What is Rubber Transfer Molding- an Overview

Rubber Transfer Molding

Rubber Transfer Molding, also known as Transfer Compression Molding, Silicone Transfer Molding, which is ideal for the production of solid silicone rubber parts, rubber parts with inserts, and parts that require bonding rubber to metal. Rubber transfer molding combines the advantages of injection rubber molding  & compression molding. Transfer molding begins by measuring the molding material and placing this uncured silicone rubber in a “pot” that is a part of the mold. The mold is then closed and a plunger compresses the material (Normally, it is 3-plates mold tool, but it doesn’t ends at 3-plate, can be 4 or 5). As heat is applied, pressure forces or “transfers” the uncured rubber through sprues into the mold’s cavities, giving this molding process its name.

How Does Rubber Transfer Molding Work?

To complete Rubber Transfer Molding process, there are steps as follows

• Make mold tools - prepare steel material, complete the product & mold design.

• Prepare raw materials - Selected the type of material you want to use and calculate the proper amount of material. If too much material, excess material will seep out of the cavity and result in big flash which would be difficult to remove.

• Set up the machine parameters - after the mold is well-done, can get ready for the sampling to ensure the part dimension and surface is what we required, need to set up the machine parameters and heat up the mold.

• Place a piece of uncured rubber into a portion of the mold.

• The mold is closed up and under hydraulic pressure, the rubber is forced through a small hole into the cavity. The mold is held closed while the rubber cures.

• The plunger is raised up and the transfer pad material is removed.

• Mold is opened and the part can be removed. Any flash is trimmed and removed during this time. (Trimming flash, a secondary process that removes unwanted extra overflow material called flash )

Notes:

There are some ways to remove the waste edge or flash from a molded rubber product:

• Tear trimming manually

• Cryogenic processing

• Tumbling

• Precision grinding (for the molded silicone rubber parts, like silicone sleeve with big parting line, we can minimize the parting line via this way)

Rubber Transfer Molding Advantages

• Less Flash: Transfer molded parts inherently have less flash (excess material that runs along the parting line of the mold) than their compression molded counterparts because the mold remains closed when the material enters the mold cavity.

• Design Flexibility,Diversity and Inclusiveness:  Transfer molding allows for sharper edges. Micro grind vents reduce the need for overflows allowing for near flash less parts, greatly reducing deflash process or allow flash limits to directly ship parts. With pot and plunger design, simplified preforms required allowing for standardization and lower cost.

• Mold with More Cavity: Transfer molded silicone rubber products require few and simple prepared material. One prepared material can fill many cavities. Compare with silicone compression molding, it is an advantage and can save a significant amount of time in the molding process, which is also an advantage over injection molding since there is not a runner to fill each cavity.

• Allow Different Materials of Inserts: One of the key advantages of transfer molding over compression molding is that different inserts, such as metal parts, plastic chips, dry composite fibers, ceramics, etc., can be placed/positioned in the mold cavity before the material is injected into the cavity. This ability makes transform molding the leading manufacturing process for integrated circuit packaging and electronic components with molded terminals, pins, studs, connectors, and so on.

• Short Production Cycle. Silicone Rubber Transfer molding offers shorter cycle times than compression molding and can also provide more consistency. The process allows for tighter tolerances and more intricate parts.

Rubber Transfer Molding Disadvantages

• Material Waste: However, transfer molding still produces more waste material than compression molding because of the sprue, the air holes and the overflow grooves that are often needed to allow air to escape and material to overflow.

• Silicone Transfer Molding Mold is more expensive than compression mold, so tooling investment can be somewhat higher.

• Transfer Molding has Slower production cycle than an LSR injection tool, sometimes limiting the overall production rate, as changeover times can be somewhat extended.

  
  

Rubber Transfer Molding for Rubber to Metal Bonding

Transfer molding is an important part of the rubber to metal bonding process or over molding, the components are loaded into a heated mold either by hand or using a loading fixture.  Material is then loaded into the well pot of the transfer mold.  Once the mold closes, the rubber is heated and then transferred through the runner. The gate sprues into the mold cavities and around the metal insert.  With the pressure and elevated temperatures, the rubber cures and vulcanizes, allowing the parts to be removed.