Rubber Molding

Rubber molding is the process by which raw rubber is melted and formed in a mold. It is the method of choice in the creation of many different kinds of complex rubber products. For example, rubber seals and rubber diaphragms, which prevent seepage of liquids or gasses, must be precisely designed in order to fit the equipment in which they are installed. The same is true for rubber grommets and rubber washers, both of which must be precisely designed, because they are used to protect equipment. Extrusion can produce seals, diaphragms, grommets and washers, but an extruded seal or grommet's construction is limited to detail in two axes. This means that an extruder can produce three-dimensional shapes, but it only has design control over the X and Y axes (the rubber product grows in the Z axis as rubber passes through the extrusion die). For this reason, extrusion is more appropriate for the production of simple rubber sheets, rubber baseboards and other long or thin rubber products. Because rubber molding processes use enclosed molding cavities instead of open dies, molding allows for design control over all three axes. Natural rubber as well as synthetic rubbers like neoprene, silicone rubber, liquid silicone rubber, EPDM and foam rubber can all be molded.

Every industry makes use of some variety of molded rubber products to some extent. Manufacturing operations that involve machinery often must contend with issues of vibration caused by moving parts. In order to protect workers, other machinery and the vibrating equipment itself, machinery that creates vibration is often secured using rubber washers or bushings that absorb vibration and prevent excessive movement. Rubber sheets are also used to absorb vibration; large, thick sheets are often placed beneath machinery that creates vibration. This also can reduce noise, which improves working conditions. Rubber grommets are used in industrial, commercial and consumer contexts. A rubber grommet is a rubber shape that fits around a hole in a surface; the grommet can protect wires or other hardware that passes through the hole, or grommets can protect the holes from becoming torn or otherwise damaged. Wire entry ports in electrical paneling are often fitted with grommets to prevent wires from becoming damaged or disconnected. Simple rubber grommets can also be used in office desks, which are often drilled with holes to allow for the passage of wires from computers and other office hardware to electrical outlets.

For every given molded rubber product application, a host of raw rubber materials are available. Natural rubber, which may be the best known rubber variety, has been in use on an industrial scale since the 19th century (rubber's first suspected uses are prehistoric, and the earliest known use of primitive rubber was by the Mayans around 1600 B.C.E.). Fueled by conflict and scarcity during the first half of the 20th century, synthetic rubber development flourished and gradually began to replace natural rubber development; natural rubber now accounts for less than half of all the rubber materials produced worldwide. Neoprene was the first synthetic rubber produced on an industrial scale, and it and other materials like it feature all of the desirable qualities of natural rubber, but few of its undesirable qualities. Synthetic rubber like EPDM, silicone and neoprene can be engineered to varying levels of resistance to corrosion, chemical inertness, heat resistance, ozone resistance, strength, durability, flexibility, rigidity and many other properties. Industries for which natural rubber products were not practical can now enjoy the benefits of rubber products without enduring the problems they can also cause. Health care facilities, for example, which make extensive use of rubber tubing, must make considerations for patients with latex allergies (natural rubber is derived from rubber latex). Silicone tubing allows for the transmission of intravenous fluids without causing allergic reactions.

There are many kinds of rubber molding, the most common of which are injection molding, liquid injection molding, compression molding and transfer molding. Each method is similar, though of the four injection molding is the most widely employed method of rubber molding. The injection molding process begins with a collection of raw rubber stock in a hopper suspended above a conveyance channel. When the stock is released, a large, turning screw within the conveyance channel forces the stock toward a mold cavity at the end of the channel. Friction caused by the turning screw heats the rubber to a molten state; by the time it reaches the end of the channel, the rubber is completely molten and ready to be formed. The mold at the end of the channel is a specially-designed opening into which molten rubber is injected. The rubber fills the cavity and forms along its contours. The combination of heat and pressure applied to the plastic ensures that it completely fills the cavity, allowing for uniform, complete distribution of rubber throughout the mold. Once the rubber has taken the shape of the mold, it is allowed to cool and harden. The newly molded rubber product is then ejected from the mold, cleansed of imperfections (like seams) if necessary and prepared for shipment or further processing.

Image Provided by Jet Rubber Company	Image Provided by Timco Rubber Products, Inc.

Image Provided by Rader Products, LLC

Rubber Molding Types

• Blow molding is a less-common process of placing a hollow tube between the two halves of a blow mold. The blow mold then closes, pinching off the bottom half of the tube, and air is injected into the top, forcing the material outwards to the walls of the blow mold.
  
