Natural Rubber
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. The plant that is most widely known for having latex in its sap is the Para rubber tree; however many other plants have latex in their sap such as a Panama rubber tree, Russian dandelion, common dandelion, rubber fig or lettuce. Natural rubber that has been obtained from plant sap is often referred to as gum rubber. Natural rubber is both an elastomer, which is an elastic polymer, and a thermoplastic, which means that it becomes soft when heated and hard when cooled. However, natural rubber can also become thermoset, meaning that is will become irreversibly hard and rigid upon heating or curing. Additionally, synthetic natural rubber, or polyisoprene, is a term that refers to natural rubber that has undergone purification. There are numerous applications for natural rubber including tires, gloves, adhesives, catheters,
diaphragms,
water seals, engineering components,
hoses, industrial bands and latex products. Almost every industry makes use of natural rubber ranging from aerospace, automotive, construction and medical to food processing, sporting goods, military and nuclear power.
Natural rubber can be cured during the process of vulcanization. During the vulcanization process, natural rubber molecules are cross-linked with each other by heating liquid natural rubber with sulfur; this creates thermoset natural rubber. The three most common methods in the natural rubber and synthetic natural rubber molding process include compression molding, injection molding and transfer molding. Each of these natural rubber molding methods is different, but all involve pouring liquid natural rubber into a mold where the rubber molecules form cross-links by heat and/or pressure curing. Compression molding squeezes the natural rubber material which has been placed within a heated mold cavity until excess material comes out as flash or sprue, at which time the mold is removed to set in open air. Transfer molding is similar to compression molding, except that the mixed resin is first placed in a heated transfer chamber until it is liquefied, then it is pressed into the heated mold by a plunger until a flash forms; natural rubber transfer molding is unique in its ability to mold other materials, such as strengthening fabric, within a rubber part. Injection molding is the latest advancement in natural rubber molding, producing high tolerance, highly repeatable parts with short production times.