From this article, you will learn what silicone rubber molding is, its products, and their uses.
You will learn:
Silicone rubber molding is a method for shaping, forming, and fabricating silicone rubber parts and products using a heated mold. The process involves compressing or injecting silicone rubber into a mold where it takes the shape and form of the mold cavity. The processes used to produce silicone rubber molded parts are injection, compression, and transfer molding each of which relies on a mold made of steel or cast iron.
The various forms of silicone rubber vary in their hardness and pliability, which makes the material an ideal choice for applications that require durability and flexibility with resistance to high and low temperatures and wear. In many applications, silicone rubber is used in place of other forms of rubber due to its ability to withstand temperatures above 100°C (212°F), a characteristic that makes it ideal for use as medical instruments and seals and gaskets for engines.
The initial purpose for silicone rubbers was to insulate generators and electric motors. As the many positive properties of silicone rubber were discovered, it evolved into a common household and industrial material that can serve nearly any purpose. The choice of the correct type of silicone rubber can enhance the performance of a product or part. Its heat resistance, which is one of its major properties, is due to its combination of silicone, hydrogen, carbon, and oxygen.
Silicone rubber is a modern elastomer that has properties that differentiates it from organic elastomers such as natural rubber, latex, and polyurethane. It is made from silicone and oxygen, which are two of the most common elements on earth and exist as silica and silicate. The silicon base for silicone rubber was isolated in the early 19th century but did not find commercial use until the middle of the 20th century.
When silicone rubber was first synthesized, it was believed that it did not have any practical or commercial use. In the 20th century, electric motors and generators were a common part of industrial operations. Since they generated a great deal of heat, they need to be covered in a heat resistant material. At the time, resin impregnated glass fibers seemed to be the solution. Unfortunately, the resin material could not endure the heat from small motors.
In the search for a heat resistant material, scientists synthesized the first silicone polymer that could withstand the heat of the motors and generators. The discovery led to the production of silicone rubber, which was initiated by Dow Chemical and Dow Corning in 1943. As engineers studied the material, they discovered its broader use for other products.
Part of the process of classifying silicone rubber involves the use of the Shore hardness scale, which measures the resistance of a material to deform under pressure using a durometer. Higher numbers on the durometer indicate the resistance of a material to indentation. Shore scales are divided into Shore A, Shore D, and Shore 00 with Shore 00 being used to measure the hardness of very soft materials. Each shore scale has a range of 0 to 100 with 0 representing soft, non-metallic materials and 100 being the hardest.
The properties of silicone rubbers, such as thermal conductivity, fire resistance, chemical stability, flame retardancy, and heat and cold resistance, can be adjusted by changing the base polymer by adding reinforcing fillers and additives. The purpose of such changes is to strengthen silicone rubber to meet the needs of an application. The classification of silicone rubbers is based on their properties.
General purpose silicone rubber has a Shore hardness of A 30 to A 70 and is used to make common products such as kitchen utensils and kitchenware. It can be used for compression molding and extrusion of a wide range of products including tubing and vehicle parts.
Low hardness silicone rubber is the softest of the classifications with a Shore hardness between A 5 and A 20. It is used to produce baby bottle nipples, food storage film, anti-slip mats, and shoe insoles. The wide use of low hardness silicone rubber is due to its stretchiness, which makes it possible for the material to be pulled and reformed to several times its original size without tearing.
High hardness silicone rubber has a Shore hardness up to A 90 after being cured. It has high elongation and tear strength and is used for molding, extrusion, and calendaring to make products that are abrasion and fatigue resistant. Products produced using high hardness silicone rubber can be transparent or translucent with exceptionally high tensile strength.
High strength silicone rubber is designed to be able to withstand extreme elongation and tearing and is highly resistant to abrasion and fatigue. As with high hardness silicone rubber, high strength silicone rubber is used for molding, extrusion, and calendaring. It has a tear strength of 29 to 49 kN/m compared to a tear strength of 9.8 kN/m for general silicone rubber.
The most common versions of silicone rubber are red, black, or gray, which are available in a wide variety of forms and shapes. Clear sheets of transparent or clear silicone rubber are also used for liquid injection molding. Transparent silicone rubber has Shore hardness of A 10 to A 70 with Shore A 40 on the durometer being the most common type. The transparency of clear silicone rubber makes it possible to use it as an invisible gasket that does not interfere in the aesthetic appearance of a cosmetic product. A common use for transparent silicone rubber is as air flow diaphragms on ventilators and respirators.
