This article contains all the information you need to know about rubber molding on the internet.
You will learn more about topics such as:
- What is Rubber Molding?
- Types of Rubber Molding
- How Rubber Molding is Made
- The Advantages of Rubber Molding
- And much more…
Chapter One – What is Rubber Molding?
Rubber molding is a process of transforming uncured rubber or an elastomer into a usable product by transferring, compressing, or injecting raw rubber material into a metal mold cavity. The applied pressure creates a chemical reaction, such as curing or vulcanization, which causes the polymer chains in the material to crosslink.
All rubber manufacturers use heat and pressure to produce their products. The most common methods for rubber molding are injection, compression, and transfer. With the constant evolution of products, rubber mold producers are continually adjusting to meet the ever growing demand for grommets, gaskets, tubing, o-rings, and hoses.
Chapter Two – Uses for Rubber Molding
Rubber, synthetic or natural, can be shaped and molded into complex forms, shapes, and configurations, which enhances its usefulness. Rubber’s flexibility and durability offers engineers different ways to design seals and cushioning components. The stability of rubber is seen in its use as a method for cushioning machinery to reduce impact and vibrations.
While natural rubber is still widely used, manufacturers have developed synthetic versions of rubber that have the same strength as natural rubber with greater durability and longevity. Manmade materials such as neoprene, silicone, ethylene propylene diene monomer (EPDM), and various types of elastomers have the same properties as rubber but have greater flexibility and endurance.
Uses for Rubber Molding
Rubber bumpers come in a variety of shapes and sizes to be used in manufacturing specific products such as recessed, mushroom, stem, grommet, and tack bumpers. Bumpers reduce vibration and movement in equipment to protect rigid parts from damage caused by vibrations and impact. They protect furniture, work surfaces, circuit boards, displays, and packaging.
The types of materials used to produce bumpers include various polymers that are a mixture of rubber and plastics.
Rubber bushings are somewhat like rubber bumpers since they are designed to absorb vibrations and impact on machinery. They are shaped and configured to fit specific applications though the most common form is as a cylinder. Rubber bushings can be placed on a machine to reduce vibrations or between two pieces of equipment to prevent vibration transfer. A particular use of rubber bushings is on automobiles, trucks, bicycles, and motorcycles as a way of containing the excessive vibrations from deformities in roads.
Rubber diaphragms are sealing materials used to prevent leakage or friction. They are designed to withstand the effects of pressure while preventing any form of liquid from escaping. Rubber diaphragms are a barrier between two enclosures or chambers to prevent contaminants from entering. They can be static or dynamic where a dynamic diaphragm can be activated by the slightest amount of pressure.
Rubber grommets are rings that are placed around metal openings to protect objects or materials that pass through the opening. A normal use for grommets is for openings where wires pass through. They are made of synthetic or natural rubber and are produced by various types of molding. Electrical cables are vulnerable to abrasions from the surface of a metal opening, which, when damaged, can short out the wiring. Rubber grommets prevent this and protect wiring.
Rubber vibration isolators are used to protect machines and appliances from damage due to vibration and keep them level and stationary. Included in the term isolator are mounts, leveling pads, bumpers, shock mounts, and bushings. The control of vibrations is an important aspect of equipment operation since vibrations can lead to equipment malfunction and poor quality products. Uncontrolled vibrations can result in unnecessary wear and damage to machine components. A common use of isolators is on residential air conditioning units to suppress noise from the unit's operation.
Rubber mounts are closely associated with isolators. The two parts of a rubber mount are the fastening mechanism and the isolator. The fastener is threaded metal. The isolator is made from synthetic or natural rubber. Much like isolators, rubber mounts control vibrations that may damage equipment. Rubber mounts come in a wide variety of sizes, shapes, and designs with the majority being custom made to fit a specific application.
Cylindrical mounts can be found as supports for conveying lines and transfer equipment. Other types of rubber mounts serve as supports for tabletop equipment such as computers and printers. The simple design of rubber mounts makes them easy to install and maintain. The metal threads are designed to quickly fasten to the device being supported and protected.
Rubber plugs are shaped and formed to fit their use. They are normally used for equipment that need their joints and edges covered. In special situations, they are used to cover wires that are part of a machine’s intake wiring. The various types of rubber plugs include plus, T, washer, and connector.
