Manville Rubber Products
Manville Rubber Products

**RUBBER 101

Familiarize yourself with rubber.  The basics: Natural vs synthetic rubber, material properties to consider for your application,  rubber vs plastic molding.

RUBBER 101

The links available from this Resources page are intended as a convenience for those seeking more information and detail related to sourcing of rubber products and engineering with rubber. It should be emphasized that since there is very little standardization in the rubber industry it is difficult to provide complete material specifications except by request as we literally use hundreds of formulations and are continually developing additional formulations to suit specific projects. Any information here should be used for reference only; please contact us if you are writing specifications. We will be updating and adding appropriate and useful content to this page. Please contact us if you are in need of any specific information or would like to see anything in particular on this page.

 

What is Natural Rubber?

Natural rubber is the rubber material derived directly from the Hevea brasilliensis tree. It is produced by collecting (tapping) the sap or latex of the tree and then stabilizing and coagulating the latex mainly through the addition of ammonia and formic acid. Chemically known as Polyisoprene, natural rubber is processed into end produces using both a latex liquid form as well as a dry gum form. At Manville Rubber Products we only process the dry gum form.

Is All Rubber Natural?

No. Actually at Manville Rubber Products the bulk of the formulations we use and develop are based on synthetic rubbers. Though natural rubber has many outstanding properties, mainly high strength and toughness, many synthetic rubbers can match these properties and also provide added benefits such as improved aging resistance or chemical resistance. Natural rubber still has its place and is the right choice for some applications, but the wide range of synthetic materials available means the rubber compounder can often better meet application requirements by using synthetic rubbers.

What Material Properties Should I Consider for my Project?

Among the many physical properties to consider Include:

  • Hardness (durometer)
  • Tensile strength
  • Tear resistance
  • Abrasion resistance
  • Resistance to attack by chemical exposure
  • Resistance to degradation by exposure to sunlight, oxygen and ozone ("rooftop" conditions)
  • Ability to maintain properties during exposure to heat or cold, or after exposure to prolonged heat or cold
  • Resistance to compression set or tension set
  • Resilience

It is not generally practical to test every possible property of a material. For this reason it is best to understand the end uses of the product and test only for relevant properties. We will be happy to assist you in determining which testing is applicable to your project.

What is Durometer?

A Durometer is a hardness measuring device used for rubber and other soft materials. It consists of a spring loaded point that is applied to the rubber surface and measures the resistance to indentation of the surface by the point. The durometer based on the ASTM D2240 standard has come to be used and known world-wide. The Type A durometer measures readings up to 100 (0 being the softest and 100 being the hardest) and covers the bulk of the products manufactured by Manville Rubber Products. The readings from the Type A durometer should be referred to with the suffix "A" to avoid any confusion with other durometer scales (00 scale for foam rubber, D scale for very hard plastic-like materials). As rough points of reference chewed bubble gum would measure about 5-10A durometer, a passenger car tire would measure about 60A durometer while 100A durometer is approaching the hardness of soft plastic.

What is Set?

Set is a very important material property to consider when designing any product that is installed in such a way as to be deformed from its original shape for long periods of time. Set refers to a material's tendency to become permanently deformed after long term exposure to a deforming force, even after that force is removed. ASTM standard tests have been developed for evaluating a material's resistance to set under compression (ASTM D395) and resistance to set under tension (ASTM D412). The tests measure delayed elastic recovery after removal of a given force over a given period of time. The tests can be performed at room temperature or reduced or elevated temperatures to better simulate application conditions. If any chemical changes occur within the material during the course of the testing, permanent deformation will be observed. This deformation is commonly referred to as a percentage of the original dimension. For Instance 20% compression set would indicate that a sample dimension returned to within 20% of its original length after compression along that dimension for a given period of time at a given temperature.

I am familiar with plastics molding, but how does rubber molding differ?

Rubber molding is markedly different from plastic molding. Plastic processing involves phase changes of the material from solid to liquid and back to solid. In theory this can be repeated indefinitely. With rubber a chemical cross-linking occurs during processing that is not reversible. Chemicals in the rubber compound are activated by heat and once the cross-linking or vulcanizing reactions occur they cannot be undone. Furthermore, molded plastic parts are almost exclusively produced using injection molding equipment whereas rubber components are molded using compression molding, transfer molding or injection molding equipment.

 
Rubber Plastic
Material is fed into the extruder cold, in the form of a continues "gum" strip Material is fed into the extruder cold, as loose pellets
The extruder warms and softens the material to a stiff, chewing gum consistency The extruder melts the material converting hard pellets into a viscous liquid
Material is held in the injection barrel awaiting injection at the highest temperature possible which minimizes cycle time Material is held in the injection barrel awaiting injection at the lowest temperature possible which minimizes cycle time
Material is injected into a mold heated to far above the injection barrel temperature Material is injected into a mold cooled to well below the injection barrel temperature
Once in the mold the material continues to heat and therefore to expand. Pressures in the mold continue to rise throughout the cycle and become tremendously high by the end of the cycle due to thermal expansion of the material Once in the mold the material instantly begins to cool. The material shrinks as a result of the cooling. Pressure in the mold is a result of the injection pressure alone as no material expansion occurs
Typical in mold vulcanizing times vary from just under a minute to several minutes Typical in mold freeze times range from seconds to tens of seconds
Due to the extremely high pressures and long in situ times within the mold cavity the rubber tends to intrude into even the smallest gaps and spaces. This makes moving features such as ejectors prone to clogging and jamming and almost always results in flash at mold parting lines requiring secondary deflashing of the molded article Because pressure within the mold cavity rapidly drops as the part quickly freezes, plastic tends not to intrude into features such as ejectors or between mold parting lines, within reason. Plastic parts normally come out of the mold free of flash and can be shipped "as molded"

 

Can Rubber Be Recycled?

 

Unlike most plastics rubber is a thermosetting material meaning it physically and chemically changes form after processing and cannot simply be "remelted". However, used or scrap rubber articles can be ground into a fine powder and added in small quantities to virgin rubber material. Unfortunately, this usually results in a loss of physical properties so reground rubber is generally only used in very low cost, non-engineering applications Ground rubber is currently being added to paving asphalt with great success. As global interest in recycleability continues to grow more research is being conducted in developing new uses for ground rubber as well as in developing alternate methods to grinding for reclaiming rubber.