Flame retardant silicone rubber is designed with high thermal conductivity and heat resistance. It is used in applications that involve fire or direct heat contact and is self-extinguishing. The production of flame-retardant silicone rubber requires the blending of a special set of compounds in specific quantities and special curing. The property of flame resistance is common to all forms of silicone rubbers, which is enhanced in flame resistant silicone rubber by the addition of blockers.
Thermal conductive silicone rubber moves heat away from sensitive areas and is used with CPUs and electronic enclosures. The rapid development of smaller electronic components has necessitated the implementation of methods to remove the excess heat they generate without damaging their electrical functions. Since silicone rubber can be produced in a wide variety of thicknesses and forms, it is the ideal material for dissipating heat in small electronic devices.
Electrically conductive silicone rubber has all of the normal properties of silicone rubber but is compounded with carbon and other materials that are electrically conductive. It is used to reduce or eliminate electromagnetic interference (EMI) and radio frequency interference (RFI) noise that is found in various electronics. Electrically conductive silicone rubber is referred to as metallized silicone, which refers to the various metals that are placed in the rubber such as Monel or aluminum wire.
One of the properties of silicone rubber that makes it applicable to a wide range of processes is its thermal conductivity, which makes it possible to transfer heat at a very low rate, a factor that makes it heat resistant. There is no change in its properties at 150°C (302°F). The heat resistance of silicone rubber is due to the stability of its chemical structure, which is based on its siloxane bonds that are tightly bound and exceptionally stable.
The polymer structure of oil resistant silicone rubber is unaffected by oils and maintains its structure after being exposed to oils. The high temperature resistance of silicone rubber combined with its oil resistance makes it ideal for use in applications that involve and require the use of oil.
Fluorosilicone rubber has the same structure as silicone rubber with fluorine added to its polymer chain. It combines the best properties of fluorocarbons and silicones and is solvent and oil resistant, able to withstand extremely low temperatures with high temperature stability, and will not deform when placed under pressure. As with other forms of silicone rubber, fluorosilicone rubber can be molded, extruded, and calendered and is widely used in aerospace and automotive applications. The main characteristic of fluorosilicone rubber, compared to regular silicone rubber, is its exceptional resistance to the effects of fuels, oils, and chemicals, which makes it more expensive.
Organic groups of silicone rubber are classified by specific materials, which include methyl, vinyl, phenyl, and fluorine. ASTM D1418 standards cover the terminology used to describe the different classifications.
Silicone, known as polysiloxane, is a polymer that has a rubber-like consistency with molecules having chains of oxygen and silicon atoms. It is different from other forms of polymers in that it does not have carbon as its backbone. The primary ingredient of silicone is silica, which is found in sand. Silicone differs from silicon in that it is flexible, softer, and resistant to the effects of extreme heat.
The manufacture of silicone begins by isolating silicon from silica or silicates, which is achieved by heating quartz sand to 1800°C (3272°F). The silica sand comes from crushing, grinding, and washing sandstone or quartzite to achieve the appropriate fine grain distribution. The pure silicon is reacted with methyl chloride to form dimethyldichlorosilane (DMDCS) that is hydrolyzed and vigorously stirred to form polydimethylsiloxane (PDMS) chains.
The PDMS is cross-linked using heat or radiation to create bonds between the PDMS chains, the result of which is a three-dimensional network structure. The various types of silicone are produced by adding phenyl, methyl, fluorine, or vinyl groups to the PDMS chains during its synthesis. Each of the different groups gives the silicone rubber different properties and characteristics. The completed compound can be molded or extruded to create different shapes.
The final step in the production of silicone rubber involves determining the final form to be used for the production of various types of rubber products. This is achieved by adding fillers and additives to enhance specific properties and add color. The steps in its manipulation are designed to produce the properties for the industry where it will be used.
Silicone rubber additives:
Silicone rubber is shaped, formed, and configured using various methods depending on the type of product or part that is being manufactured. Once shaped, the rubber product undergoes vulcanization, which involves a curing agent that heats the product to cross link the polymer chains. High temperature vulcanization (HTV) is completed at 150°C up to 200°C (302°F up to 392°F) while room temperature vulcanization (RTV) is completed at room temperature
HTV vulcanization is completed in a heat press for several minutes and is used to produce high strength silicone rubbers for industrial use. RTV curing happens rather slowly using a platinum catalyst, about 24 hours, and is used to produce soft, pliable silicone rubber products that are very soft or medium soft.
