Material and Properties

Liquid silicone rubbers are molecules formed from a basic silicone oxide backbone.

By varying certain side groups [R] (in this case CH3) of the structure, the material may be tailored to meet specific functions or requirements. Therefore, there can be many different LSR materials, each with different characteristics though some characteristics will remain the same for all liquid silicone rubbers. LSRs have an extreme temperature range performance. At their pure state they will be stable from -50˚C to 180˚C. By the addition of stabilizers the material can remain stable up to temperatures over 250˚C and by use of special compounds a low temperature flexibility of -90˚C. LSRs are naturally transparent but may be easily colored by use of non-hazardous pigments. LSRs have excellent chemical and oil resistance, are very stable, highly resistant to oxidation, light, UV and X-rays.

Liquid silicone rubbers do not contain any plasticizers, stabilizers or promoters. Their exceptional flex characteristics are due to their unique chemistry. As a crosslinked material it will withstand extreme temperatures with no heat ageing and will maintain its mechanical properties and resilience in time. The material is cured through a process that contains non-hazardous substances therefore, even in an under cured part, there is no threat of hazardous chemicals being released.

There are many different LSR grades available. Some examples include: electrically conductive, medical grade, FDA compliant, self-adhesive, self-lubricating and combinations of these.

Electrically conductive grades are usually carbon filled to desired conductivity. Medical grade materials are prepared under rigorous testing and regulations to assure superior quality. There are many different kinds of self-adhesive LSRs. In general, LSRs have outstanding compatibility with thermoplastics but many formulations are available so as to fit many different applications. The most general formulations provide direct strong bonds onto most thermoplastic surfaces and metals including aluminum, magnesium and steel. When overmolding to thermoplastics, due to the high curing temperatures, glass-filled temperature resistant thermoplastics such as Polymides, PBTs and PCs are preferred.

Properties of liquid silicone rubbers make them stand out over any other material in the market. LSRs are translucent, odorless and tasteless. They are considered hypoallergenic and a material that will prevent bacteria or mold growth due to their inorganic molecular backbone. Their excellent heat range and weather resistance go along with its low ageing. LSR parts will not become brittle in time.

Silicone Rubbers stand out with their outstanding mechanical properties:

Superior Elongation and Tear Resistance

Silicone rubbers are cured during processing to create a strong intermolecular crosslinked structure. This structure will provide extreme elongation of the material with no permanent deformation and a higher tear resistance—a high resilience material.

Very low compression set and stress relaxation

LSR is a high resilience material both in elongation and compression. When the material is compressed it will create an equal opposing force which will not change in time making it great for seals and gaskets. LSR materials are crosslinked which will prevent any creep or stress relaxation problems.

Many variable options for material properties

Typical ranges for properties have been provided on the following table as a reference. Ranges shown are approximate and depend on the individual materials but these values are the most commonly seen in the market today.

 

Our advanced molding technology, high precision molds and cold runner systems lead to flash-free parts. Our expertise in the technology and materials allows us to optimize the process leading to lowest possible cycle times and best product quality.

When comparing our LSR injection molding process with other processes for making rubbers, LSR high precision injection molding stands out in volume, purity, and quality. LSR purity stands-out over any thermoplastic elastomer, polyurethane or natural rubber. There is no ash content on cured parts and parts may be made completely and perfectly transparent. Unlike with peroxide based vulcanizations there is perfect batch-to-batch consistency. This is made possible because of the simplicity of the materials to be mixed and the use of advanced automated mixing systems. High-cavitation molds provide high volume of parts in shorter periods of time.

Multi-Component / Overmolded Parts

Multi-component or multi-material injection molding is a process that has developed greatly in the last decade. The process has grown not only due to technological advancements but due to OEM’s searching for new ways to gain advantage over competitors in quality and value of products. Multi-component molding may be described as the integration of multiple parts into one component directly in the molding process—a process that previously required separate manufacturing and assembly steps but that now has been streamlined into a single multi-part solution.

There are many names for all the different multi-material injections molding processes such as two-component (2K), overmolded, co-injection, or multi-component; we will refer to multi-component injection as the process of making a part out of more than one material in the same manufacturing process.

The basic reason multi-component processes are done is to reduce costly or complicated assembly steps; for example, assembling gaskets with complicated geometries, although there are many other benefits.  When materials are overmolded (placed one over the other) you obtain a combination of the different properties of the materials used in the part. For example, when overmolding onto metals you may be looking for the stiffness and shine of a metal but the softness, smoothness and transparency of a liquid silicone rubber.

Another benefit of overmolding with LSRs is that grades have been developed to offer prime-less adhesion to a wide range of substrates including metals and engineering plastics. Many materials require the use of primers or surface treatments which leads to extra steps in the manufacturing process but LSRs will bond to most surfaces through direct bonding. Due to the high compatibility between materials, full bond strength is obtained immediately after de-moulding. Parts can be bonded through chemical and mechanical adhesion leading to the strongest possible bond between materials. Other benefits for multi-component molding of LSRs include:

 

• The ability to mold complex designs and geometries which could never be done by a separate assembly step.

• The possibility of integrating bonding and non-bonding materials to generate movable parts that sometimes could never be assembled in
   such a way.

• Superior surface finish with the best bonding characteristics.

• Clean surfaces and consistent thicknesses throughout the parts.

• Perfect alignment of seals, gaskets, or fittings, consistent for every part made.

• Competitive pricing – by reducing assembly steps, production time and material waste.

 

Supporting Research

	Characterization and Modeling of the Curing Process of Silicone Rubber (PDF)
	Modeling the Vulcanization Process of High Consistency Rubber and Liquid Silicone Rubber (PDF)
	A Study of Injection Molded Liquid Silicone Rubber for Syringe Applications (PDF)

Technical Bulletins

"Assembly Injection Molding", SIMTEC Technical Bulletin Vol.1, Issue 1 (PDF)