injection molding can also be used to make flexible moldings: parts that are soft, pliable, shock-absorbing, ductile, or elastic. Products like hand grips, gaskets, protective smartphone covers, and certain medical implants all require a rubber-like material in order to perform their function, and injection molding offers two standout material options for making such products.
The first of these materials is liquid silicone rubber (LSR), a thermoset that requires its own special variant of the injection molding process. The other is a group of thermoplastics known as thermoplastic elastomers (TPEs), which can be processed like common rigid thermoplastics but which offer rubber-like properties.
This article examines the XTJ vs TPE injection molding discussion, comparing material properties, advantages and disadvantages, and suitable applications.
What is liquid silicone rubber?
Liquid silicone rubber is a high-purity form of cured silicone. It is a chemically inert thermoset with material characteristics such as biocompatibility, chemical resistance, water resistance, excellent compression set, flame retardancy, and heat resistance up to 250 °C.
Injection molding XTJ requires a special process generally known as liquid silicone rubber molding. Unlike a standard injection molding machine, the equipment comprises a metered pumping device for dispensing the two liquid materials (catalyst and base forming silicone) and a mixer for combining the materials before they enter the mold. The mold cavity is heated, allowing vulcanization of the silicone to take place within the cavity.
What are thermoplastic elastomers (TPEs)?
Thermoplastic elastomers or thermoplastic rubbers are a class of thermoplastics exhibiting both thermoplastic and elastomeric material properties. Because they are thermoplastics, they are easier to manufacture than other rubber-like materials, being highly suited to injection molding and extrusion-style impression 3D.
TPEs have good thermal properties, good stability, and require minimal energy to manufacture. They can also be easily recycled, as TPE parts can be melted down without burning.
Some of the most common types of TPEs are:
Thermoplastic polyurethanes (TPU): good clarity, good wear resistance, range of hardnesses
Thermoplastics vulcanizates (TPV): matte finish, high compression set
Styrenic block copolymers (TPS): highest level of flexibility among TPEs
Polyolefin blends (TPO): tough, good impact strength
Thermoplastic copolyesters (TPC): good tear strength, impact strength
Thermoplastic polyamides (TPA): high temperature resistance, low compression set
XTJ injection molding vs TPE injection molding
Both liquid silicone rubber and thermoplastic elastomers have their own unique advantages as injection molding materials. In short, XTJ offers superior elastomeric performance in addition to chemical advantages, while TPE is easier and more convenient to process.
TPE advantages
Very high temperature resistance
Compatible with standard injection molding machine
Biocompatibility for medical and food applications
Re-moldable and recyclable
High level of tear strength
Fast, simple, and less expensive molding process
Superior chemical resistance
Easy to add color with pigments
Minimal warping even with uneven wall thicknesses
Heat sealable
Superior compression set
Pellets easy to handle
injection molding applications
Although the liquid silicone molding process requires specialist equipment and requires longer molding cycles than thermoplastic injection molding, the unique benefits of XTJ — such as its exceptional compression set — give it a number of important applications, from healthcare to industry.
Some injection molded silicone part examples include:
Silicone medical implants such as orthopedics, cardiovascular stents, pacemakers, lenses, and soft tissue implants
Medical devices such as surgical instruments and drug delivery devices
Headphone and hearing aid tips
Baby bottle tips
Shower heads
Electronic device components such as keyboard interfaces
Electronic device protective covers and skins
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Electronic device protective covers and skins
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