Custom injection molding is a viable solution for many projects, but there’s often hesitation in using it because of confusion about which material matches the job. While “thermoplastic” and “thermoset” sound similar and both are appropriate for a wide range of applications, the material properties of these two resin categories and how they behave during processing ultimately reveal the best choice for your injection molding project.
For a better understanding of similarities and differences, let’s compare one material from each group: thermoplastic elastomer (TPE) — a thermoplastic — and liquid silicone rubber — a thermoset. Both are frequently considered for projects that require complex designs and secondary overmolding.
What is TPE and LSR?
TPE, or thermoplastic elastomer, is synthetic resin that melts upon heating and hardens on cooling, without changing plastic chemistry. LSR, or liquid silicone rubber, is synthetic resin wherein polymers are joined and structured by chemical bonds that harden permanently after one application of heat.
It quickly becomes evident that the main difference between the two is how they react to heat. After LSR is heated, its chemical and physical properties cannot be altered. For TPE, reprocessing is possible and can be done multiple times, if necessary.
The properties and behaviors of TPE and LSR during heating are what drive moldability:
- TPE: Plastic pellets are heat-liquified and pressure-molded into components that strengthen and hold shape upon curing without using a chemical bond, so molding can be reversed (reground/melted) to correct defects and the resins reused multiple times
- LSR: Viscous plastic is heated to cause polymer cross-linking, and the resulting chemical bond provides irreversible strength and shape after curing. Another common way to process LSR is to mix two components prior to injection into the mold. One component is a catalyst that initiates cross-linking
Advantages and disadvantages
Selecting either TPE or LSR brings with it both advantages and disadvantages that may impact certain aspects of moldability and project outcomes. Therefore, it’s important to understand the benefits and drawbacks of each:
- TPE advantages include recyclability and less energy consumption for production; re-molding opportunities without chemical change; easier molding than thermoset LSR; shorter molding cycles; less expensive molding process; heat sealable; easily colored by most dyes; greater number of two-shot molding options
- TPE disadvantages include melting if exposed to high temperatures after curing; “creeping” and deformation if under sustained pressure or other stress; tooling can be costly
- LSR advantages include better resistance to high temperatures than thermoplastics; design flexibility, including thick to thin wall constructions; high tear strength; biocompatibility; high chemical resistance; superior compression set
- LSR disadvantages include no recyclability; inability to be re-molded after curing; possibility of burning if heated after curing; lengthy curing period, which adds to production time and cost; can cause production delays as equipment must be disassembled and cleaned if early cure occurs; liquid state of material can make it difficult to handle; can be bulky or thick in appearance
After weighing this information, you may be wondering what material is right for your particular complex project, overmolding, or metal-to-plastic conversion. While an experienced injection molder like Kaysun evaluates every project on its own merit, here are some typical applications well-suited for TPE and LSR:
- TPE is ideal for soft-touch grips, seals, impact-resistant devices, and component housings; sealing rings; some food-contact-approved applications like bottle cap and closure liners, baby bottles, and toddler cup spouts (provided proper FDA regulatory compliance is met); alternatives to latex, silicone, PVC, or rubber in some medical/healthcare applications like gloves and non-invasive equipment components
- LSR is ideal for medical implantables; gaskets and hardware in consumer appliances like microwaves; electronic interfaces on device keyboards or touchpads; o-rings, wire harnesses, coverings, stoppers, and other plastic automotive parts consistently exposed to harsh chemicals and high heat
To learn more about overmolding, a popular injection molding process that TPE and LSR are both commonly considered for, read our Overmolding Fundamentals Whitepaper. Click the button below to download your free copy.