What’s Transfer Molding? Process, Variations, and Uses

Did you know transfer molding can embed ready parts within the molded material? Electronics, medical, automotive, and other industries rely on this molding technique for regular and over-molded parts. This method involves heating raw material and injecting it into a transfer mold by compressing it with a plunger to create a final part. So, it combines the principles of injection and compression molding.

Moving further, we will discuss transfer molding processes, the history of technology, pros & cons, and application. At the end, we will also compare this with other molding types.

History of Transfer Molding

The invention of transfer molding technology is related to competition in marine vessel design and manufacturing after World War II. At first, 28-ft long glass fiber/polyester personnel boats were made for the US Navy force with this technique in 1946. However, this credit goes to a private contractor. Then, it is continuously evolving till today.

Here is the development history throughout the decades;

What’s Transfer Molding?

It is a prevalent manufacturing approach that primarily applies to thermosetting plastics and rubbers. The process involves placing a pre-measured molding charge into the upper part of the mold(known as the transfer pot), where the heating mechanism heats the material to a liquid or semi-fluid state. Then, the plunger compresses the charge and injects it into the mold cavity (lower part). Following that, a mold heating process ensures the proper cure necessary for setting. Finally, it creates the exact part while opening the mold.

How transfer molding process is distinct from compression and injection molding? It involves placing the material into a separate chamber before injecting it into mold cavities. In contrast, the liquid material is injected into the mold cavity during the injection molding process, and the material goes in the open mold during the compression molding before compressing with a press.

Furthermore, it is a precise and cost-effective way to mold thermoset resins and rubbers. The application is diverse, from electrical connectors and actuators to seals and gaskets for an aircraft.

Advantages of Transfer Molding

Since it combines compression and injection molding capabilities, you can find several advantages in precision, time, cost, and design flexibility.

Here is the list of transfer molding advantages;

• High compression pressure makes it more precise than compression molding.
• Although not as fast as the injection molding method, transfer molding has a faster production cycle than compression.
• It allows for the parts with more Shaper Edges because material transformation can fill even the tiny and most complex features cavity.
•  Inserts or embed parts are possible. For example, metal pins on an epoxy body to create an electrical connector.
• A simple pot and plunger setup makes the tooling cost cheaper.

Disadvantages of Transfer Molding

• The overflow grooves eject the surplus charge or feed while transforming from pot to cavities. As a result, it leads to material wastage.
• The additional charge transfer step makes it a longer process than injection molding.
• The transfer molded parts are more prone to void or air pockets due to the possibility of air trapping during the material transformation stage.

How Does Transfer Molding Work?

The transfer molding process is simpler than other manufacturing processes. As previously mentioned, it involves placing and heating the raw material into a separate chamber (generally the upper part of the mold) and then pressing it into the mold.

So, first, let’s understand the setup of transfer molding.

Furthermore, how does the process take place, or how transfer molding works by using this setup? The answer lies in the several chronological steps.

Step 1: Preparation of the Molding Compound

The process starts with feed or molding compound preparation. It involves the conversion of chosen resin into powder, small pellets, or granules form. Meanwhile, mixing colorants, plasticizers, stabilizers, or other additives can help to achieve desired properties or aesthetics. Additionally, the molding compound is heated to a predetermined temperature.

Step 2: Loading the Compound into the Transfer Pot

Next, the molding compound goes into the transfer chamber or pot with a gravity feed mechanism. However, ensuring the clean mold and proper releasing agents on the cavities wall is essential at this stage.

Step 3: Closing the Mold

The plunger pushes the molding charge into cavities with the help of a press(generally hydraulic). In this step, the material in the chamber goes into cavities through the sprue, followed by the closing of the mold.

Step 4: Applying Heat and Pressure

The continuous pressure and heat keep the mold close to cure the loaded resin. However, only uniform heating across the mold can result in consistent curing.

Step 5: Filling the Mold

The heat and pressure on the mold force the molding compound to flow across the cavities that fill the mold. At the same time, the goal is to achieve a uniform distribution of the material within the mold for consistent wall thickness and part strength.

Step 6: Curing the Material

Here, the heated and pressurized material undergoes a chemical transformation (cross-links and polymerizes) to take the shape of cavities. The essential parameters for curing are cure time, pressure, and temperature control.

Step 7: Opening the Mold

After curing, the plunger returns to its initial position (mostly with hydraulic control), releasing the pressure from the mold. It facilitates the opening of two mold halves. Here, the careful opening ensures the damage-free part.

Step 8: Ejecting the Finished Product

At last, ejector pins or a stripper plate facilitate the transfer of part ejection from the mold. The ejector pins’ position is also in a way that they push the part out of the mold without causing damage. Next, parts go for inspection and quality check.

That is how the whole process works!