A special use for silicone rubber is the formation of silicone molds used for mold making using various materials including resins, gypsum, wax, and other materials. Silicone molds make it easy to create custom products and designs using silicone molds that are durable, long lasting, and have exceptional repeatability. The inorganic nature of silicone rubber enables it to resist the effects of temperature changes, exposure to chemicals, and natural contaminants such as bacteria and fungus.
A common use for silicone molding is the creation of prototypes in order to examine a product design. Since the creation of metal molds is time consuming, labor intensive, and costly, silicone molds provide a cost effective alternative. Aside from their use in prototyping, they assist in the creation of beta units for market testing and consumer input. Although 3D printing is ideal for quickly creating one off samples, silicone molds are capable of short production runs for testing.
One of the reasons that manufacturers choose silicone molding is that it is possible to quickly and easily create a mold of a component without the need for machining or special tools. The initial decision is whether the mold will be one piece or two pieces. The selection is determined by the design of the master with two part molds taking longer to create.
The one part open face mold is the simplest form of silicone rubber mold. It is ideal for flat back masters without undercuts and is good for molding rigid materials like ceramics and silicone rubber.
The beginning steps for the creation of a two part silicone mold are the same as those for a one part silicone mold, which is the creation and selection of the master.
The use of silicone rubber is an ideal solution for mold making due to the resilience and strength of silicone rubber. Rubber molds made of silicone can be used multiple times to produce parts, components, and decorative items. The rubber mold making process is quick, easy, and can be completed quickly to fit customer requirements, which is essential in modern industry.
With two part silicone rubber molds, the tight seal between the halves limits the creation of a parting line in the casting process. One part silicone molds, due to their structure, do not have the potential for any form of parting line. In both cases, the created molds have all the properties required for the creation of high quality castings.
Silicone rubber is an essential part of a wide range of products and is manufactured using a variety of methods, each of which is capable of producing parts that meet the expectations and requirements of an application. The choice of fabrication method varies between molding processes, extrusion, and calendaring with each method able to produce parts with the correct hardness, heat resistance, and electrical conductivity.
The extrusion of silicone rubber parts includes heating and compressing the raw silicone material through a die that has the cross-sectional shape of the final product. The raw silicone rubber is loaded into a hopper that is connected to the barrel of the extruder. The barrel contains one or more screws that move the raw material along the barrel to the die. During the movement toward the die, the raw material is heated, melts, and is compressed. As the material is forced through the die, it takes the desired shape after which it is cooled, cured, and cut to the desired lengths.
Extrusion takes many forms including film and sheet extrusion, each of which is used to achieve different benefits. The extrusion process is low cost, allows for high volume production runs, and is ideal for producing tubing, gaskets, seals, and wire insulation.
Calendaring is used to produce thin sheets of silicone rubber through the use of rollers or bowls with each roller rotating in the opposite direction of its opposing roll at an even and controlled speed. One roll in each pair has a nip adjustment to control the thickness of the silicone rubber material. A silicone rubber calendar can contain two or more rolls. The silicone rubber material is heated and fed into the rolls in strip or pig form on one side of the nip and is squeezed to form sheets of silicone rubber.
Injection molding is the most common process for forming silicone rubber parts that vary in size, complexity, and final application. The process involves the use of a screw type plunger that is similar to that of an extruder. As with an extruder, the raw silicone material is loaded into a hopper connected to the barrel and screw. The material is heated as it moves along the barrel toward a tightly sealed die that has the shape of the final part. The pressure created by the movement of the screw forces the raw material into every portion of the die.
The shot, the amount of material injected into the die, increases in density as the pressure of the injection machine rises and fills the die cavity. The key to the increasing pressure is the slow movement of the screw that helps to sustain and maintain continuous, even pressure. The silicone rubber cools during injection, which accelerates the solidification process and makes up 80% of the molding cycle.
Liquid injection molding involves the use of liquid silicone rubber that is injected into the mold cavity by a nozzle. The difference between injection molding and liquid injection molding is the mechanical mixing of the silicone rubber material prior to it being injected into the mold, which is completed by two plungers. One of the plungers contains the base material reinforced with additives and fibers. The second plunger has the catalyst compound that activates the mixing.