Plus plugs hold equipment together, T-plugs are designed to be inserted into various types of tubing, washer plugs function like T-plugs but extend out to cover the area surrounding the hole where they are inserted, and electrical connector plugs protect female threaded connectors from contamination by moisture or dust.
Rubber seals are used to seal gaps, insulate spaces from water or liquids, and protect surfaces. Since sealing material is an essential part of a wide variety of equipment and processes, they are easily customizable and can be shaped and fitted to be installed as a component in multiple types of equipment. An important use of rubber seals is in processes that involve pressurized liquids or gasses. A rubber seal can be a gel, film, putty, or strip and be able to adhere to glass, ceramics, concrete, paper, and other materials or porous surfaces.
Rubber pads serve the same purpose as bumpers, mounts, and isolators as a means of suppressing noise and controlling vibrations. They are specially designed to insulate machinery against vibrations, reduce noise, and protect equipment against damage. Rubber pads are very versatile and can be used with small pieces of equipment such as table top mixers or larger machines such as power generators.
In factories, where there are multiple pieces of equipment, rubber pads serve as an efficient and economical method of noise suppression. They are normally made from various forms of synthetic rubbers such as neoprene, polychloroprene, or nitrile rubber. An additional benefit, aside from their low cost, is how easy they are to maintain.
Rubber Suction Cups
Rubber suction or vacuum cups are used as a handling system for manipulating items with a flat surface such as glass, metal sheets, wood paneling, molded pieces, or tubes. They come in several sizes, shapes, diameters, and types of rubber material.
Two types of suction cups are flat and bellows. Flat rubber suction cups are used for plain or flat surfaces. Bellows suction cups are able to handle irregular surfaces such as vehicle panels, tubes, and various types of molded products. They are more adaptable and versatile than flat suction cups.
Rubber washers are a flat disk with a hole in the middle that is used as a way to distribute a load, spacers, wear pads, method of locking a seal, or reduce vibrations. The dimensions of a rubber washer are determined by their outer diameter (OD) and inner diameter (ID), which vary according to the use of the washer.
When rubber washers are used for pipe connections, they generally serve as a means of preventing leaks and help to secure the joint or connection. Rubber washers are chosen for a variety of conditions because they form a tight seal.
Rubber O-rings are round ring shaped rubber seals used to prevent leaks of lubricants, liquids, and substances from mechanical devices. They are used to seal gas lines and chemical feeding lines. Aside from their ability to prevent leaks, they serve as a way of sealing connections. O-rings are widely used by aerospace, hydraulic and pneumatic equipment, vacuum flow lines, and petrochemical piping.
Chapter Three – How Rubber Moldings are Made?
Rubber products and components are produced, shaped, and formed using the molding process. The reason for using molding, aside from shaping parts, is to subject the rubber material to chemical processes that toughen or harden the polymer chains of the raw rubber. The chemical reaction forces the long polymer chains to crosslink, which prevents them from moving independently. The rubber material expands and contracts as stress is applied or eliminated.
In the majority of cases, heat is involved in rubber molding to reduce curing time. The three most common types of rubber molding are compression, injection, and transfer. A fourth process, extrusion, is also used to produce rubber parts and components but does not involve using a mold.
How Rubber Moldings Are Made
There are several steps that need to be taken prior to initiating the molding process.
Designing: Rubber can be molded into complex shapes, geometries, and designs. During the design process, configurations are included in the rubber geometry that are used to make the production of the final product more economical. In molding part design, engineers have to account for the parting line and its location, which needs to have a minimum draft angle. The design of the mold is the foundation of part development and can determine the success or failure of the parts production.
Producing the Mold: Once the design is completed, it is transformed into a metal mold. The manufacturing and producing of molds involve the use of hardened steel, beryllium hardened copper, or some forms of aluminum. Steel is the most common material used to make molds since it lasts the longest, which makes it an economical choice even though, initially, it is more expensive. Hardened steel measures 38 to 45 on the Rockwell hardness scale. Included in the construction of the mold are the mold cavity and surface finish of the cavity that has to be correctly adjusted for the proper finish on the final part. The creation of the mold involves the use of computer programs that precisely produce molds that exactly match the parameters of the design. The computerization process cuts down on the amount of time necessary to produce the rubber mold for production. Once the mold has been cast and made, it is tested for dimensional accuracy and compliance with the original design.
