Transfer Molding

Transfer molding is a unique molding process that has evolved substantially since its invention. Its uniqueness is due to its hybrid nature, as it combines the fundamentals of both injection and compression molding. 

The transfer molding technique is a versatile and efficient manufacturing process that can be employed for various applications. In this article, we discuss the basics of the transfer molding process, its variations, advantages, and challenges in detail. 

Table of Contents

What is Transfer Molding?

Transfer molding is a type of manufacturing process that involves pressing a casting material into a closed mold. It includes creating a product by filling a mold cavity with raw material. The raw material is first injected into the mold cavity, after which the raw material solidifies and creates the casting. The casting is further, ejected from the mold cavity, which is the product. The mold used in this technique is hollow. The hollow cavity of the mold has an inside surface that defines the product shape.  

The transfer molding process is a precision manufacturing process as it involves the transfer of a pre-measured amount of heated material into the mold cavity. 

This particular manufacturing process applies generally to thermosetting plastics and rubbers. Though this process works best with thermosets, it can work with some thermoplastics as well.

Types of Transfer Molding Processes

transfer molding types

Resin Transfer Molding (RTM)

Resin transfer molding is one of the most common transfer molding variations. In this process, liquid resin is injected into a closed mold that contains pre-placed reinforcements which are then cured to form a solid composite part. This process primarily involves incorporating dry reinforcement composites, such as fiberglass, carbon fiber, or Kevlar for molding. The raw materials also include additives and curing agents. 

The initial setup and tooling costs of RTM can be high, however; RTM processes versatile thermosets, and is fast and repeatable.

Vacuum-Assisted Resin Transfer Molding (VARTM)

VARTM combines the principles of resin transfer molding, however; it incorporates a vacuum chamber. In this particular process, dry reinforcement materials are placed and the mold is then sealed within a vacuum chamber. The vacuum eliminates any type of trapped air in the cavities by compacting the resins. This particularly avoids any type of void formation.

VARTM provides more design flexibility and addresses potential structural and strength issues.

High-Pressure Resin Transfer Molding (HP-RTM)

HP-RTM is another variant of RTM that aims to reduce resin injection times and improve part consolidation by incorporating higher injection pressures. In the case of HP-RTM, the resin injection time is typically around 1-5 minutes which is significantly shorter than the resin transfer molding process, which can take between 30-60 minutes.

Microtransfer Molding

Micro Transfer Molding refers to manufacturing products that are small in size and weight via the transfer molding process. For instance: Tiny electric connectors, microfluidic devices, micro-scale medical implants, and some micro-optical components. These components can weigh a few grams and size can be in millimeters thus, the size of transfer molds, setups, and other equipment is also small. 

Micro Transfer Molding enables the manufacturing of small, complex parts with precision.

Transfer Molding Materials


Epoxy resins are thermosets that undergo an irreversible chemical reaction upon curing. 

Epoxy resins are versatile and exhibit low viscosity, excellent mechanical properties, electrical insulation properties, and high chemical resistance. Its low viscosity allows it to easily flow during the transfer molding process, thereby ensuring precision and accuracy during replication.


Silicone is known for its flexibility, durability, thermal stability, weather resistance, temperature resistance, and bio-compatibility. It is thus employed for various applications.


Polymers such as polyurethane and polyesters are apt for transfer molding due to their versatility and various properties. 

Polyurethane demonstrates excellent flexibility, strength, and chemical resistance thereby, making it ideal for applications where durability and elasticity are a priority. 

Polyester demonstrates good heat resistance and mechanical strength and they can be easily formulated with various additives, fillers, and reinforcements to meet specific application requirements.

Rubber / Elastomers

Natural rubber or synthetic elastomers are easily processed via the transfer molding process. They are recognized for their elasticity and resilience. You should note that post-curing or vulcanization processes may be required to achieve the required properties.


Some resins such as ABS can’t be used as they demonstrate limitations in terms of temperature resistance and dimensional stability but various plastics such as polypropylene and polycarbonate are considered for transfer molding for various applications. Polypropylene demonstrates excellent flow properties, chemical resistance, and dimensional stability and Polycarbonate demonstrates high impact strength, transparency, and heat resistance.

Transfer Molding Versus Injection Molding and Compression Molding

transfer molding vs injection molding

Transfer Molding Versus Injection Molding

Transfer molding and injection molding are both popular manufacturing techniques but they differ in various aspects. Let’s understand these aspects further: 

  • In transfer molding, the material is fed into a heated transfer chamber after which the plunger forces it into a mold cavity whereas in injection molding, the molten material is injected directly into a mold cavity. 
  • Transfer molding is applied for low to medium-volume production runs of simple to moderately complex molds whereas injection molding is best suited for high-volume production runs that can support simple to extremely complex mold geometries. 
  • In injection molding, the molten material moves directly into the mold cavity and thus, in most cases, it takes between a few seconds to a minute to manufacture one product whereas in transfer molding, the time taken to manufacture 1 product is significantly more. In the case of transfer molding, the material is preheated, after which it moves from the pot into the sprue and then cures inside the mold. 
  • Injection molding demonstrates better precision than transfer molding. 
  • The tooling cost in injection molding is considerably higher than the tooling cost of transfer molding.  
  • Injection molding is preferred for larger thin-walled parts than transfer molding.
  • Injection molding injects molten material directly into the mold at high speed which is why the cycle times are shorter than transfer molding. 
  • Transfer molding is particularly suitable for thermosets although they can work with some thermoplastic materials as well whereas, injection molding is more suitable for thermoplastics but can also mold a few thermosets. 
  • Injection molding and transfer molding processes differ in terms of material compatibility and machine configurations, thus, the applications and use cases are different from each other. Transfer molding is best suited for manufacturing small rubber parts such as seals, pins, and connectors whereas, injection molding is suited for manufacturing various plastic parts across a range of industries of small to large size.
  • All manufacturing processes have setup costs, operational costs, and maintenance costs. In terms of costs, an injection molding machine typically ranges between $50,000 to $100,000 and more whereas, transfer molding machines are relatively cheaper, costing between $10,000 to $50,000.

