How to use liquid silicone rubber

How to use liquid silicone rubber

Liquid silicone rubber (LSR) has become an increasingly popular material in recent years due to its many beneficial properties. LSR offers high tear strength, excellent flexibility and compression set resistance, good electrical insulation, and biocompatibility. It can be easily injection molded into complex geometries with tight tolerances. LSR also provides a smooth, non-tacky surface and can withstand high and low temperatures. With so many advantages, it's no wonder LSR is used across a variety of industries including automotive, medical, consumer goods, and more. This article will provide an overview on liquid silicone rubber - what it is, how to use it, and the many applications it is suitable for.

What is liquid silicone rubber?

Liquid silicone rubber refers to a two-part silicone that vulcanizes at room temperature. It consists of a silicone polymer mixed with a cross-linking agent and catalyst that causes the rubber to cure into a flexible, durable elastomer. LSR has a low viscosity allowing it to easily flow into complex molds before rapidly curing. It is well-suited for high volume injection molding of detailed parts.

LSR is different from solid silicone rubber which requires heat to vulcanize the material after it has been molded. The injection molding process minimizes waste since LSR can be mixed immediately before injection and only the required amount needs to be used. LSR also provides fast cycle times, facilitating high volume production.

Key properties of liquid silicone rubber include:

High tear strength and flexibility
Excellent temperature resistance from -55°C to 200°C
Good chemical and weathering resistance
Low compression set and high elasticity
Electrically insulating
Excellent biocompatibility and non-toxicity
Low viscosity for intricate molding
Rapid room temperature cure

These characteristics make LSR suitable for a wide range of applications which will be explored in more detail throughout this article.

How to use liquid silicone rubber

Using liquid silicone rubber involves a two part injection molding process.

First, the LSR must be mixed thoroughly to blend the two components together. This is often done using a static mixer that combines the ingredients right before injection into the mold. Proper mixing is critical to ensure the material cures uniformly without defects.

Next, the LSR is injected into the mold using an injection molding machine. The viscosity of LSR allows it to easily flow and conform to the detailed mold geometry. Pressure and temperature are controlled during injection to fill the mold cavity completely.

Once injected, the LSR crosslinks and cures at room temperature aided by the presence of the catalyst. Cure times are generally short, often under 2 minutes. The mold remains closed as the elastomer solidifies so it retains the proper shape.

After curing, the silicone part is removed from the mold. Any flash or excess material can be easily trimmed away. Post-curing may be done to fully stabilize properties but is not required. The silicone molded parts can also undergo secondary operations like assembly or decorating.

Compared to solid silicone rubber, liquid silicone provides faster production times, lower labor costs, and less waste since compression or transfer molding is avoided. Automating the process also improves consistency versus hand-mixing and molding. With the proper equipment and process controls, high quality silicone parts can be reliably mass produced.

What is the purpose of liquid silicone rubber?

Liquid silicone rubber serves several key purposes across industries:

Sealing - LSR's flexibility and temperature resistance allows it to serve as gaskets and seals in valves, pumps, and electrical connections. It maintains a tight seal across a wide temperature range.

Isolation - The electrically insulating properties of silicone rubber enables it to isolate electrical components and prevent short circuits. LSR has high dielectric strength for electronics applications.

Biocompatibility - With its inertness and non-toxicity, LSR is ideal for medical devices and implants that interface with the human body, such as O-rings, tubing, and masks.

Shock absorption - The high tear strength and elasticity of silicone can be used to provide impact and vibration dampening in pads, handles, and sporting goods.

Component molding - Intricate shapes, micro-features, and thin walls can be molded with LSR to create seals, diaphragms, valves, and precision components.

Bakeware - LSR's heat resistance allows it to withstand oven and freezing temperatures for bakeware applications. It provides a non-stick surface and easy release.

Consumer goods - Silicone rubber is common in kitchen utensils, anti-slip mats, and baby products where its safety, flexibility, and moldability are beneficial. LSR enables mass production of these goods.

The room temperature cure and injection moldability of LSR makes it efficient for molding detailed, high performance silicone parts suited for its many purposes across industries.

How long does it take for liquid silicone rubber to cure?

The cure time for liquid silicone rubber can range from less than 30 seconds up to 5 minutes depending on the formulation and part thickness. Typical cure times are between 1 to 2 minutes.

LSR cure speed is affected by:

Chemistry

Platinum catalyzed LSR cures fastest, followed by peroxide cure systems. The catalyst and crosslinker concentration also impacts rate.

Temperature

Curing is faster at higher mold temperatures up to 170°F (75°C). Heat accelerates the reaction speed.

Part size

Thicker parts require more time for the heat generated during cure to dissipate, slowing the process. Small, thin parts cure fastest.

Filler content

Unfilled LSR can cure in under 30 seconds while highly filled formulations take longer due to increased viscosity and thermal conductivity.

Mold design

Well-vented molds with greater surface area speed heat dissipation and curing. Minimal runners are optimal.