Compression: Compression molding is the simplest type of rubber molding processes. A two part clamshell style mold is used to shape the final part. Once unvulcanized rubber is placed in the mold cavity, the mold is closed and heated. Pressure is applied using a hydraulic press. As the rubber in the mold heats up, it fills the mold cavity. When the proper temperature is reached, the rubber vulcanizes or hardens, which ensures that the rubber will keep its shape as it cools in the mold. The two videos below from REDCO Rubber Engineering and Development Company show the steps of the compression molding process.
The First Part of Compression Molding
The Demolding Process of Compression Molding
Transfer Molding: The transfer molding process is similar to the compression molding process but uses a transfer system that is on top of the mold that feeds raw rubber into multiple mold cavities after the press has been closed. Before being forced into the mold cavity, the rubber compound is placed in the part of the mold called the pot. From the pot, the compound is forced by a heated plunger or piston into the mold cavity through a small section of the mold called the sprue. Transfer molding is a highly efficient process with short tool loading times. The process allows for high production runs though parts do not have tight tolerances. When curing is completed, the mold is split, and the parts are released. The two videos below from REDCO Rubber Engineering and Development Company provide a short demonstration of the transfer molding process.
Injection Molding:The injection molding process is used for the production of products that require tight tolerances with high precision and accuracy. The injection process begins by heating raw rubber to form a liquid such that it flows more freely and readily into the mold cavity. The preheating process shortens curing time and allows for higher curing temperatures. The nature of injection molding requires that the mold be tightly clamped, which helps to avoid or limit flashing. The rubber for injection molding is supplied to the mold through a nozzle and barrel assembly that provides a continuous flow. The consistency of the rubber is referred to as gum stock, which enters the mold cavity through a gate after having travelled through a system of sprues and runners. The liquified rubber fills the mold cavity and packs tightly to the walls of the mold to form the desired shape.
Though injection, compression, and transfer molding are the most common forms of rubber molding, each manufacturer has a different way of completing the process. Regardless of any variations, all rubber molding involves the use of heat and pressure.
Chapter Four – Types of Rubber Used for Rubber Molding
There is a very long and ever growing list of the different types of rubber that is used for rubber molding. There is a wide variation in the types of rubber used for the rubber molding process, the majority of which are synthetic and formed by a chemical process involving polymers, which enhance the durability and elasticity for the forming and shaping process.
To develop a complete understanding of the materials used in the rubber manufacturing process, it is important to know the difference between natural rubber and synthetic rubber. Natural rubber comes from the rubber tree plant from which latex is taken that is used to produce rubber.
Synthetic rubber is made artificially by the polymerization of monomers into polymers. The process can be done by solution or emulsion polymerization. The solution method involves dissolving the monomer on a solvent that contains a catalyst with the resulting effect being a polymer. The emulsion process uses an emulsion that contains water, a monomer, and surfactant, a compound that lowers surface tension.
Types of Rubber Used for Rubber Molding
Natural rubber comes from the latex produced by the rubber tree. It has high tensile strength with abrasion and surface friction properties, which makes it ideal material for vibration dampers, seals, o-rings, mountings, diaphragms, and bumpers. Since natural rubber bonds well with metal, it can be used as a coating.
Nitrile or Buna-N (NBR)
Nitrile rubber is made from acrylonitrile and butadiene. It is made using the emulsifier process where acrylonitrile, a butadiene monomer, an activator, and catalyst are mixed in a polymerization container. For the various compounds to mix, the container has to be heated to 40o C. NBR is the most used of the synthetic rubbers in the rubber molding process due to its resistance to water, oil, various fluids, and solvents. It is resistant to abrasion and tearing and has excellent mechanical properties.
Hydrogenated Nitrile Butadiene (HNBR)
HNBR is produced by the hydrogenation of NBR, which removes the olefinic that are vulnerable to degradation. It is known for its strength and resistance to heat. HNBR rubber compounds are resistant to petroleum based oils and fuels, aliphatic hydrocarbons, vegetable oils, silicone oils and greases, ethylene glycol, water and steam, dilute acids, bases, and salt solutions. It is extensively used in the auto industry.
Ethylene Propylene Diene Monomer (EPDM)
EPDM is made from combining ethylene, propylene, and a diene comonomer that allows for crosslinking. The structure of EPDM makes it resistant to heat, light, and ozone as well as capable of withstanding temperatures up to 150o C. EPDM is highly durable and lasts in an application for a long time. It is used for steam systems, panels on cars and trucks, and braking systems.
