What is Rubber Molding? Types of Rubber Molding & Rubber Properties
Lake Erie Rubber / March 9, 2022
What is Rubber Molding?
Rubber molding is a manufacturing process used to transform uncured rubber into a usable product. This is done using various processes that involve heating and reshaping rubber in a metal cavity. The three main types of molding are injection molding, compression molding, and transfer molding.
What is Injection Molding?
Rubber Injection molding is done by injecting uncured rubber into a mold. The rubber stays in the mold for a set amount of time until it vulcanizes into its final shape. While injection molding is a type of rubber molding, it can also be used with many different materials, such as metals and glass.
Rubber compression molding is done by placing preformed shapes of uncured rubber into the cavities of a heated mold, which is then closed using hydraulic pressure. The rubber is held in the heated cavity until it is molded to its final shape.
Rubber transfer molding is done by loading a measured amount of uncured rubber into a transfer pot. The ram is then closed, pushing the preform through the runner and gate system into the part cavity. The rubber is held in the heated cavity for a specific amount of time to cure the rubber into its final shape.
Rubber has unique material properties that allow it to be strong, durable, flexible, abrasion resistant and extremely waterproof. Natural rubber is derived from the latex from rubber trees. Tapping the bark of these trees allows the latex (sticky, milky colloid) to be drawn off by making incisions in the bark and collecting the fluid in vessels. The latex then is refined into rubber ready for commercial processing. There are numerous types of rubber to suit applications, and these rubber types have specialized properties that range from vibration elimination, to electrical insulation.
Natural vs. Synthetic Rubber
While there are many different types of rubber, there are only two main types of rubber: natural rubber and synthetic rubber. Natural rubber comes from the hevea tree, also known as the Pará rubber tree or simply the rubber tree, and has been used by people to make rubber since the 1800s. Over time, as demand for rubber grew, synthetic rubbers were developed. There are many more specific types of rubber within the two larger categories, and both are developed by our chemists using select ingredients based on the application. Functionally speaking, one is not better than the other. Which one is used for a certain application, depends on the specific properties that are needed.
NOTE: Many “rubber” materials today are actually synthetic elastomers that have been developed to offer similar properties but enhanced for specific applications. Contact us for help determining if an elastomer would better fit your application.
How to Choose the Right Rubber
Choosing the right rubber for your application can be difficult and confusing. To help you with that decision, consider the following three aspects when picking the perfect polymer for your application:
What temperatures will your product be operating at? What is the highest and lowest temperature this product will be exposed to? Will it be exposed to a range of varying temperatures?
What kind of environmental stresses will your product be exposed to? Will it be exposed to ozone, UV rays, pollutant gasses, or acids? Knowing this allows us to choose a base polymer and add specific ingredients to resist these environmental stresses.
What kind of physical forces will be put on your product? Will it be in constant tension? Will it be compressed? Will the forces on it be more dynamic and changing?
Sharing the answers to these important questions will allow us to select the perfect compound for your application.
Basic Rubber Properties
Rubber has many properties that makes it a very useful material across almost every industry. These are some of the basic properties of rubber that make it a useful and popular material:
Elasticity. Rubber is best known for its ability to stretch, bend, twist, and return to its original shape. Additionally, elasticity allows for rubber products to bounce, as seen in a rubber ball.
Durability. Rubber is much more durable than other materials, like plastic. This makes it a popular material for many different applications where durability is needed.
Water and Chemical Resistant. This makes it useful for a variety of applications that need to be water resistant.
Temperature and Electrical Isolation. Rubber is an excellent material when it comes to isolating temperature away from a product, as well as making a great material for electrical insulation.
High Coefficient of Friction. This is very useful for keeping this in place and making sure that parts are not moving around when they should be held in place.
Extreme Abrasion Resistance. Rubber is a great material when it comes to being able to withstand wear and tear without getting scratched or scraped. This is why it is frequently used in dog toys.
Wide Temperature Operating Range. Rubbers can operate at very high or very low temperatures. Some rubbers are able to withstand both high and low temperatures.
Contraction by Heat. While most other materials expand when heated, rubber is one of the few that contracts. This property can be useful in specific applications.