Ejection

Parts should remain in the mold until fully cured to obtain full properties and avoid deformation.

The injection molding machine parameters can be adjusted to ensure sufficient cure time prior to opening the mold and ejecting the parts. Adequate cure must be confirmed through testing before demolding parts. The short LSR cure times enable high production rates.

What is silicone liquid used for?

Silicone liquid has a number of uses:

Mold making

Adding silicone liquid to silicone rubber creates a pourable, brushable material for making molds of various objects. Condensation cure silicones typically used.

Casting resin

Low viscosity addition cure silicone resins can be used to cast intricate shapes and replicas. Cures with little shrinkage.

Lubrication

Silicone oils and fluids are excellent lubricants and dampening fluids due to high temperature stability, low volatility, and water repellency.

Heat transfer

Liquid silicone can fill thermal pads and gaps to aid heat dissipation thanks to high thermal conductivity and dielectric strength.

Waterproofing

Silicone sprays and coatings provide waterproof, protective barriers on various substrates while retaining flexibility.

Release agent

Silicone oils help release baked goods and ice from molds. Also useful for releasing parts from metal molds.

Anti-foam

Small amounts of silicone oil can prevent excessive foaming and bubbling in liquids.

Personal care

Light silicone fluids improve spreadability in cosmetics as well as provide silky, smooth feel to lotions.

Textiles

Silicone oils provide softness to fabrics and act as antistatic and water repellent treatments.

The inertness and fluidity of silicone liquid makes it a versatile ingredient across many industries.

What can you use silicone rubber for?

Silicone rubber has many uses:

Seals and gaskets - O-rings, sealing membranes, valve seats. Silicone's flexibility and temperature resistance are ideal for sealing.
Tubing and hoses - Excellent purity and biocompatibility makes silicone rubber suitable for food, beverage and medical fluid transfer tubing.
Bakeware - Can withstand oven and freezing temperatures. Provides non-stick performance and easy food release.
Molds - Used to create detailed molds for casting various materials including resins, rubbers, chocolate, wax, concrete, etc.
Electrical insulation - Isolates electrical components due to high dielectric strength. Protects from short circuiting.
Cookware handles - Provides improved grip and heat resistance compared to other rubbers.
Infant products - Used in bottle nipples, pacifiers, teethers. Silicone is safe, hygienic, flexible and durable.
Medical devices - Biological inertness allows use in catheters, implants, tubing. Can be sterilized.
Sporting goods - Provides impact absorption and waterproof cushioning for goggles, pads, swimwear.
Automotive - Resists fuels, oils and temperature extremes. Used for O-rings, grommets, spark plug boots.

Silicone is highly versatile for products requiring flexibility, heat resistance, electrical insulation, moldability, and biocompatibility. It can be formed into a wide array of custom parts.

How to use liquid silicone rubber for molding?

Here are some tips for using liquid silicone rubber for molding:

Select a two-part, platinum-cure LSR formulated for mold making that offers good tear strength and dimensional stability. Condensation cure silicones can also be used.
Prepare the mold surface properly - apply a release agent and perform trial casts to ensure proper release.
Mix LSR base and catalyst thoroughly (typically 10:1 ratio) either by hand or using an automated mixer. Avoid introducing air bubbles while mixing.
Pour the LSR into the prepared mold cavity, allowing air to escape. Optionally apply vacuum to remove air pockets.
Allow silicone to cure at room temperature based on material specifications (typically 30 mins to 24 hours).
Once fully cured, carefully separate the silicone from the mold. Trim away excess material with a knife.
Perform a post-cure if specified by manufacturer to stabilize properties. Oven-cure according to guidelines.
Clean the cured silicone rubber mold as needed using mild detergent. Avoid harsh chemicals that could degrade surface.
Test mold first with non-critical castings to confirm material fully releases from the silicone before use.
Secure mold while casting material is poured in to prevent distortion.

With proper procedures, liquid silicone rubber can be used to create detailed, reusable molds for casting resin parts, chocolates, waxes, concrete and more.

Liquid silicone rubber for mold making

Liquid silicone rubber (LSR) is an excellent material for mold making applications due to its easy pour and de-mold characteristics. LSR offers many advantages when used to produce molds:

Intricate details

Low viscosity LSR easily flows into complex mold geometries, capturing very fine details and contours. Great for reproducing prototypes.

No shrinkage

LSR exhibits minimal shrinkage during or after curing, producing dimensionally stable molds.

Fast production

Room temperature cure enables quick turnaround. Parts can be demolded in just 5-10 minutes after pouring.

Simple process

LSR does not require adding thickener like silicone RTVs, decreasing preparation time. No oven or heat cure needed either.

Reusable

Silicone molds have excellent tear strength and can last for many castings if properly handled and stored.

Release properties

Cured silicone naturally releases from many casting resins and materials without added release agents.

Heat/cold resistance

Silicone retains elastomeric properties from -45°C to 200°C enabling use with various casting materials.

