Rubber Injection Molding

Chapter 1: Principle of Rubber Injection Molding

This chapter will discuss the definition of rubber injection molding, the types of rubber used, and how the rubber is made.

What is Rubber Injection Molding?

Rubber injection molding is when uncured rubber is transformed into a usable product by injecting raw rubber material into a mold cavity made of metal. The applied pressure produces a chemical reaction like vulcanization or curing, which causes the cross-linking of the polymer chains inside the material.

For the manufacturers to be able to come up with rubber products, they make use of pressure and heat. With the continuous evolution of the products, rubber mold producers are increasingly adjusting to meet the expanding demand for gaskets, hoses, O-rings, tubing, and grommets.

How Rubber Moldings Are Made

The components and products of rubber are shaped, formed, and produced by the molding process. The molding process is used instead of shaping the parts by hand. This is to subject the material to chemical processes that harden or toughen the chains of the polymers that are inside the raw rubber.

The effects of the chemical reaction force the long polymer chains to cross-link and thus prevent them from independent movement. The rubber material expands or contracts as the stress is removed or applied. In many cases, heat is used to reduce or minimize the curing time. The rubber can be made into complex designs, shapes, and geometries.

During the initial design process, there is a consideration of configurations in the geometry of the rubber. These are used to make the production of the end product less expensive and more economical. During the part design and molding process, the engineers have to consider the parting line and its location. This is because it needs to have a minimum draft angle. The design of the mold is considered the foundation of part development. The design of the mold can also determine the failure or success of the production parts.

Once the mold is completed, it is transformed into a mold of metal. The production and the manufacturing of these molds involve the use of some forms of aluminum, hardened steel, or beryllium-hardened copper. The most-used material in making these molds is steel because it is long-lasting, making it a choice that is expensive at first but economical in the long run. The manufacturing of the mold is helped by making use of specific programs on computers that precisely make molds that perfectly match the parameters of the design.

The use of the computers helps to reduce the time taken to produce the mold meant for the production process. When the mold has been made and cast, it undergoes testing for dimensional accuracy as well as to see if there is compliance with the original design.

Injection Molding

The injection molding process is mostly used when there is a need to produce an end product with requirements involving tight tolerances with very high accuracy and precision. First, the process of injecting involves the heating of the raw rubber; the rubber is heated to form a liquid, and this liquid is made to flow more readily and freely into the cavity of the mold. The gum stock is a phrase that refers to the consistency of the rubber.

The preheating process reduces the curing time, making it possible for higher curing temperatures. The nature of the injection molding has a requirement that the mold must be clamped tightly, which helps to limit or to avoid flashing. The rubber used for injection molding is made available to the mold through a barrel and nozzle assembly that provides continuous flow.

The rubber enters the mold cavity by a gate after it has been fed through a system of runners and sprues. Afterward, the liquefied rubber fills the mold cavity. It then packs very tightly to the walls of the mold to form the specific shape that is desired.

Chapter 2: Types of Rubber & Processes in Rubber Injection Molding

This chapter will discuss the types of rubber used in injection molding and the types of rubber injection molding processes.

Types of Rubber Used In Rubber Injection Molding

The types of rubber used in rubber injection molding include:

Natural Rubber

Natural rubber is a type of rubber that comes from latex, which is produced by rubber-producing trees. Its tensile strength is very high, and it has friction and abrasion properties that make it an appropriate material for seals, O-rings, diaphragms, dampers, and bumpers. Natural rubber can be used as a coating because it bonds very well with metals.

Nitrile or Buna-N

Nitrile rubber is a product of butadiene and acrylonitrile. The manufacturing process of nitrile rubber involves the use of an emulsifier process: the acrylonitrile, a butadiene monomer, and an activator are brought together and mixed with a catalyst in a polymerization container. The polymerization container has to have a temperature of about 104 °F (40 °C) for the different compounds to be able to mix well.

The mechanical properties of nitrile rubber are excellent, and nitrile rubber is one of the most used rubbers in the process of rubber molding because it has a very high resistance to various fluids, oil, solvents, and water. Another property of nitrile is that it has a high resistance to abrasion and tearing.

