All About Transfer Molding

An Intro to Transfer Molding

Transfer molding is a manufacturing process that entails pressing a casting material into a closed mold. Transfer molding is distinct from compression molding in that it uses an enclosed mold. It involves transferring a measured amount of material, in a preheated and softened state, into a closed mold cavity under pressure. The material is then cured or solidified to form the shape you want. 

Transfer molding is primarily used to encase electronic components in rubber or plastic. It enables the fabrication of plastic parts with metal inserts, such as prongs or semiconductor chips. Pins, studs, connectors, and molded terminals can all be produced using this method.

Furthermore, transfer molding allows for the production of parts with sharper corners and edges. This feature is particularly advantageous in industries such as hydraulics, where the sealing of fluids is crucial. Lip seals, which are used to prevent fluid leakage in hydraulic systems, often require precise edges to ensure effective sealing. Transfer molding enables the creation of sharp and well-defined edges, which enhances the performance and reliability of such seals.

The Transfer Molding Process

Transfer molding is a process that combines compression and transfer of the polymer charge. It involves transferring a pre-weighed polymer charge to a mold cavity from a heated transfer pot. The charge is fed into the transfer pot by gravity feed, but it is not directly injected into the mold cavity. Instead, it is introduced into the mold cavity under the pressure of a plunger. The molded part is then ejected using an ejector pin after the resin has had time to cure. Normal post-processing requirements include trimming off the sprue and gate. Figure 1 below illustrates the basic process of transfer molding:

The duration of the transfer molding process can vary depending on several factors, including the size and complexity of the part, the type of material being used, and the specific production requirements. Compared to compression molding, transfer molding generally offers faster cycle times due to its ability to inject the material into the mold under higher pressure. On the other hand, injection molding typically has shorter cycle times than transfer molding, as it involves injecting molten material directly into the mold at high speed.

RTM (Resin Transfer Molding) is a process where liquid resin is injected into a closed mold containing pre-placed reinforcements and then cured to form a solid composite part. HP-RTM (High-Pressure Resin Transfer Molding) is a variant of RTM that utilizes higher injection pressures to reduce resin injection times and improve part consolidation. However, in the case of high-pressure resin transfer molding (HP-RTM), the resin injection time is typically around 1-5 minutes. This is significantly shorter than traditional resin transfer molding (RTM), which can take approximately 30-60 minutes. This shorter production cycle time makes HP-RTM an attractive manufacturing process for large composite parts in the automotive industry.

Transfer Molding Vs. Injection Molding: Similarities and Differences

Transfer molding and injection molding are both popular techniques for manufacturing plastic parts, but they differ in several ways. In transfer molding, the material is typically fed into a heated chamber through a screw, and then a plunger forces it into a mold cavity. Injection molding, on the other hand, uses a reciprocating screw to melt and inject the material directly into the mold cavity. Transfer molding is commonly employed for encasements and low-volume production of simpler molds, while injection molding is well-suited for larger, thin-walled parts. Injection molding offers higher production rates and better precision, while transfer molding offers lower tooling complexity. Transfer molding necessitates preparing the raw material before it is forced into the mold, increasing processing time and raising costs. In contrast, injection molding instantaneously mixes and readies the material, allowing for immediate production. This disparity in material preparation leads to differing production timelines and cost structures. Making the choice between transfer molding and injection molding is dependent on specific requirements and constraints. For more information, see our guide on Injected Plastics.

Materials Used in Transfer Molding

Some of the materials used in transfer molding include:

1. Epoxy

Epoxy is a versatile thermosetting polymer that exhibits excellent electrical insulation properties and high chemical resistance. Its low viscosity allows for easy flow during the transfer molding process, ensuring precise replication of mold details. Epoxy can be brittle and may require post-curing processes for optimal mechanical strength.

2. Silicone

Silicone is a versatile polymer renowned for its flexibility, durability, temperature resistance, and biocompatibility, making it highly desirable for various applications. However, compared to other materials, silicone can be relatively more expensive due to its unique properties and production processes. Additionally, the curing time of silicone can be longer compared to materials such as epoxy resin.

3. Polymers

Polymers like polyurethane and polyester are suitable for transfer molding due to their desirable properties. Polyurethane offers excellent strength, chemical resistance, and flexibility, making it ideal for applications requiring durability and elasticity. Polyester, on the other hand, provides good heat resistance and mechanical strength. These characteristics enable both polyurethane and polyester to withstand the heat and pressure involved in transfer molding processes. 

4. Rubber

Rubber refers to elastomers with high elasticity and resilience. Rubber materials, whether natural rubber or synthetic elastomers, can be easily processed using transfer molding. Depending on the type of rubber, post-curing or vulcanization processes may be required to achieve optimal properties. For more information, see our guide on TPR Material.

5. Plastics

Plastics like polypropylene and polycarbonate are suitable for transfer molding due to their favorable characteristics. Polypropylene exhibits excellent flow properties, allowing it to fill intricate mold cavities easily. It also offers good chemical resistance and dimensional stability. Polycarbonate, on the other hand, possesses high impact strength, transparency, and heat resistance, making it ideal for applications requiring these properties. Some plastic materials may have limitations in terms of temperature resistance, dimensional stability, or strength, requiring careful selection for specific applications.

Products made by transfer molding are renowned for their exceptional durability. Because they have a good strength-to-weight ratio, they can be used for many different things, including heavy objects. The addition of reinforcing fibers and the high strength-to-weight ratio that is made possible with a fiber load of 25–50% also contribute to transfer molding's improved quality and durability, particularly in thicker-walled parts.

Transfer-molded products can be heat resistant, depending on the specific material used in the process. Certain materials, such as silicone and certain thermosetting plastics, exhibit excellent heat resistance. They can withstand high temperatures without deformation or degradation. It's important to keep in mind that not all thermoplastics are suitable for transfer molding, even though certain materials can provide heat resistance when used in this process. Polyethylene is one type of thermoplastic that has a reputation for being less suitable for transfer molding because of its weaker heat resistance. Due to its low melting point, polyethylene might not be able to withstand the high temperatures and pressures used in transfer molding. To ensure the desired heat resistance for a specific application, it is advised to consult the material specifications and carry out the necessary testing.

Common Applications of Transfer Molded Products

Transfer molding finds applications in various industries:

1. Electrical Industry: Connector seals can be molded around wires, with spark plug wires being a common example.
2. Natural Gas Industry: Transfer molding is used to create metal-to-rubber face seals, providing a reliable interface for gas valves.
3. Medical Industry: In the medical field, transfer molding is extensively utilized for silicone overmolding, particularly for medical device handles and surgical instrument components.

The Pros of Transfer Molding

Advantages include:

1. Allows for the easy integration of inserts, such as metal components of electronic parts, into the molded product
2. Allows for intricate designs with sharper edges, providing greater design flexibility compared to other molding methods
3. Produces parts with minimal or no flash, eliminating the need for additional deflashing processes
4. Typically involves simpler pot and plunger designs, resulting in lower tooling and equipment costs compared to other molding techniques

The Cons of Transfer Molding

Disadvantages include:

1. Has the potential for material wastage, particularly during the preparation and transfer stages. This is primarily due to the use of sprue and overflow grooves, which can result in excess material that is discarded. Additionally, transfer molding is not conducive to recycling thermosetting polymers into the process, further increasing production costs and environmental impact.
2. Generally has a slower production rate compared to injection molding due to the additional steps involved in material preparation and transfer
3. Air can get trapped in the mold during the transfer process. This can lead to defects in the final product and require additional measures to ensure air is properly evacuated

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