Silicone rubber is a rubber like material composed of silicon combined with carbon, hydrogen, and oxygen. The term silicone rubber covers several types of silicone rubbers that are differentiated by their organic grouping and chemical structure. The different types include methyl, methyl phenyl, methyl vinyl, methyl phenyl vinyl, and fluoro vinyl methyl. Silicone rubber is widely used for its resistance to extreme temperature variations, flexibility, and weather resistance.
Fluorosilicone is one of the variations of silicone that contains trifluoropropyl, which enhances its resistance to chemicals, non-polar solvents, fuels, oils, acids, and alkaline. Like silicone, it is highly adaptable across a wide range of temperatures. It is this final quality that has made it extensively used in the aerospace industry as well as its longevity and resistance to petroleum products.
Neoprene rubber, which is also known as polychloroprene or chloroprene, is produced by the emulsion polymerization of chloroprene using a potassium catalyst. It is a multipurpose material that has weather, temperature, fire, and abrasion resistance properties. Since neoprene resists degradation, it is used for gaskets, hoses, and various forms of coating. The versatility of neoprene makes it ideal for use in rubber molding regardless of the type of molding being used.
Styrene Butadiene (SBR)
SBR rubber is produced from mostly butadiene with a small percentage of styrene, which are copolymerized using the emulsion process. It has exceptional abrasion, crack, and aging resistance. The many positive characteristics of SBR has made it ideal for rubber molding of diaphragms, seals, and gaskets.
FKM is made from VITON or fluororubber and are classified by their chemical composition, fluorine content, or method of cross linking. The classification of FKMs goes from one to five with each type having a different type of copolymer. There are three types of crosslinking materials for FKMs, which are diamine, dihydroxy, and peroxide. The characteristics of FKM makes it resistant to chemicals and extreme high temperatures making it ideal for the molding of hoses, O-rings, gaskets, and fuel lines.
Butyl or Isobutylene Isoprene (IIR)
IIR rubber is made by copolymerizing isobutylene with isoprene, which are two compounds that come from natural gas or crude oil. To combine the two materials, they are cooled to -100o C or -150o F. IIR is not permeable by gas, which makes it ideal for the production of seals for high pressure and vacuum applications. Additionally, it has sound dampening qualities to use as a bumper for equipment.
Urethane or polyurethane is made by reacting polyol with a diisocyanate or polymeric isocyanate after being placed with a catalyst. Since there are several different types of polyols and diisocyanates, there are several different types of urethane rubber materials that can be made from the different combinations of the compounds. Urethane rubber is used in applications where there is a need for materials that can withstand abrasion and has excellent tensile strength.
Chapter Five – Advantages of Rubber Molding
Rubber molding is an essential part of the rubber production industry since components, parts, configurations, and designs can be produced to exactly fit the needs of an application. The main reason for choosing rubber as a central part of an application is its unique durability and endurance since rubber can withstand abuse for long periods of time without failing.
Advantages of Rubber Molding
Injection molding heats the rubber material and places it under pressure such that it fills the mold cavity. The process of injection molding is a labor saving method for molding rubber parts since it is fully automated, which cuts down on labor costs.
Advantages of Injection Molding
- Efficient process
- Fully automated with insertion of raw material and ejection of final product
- High precision products
- Very repeatable with every part being the same
- Able to mold complex and intricate geometries
- Allows for molding of complex geometries
- Eliminates preforming labor costs
- Produces little to no flash
- Rapid cycle times
- Significantly reduces waste
- Products that are produced are recyclable
- Tighter dimensional tolerances
- Uniformity of parts
- Reduced tooling time
Compression molding uses a preformed piece of rubber that is placed in a mold. The mold is heated such that when it is closed, the preformed rubber piece is compressed to the sides of the mold. Products are produced under high pressure and temperature, which activates the curing process.
Advantages of Compression Molding
- Cost efficient
- Less expensive tooling
- Increased cavity count
- Perfect for larger pieces that need extended curing time
- Accepts all forms of elastomers
- Eliminates the need for runners, sprues, or gates
- Simplified process
- Little to no waste
Transfer molding is a closed mold system that uses rubber pellets and preformed rubber. The pellets or preformed rubber are placed in a pot that is located between the top plate of the mold and the plunger. In the pot, the pellets are heated by the plunger and forced by it down the sprue into the mold cavities. The plunger remains in place until the material takes the shape of the mold and cures.