Hydrogenated Nitrile Butadiene

The hydrogenation of NBR is what manufactures hydrogenated nitrile butadiene. It is responsible for removing the olefinic, which is very vulnerable to degradation. The hydrogenated nitrile butadiene rubber is very resistant to petroleum-based oils. They are also resistant to aliphatic hydrocarbons, silicone oils, petroleum fuels, vegetable oils, salt solutions, greases, water, dilute acids, ethylene glycol, bases, and steam. They often find applications in the automotive industry.

Ethylene Propylene Diene Monomer

This type of rubber is a combination of propylene, ethylene, and diene comonomer that allows room for cross-linking. The structure of the ethylene propylene diene monomer enables it to be much more resistant to light, ozone, and heat, and it is also capable of withstanding high temperatures. It is very durable and lasts for a long time in an application. It’s found in steam systems, panels on cars, braking systems, and trucks.

Silicone

Silicone rubber is a rubber-like material made up of silicon combined with oxygen, carbon, and hydrogen. The phrase “silicone rubber” is generally used to cover the various silicone rubbers separated by their chemical structures and organic classification.

There are various types of silicone rubbers, and these include methyl, fluoro vinyl methyl, methyl phenyl, methyl phenyl vinyl, and methyl vinyl. The silicone rubbers have high resistance and can cope with extremely high temperatures. They are also flexible and are widely used for their weather resistance.

Fluorosilicone

Fluorosilicone is a rubber type that is a variation of silicone. It contains trifluoropropyl. This enables it to enhance such high resistance to furls, chemicals, oils, non-polar solvents, alkalines, and solvents. Just like silicone, fluorosilicone rubber is highly adaptable across different temperatures. Its application is found in the aerospace industry. The fluorosilicone rubber has a high resistance to petroleum products.

Neoprene

Another name for neoprene rubber is polychloroprene or chloroprene. It is manufactured by the process of emulsion polymerization of chloroprene. The process is aided by the use of a potassium catalyst. Neoprene is used for multiple purposes. Its properties include fire, weather, temperature, and abrasion resistance. Its uses include gaskets, various forms of coating, and hoses. Neoprene is ideal for use in the molding process of rubber due to its versatility, and this does not consider the type of molding being used.

Styrene-Butadiene

Styrene-butadiene rubber is made mostly from butadiene and a very small fraction of styrene. They undergo copolymerization by the use of the emulsion process. The rubber has exceptional aging, abrasion, and crack resistance. The rubber has numerous positive characteristics, and this has made it ideal for the rubber molding of seals, gaskets, and diaphragms.

Fluorocarbon

Fluorocarbon is a rubber that is made from Viton or fluororubber. They are classified by their fluorine content, chemical composition, or by method of cross-linking. The grouping of the fluorocarbon rubber ranges from one to five. Each of the groups has a unique type of copolymer. For the fluorocarbons, there are three categories of materials for cross-linking. These cross-linking types are diamine, peroxide, and dihydroxy. The fluorocarbon rubber has characteristics that make it excellent for resisting chemicals and very high and extreme temperatures. This makes the rubber much more ideal for molding fuel lines, hoses, and O-rings.

Butyl or Isobutylene Isoprene

Butyl rubber is produced by the copolymerization of isobutylene together with isoprene. These two compounds originate from crude oil or natural gas. In order for the two materials to be combined, they have to be cooled to very low temperatures of about -238 °F (-150 °C). The butyl rubber is not permeable by gas. This makes it ideal for the manufacture of seals which are used for high pressure and vacuum uses. The butyl rubber consists of sound dampening qualities, which can be employed as a bumper for different equipment.

Urethane

Urethane rubber is also called polyurethane rubber. It is manufactured by the reaction of a polyol with polymeric isocyanate or diisocyanate together with a catalyst. There are many types of polyols and diisocyanates; therefore, various types of urethane rubber materials can be made from the various combinations of the compounds. Their application is found where there is a high need for materials with high tensile strength or materials with good abrasion resistance.

