacturing services to fit all your needs. We understand that choosing the best material and production method for your part can be difficult. If you need assistance, our expert engineers are happy to guide you based on your part’s design and intended application. For more information, contact us today.