Advantages of Transfer Molding
- Economical process
- Cost effective tooling
- Shorter cycle times
- Highly precise parts
- Tight dimensional tolerances
- Long curing time for larger products
- Accepts all types of elastomers
- Multiple parts produced during each cycle
- Greater design flexibility allowing for sharp edges
- Reduced flash
- Simple process with pot, plunger, and mold
Chapter Six – Industries Served by Rubber Molding
There are limitless uses for rubber molded products, which makes them a valuable part of a wide variety of industries from auto manufacturing to aeronautics and space craft producers. The guaranteed durability of rubber molded components has made them an essential part of several industrial processes.
Industries Served by Rubber Molding
Aerospace: Rubber components for the aerospace industry must be dependable and meet exacting performance standards that are not required by other industries. Parts must be highly durable, have excellent weathering ability, and able to withstand sudden variations in temperature. For passenger and aircraft safety, gaskets and seals are resistant to oil, gases, and fire.
Automotive: The auto industry uses rubber parts in many sections of cars from brakes and clutch seals to gaskets for the battery. The noise suppression capabilities of rubber parts make them ideal for consoles and other parts of a car’s interior.
Print: An important use of molded rubber products in the printing industry is rubber rollers that have three parts, which are its metal core, rubber bonding material, and rubber covering. These rollers are used in graphic arts and printing and are important for passing ink onto printing plates. Rubber pads are widely used for shock absorption along with rubber mountings.
Home Applicances: Home appliance rubber parts are subjected to constant use and extreme temperature changes. Rubber molded parts have to be durable and strong enough to take the intensive wear.
Electrical: An important use for rubber molded products for the electrical industry is safety and protection. The types of products can include outlet covers, protection from shock, chemical resistance, methods for reducing electromagnetic interference, and sealing connections from dust and moisture. Much like all industries, parts have to be durable, resilient, and easy to use. The main concern for the electrical industry is meeting the standards and regulations that are designed to protect workers and ensure safety.
Agriculture: The demands of the agriculture industry may seem to be dissimilar to those of other production and manufacturing industries. What is important for agriculture is protection from leaks and seepage but has the ability to withstand harsh weather conditions that all agricultural products must endure. A major part of agricultural production is the use of chemical fertilizers and bug repellents, which have to be handled with great care. Rubber molding produces gaskets and seals that give the necessary protection and safety that are required for handling the variety of chemicals used for agricultural production.
Medicine: There are countless ways that rubber molded products are used in the medical field from seals and gaskets on equipment to tubing to provide fluids. The nature of rubber and its durability makes an ideal partner for medical use. Whether it is medical research or patient treatment, rubber molded parts and products can be found in every aspect of medical treatment. The most important feature of rubber molded products is their ability to be recycled since most medical applications allow instruments to be used once and then disposed of. Another factor of concern with medical instruments is the requirement to meet the exacting standards that have been set for patient treatment. Manufacturers strictly adhere to the legislated requirements and restrictions set for the protection of the public.
Food and Beverage: The standards for rubber molded products for the food and beverage industry have to adhere to the regulations of the Food and Drug Administration (FDA) of the Federal Government. Products are required to be designed for exceptionally hygienic conditions and have the ability to endure regular cleaning and sanitization. The FDA specifies that types of acceptable rubber compounds that can be used in the production of parts and components that can be used by the food and beverage industries. The main concerns are that materials be non-toxic and non-carcinogenic.
Chemical: The chemical industry produces a variety of products that include chemicals for agriculture, pharmaceutical use, paints, and the petroleum industry. The many varieties of products require a wide assortment of rubber products since rubber is chemically resistant. Hard rubbers are used for products for the chemical industry because they are made with a high percentage of sulfur, which makes them chemically inert and resistant to corrosion.
- Rubber molding is a process of transforming uncured rubber or an elastomer into a usable product by transferring, compressing, or injecting raw rubber material into a metal mold cavity.
- The most common methods for rubber molding are injection, compression, and transfer.
- Rubber’s flexibility and durability offers engineers different ways to design seals and cushioning components..
- The purpose of the molding process, aside from shaping parts, is to subject the rubber material to chemical processes that toughen or harden the polymer chains of the raw rubber.
- Rubber molding is an essential part of the rubber production industry since components, parts, configurations, and designs can be produced to exactly fit the needs of an application.