Types of Rubber Injection Molding Processes

Many specs are used to determine the kind of molding process needed or required for a specific application. The less expensive solutions for the molded rubber depend on the right fit for the weight, size, geometry, and material being used. The injection molding process has become a very popular and constantly used procedure. The injection molding process has its roots in plastics. The engineers were able to manipulate rubber with much better accuracy, and they came up with offshoots of the injection molding process discussed briefly below. These offshoots are organic rubber injection, liquid injection molding, and thermoplastic rubber injection.

Organic Rubber Injection

The process of organic rubber injection relies greatly on the preparation that is done on the materials. After the rubber is stripped and mixed, it enters a screw in measured quantities. When there is enough quantity of the rubber in the barrel, it is driven inside the cavities of the mold, where afterward it cures. An advantage of this method is that it removes the need for operators to put pre-forms, and it preheats the rubber. This has the effect of increasing the time cycle.

Liquid Injection Molding or LSR Injection

The LSR injection is the second type of injection molding. The process involves a compound of silicone in liquid form being placed into a mixer. Afterward, the mixer is cured and placed inside an injection unit. The flow of the prepared rubber compound is directed by the runners and gates, which are built into the mold cavity. All of the material parts of the rubber cure when they are inside the mold. Afterward, they are ejected before the next molding process initiates. An outstanding advantage of this process is that there is a limitation to contamination by the use of closed systems. Another benefit is that the cycle times are accelerated greatly. Mostly the manufacturer of products used in the medical industry benefits from and makes use of liquid injection molding for rubber injection.

Thermoplastic Rubber Injection

Thermoplastic rubber injection is the third type of rubber injection. The procedure involves the processing of elastomers like plastics. They have performances that are similar to those of rubbers. The procedure for injecting molding is also the same as that of plastic injection molding. There is an elimination of the effort and time of mixing or of vulcanizing if the actual rubber material is not used. The thermoplastic elastomers are capable of being recycled, and they can take different colors meaning they can be colored inside an injection mold where they work. However, they have the disadvantage of being only useful in applications that require high-temperature uses. The mentioned offshoots of injection molding are very helpful and available as standard application processes. There are options available for custom rubber injection, which are made to meet the specified requirements perfectly.

Custom Rubber Injection Molding

Various manufacturers and businesses have different needs for the parts. The issue that most engineers try to resolve is to innovate and come up with the desired product at a lower cost and at its highest efficiency. Mostly the solution is a custom-made product. Thus, the concept of custom rubber injection molding came into place.

Chapter 3: Applications, Uses, and Advantages of Rubber Injection Molding

The injected rubber, natural or synthetic, can be molded and shaped into many forms, configurations, and shapes, which help to enhance its usefulness. The flexibility and durability of rubber allow the engineers to come up with different ways for geometries and cushioning components. The stability of rubber is made useful in procedures for cushioning machinery so as to minimize vibrations and impact.

Although natural rubber is mostly used, the manufacturers were able to come up with synthetic versions that consisted of the same strength as that of natural rubber with much greater longevity and durability added.

Applications and Uses for Rubber Injection Molding

The different applications and uses of rubber injection molding include:

Rubber Bumpers

Rubber bumpers come in many shapes and sizes. These are used in the manufacture of certain products like stem, mushroom, tack bumpers, recessed, and grommet. The rubber bumpers minimize the vibration as well as the movement inside the equipment. This protects the parts which are rigid from damage that is caused by the impact and vibrations.

Rubber bumpers offer protection on work surfaces, circuit boards, furniture, displays, and packaging. The type of material used to manufacture the rubber bumpers includes many polymers resulting from the mixture of plastics and rubber.

Rubber Bushings

Rubber bushings are similar to rubber bumpers due to the fact that they are made in such a way that they absorb most of the vibrations and impact on the machinery. Their shape and configurations are made in such a way that they fit specific applications; however, the most commonly used form is a cylinder. The use of rubber bushings is in machines where they minimize vibrations.

