Liquid Silicone Rubber or LSR is a thermoset elastomer based on silicone dioxide (sand). And in recent years, use of LSR is growing in usage and popularity. This installment ofAsk the Engineer addresses some questions on uses of LSR in medical devices, how it can be injection molded, and 2K molding (multi-component LSR molding).

Q: Several of my customers have specified LSR for their new medical devices. What is LSR and why is it growing in usage and popularity?

A: LSR (Liquid Silicone Rubber) is a thermoset elastomer based on silicone dioxide – sand. Silicones consist of alternating silicon and oxygen molecules that are reacted with methyl chloride to create polydimethylsiloxane (PDMS). Due to their unique molecular structure, silicone materials exhibit both organic and inorganic properties that, depending on their combination with other chemical groups, can exhibit a wide range of forms and functional behaviors. Silicones are typically grouped into fluids, rubbers, liquid rubbers, resins and silanes.

LSRs are extremely pure and contain no plasticizers, curing agents, stabilizers, or promoters. They are cured, or cross-linked, using a platinum-catalyzed reaction that generates no by-products. LSRs are formed using two independently stable materials that may range in consistency from liquids to thick pastes. When Component A (containing the catalyst) and Component B (cross-linking constituent) combine, a chemical reaction occurs and curing begins. There are several different types and grades of LSR and other silicone rubbers that offer a wide range of properties for use in many applications and industries like automotive, medical, and advanced electronics.

LSR is enjoying increased popularity for medical device applications due to both its broad process flexibility and to its unique combination of biocompatibility and performance attributes:

• Chemically inert, odorless, tasteless, and stainless;
• Hydrophobic – repels water for exceptional sealing;
• Bacterial resistant, easy to clean, and able to withstand sterilization processes;
• Biocompatible to ISO 10993-1 and USP Class VI tests for medical usage;
• Lubricious for less invasive biological applications;
• Gas permeable;
• Thermally stable over a broad temperature range for heat resistance and extreme low temperature flexibility;
• Fatigue, tear and compression set resistant;
• Elongatable (100 to 900 percent); and
• Dialectric with superior insulating characteristics.

LSR can be used to create complex injection-molded parts with extremely fine detail and tight tolerances. Typical applications include devices that were previously manufactured using metal, latex, high-consistency silicone rubber (HCR), TPE, and other elastomers. Relevant products include: wound drain bulbs, masks, valves, seals and sealing membranes, gaskets, medical implants, urological and gynecological devices, electrical connectors, infant and baby care products, and even disposable medical products.  

Q: How is LSR injection molded?

A: LSR injection molding requires specialty machinery, equipment and tooling compared to conventional thermoplastic molding. While thermoplastics must be heated to their melting points in the injection barrel and then cooled in the mold to a solid state, the reverse occurs in LSR injection molding. LSR starts as two independently stable liquids, gels or pastes (depending on the durometer of the material) that are heated in the mold to initiate curing and solidification. Table 1 illustrates the key differences between the processing of thermoplastic resins and LSRs.

A water-cooled injection barrel is used in LSR molding to keep the temperature of the material below the point where the curing reaction will occur – usually in the range of 60ºF to 100ºF. Cold runner systems, rather than the hot runner systems used in thermoplastic molding, are used to ensure that the silicone remains uncured it enters the tool cavities – usually heated to 355ºF to 390ºF. Since LSR flows quickly and easily through the cavities, mold makers must pay great attention to parting lines and vents to prevent flash.  However, unlike thermoplastics, LSR flows easily into long, thin sections to form complex shapes with aggressive undercuts and little to no draft.

In processing thermoplastics, high clamp forces are needed to allow the material to “pack-out” or fill the entire mold cavity. The clamp force required for LSR injection molding is significantly less. LSR expands considerably during cure as a result of cross-linking; thus clamp force must be sufficient to ensure proper engagement and force distribution of the tool and to prevent the tool from opening during cure. Since LSR also contracts upon cooling, hold pressure is important to ensure control of final dimensions.

