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Injection molding of liquid silicone rubber (LSR) is a process to produce pliable, durable parts in high volume.
Liquid silicone rubber Silicone rubber is an elastomer (rubber-like material) composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formu ...
is a high purity platinum cured
silicone A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cookin ...
with low compression set, good stability and ability to resist extreme temperatures of heat and cold ideally suitable for production of parts, where high quality is required. Due to the
thermosetting In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...
nature of the material, liquid silicone
injection molding Injection moulding (U.S. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals (for ...
requires special treatment, such as intensive distributive mixing, while maintaining the material at a low temperature before it is pushed into the heated cavity and
vulcanized Vulcanization (British: Vulcanisation) is a range of processes for hardening rubbers. The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to inclu ...
. Chemically, silicone rubber is a family of
thermoset In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer ( resin). Curing is induced by heat or suitable radiation ...
elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and Elasticity (physics), elasticity) and with weak intermolecular forces, generally low Young's modulus and high Deformation (mechanics), failure strain compared with other mate ...
s that have a backbone of alternating silicon and oxygen atoms and
methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ...
or
vinyl Vinyl may refer to: Chemistry * Polyvinyl chloride (PVC), a particular vinyl polymer * Vinyl cation, a type of carbocation * Vinyl group, a broad class of organic molecules in chemistry * Vinyl polymer, a group of polymers derived from vinyl ...
side groups. Silicone rubbers constitute about 30% of the silicone family, making them the largest group of that family. Silicone rubbers maintain their mechanical properties over a wide range of temperatures and the presence of methyl-groups in silicone rubbers makes these materials extremely
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...
, making them suitable for electrical surface insulations. Typical applications for liquid silicone rubber are products that require high precision such as seals, sealing membranes, electric connectors, multi-pin connectors, infant products where smooth surfaces are desired, such as bottle nipples, medical applications as well as kitchen goods such as baking pans, spatulas, etc. Often, silicone rubber is overmolded onto other parts made of different plastics. For example, a silicone button face might be overmolded onto a
Nylon 6,6 Nylon 66 (loosely written nylon 6-6, nylon 6/6, nylon 6,6, or nylon 6:6) is a type of polyamide or nylon. It, and nylon 6, are the two most common for textile and plastic industries. Nylon 66 is made of two monomers each containing 6 carbon atoms ...
housing.


Equipment

In order for the liquid injection molding process to fully occur, several mechanical components must be in place. Typically, a molding machine requires a metered pumping device in conjunction with an injection unit—a dynamic or static mixer is attached. An integrated system can aid in precision and process efficiency. The critical components of a liquid injection molding machine include: Injectors. An injecting device is responsible for pressurizing the liquid silicone to aid in the injection of the material into the pumping section of the machine. Pressure and injection rate can be adjusted at the operator's discretion. Metering Units. Metering units pump the two primary liquid materials, the catalyst and the base forming silicone, ensuring that the two materials maintain a constant ratio while being simultaneously released. Supply Drums. Supply drums, also called plungers, serve as the primary containers for mixing materials. Both the supply drums and a container of pigment connect to the main pumping system. Mixers. A static or dynamic mixer combines materials after they exit the metering units. Once combined, pressure is used to drive the mixture into a designated mold. Nozzle. To facilitate the deposition of the mixture into the mold, a nozzle is used. Often, the nozzle features an automatic shut-off valve to help prevent leaking and overfilling the mold. Mold Clamp. A mold clamp secures the mold during the injection molding process, and opens the mold upon completion.


Characteristics of LSR

Biocompatibility: Under extensive testing, liquid silicone rubber has demonstrated superior compatibility with human tissue and body fluids. In comparison to other elastomers, LSR is resistant to bacteria growth and will not stain or corrode other materials. LSR is also tasteless and odorless and can be formulated to comply with stringent FDA requirements. The material can be sterilized via a variety of methods, including steam autoclaving, ethylene oxide (ETO), gamma, e-beam and numerous other techniques, meeting all required approvals such as BfR XV, FDA 21 CFR 177.2600, USP Class VI. Durable: LSR parts can withstand extreme temperatures, which makes them an ideal choice for components under the hood of cars and in close proximity to engines. Parts fabricated via liquid silicone rubber injection molding are fire retardant and will not melt. Chemical resistance: Liquid silicone rubber resists water, oxidation and some chemical solutions such as acids and alkali. Temperature resistance: Compared to other elastomers, silicone can withstand a wide range of high/low temperature extremes. Mechanical properties: LSR has good elongation, high tear and tensile strength, excellent flexibility and a hardness range of 5 to 80 Shore A. Electrical properties: LSR has excellent insulating properties, which offer an appealing option for a host of electrical applications. Compared to conventional insulating material, silicone can perform in far higher and lower temperatures. Transparency and pigmentation: LSR possesses a natural transparency. This attribute makes it possible to produce colorful, custom, molded products