Common Thermosetting Resins Used in Transfer Molding

Transfer molding processes are compatible with thermoplastics, thermosets, and rubbers, although thermoset resins are the most common.

Here are some transfer molding materials;

Epoxy Resins

Epoxy resins are the thermoset material that cures (hardens) while mixing with a curing agent, undergoing an irreversible chemical reaction. They provide excellent mechanical properties, low viscosity, electrical insulation, and chemical resistance.

Phenolic Resins

These are synthetic thermosetting resins made with the reaction of phenol with formaldehyde, known for their high thermal and mechanical stress-bearing capabilities. Next, phenolic resins show excellent electrical inertness and flame retardancy. Some uses of molding phenolic resins are heat shields and thermal insulators for automotive.

Polyester Resins

These resins are popular in the transfer molding process due to their versatility in formulation. You can easily formulate it with various additives, fillers, and reinforcements to meet specific application requirements. Consequently, polyester resins are suitable for durable (impact and abrasion resistant, chemically inert, and high-strength transfer molded parts.

Melamine Resins

Melamine resins consist of two elements: Melamine and formaldehyde. They are renowned for durability, surface hardness, fire retardancy, and heat resistance properties.

Silicone Resins

Various forms of silicone are used in transfer molding, such as fluids, elastomers, and hard resins. Some performance advantages of silicone resins are bio-compatibility, thermal stability, weather resistance, and electrical insulation. Furthermore, silicone transfer molding products are widely popular in sealing applications.

Applications of Transfer Molding

Transfer molding serves the various manufacturing purposes of different industries. The reason is its materials and complexity handling capability, insert molding feature, precision, and surface finish.

Let’s look at some specific transfer molded products across the industries.

Transfer Molding Variations

Resin Transfer Molding (RTM)

It is the most common variation of transfer molding, which involves using dry reinforcement composites, such as fiberglass, carbon fiber, or Kevlar for molding. At the same time, the raw materials also contain suitable additives and curing agents. After that, the process follows the standard transfer molding.

Next, the advantages are: The RTM method reduces the emission like styrene, it is fast and repeatable, processes versatile thermosets, and allows intricate details. Also, automation can provide precise and consistent parts. However, initial setup and tooling costs can be high.

Vacuum-Assisted Resin Transfer Molding (VARTM)

This process combines the principles of transfer molding (RTM) with vacuum assistance. Like regular resin transfer moldings, it involves placing dry reinforcement materials. However, the difference is that the mold is then sealed with a vacuum chamber or bag. Here, the vacuum assistance eliminates the trapped air in cavities. As a result, it helps to compact the feed resins and avoid void formation.

This transfer molding category addresses the potential structural and strength issues in molded parts and provides more design flexibility. As a result, it fits robust parts for automotive, aerospace, and industrial equipment.  

Micro Transfer Molding

This refers to the transfer molding of parts or products that are small in weight and size(a few grams and mm). The size of transfer molds, setups, and other equipment is also small. It enables the production of tiny and complex parts with precision, maintaining structural integrity.

The micro-transfer molding is suitable for manufacturing tiny electric connectors, actuators, microfluidic devices, micro-scale medical implants, drug delivery systems, and some micro-optical components.

Comparison with Other Molding Processes

Since the molding technology has other processes besides transfer molding, let’s compare it with two other approaches: injection and compression molding processes.

Injection Molding vs Transfer Molding

The injection molding process injects the molten form of material into a mold. In contrast, the transfer mechanism places the pre-measured resins in the transfer chamber and passes them to the mold. Thus, injection is suitable for thermoplastics and large volumes, while transfer is for thermoset composites and small to medium volumes. Subsequently, injection molding is a favorite for larger thin-walled parts, and compression is for thick parts.

Compression Molding vs Transfer Molding

The material is directly placed in an open mold during compression molding without heating it. So, this is the key difference between the two. Furthermore, the transfer molding process can create more precise and intricate detailing than compression. Next, compression is popular in thermoset and composite molding. However, the transfer approach is mainly for thermoset materials.

Table: Compression vs Injection vs Transfer Molding

Choose RapidDirect Molding Services for Plastic Parts

The transfer molding process involves many special considerations and chronological steps. It demands expertise and proper tooling to produce quality plastic parts. This is where RapidDirect’s expertise shines. Our team of seasoned engineers leverages extensive experience and advanced plastic molding facilities to transform your unique design specifications into functional, high-quality plastic parts.

At RapidDirect, we believe in the power of precision and the importance of custom solutions. Whether you’re in the automotive, healthcare, consumer electronics, or any other industry, our injection molding and comprehensive plastic part services are designed to cater to your specific needs. Our approach is collaborative and focused on delivering results that not only meet but exceed expectations.

Embarking on a project with us is effortless. Start by sharing your design vision, and let us take care of the rest. Request a quote today, and explore how our plastic molding services can bring unparalleled value to your next project.

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