They can also be placed between two pieces of equipment in order to stop the transfer of vibration from one end to the other. One particular application of the rubber bushings is found in automobiles, bicycles, trucks, and motorcycles, where they contain the built-up vibrations from roads with deformities.

Rubber Diaphragms

Rubber diaphragms are materials that are mostly used to prevent friction or leakage. Their design and geometry are in such a way that they can withstand the effects of pressure while at the same time preventing any form of leakage of the liquid.

Rubber diaphragms are also defined as barriers between two chambers or enclosures that prevent contaminants from entering. In form, they can be dynamic or static. A dynamic rubber diaphragm can be activated by a very slight amount of pressure.

Rubber Grommets

Rubber grommets are ring-shaped materials put around metal openings to protect the materials or objects that pass through the opening. One normal use for rubber grommets is found in openings where a wire passes through. Rubber grommets are made up of natural rubber or synthetic rubber.

They are manufactured and produced by various types of molding processes. Since the use of electrical cables meets a challenge of vulnerability to abrasions from the surface of a metal opening, rubber grommets prevent the short-out of the wiring and protect the wire.

Rubber Isolators

Rubber isolators are made use of in the industry mostly. They protect machines and appliances from the damage that is caused by the vibration. They are also responsible for keeping the machinery stationery at its position and level. Rubber isolators include mounts, bumpers, leveling pads, bushings, and shock mounts. Controlling the vibrations is considered a very important aspect of the operation of equipment.

The reason is that vibration can lead to the malfunction of the equipment and the production of products that are of very poor quality. If not checked or uncontrolled, the vibrations will result in unnecessary damage and wear. Rubber vibration isolators are commonly used on residential air conditioning devices. Their use is to suppress the noise that is made during the operation of the unit.

Rubber Mounts

Rubber mounts are closely associated with the above-mentioned isolators. Rubber mounts consist of two parts which are the isolator and the fastening mechanism. The fastener used is in the form of threaded metal. The materials that make up the isolator are natural rubber or synthetic rubber. Just like the rubber isolators, rubber mounts come in many different shapes, designs, and sizes.

However, the majority of the rubber mounts are custom made in order to fit a specific requirement or application. For transfer equipment and conveying lines, cylindrical mounts are made use of as supports. The other rubber mounts serve as supports for tabletop equipment like printers and computers. Rubber mounts have a very simple design, making them easy to maintain and install. The metal threads are made in such a way that they quickly fasten to the unit being protected and supported.

Rubber Plugs

Rubber plugs are designed in a way that they fit their specific use. Usually, the rubber plugs are used for equipment that requires their edges and joints to be covered. In special situations, rubber plugs cover up wires that are part of a unit’s intake wiring. The rubber plugs come in various types, and some include the connector, the T, the plus, and the washer. The main use of the plus rubber plug is to hold the equipment together. On the other hand, the T rubber plug is made to be put into many types of tubing.

The washer rubber plugs have a similar function as the T plug, but they extend to cover the whole area surrounding the hole where they are put. Lastly, the connector rubber plugs are used for the protection of the female threaded connectors from contamination by dust or by moisture.

Rubber Seals

The use of rubber seals protects surfaces, seals gaps, and insulates all the spaces from the liquids or water. The sealing material is an essential material for many equipment and processes; therefore, it can be customized easily. They can be fitted and shaped so that they are installed as a single unit in many types of equipment being used.

One important use of rubber seals is found in a process involving the use of pressurized gasses or liquids. Rubber seals come in various forms; for example, they can be a gel, strip, putty, or film and be able to stick and adhere to ceramics, paper, glass, concrete, porous surfaces, and many other different materials.