Injection molding process

Liquid
silicone rubber Silicone rubber is an elastomer (rubber-like material) composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulation ...
s are supplied in barrels. Because of their low viscosity, these rubbers can be pumped through pipelines and tubes to the
vulcanization Vulcanization (British: Vulcanisation) is a range of processes for hardening rubbers. The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to includ ...
equipment. The two components are pumped through a static mixer by a
metering pump A metering pump moves a precise volume of liquid in a specified time period providing an accurate volumetric flow rate. Delivery of fluids in precise adjustable flow rates is sometimes called metering. The term "metering pump" is based on the ap ...
. One of the components contains the
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
, typically
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...
based. A coloring paste as well as other additives can also be added before the material enters the static mixer section. In the static mixer the components are well mixed and are transferred to the cooled metering section of the injection molding machine. The static mixer renders a very homogeneous material that results in products that are not only very consistent throughout the part, but also from part to part. This is in contrast to solid silicone rubber materials that are purchased pre-mixed and partially vulcanized. In contrast, hard silicone rubbers are processed by
transfer molding Transfer molding ( BrE: transfer moulding) is a manufacturing process in which casting material is forced into a mold. Transfer molding is different from compression molding in that the mold is enclosed rather than open to the fill plunger resu ...
and result in less material consistency and control, leading to higher part variability. Additionally, solid silicone rubber materials are processed at higher temperatures and require longer vulcanization times. Liquid silicone has a very low viscosity index and requires perfect seals of the mould cavity in order to guarantee a burr-free finished product. As injections are carried out at high temperature, steel dilation and natural shrinkage of materials must be considered at the design stage of the LSR injection tooling. From the metering section of the injection molding machine, the compound is pushed through cooled sprue and
runner Running is a method of terrestrial locomotion allowing humans and other animals to move rapidly on foot. Running is a type of gait characterized by an aerial phase in which all feet are above the ground (though there are exceptions). This is ...
systems into a heated cavity where the
vulcanization Vulcanization (British: Vulcanisation) is a range of processes for hardening rubbers. The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to includ ...
takes place. The cold runner and general cooling results in no loss of material in the feed lines. The cooling allows production of LSR parts with nearly zero material waste, eliminating trimming operations and yielding significant savings in material cost. Liquid silicone rubbers are supplied in a variety of containers, from tubes to 55 gallon drums. Because of their viscous nature, these liquids are pumped at high pressures (500 - 5000 psi) based on the durometer of the material. The raw materials are shipped in two separate containers (known in the industry as a kit) identified as "A" and B" compounds, with the "B" side usually containing the catalyst, but may vary based on the brand of silicone used. The two (A and B) compounds must be mixed in a 1 to 1 ratio, usually by way of a static mixer, adding pigment during the mixing process before the curing process begins. Once the two components come together the curing process begins immediately. A chiller supplying cold water to jacketed fittings is typically used to retard the curing process prior to the materials introduction to the mold. A color pigment can be added via a color injector used in conjunction with the material pump (closed loop metering system) before the material enters the static mixer section. In a cold deck scenario, the 1 to 1 mixed compound is pumped through cooled sprue and
runner Running is a method of terrestrial locomotion allowing humans and other animals to move rapidly on foot. Running is a type of gait characterized by an aerial phase in which all feet are above the ground (though there are exceptions). This is ...
systems into a heated cavity where the vulcanization takes place. The cold runner and general cooling results in minimal loss of material as the injection occurs directly into the part or cavity, saving on overall material costs and using high consistency rubber. The cooling allows production of LSR parts with nearly zero material valve gate waste, however this does not guarantee a "flash free" finished part. Molds and tooling are varying in design, execution and cost. A good cold runner is expensive as compared to conventional hot runner tooling, and has the potential to provide a high level of performance.


Advantages of liquid silicone injection molding

*Batches stability (ready-to-use material) *Process repeatability *Direct injection (no waste) *Short cycle time *‘Flashless’ technology (no burrs) *Automated process *Automated demolding systems


References


Further reading

*{{cite book , author=Osswald, E. Baur, title=International Plastics Handbook , publisher=Hanser Publishers , year=2006 , isbn=1-56990-399-9, display-authors=etal Elastomers Plastics industry Rubber Silicon Injection molding