Q: Can LSR be molded onto other substrates? When and why is this desirable?

A: Multi-component or “2K” molding is a rapidly growing technical segment of LSR molding. In 2K LSR molding, uncured LSR is applied to a base material (or substrate) and cured to shape while in contact with that material. During molding, the LSR adheres to the substrate surface through chemical bonding, mechanical bonding, or a combination of both. Chemical bonding is the strongest mechanism of attachment and is generally more reliable since it takes place at the molecular level. Mechanical bonding requires a series of undercuts around and interlocks through which the liquid LSR flows during molding. Once cured, these design features provide positive retention between the two components.

The advantages of 2K LSR molding are numerous and significant:

• Design flexibility. Unique material combinations create more design and performance options.
• Reduced assembly cost. The often “tacky” surface of silicone parts makes them especially difficult to assemble. By molding the LSR directly onto the surface, the operation is quicker and easier.
• Reduced material cost. “Skinning” a less expensive substrate with LSR can provide the finished product with the desired silicone properties (biocompatibility, insulation, “soft-touch”, etc.) at a more economical cost.
• Increased reliability and quality. The bond strength between the two materials is often greater than that which could be achieved with other adhesive or non-adhesive methods of assembly. Most LSR 2K molded components exhibit only cohesive failure – the substrate tears before the bond breaks.

While LSR can be injection molded onto many different substrate materials, the most common of these are silicone, metal and thermoplastics. Silicone-to-silicone molding is the easiest, and frequently used to achieve multiple color or durometer combinations or to “skin” a less expensive silicone with a more expensive, property-rich LSR.

Metal is the next easiest material for 2K LSR molding. Most metals require a primer or barrier to promote chemical adhesion and prevent leaching of chemicals, such as sulfur, that act as inhibitors to silicone cross-linking. Metal surfaces can also be sandblasted, peened or textured to provide more bonding surface area.

LSR-to-thermoplastic molding is the fastest growing segment of 2K LSR molding. It allows the designer to combine the inherent material properties of silicone – service temperature, compression set, and biocompatibility – with the specialized properties of engineered thermoplastic resins.

Unfortunately, thermoplastic materials also present the most difficult processing challenges because: 1) the glass transition (melt) temperature of many thermoplastics is lower than the temperature required to properly cure LSR, and 2) not all thermoplastics bond to silicone on a molecular level. With regard to the first challenge of temperature, it is important to select thermoplastics with glass transition temperatures greater than 300ºF-350ºF to avoid substrate deformation. To overcome the second challenge of material compatibility, there are several options available:

• Select materials with proven compatibility and demonstrated molecular bondability. Proven thermoplastic materials include polycarbonate, nylon, PBT, polyester, and PEEK;
• Avoid thermoplastics that contain release agents or chemicals which inhibit silicone cross-linking;
• Pretreat the thermoplastic substrate surface with chemical primers, plasma, corona, or UV radiation; and
• Select a selective, self-bonding LSR.

It is important to keep in mind that there may be cost and quality tradeoffs associated with each of these options.

2K LSR/thermoplastic molding is generally executed using two separate machines. Demolding and transfer of the thermoplastic part from the thermoplastic mold machine into the LSR mold machine occurs either manually or automatically. The primary advantage of this method of 2K molding is that both machines are available for independent use on other projects. It is also easy to control the thermal conditions of each process individually.

Multi-material LSR parts can also be manufactured through co-injection molding (or 2-shot molding) where one injection mold machine is outfitted with two injection units and the mold contains cavities for both the thermoplastic substrate and the LSR. Successive injection of the two materials takes place by rotating the mold plate or the injection machine platen. Though this method of 2K molding usually requires greater initial investment in capital and tooling, these costs are generally recouped on medium to higher volume applications. The advantages associated with LSR co-injection molding include quality improvements, cost savings and speed to market through: elimination of one tool and its associated validation costs; improved part performance and more consistent part quality (no insert misalignment and less part handling); WIP storage and packaging savings; overall cycle time savings; and material savings.