Rubber molding is a standard way to give rubber shape in the industry. At rubber molding, the rubber is formed into an accurate shape by applying heat and pressure. Recent years have seen increased popularity of 3D printing as a manufacturing method yet its usage remains far below rubber molding and it faces several restrictions.
This blog will examine the different rubber molding methods used in designing and manufacturing rubber seals and how they differ.
Rubber molding manufactures a particular shape from raw rubber to a specific dimension and produces the final rubber component of the assembled machinery. The industry has three molding methods for rubber seals.
● Compression molding
● Transfer Molding
● Injection molding
All three methods follow the same main principle of rubber molding. All methods involve heating the rubber and stuffing it into a mold die, where it solidifies in the desired shape. The rubber is then vulcanized during molding, so it acquires an elastic property and retains its molded shape even after stretching.
This technique uses raw or unvulcanized rubber in the mold cavity for vulcanization or curing. The oldest method is compression molding, invented by Harvey Firestone in the 1890s.
● Fixed mass batches of raw rubber are mixed.
● Once the product has been conceived, the raw rubber material is shaped into a finished product called a preform.
● Two identical halves on an open mold are filled with preforms.
● By closing the two halves, they close the mold.
● In this step, the rubber takes on the mold shape and is vulcanized (heated and pressure applied to it so it becomes a solid).
● Before that, the rubber is removed from the mold by hand to show the final product.
● It is cheap to manufacture the tools and apparatus.
● The processing time is short.
● Compression molding has no significant waste of raw material.
● The technique is suitable for processing stiff rubber materials.
● The time-consuming part of taking out the mold by hand slows down the lead time and pace of manufacturing.
● Rubber extrusion is one extra step; process time slows manufacturing efficiency.
Compression molding is a molding method used for prototyping and small-scale production. This method is not used to mass-produce molded seals; it is only used to tool up a prototype design.
In transfer molding, a preform is placed in the mold chamber. The raw rubber is then placed into a cavity for curing. This method occurs as above, with the ‘preform’ rubber material put into the mold, but with more sophisticated tooling and molds having a more complicated shape.
● In fixed mass batches, raw rubber is mixed.
● It is put into a pot contained in a molding chamber.
● A plunger pushes the preformed material into the part cavity.
● Then, the rubber takes the shape of the mold and is vulcanized using heat and pressure.
● When the completed, it should be taken out of the mold.
● Multiple cavities are used simultaneously, leading to a short lead time.
● A few forms that can fill hundreds of molding cavities are needed.
● It is a more precise dimensional tolerance
● However, rubber components with custom colors are easier to produce.
● Higher precision in shapes.
● There is also a lot of raw material waste associated with this technique.
● The problem is that the rest of the rubber hasn’t been transferred out, thus left in the pot and must be binned or recycled.
● Tooling costs more.
This molding method has the significant advantage of being perfect for low-volume manufacturing and producing parts with precise dimensions. Transfer molding can make more finished products instead of being limited to one specific application. It is ideal for bonding rubber to delicate metal parts, sealing composite materials with rubber, etc.
It is widely thought to be the most efficient way to mold rubber and involves the injection of molten polymer at high temperature and pressure into a closed mold cavity. We say that this method was invented more recently, about in the 1960’s
Fixed mass batches of raw rubber are introduced.
After the mixing, the raw material is cut to uniform dimensions, for example, 1.25-inch strips 0.375-inch thick.
The strip will then be placed on screws on injection molding machines. The screw refills the barrel with the right amount of material.
After the mold is filled, it is shut down.
The barrel is injected with the material to be supplied into the mold cavity.
The rubber is entered into a heat and pressure cure.
Once it’s ready, the finished product will be de-molded.
● High production rates
● It can use different rubber materials like silicone, neoprene, nitrile, and EPDM.
● Minimal waste
● Other critical fast items include cycle and time to production.
● There is no need for preforms, eliminating the time and labor needed to produce them.
● Eliminating the option for the operator to perform altogether.
● The injection screw heats the material before injecting it into the cavity, reducing the viscosity of the material, and therefore it is injected into the cavity.
● Cure time might not be as long as many believe because the rubber is preheated.
Potential for shorter curing time that resulted from heat when the screw is being charged.
● Setup is expensive, and changeover is costly
● Not able to adapt to the low volume production.
Rubber injection molding should not be used for prototyping because a mold would cost money. However, it is appropriate for large-scale rubber-finished items, such as consumer goods, gaskets, seals, and electronic components. Automobile, medical, and aerospace goods are all made by injection molding.
There are three major processes: compression molding, transfer molding, and injection molding. All the methods have some advantages and should be used under certain circumstances. Molding is commonly used to form relatively large-volume rubber seals.
Transfer molding has the advantage of better control of the seal shape or contour. Injection molding is the best method to manufacture intricate seals with acceptable tolerance. If one knows the above essential molding methods for rubber seals, one can choose the appropriate one according to the size of the rubber seals and the application.
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