No contamination

LSR does not contain sulfur or plasticizers which could leach out and contaminate castings.

Low toxicity

Unlike polyurethane resins, LSR has very low VOCs and toxicity, making it safer to work with.

The excellent fluidity, moldability, and reusability make liquid silicone rubber ideal for quickly producing detailed molds for casting prototypes, models, toys, and more.

How to make liquid silicone rubber at home?

Making liquid silicone rubber at home is possible on a small scale using the following method:

Materials Required

Two-part platinum cure silicone rubber (polydimethylsiloxane + curing agent)
High accuracy gram scale (0.01g resolution)
Containers and sticks for mixing
Popsicle sticks or tongue depressors for mixing

Steps

Weigh out 100 parts of the silicone base material and 10 parts of the curing agent by weight in separate containers. The typical ratio is 10:1 base to curing agent.

Mix the curing agent into the base material very thoroughly for 3-5 minutes, scraping the sides and bottom to ensure homogeneity. Avoid introducing air bubbles.

Optional - Place mixture in vacuum chamber to remove any air entrained during mixing. Air bubbles can leave voids in the cured silicone.

Pour the degassed silicone rubber slowly into molds, allowing air to escape. Optionally apply vacuum in mold to prevent trapped air.

Allow silicone to fully cure at room temperature based on manufacturer specified cure time.

Carefully demold the silicone part after full cure is reached. Trim away excess flash material with a knife.

Post-cure the silicone further if specified by manufacturer to fully stabilize properties. Follow cure schedule.

Safety precautions are required when handling silicone curing agents. Accurate measuring of the two components is critical to achieve proper cure and final rubber properties. With care, high quality silicone parts can be produced.

Liquid silicone for skin

Liquid silicone can be used on the skin for several purposes:

Medical grade silicone - Used for wound care, scar treatment, and dermatology. Creates protective barrier, reduces inflammation, and improves hydration. Requires careful application.

Personal lubricants - Silicone-based lubricants provide longer lasting, hypoallergenic lubrication for intimacy. Unlike oil-based lubricants, silicones won't degrade latex.

Cosmetics - Silicone oils improve spreadability in cosmetics. Give silky, smooth feel to lotions and serums. Act as conditioning agents on skin and hair. Impart water repellency.

Sunscreen additive - Dimethicone and other silicone fluids help evenly distribute sunscreen active ingredients. Improves application, water resistance, and skin feel.

Hair styling - Silicone oils coat and seal the hair cuticle to reduce frizz and smooth flyaways. Also impart shine and reduce drying time.

Shaving/grooming - Silicone-based shaving creams, gels, and oils allow razors to glide smoothly over skin, reducing irritation and friction.

Massage oils - Light silicone fluids can be added to massage oils. Reduce drag and friction for easier application during massage. Provide silky after-feel.

Liquid silicones should only be applied to skin after performing a patch test to check for any reactions. Only use high purity medical or cosmetic grade silicone oils and lubricants designed for skin contact.

Silicone liquid uses

Here are some common uses of silicone liquid:

Lubricating oils - Used to reduce friction in machinery, automotive systems, bearings, hinges. Withstands heat and provides durable lubrication.
Hydraulic fluids - Silicone fluids retain viscosity characteristics across wide temperature range for use in hydraulic systems and dampeners.
Heat transfer fluids - Liquid silicone's thermal stability makes it ideal for removing heat in electronics and solar thermal systems.
Release agents - Coating molds with silicone oil allows easy demolding of rubber and plastic parts with a glossy finish.
Defrosting fluids - Used to rapidly defrost frozen foods due to good thermal conductivity and flow at freezing temps.
Water repellents - Reacts with materials to create hydrophobic coatings that repel water, like on fabrics or ceramics.
Anti-foaming agents - Small amounts can prevent foaming during lubricant circulation and in coating chemistry processes.
Polishes and cleaners - Low chemical reactivity makes silicone suitable for shining delicate materials like jewelry or glass.
Resin additives - Mixed with epoxies or polyester resins, silicone oils reduce shrinkage and bubbles during cure.
Personal care - Added to cosmetics and lotions, silicones enhance spreadability and provide silky sensory properties.

Silicone's unique properties make it a versatile liquid additive across many industrial and consumer applications.

Conclusion

Liquid silicone rubber is a game-changing material for molding high performance elastomeric parts. With its facile room temperature injection molding process and rapid cure, LSR enables efficient mass production of intricate silicone parts. LSR offers an exceptional balance of physical properties from its high tear strength and flexibility to its thermal stability and electrical insulation. These characteristics make liquid silicone suitable for seals, medical devices, bakeware and many other applications across diverse industries. Understanding how to properly use and process LSR unlocks its full potential. With care in mold design, pre-processing, and curing, high quality silicone molded components can be consistently manufactured. As LSR continues gaining popularity, its uses and impact will certainly continue evolving.