Rubber Pads

The use of rubber pads is the same as that of bumpers, isolators, and mounts: to suppress the noise being produced by the machine and control the vibrations. Rubber pads are designed specially to insulate the machinery to protect the equipment against any possible damage that can be encountered, minimize the noise produced by the equipment unit, and control the vibrations as much as possible. The physical properties of rubber pads are that they are very versatile, and therefore they can be made use of with very small pieces of equipment, like tabletop mixers. Rubber pads can be used with larger machines like power generators. Rubber pads function as an economical and very efficient method of suppressing noise in factories involving many pieces of equipment. The rubber pads' materials are usually forms of synthetic rubbers like nitrile rubber, neoprene, and polychloroprene. An added advantage besides the fact that rubber pads have a very low cost is that they are very easy to maintain.

Rubber Suction Cups

Rubber suction cups are also called vacuum cups. Their use is as a handling system for manipulating items involving a flat surface like metal sheets, glass, tubes, wood paneling, or molded pieces. Rubber suction cups are available in many shapes, sizes, diameters, and types of rubber material. There are two main types of rubber suction cups, the bellows and the flat.

The flat rubber suction cups are used mostly for flat or plain surfaces. On the other hand, the bellow rubber suction cups are capable of handling frames that have irregular faces like tubes, panels of vehicles, and many types of molded products. The bellow suction cups are more versatile and adaptable when compared to flat rubber suction cups.

Rubber Washers

Rubber washers are defined as disk-like devices with flat surfaces and holes in the middle. The hole is a means to distribute the weight or load evenly, wear pads and spacers, and minimize the vibrations or method of locking a seal. The inner and outer diameter of the rubber washer varies with respect to how the washer is used and, therefore, determines the rubber washer's dimensions.

During the use of rubber washers in connections involving pipes, they usually serve as a way of stopping the leakages, and they also help to secure the connection or the joint. Due to the fact that they form a tight seal, rubber washers are chosen for use in a variety of applications and conditions.

Rubber O-Rings

These are round ring-shaped rubber seals. These rubber O-rings are mainly used to stop leaks of liquids, lubricants, and many different substances from the mechanical unit. Their use is also found in chemical feeding lines. Rubber O-rings are used to seal gas lines. Besides the ability of the rubber O-rings to stop leakages, they do serve as a way of sealing connections. Rubber O-rings have various applications in many industries. However, they are mostly used in the aerospace industry, vacuum flow lines, petrochemical piping, and hydraulic and pneumatic equipment.

Advantages of Rubber Injection Molding

Rubber molding is a very important part of the rubber production sector. The reason for its significance is that the configurations, components, designs, and parts can be manufactured and produced to fit a specific application's requirements. Rubbers are taken as a central part of an application because they have very unique endurance and durability properties. They can withstand extreme or harsh conditions for quite a long time without failing.

The process of rubber injection molding saves labor greatly because it is automated fully, and this, as a result, cuts down on the cost of labor. Rubber injection molding comes with many benefits and advantages, and some of them are listed below:

• The process is very efficient
• The process is automated fully from the initial insertion of the screwed rubber to its ejection as a finished end product
• Production of high precision products
• The procedure can be repeated with each part being the same
• Intricate and complex geometries can be mold
• There is a reduction in the performing cost of labor
• There is no or very little flash produced
• There are rapid cycle times
• There is a significant reduction in waste
• The finished end products can be recycled again
• Dimensional tolerances are tighter
• The parts have uniformity
• The tooling time is reduced

Industries Served by Rubber Molding

The application and use of rubbers have a very wide range across different sectors:

• The automotive industry
• The aerospace industry
• Home appliances
• Electrical
• Agriculture
• Medicine
• Food and beverage
• Chemical industry

Conclusion

Rubber injection molding is defined as a process where uncured rubber is transformed into a usable product by injecting raw rubber material into a mold cavity made of metal. The flexibility of rubber and its durability offers manufacturers and engineers ways to design cushioning components and seals. The rubber molding process is crucial in the rubber manufacturing industry. The reason for its importance is that the designs, configurations, components, and parts can be produced to fit the exact requirements of an application. Over the century, rubber molds and products have been of great significance to humanity. Its application in many sectors and industries has made it safer for people and the machines and appliances they work with. Rubbers have had a great impact on the working industry. New molding processes are being tested to come up with much better and more cost-efficient ways that reduce tooling time and have highly precise results.