Transfer Molding OEM filter plates in Mexico

What’s the difference between Compression Molding and Transfer Molding?

Transfer molding and compression molding are related processes, but they are not the same. However, transfer molding can be considered a variation of compression molding with some key differences. Let's clarify the distinctions between these two processes, and watch a video that shows our compression transfer molding process in action.

First: There are two types of transfer molding. Injection transfer molding, and compression transfer molding. For the purpose of this article, we’ll focus on compression transfer molding, and the way it is distinct from compression molding. Here we go.

Compression Molding

1. Process: Compression molding is a traditional molding process where a pre-measured amount of material, often in the form of a pre-cut or pre-shaped piece (a "charge"), is placed in an open mold cavity.

2. Mold Closure: The mold is closed manually or by a hydraulic press, which squeezes the material to fill the mold cavity. The material is usually in a solid or semi-solid form.

3. Pressure and Heat: Pressure and heat are applied to the mold to facilitate material flow and curing. The pressure is usually maintained throughout the curing process.

4. Material Flow: In compression molding, the material flows and conforms to the shape of the mold cavity through the force of compression. It is not forced into the cavity through a sprue or runner system.

5. Flash: Flash, which is excess material that escapes from the mold cavity during molding, is common in compression molding. It needs to be trimmed or removed in post-production.

Transfer Molding

1. Process: Transfer molding is a closed-mold compression process that combines elements of both compression molding and injection molding.

2. Material Placement: A pre-measured amount of material (similar to a charge in compression molding) is loaded into a heated chamber called a "transfer pot" or "chamber."

3. Material Transfer: The material in the transfer pot is softened by heat, and a plunger or piston forces it through a sprue and runner system into a closed mold cavity - sometimes through a fabric ‘transfer pad’.

Fabric Transer Pad used in Transfer Molding

Side note: What’s the point of the fabric pad?

The fabric pad helps to allow for easy removal of the sprues from the mold. You can see this at 0:52 in our video, below.

4. Pressure and Heat: Like compression molding, transfer molding applies pressure and heat to facilitate curing. However, the pressure in transfer molding is primarily used to force the material into the mold cavity rather than just compressing it.

5. Flash: Transfer molding generally produces less flash compared to traditional compression molding because the closed-mold system helps contain the material. This results in less waste and reduced post-production trimming.

What about *Injection* Transfer Molding?

At Morgan Polymer Seals, we use both compression transfer molding and injection transfer molding to manufacture rubber gaskets for our OEM customers. The main difference to note is that compression transfer molding involves preheating and transferring rubber material to a mold cavity, typically resulting in longer cycle times and manual operations, while injection transfer molding uses automated, direct injection into the mold cavity.

In summary, while both transfer molding and compression molding involve the use of pressure and heat to mold rubber or plastic materials, transfer molding distinguishes itself by its closed-mold system, precise material placement through a sprue and runner system, and typically reduced flash generation.

Click below to watch a 2-minute video that shows our transfer molding operations in Mexico.

Video: Transfer Molding rubber gaskets and seals in Mexico

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Compression Molding vs. Transfer Molding