Liquid silicone rubbers

Technical Information

Material and Properties

Liquid silicone rubbers aremolecules formed from a basic silicone oxide backbone.

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

Liquid silicone rubbers do notcontain any plasticizers, stabilizers or promoters. Theirexceptional flex characteristics are due to their unique chemistry.As a crosslinked material it will withstand extreme temperatureswith no heat ageing and will maintain its mechanical properties andresilience in time. The material is cured through a process thatcontains non-hazardous substances therefore, even in an under curedpart, there is no threat of hazardous chemicals beingreleased.

There are many different LSRgrades available. Some examples include: electrically conductive,medical grade, FDA compliant, self-adhesive, self-lubricating andcombinations of these.

Electrically conductive gradesare usually carbon filled to desired conductivity. Medical gradematerials are prepared under rigorous testing and regulations toassure superior quality. There are many different kinds ofself-adhesive LSRs. In general, LSRs have outstanding compatibilitywith thermoplastics but many formulations are available so as tofit many different applications. The most general formulationsprovide direct strong bonds onto most thermoplastic surfaces andmetals including aluminum, magnesium and steel. When overmolding tothermoplastics, due to the high curing temperatures, glass-filledtemperature resistant thermoplastics such as Polymides, PBTs andPCs are preferred.

Properties of liquid siliconerubbers make them stand out over any other material in the market.LSRs are translucent, odorless and tasteless. They are consideredhypoallergenic and a material that will prevent bacteria or moldgrowth due to their inorganic molecular backbone. Their excellentheat range and weather resistance go along with its low ageing. LSRparts will not become brittle in time.

Silicone Rubbers stand out withtheir outstanding mechanical properties:

Superiorelongation and tear resistance

Silicone rubbers are curedduring processing to create a strong intermolecular crosslinkedstructure. This structure will provide extreme elongation of thematerial with no permanent deformation and a higher tearresistance—a high resilience material.

Very lowcompression set and stress relaxation

LSR is a high resiliencematerial both in elongation and compression. When the material iscompressed it will create an equal opposing force which will notchange in time making it great for seals and gaskets. LSR materialsare crosslinked which will prevent any creep or stress relaxationproblems.

Manyvariable options for material properties

Typical ranges for propertieshave been provided on the following table as a reference. Rangesshown are approximate and depend on the individual materials butthese values are the most commonly seen in the market today.

Process Description

Injection molding is a processthat is used for high volume parts, complex variable geometries,and when tight tolerances are required. The process of injectionmolding liquid silicone rubbers, is a very complex process, andinvolves much more variables than when dealing with thermoplastics.LSRs have to be perfectly mixed, injected, and cured inside themold cavity immediately after injection. Mold cavities and runnershave to be designed to only heat the material in the part cavity.Sufficient heating time has to be provided to the parts to assurefull conversion. Parts which are not fully cured will not haveoptimal properties.

Our process consists of a fullyautomated injection molding of liquid silicone rubber. Allvariables in the process are taken into account simultaneously.Automated mixing of batches and pigments followed by a controlledmaterial injection. Similar to thermoplastics where cooling is thedeterminant step in the process, heating time for cure is thedeterminant step for LSRs. During the curing stage material is setfor next injection. De-molding of the material is also wellcontrolled and done in the fastest possible way. Formulti-component injection molding, overmolding or in-mold assemblyof parts automated robotic transfer or rotation of part into themolding cavity is done to assure precise and consistent results foreach and every part. There is no variation from part to part andprocess time is greatly reduced leading to complex multi-materialparts right out of the machine.

Our advanced moldingtechnology, high precision molds and cold runner systems lead toflash-free parts. Our expertise in the technology and materialsallows us to optimize the process leading to lowest possible cycletimes and best product quality.

When comparing our LSRinjection 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 thermoplasticelastomer, polyurethane or natural rubber. There is no ash contenton cured parts and parts may be made completely and perfectlytransparent. Unlike with peroxide based vulcanizations there isperfect batch-to-batch consistency. This is made possible becauseof the simplicity of the materials to be mixed and the use ofadvanced automated mixing systems. High-cavitation molds providehigh volume of parts in shorter periods of time.

Multi-Component / Overmolded Parts

Multi-component ormulti-material injection molding is a process that has developedgreatly in the last decade. The process has grown not only due totechnological advancements but due to OEM’s searching for new waysto gain advantage over competitors in quality and value ofproducts. Multi-component molding may be described as theintegration of multiple parts into one component directly in themolding process—a process that previously required separatemanufacturing and assembly steps but that now has been streamlinedinto a single multi-part solution.

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

The basic reasonmulti-component processes are done is to reduce costly orcomplicated assembly steps; for example, assembling gaskets withcomplicated geometries, although there are many otherbenefits.  When materials are overmolded (placed one over theother) you obtain a combination of the different properties of thematerials used in the part. For example, when overmolding ontometals you may be looking for the stiffness and shine of a metalbut the softness, smoothness and transparency of a liquid siliconerubber.

Another benefit of overmoldingwith LSRs is that grades have been developed to offer prime-lessadhesion to a wide range of substrates including metals andengineering plastics. Many materials require the use of primers orsurface treatments which leads to extra steps in the manufacturingprocess but LSRs will bond to most surfaces through direct bonding.Due to the high compatibility between materials, full bond strengthis obtained immediately after de-moulding. Parts can be bondedthrough chemical and mechanical adhesion leading to the strongestpossible bond between materials. Other benefits for multi-componentmolding of LSRs include:

• The ability to mold complexdesigns and geometries which could never be done by a separateassembly step.
• The possibility of integratingbonding and non-bonding materials to generate movable parts thatsometimes could never be assembled in such a way.
• Superior surface finish withthe best bonding characteristics.
• Clean surfaces and consistentthicknesses throughout the parts.
• Perfect alignment of seals,gaskets, or fittings, consistent for every part made.
• Competitive pricing – byreducing assembly steps, production time and materialwaste.

All these benefits make liquidsilicone rubber multi-injection molding the optimal choice forcost-efficient manufacturing of high volume complex parts.