• Compression molding is a process that compresses the rubber material in a mold under heat and pressure to achieve the desired shape.
• EPDM, or Ethylene-Propylene-Diene Monomer, an elastomer, is a type of synthetic rubber.
• Foam rubber is rubber that was manufactured with the addition of a foaming agent in order to create a flexible, air-filled substance.
• Injection molding involves melting rubber in an injection unit and then injecting it into the mold where it stays until after cooling when the finished product is ready.
• Liquid silicone rubber is the liquid form of a synthetic, two-component, elastomeric polymer that is made from silicone elastomers.
• Molded rubber is formed through pressing melted rubber into dies.
• Natural rubber is an elastic hydrocarbon polymer that was originally developed from a milky substance, known as latex, which can be found in the sap of some plants. 
• Neoprene is a brand name for a type of synthetic rubber that is also known as polychloroprene.
• Rubber baseboard is a molded rubber product used to cover the joint that is formed when a wall meets the floor.
• Rubber diaphragms are flexible seals that are resistant to a variety of media at different pressures.
• Rubber grommets are rubber rings inserted into a hole in sheet metal to protect cords or electrical wires from the abrasion.
• Rubber seals are used to prevent leakage at joints.
• Rubber sheets are flat pieces of rubber used for a variety of purposes.
• Rubber tubing refers to long, hollow cylinders used to transport liquids and gases.
• Rubber washers are primarily used to support the weight of a threaded fastener, but also are also used in taps or valves to control the flow of liquids or gases.
• Silicone rubber is a synthetic, two-component, elastomeric polymer that is made from silicone elastomers that can be cured at room temperature into a solid elastomer for use in molding; however, it is usually molded from a liquid form.
• Transfer molding involves building a "piston and cylinder"-like device in the mold and squirting the rubber into it through small holes. The mold is then closed and under hydraulic pressure the rubber or plastic is forced through a small hole into the cavity where it cures.

Rubber Molding Terms

Abrasion Resistance - A rubber compound's capability to withstand mechanically caused deterioration.
 
Accelerated Life Test - A test made to replicate in a short period of time the breakdown resulting from normal working conditions.
 
Accelerator - A substance that increases the speed of vulcanization when used in small quantities in conjunction with vulcanizing agents.
 
Activator - A compound used to increase the effectiveness of an accelerator, small amounts at a time.
 
Adhesion - Tendency of rubber to cling or bond to any surface it contacts.
 
Aftercure - The continuance of vulcanization, even after the energy source has been taken away.
 
Air Checks - Depressions and marks on the surface of rubber, caused by air trapped during the molding process.
 
Autoclave - Uses steam under pressure to vulcanize rubber products.
 
Backrind - A defect in the molding process, where the rubber near the parting line sinks below the surface and the parting line ends up ragged and torn.
 
Batch - The result of a mixing operation.
 
Blank - Rubber compound that fills a mold.
 
Bloom - A discoloration of rubber, caused by a liquid or solid migrating towards the surface.
 
Breakout Friction - The necessary force to start the sliding between a rubber seal and its mating surface.
 
Chalking - The development of a powdery residue on a rubber surface as a result of surface breakdown.
 
Checking - Small cracks on the surface of rubber, usually from environmental damage.
 
Compression Set - The permanent deformation of rubber after removing the compression.
 
Conducting Rubber - Rubber that is able to conduct electricity.
 
Crosslink - A chemical bond between polymer chains.
 
Cure Date - The date of completion of the molding process for a rubber product.
 
Deflashing - Any of a variety of processes for waste edge removal from molded rubber parts.
 
Dispersion - The application of force used to evenly disperse various compounds through rubber.
 
Durometer - An instrument that measures the hardness of rubber.
 
Dusting - Applying powder to rubber to prevent adhesion to something else.
 
Dynamic Seal - A seal necessary for the prevention of leaks beyond parts that are in relative motion.
 
Elasticity - A characteristic of rubber, describing its tendency to return to its initial shape after warping.
 
Elongation - Extension of rubber when exposed to stress.
 
Extruder - A machine that forces rubber through a hole that shapes it into the finished product.
 
Fatigue Breakdown - The wearing out of elastomers after repeated deformations.
 
Flexural Strength - The flexing capability of a material with no permanent deformation or breakage.
 
Flow - Capability of uncured rubber to move in the mold and runner system in the molding procedure.
 
High Consistency Rubber (HCR) - Rubber processed on a rubber mill that has a much greater viscosity than liquid silicone rubber.
 
Hysteresis - The process of mechanical energy changing to heat in rubber under strain.
 
Insert - A material that rubber is chemically or physically bonded to during the molding process.
 
Logy - Slow recovery rate of rubber after stress.
 
Mastication - The softening of raw rubber by mechanical and atmospheric forces.
 
Monomers - A chemical compound that is able to endure polymerization.
 
Non-Fill - A defect that occurs when the rubber does not completely fill out the mold.
 
Parting Line - A line on the surface of rubber resulting from where the two halves of the mold met.
 
Peptizer - A material that is used to quicken the softening of rubber compounds under heat or mechanical action.
 
Pigment - An insoluble compound that gives rubber its color.
 
Plasticity - The degree to which rubber will retain deformation.
 
Polymerization - Chemical reaction in which one or more simple materials are transformed into complex materials that have different properties from the originals.
 
Reinforcing Agent - A substance that is added to rubber to increase its resistance to the harms of the vulcanization process.
 
Scorch - The result when rubber vulcanizes too quickly.
 
Spew - Extra material that leaks from the mold as it closes.
 
Splice - The term for the uniting of two parts of vulcanized rubber to make a whole.
 
Vulcanization - A thermo-setting reaction that involves the use of pressure and heat, and results in highly increased elasticity and strength of materials like rubber.