Rubber molding is a method in which a functional rubber product is manufactured. Rubber molding produces molded rubber portions by pressing a chunk of rubber into a rubber molding metal chamber, then exposed to heat to activate the chemical reaction. Rubber companies use rubber molding in the manufacturing of rubber products.
Rubber Molding History
Charles Macintosh a Scottish chemist discovered that by applying rubber to a cloth by use of a coal-tar byproduct to cement fixed cloth with an elastic surface. During the Victorian era, the demand for rubber grew in the USA and European countries.
During the early days of rubber production, the rubber products where of low quality hence the demand for rubber declined but with time the demand increased after rubber products such as shoe soles and rubber erasers.
In 1839, Charles Goodyear discovered vulcanizing. Vulcanizing is the process of adding sulfur to rubber to enhance toughness by stabilizing polymers in the rubber. This invention was not used instantly but took several years for manufacturers to accept it. In 1889, vulcanization became popular which led to the development of automobiles and bicycle tires.
There are three common techniques used in rubber molding;
- Transfer molding
- Rubber Injection Molding
- Compression molding
Rubber transfer molding is a method permitting manufacturers to produce both rubber items and molded rubber parts. In this technique, manufacturers insert the molten substance into a cavity located above the pot and then use ram to force the mold through a runner and gate system as it melts upon the application of heat and pressure.
Rubber transfer molding is related to injection molding since the rubber goes in the cavity after the mold is locked. Rubber products by this technique are less costly than a rubber product by compression molding.
Advantages of Transfer Molding
- Flexible Design. Rubber transfer molding technique permits sharp edges. The transfer pots and plunger design, easier preforms required agreeing to standardization and is cheap.
- Short Manufacturing Phase. The period of production using transfer molding is shorter compared to the other rubber molding techniques. It is more consistent and allows for constricted tolerances and more complex parts.
- Extraordinary Cavity Amount. Transfer molding products require less and simple preforms.
- Ability to produce over molded constituents.
- A cost-effective method for high and medium accurate constituents.
Disadvantages of Transfer Molding
- Material Wastage. Pots produce a large number of material wastage. This yields an enormous pad of sprues. Scraps are not recyclable because the polymers are thermosetting.
- Mold Repairs.Inserted transfer molding tools need repairing overtime. Inserts also require removal and reset maintenance.
- Intricate Molds.Designing and molding are complex hence becoming costly.
Rubber Injection Molding
Rubber Injection Molding was derived off a process that was initially intended for plastic molding. Making its debut in the mid-1960s, Rubber Injection Molding has since enabled manufacturers to create variations of molded rubber products. This cutting-edge technique was cropped out of the plastic process through an altercation of significant heating.
Rubber Injection Molding and Transfer Molding processes both begin with efficient preparation of material. First off, the material is mixed in bulk and immediately stripped off into continuous strips. The strips are fed into a screw with a pre-determined amount of rubber into a barrel.
Advantages of Rubber Injection Molding
- Elimination of pre-forms placement. Rubber Injection Molding removes the necessity for placing pre-forms in a compression molding or transfer molding by operators.
- Pre-heating materials. The technique apparently forces materials into cavities. Pre-heating materials allow for free flow into the cavities because of decreased viscosity.
- Complete dissemination of pre-forms. Pre-forms involves a lot of labor. Rubber Transfer Molding completely eliminates pre-forms thus removing the probability of affecting the final product. Pre-forms pose both weight and shape variability.
- Cuts down wastage of materials.
- Economical for both high procession components and high volumes of medium.
Disadvantages of Rubber Injection Molding
- High machinery cost. Despite flashing tooling, the Injection Molding process involves high initial tooling.
- Restrictions on parts design.
- Costly in small runs of parts.
Compression Molding is a method that allows manufacturers to create pre-forms in the shape of the end product. In this technique, manufacturers take either rubber compounds or mixed raw material and create pre-forms in the shape of the end product. This specification allows for total cavity fill due to a surplus of material provided by the pre-forms that need to fill the cavity.
Compression molding takes its toll when the mold is closed behind the pre-form. The preform is heated and pressure applied to it in order to fill the cavity. Excess pre-form material spills out into overflow grooves once the cavity is filled. The compression process paves way for de-molding of rubber. Usually, manufacturers de-mold the rubber by hand.
Once the rubber is de-molded manufacturers finally obtain a molded rubber product. Medium hardness compounds are normally a resolve of the compression process. Compression Molding is used also in applications requiring expensive materials and in low volume productions. This is due to minimal overflow amount/ flash that builds up during the molding process.
Applications of Compression Molding
- Simple O-ring drive belts.
- Complex graph diaphragms with more than 10 inches in diameter.
- Facilitates Timco to offer a variety of other molded products.
Advantages of Compression Molding
- Compression Molding is cost-effective for manufacturers in two instances; where tooling is already in existence and where the cross-section of the part is large thus requires a long time to cure.
- Maximum cavity count. Compression Molding allows for total cavity fill. This is due to the creation of pre-forms in the basic shape of the end product. The surplus pre-forms material fills the cavity.
This allows for maximized cavity count because pre-form material fills out the cavity before pouring out to overflow grooves.
- Economical for medium precision for medium hardness compounds.
- Cost effective tooling.
Disadvantages of Compression Molding
- Greater waste proportions. Once the cavity is overfilled, preforms material fills out in the overflow grooves.
- Slower process time
- High labor cost
Rubber Molding is a different kettle of fish. Its roots are traced back to Plastic Molding and its discovery through an altercation has since been resourceful to manufacturers.