Common Use Cases
EPDM: Rubber for outdoor applications
EPDM or Nitrile: Dust cover applications
Natural Rubber: Durable, abrasion resistant, vibration dampening or anti-vibration, and lower cost
EPDM: Rubber for flexible duct, connector, or bellow
Nitrile: Rubber for oil, gasoline, and abrasion resistance
Natural Rubber: Rubber bumper application
Viton: Rubber for extreme temperature and fuel resistance
Neoprene: Reasonable cost with good all around performance
EPDM Rubber: Rubber caps or plugs
Polyisoprene: Best rubber if color is important
Silicone: Medical applications or food grade applications
HNBR (Saturated Nitrile)
The saturation of nitrile in HNBR improves its heat and ozone resistance. HNBR offers extraordinarily good mechanical properties and abrasion resistance. Applications for HNBR include seals, hoses and belts. Low Temperature: -22°F/-30°C High Temperature: 284°F/140°C and for a short period of time up to 320°F/160°C
Common applications of EPDM include water gaskets/O-rings, grommets, electrical insulation and weather stripping. It is resistant to UV exposure, ozone, aging, weathering, and many chemicals. EPDM works well for outdoor applications. Low Temperature: -20°F/-29°C to -60°F/-51°C High Temperature: 350°F/177°C
Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures. Common applications include voltage line insulators, extreme industrial applications; cooking, baking, and food storage products. Low Temperature: -150°F/-100°C High Temperature: 480°F/250°C
Common applications are static seals and gaskets in fuel applications due to excellent resistance to chemicals, fuel and oil. Static applications are best due to poor resistance to abrasion. Low Temperature: -100°F/-73°C High Temperature: 350°F/177°C
FKM (Fluorocarbon, Viton®)
FKM has excellent heat resistance, excellent resistance to aggressive fuels and chemicals. Common applications are seals and gaskets or other rubber components in contact with biofuels or other extreme fuel or chemical environments. Low Temperature: -15°F/-26°C High Temperature: 350°F/177°C
Common applications are hoses, grommets, seals and gaskets. NBR has oil and solvent resistance, low temperature flexibility and abrasion resistance. Low Temperature: -40°F/-40°C High Temperature: 226°F/108°C
Common applications of Neoprene are refrigeration seals, Freon/air conditioning, motor mounts, engine coolants, petroleum and chemical tank linings, automotive gaskets and seals, and weather stripping. It provides good resistance to ozone, sunlight, oxidation, weather, oils, gasoline, greases, solvents, petroleum oils, animal and vegetable oils, compression set, silicone oil, refrigerants, ammonia, carbon dioxide, water, and steam. Low Temperature: -50°F/-46°C High Temperature: 275°F/135°C
Butyl rubber (Isobutene-isoprene)
Common applications are medical, aerospace, military, and more. Butyl has good flex properties, exceptional resistance to gas and moisture (water and steam) permeation, excellent resistance to oxygenated solvents, alkalis, flexing, and abrasion, good resistance to sunlight, ozone, heat aging, animal and vegetable oils, oxidizing chemicals, silicone fluids and greases, ammonia, hydrazine, and phosphate ester type hydraulic fluids. Low Temperature: -75°F/-59°C High Temperature: 250°F/121°C
NR (Natural rubber)
Common applications of natural rubber are refrigeration seals, Freon/air conditioning, motor mounts, engine coolants, petroleum and chemical tank linings, automotive gaskets and seals, and weather stripping. It provides good resistance to ozone, sunlight, oxidation, weather, oils, gasoline, greases, solvents, petroleum oils, animal and vegetable oils, compression set, silicone oil, refrigerants, ammonia, carbon dioxide, water, and steam. Low Temperature: -50°F/-46°C High Temperature: 275°F/135°C
Most polyurethanes are thermosetting polymers that do not melt when heated. Common applications are durable elastomeric wheels and tires (such as roller coaster and escalator wheels), automotive suspension bushings, electrical potting compounds, hoses and more. Low Temperature: -22°F/-30°C High Temperature: 230°F/110°C and for a short period of time up to 320°F/160°C
SBR (Styrene butadiene)
SBR has superior water resistance, heat resistance, abrasion resistance, low-temperature flexibility, and heat aging. SBR also provides good electrical insulation, alcohol resistance, oxygenated solvent resistance, and mild acid resistance. Common applications are plumbing and HVAC gaskets/seals/O-rings, pneumatic tires and shoe soles.
Synthetic polyisoprene exhibits excellent resilience and good tensile strength. Its color uniformity from one batch to another is easy to maintain because of the uniformity of the polymer. Common applications include those requiring uniform quality and performance, especially where color is concerned.
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