The advantages of liquid injection molding include shorter curing time, the ability to produce intricate and complex designs with extremely close tolerances, high volume production, automated process design, and the use of thermal imaging to prevent molding errors.
Compression molding includes the use of heated molds that contain the precise amount of silicone rubber, referred to as the charge, to form the final product. A heated plunger that has the upper half of the mold attached to it applies 15,000 psi to 20,000 psi of pressure to drive the silicone material to every corner of the upper and lower halves of the mold cavity. The pressure is maintained until the silicone rubber component completely cures.
The major benefit of compression molding is its cost, which makes it ideal for producing low volume production runs. The process is capable of producing small, minute parts up to large and complex ones with a short set up time.
Transfer molding involves the use of a mold, plunger injection machine, and silicone rubber material. The process begins by placing the raw silicone rubber material or charge into the transfer pot that is below the plunger and above the mold. Under intense hydraulic pressure, the plunger forces the silicone rubber through a small hole or sprue at the top of the mold into the mold cavity where it is tightly held and allowed to cure.
The process of transfer molding is very precise and produces components with less flash and waste. Transfer molding is able to produce parts with small tolerances and complex shapes with tighter control of dimensional tolerances. Although it has similarities to compression molding in that it uses compression to mold parts, the mold cavity is not divided into halves, and the raw silicone rubber material charge is located in the pot and not the lower half of the mold.
The versatility and workability of silicone rubber makes it one of the most widely used industrial and commercial materials. Along with its many positive properties, silicone rubber is a long-lasting, durable material that seldom needs to be replaced. It is used for kitchen utensils and toys as well as gaskets, seals, and automotive parts.
Silicone rubber is quite commonly used as a wire and cable insulator, connector seal, and switch boot. Its high temperature resistance and ability to withstand extreme weather conditions makes it an ideal insulation material. Silicone rubber insulation remains flexible, waterproof, and resistant to arcing and cracking even after several years of use. The strength and durability of silicone rubber insulation is capable of protecting the most sensitive components.
Hoses, gaskets, seals, and other vehicle components are essential for the safe and continued performance of a vehicle. They have to be able to endure the harsh treatment they have to withstand during the operation of a vehicle’s engine. It is for these reasons that silicone rubber is the material that is used to produce the most critical components of an engine. It has the strength, toughness, and impact resistance required and is long lasting.
Parts and components for use in aerospace applications have to be exceptionally durable and capable of withstanding constant abuse. Critical factors in aerospace manufacturing are the ability to endure radical changes in temperature, radiation, and routine exposure to various chemicals. The high performance of silicone rubber makes it perfectly suited for the needs of aerospace conditions.
The food and drug administration has established stringent guidelines in regard to the types of materials that can be used to produce medical equipment due to the sensitive nature of patient care. Included in the regulations for materials are being chemically inert, biocompatible, and being able to be sterilized. Unlike materials for airplanes and vehicles, medical devices have to perform in sanitary conditions and be in constant use, such as catheters and tubing.
Silicone rubber is an ideal material for medical applications due to its flexibility, transparency, and lubricity. Aside from being used for tubing and protection of wounds, it is used for orthopedic padding and other devices that come in contact with the body. Silicone rubber parts are implanted in the body as body part replacements due to their durability and biocompatibility.
Silicone rubber is used in the production of food and cookware due to it being non-toxic and food safe. As a cooking material, it satisfies the standards required by the food and drug administration (FDA) and can be made in aesthetically pleasing colors. Silicone rubber is widely used in bakeware since it does not leach and damage the taste or odor of the food being prepared.
Utensils made of silicone rubber are non-sticking and easy to clean. When used in the preparation of food, they require less oil, which makes the food healthier and more nutritious. The many shapes of silicone cookware, whether it is baking pans or mixing bowls, are dishwasher safe or can be washed by hand without being scratched or damaged.
The unique molecular structure of silicone means that it can be used as a liquid, solid, semi-viscous paste, oil, grease, and rubber. The versatility of silicone rubber makes it possible to use it in several environments including as a pleasing visual and aesthetically pleasing addition.
