Operator removes transfer pad from open tool in Mexico

We often quote a “wasteless” transfer molding process for OEM gasket programs, especially for gaskets and seals made with HCR silicone rubber.

But what is this distinction of “wasteless” and why does it matter?

First let’s take a look at the Transfer Molding process, and how it differs from injection molding and compression molding.

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.

Perimeter Seal molded with Orange HCR Silicone

Now that we’ve outlined transfer molding, what makes *wasteless* transfer molding a specialized process?

Wasteless tools are tools that do not require any post-processing to finish the part after molding. Parts are molded with no tear-trims, no flash, and no sprue/gate cleanup by the operator. The only waste is in the sprues themselves, which are pulled out when the tool is opened.

Historically, transfer molding was high-waste using a hot pot with tear trim. The pot, sprues, and cavity is cured with every cycle. This process involved very simple tools with large tear trim.

Eventually the process improved to utilize a cold pot with tear trim. Only the material in the spures and cavity is cured, and pot is kept below cure temp.

Minimum waste (aka wasteless) molding has a cold pot with no tear trim and includes other fill/venting features.

What types of parts or programs make the best candidates for wasteless transfer molding?

The requirements for transfer molding are based on three factors: part size, design, and material.

Smaller parts are easier to reject air out of without using an excessive number of sprues (and therefore waste). The design of the part is important too, with many different features that dictate wasteless is possible. The material is also important, with lower shear rate forces help with filling the cavities cleanly.

If the part can't meet these criteria, then tear trim and other features are typically added to assist with the molding process.

Compared to non-wasteless transfer molding, what are the main benefits to our customers for molding with a wasteless transfer process?

There are two main benefits to this process: reduced process cost and increased process reliability.

The reduced process cost is a combination of lower waste, reduced operator work, and improved cycle times.

The need for manual removal of material of the tear trim leaves intentional flash on the part, and careful inspection must be done to guarantee that all of it is gone. There's always a risk of problems like parts having some trim left on, or the flash being too large and causing functional issues, etc.

How does wasteless transfer molding compare to LSR molding, which is perfectly wasteless / flashless?

Injection tools are controlled by an injection valve to inject the material into the cavities. From this one location, the material goes through runners to multiple cavities. The operator then has to remove that runner along with the parts. More valve gates can be added to use fewer runners (or no runners at all), but the tooling cost is typically very prohibitive to do so.

On HCR injection tools, this single injection point process requires long runners, careful balancing, and extra features to eject the air out of the cavity to prevent non-fills. That typically means we have to include a tear-trim and larger gates/sprues into the parts.

For LIM tools, the runners can possibly be removed automatically, but it is part dependent. They usually don't require tear-trim due to the easy fill.

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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What is "Wasteless" Transfer Molding?