Silicone rubber offers exceptional high temperature resistance, which is one of the reasons that it is so widely used in industrial equipment as gaskets, seals, and plugs. It is capable of withstanding high and low temperature changes without any changes to its properties. For long periods, silicone rubber maintains its structure at 200deg;C (392deg;F) and can withstand temperatures over 350deg;C (662deg;F) for short periods of time.
Along with its resistance to high temperatures, silicone rubber can endure cold temperatures without becoming brittle or cracking. It retains its elasticity at -70deg;C (-94deg;F), which are temperatures that cause organic rubbers to become brittle. Silicone rubber retains its properties and structure over a wide range of temperatures in any conditions.
Silicone rubber is non-toxic and easy to sterilize. It is non-porous, which makes it impossible for bacteria or viruses to build up and grow. This aspect of silicone rubber makes it ideal for the manufacture of medical equipment, food processing devices, and cookware. When silicone items are cleaned, solvents easily remove any surface contamination. Additionally, the sanitary property of silicone rubber makes it the ideal material for use in cleanrooms and environments that require a high standard of cleanliness.
Silicone rubber does not experience any change in its properties when exposed to oils, solvents, and assorted other chemicals. When it is submerged in water, it retains its mechanical strength and electrical properties. It is used in motors due to its ability to resist the effects of oil at high temperatures and is unaffected by polar organic compounds and dilute acids.
UL 94 HB is a test for the horizontal burning rate of a substance or material that has been released by Underwriters Laboratories (UL). It is used to classify materials as to how they burn in different orientations and thicknesses. The six classifications of materials range from 5VA to HB with HB being the slowest burning rate. Silicone rubber has a UL 94 HB rating. When it burns, it produces limited black smoke and no noxious gases.
Any material that is chemically or physiologically inert does not react when exposed to other materials and refers to the arrangement of electrons on a materials outermost shell. This unique property of a group of materials is referred to as group 0 because of their zero reactivity. Silicone rubber falls into this group because of its nonreactivity when exposed to other materials. It is for this reason that it is used with living tissue and is biocompatible.
The most important advantage of silicone rubber is its versatility. By adjusting the elements used in its manufacture or changing the curing agent, silicone rubber can have a lower compression set, be more resistant to flames, water, steam, and temperature changes. It is manufactured translucent and colored and is a dependable, reliable, and durable material that lasts for decades without losing its properties.
As with many forms of industrial equipment, the top suppliers of silicone molding machines are located in Asia. The various manufacturers produce a wide range of silicone manufacturing equipment that include injection and compression molding designs.
The JD-RL Series is an injection molding machine capable of molding all forms of rubber, synthetic or organic, using a low bed structure. A PLC comes standard with microcomputers being optional. JD-RL machines have direct and indirect clamping with a special nozzle design that prevents overflow. Although many features of the JD-RL Series come standard, it can be adjusted and configured to fit specialized and unique requirements.
The KTVK 85-300T is designed for flexible and adaptive production. It is specially engineered for silicone injection molding and used for large, high-volume production. The KTVK 85-300T is described as a compression injection molding machine. It offers all the features necessary for a company to manufacture HTV silicone using compression and includes a LSR dosing system. HTV molds can be used with LSR for increased efficiency.
The YL3-V440L is a fully automated molding machine that has new core technology for more precision and reliability. It has a smart operating system with less maintenance and easy use. The YL3-V440L provides comprehensive data support for modern technologically based manufacturing. It has a L3R injection system with a D1 clamping structure and ID card reader that can be set for different levels of permission.
The L90 – 160T is capable of molding liquid silicone rubber and silicone rubber. It has an energy saving capacity of 58 tons up to 3800 tons and clamping force of 1600 kN. A unique feature of the L90 – 160T is its ability to mold LSR materials of differing hardness with the same efficiency and quality. The range of products produced by the L90 – 160T includes toothbrushes, oxygen masks, catheters, mobile phone cases, and valves and seals.
The Allrounder Hidrive hybrid injection molding machines from Arburg are technically advanced to provide exceptional performance. Their servo-electric clamping unit and hydraulic injection unit use hydraulic accumulator technology that automatically adjusts the pressure level to meet the requirements of a product. The self-adjusting hydraulic accumulator is designed to save on energy as well as ensure product quality. Along with its high efficiency, the Allrounder Hidrive injection system has been engineered for maximum energy savings to make it a cost saving production solution.
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