Transfer Molding Versus Compression Molding

Transfer and compression molding are quite similar but they distinctly differ in some areas. 

  • The first difference is how the molding material is pressurized. In the case of transfer molding, the material is preheated and pressurized in a separate chamber and after that is forced into a mold cavity whereas in compression molding, the material is directly placed into the mold cavity, after which, pressure is applied to the entire mold. 
  • Transfer molding technique creates more precise and intricate detailing than compression molding. Compression molding is often used for simpler shapes.
  • Compression molding is more popular for thermosets and composites whereas transfer molding is mainly applied for thermoset materials.
  • The transfer molding technique may not be as fast as injection molding but, it typically offers faster cycle times than compression molding due to its ability to inject the molten material into the mold under higher pressure. 

Transfer Molding Process Characteristics

Durability of Transfer-Molded Products

The lifespan of transfer molded products is dependent upon a variety of factors such as the material used, its environmental conditions, and its intended application but they typically demonstrate increased durability as they have a good strength-to-weight ratio. 

The specific durability of a transfer molded part depends upon its characteristics and usage conditions but it is typically between 5 to 20+ years.

Resistance to Heat

The heat resistance of transfer-molded parts is largely dependent upon the material type such as silicone rubber and phenolic resins that demonstrate excellent heat resistance as they can withstand extremely high temperatures without degradation. 

You should note that not all thermoplastics are suitable for transfer molding. For instance: Polyethylene is a type of thermoplastic that isn’t suitable for transfer molding due to its low melting point. 

Transfer Molded Parts are Stretchable

The stretchability of transfer-molded products is determined by the type of material used and the part geometry. Flexible polymers, elastomers, silicones, and polyurethane thermosets create stretchable products via transfer molding.

Transfer Molding Process Applications

The technique of transfer molding can be applied to various applications due to its materials and complexity handling capability, its precision, surface finish, and insert molding features.

Electronic Industry

The transfer molding process is ideal for encapsulating delicate components thereby providing insulation and protection against various environmental factors. It is thus, popularly applied to manufacture insulators, connectors, circuit boards, switches, and transformers in the electronic and electrical industry.

Medical Device

In the medical device industry, the transfer molding technique is employed especially for silicone overmolding which is particularly applied for manufacturing handles of medical devices and surgical instrument components. The transfer molding process helps to enable the production of sterile, and precise biocompatible components. 

Rubber Products

Rubber transfer molding technique is applied to manufacture Grommets, Washers, and O-rings due to its ability to create detailed and complex shapes with precision and consistency.

Natural Gas Industry

The transfer molding technique is often applied to create metal-to-rubber face seals that are particularly used in the natural gas industry to create interfaces for gas valves.

Automotive Parts

The transfer molding technique manufactures parts that demonstrate high dimensional accuracy and can withstand harsh conditions. They are thus, popularly used to manufacture various automotive parts such as seals, gaskets, dashboards, door handles, and engine components.

Hydraulic Industry

Transfer molding is often used to manufacture lip or U-cup designs in the hydraulic industry as it allows sharper cutoffs during molding. A defined lip edge leads to sharp lip seals thereby creating flash-free parts.

Transfer Molding Process

Transfer Molding Process Setup

transfer molding process setup
  • Transfer Chamber – The transfer chamber constitutes the pot which holds the material.
  • Heater –  The heater surrounds the pot or the transfer chamber to heat the material. 
  • Plunger – This particular component applies pressure to the molding compound and pushes the material into the mold cavities. It is positioned above the transfer chamber.   
  • Upper and Lower Mold Parts – The two halves of the mold come together to create the mold cavity. The material enters from the upper mold part and the lower part forms the core. 
  • Hinge – The hinge component of the transfer molding system allows the feed to transfer into the mold after heating. 
  • Ejector Pin – This particular component facilitates the ejection of the final product from the mold.

Transfer Molding Process Steps

transfer molding process steps

Molding Compound Preparation

The transfer molding process begins with the preparation of the molding compound. At this stage, the resin is converted into powder, pellets, or granules. In addition, if it’s required, colorants, plasticizers, stabilizers, or other additives are added to help, achieve a particular property or aesthetics.

Material Loading into the Transfer Chamber

In this step, the prepared compound is now placed in the transfer chamber through a gravity feed mechanism. It is critical to ensure that the mold is cleaned thoroughly at this stage.

Mold Closing

In this step, the material moves from the transfer pot to the cavities after which, the mold is closed. Here, the plunger pushes the material into the cavities, generally, with the help of a hydraulic press.

Apply Heat and Pressure for Mold Filling

A continuous pressure is applied to keep the mold closed. Along with the pressure, heat is also applied to cure the material. You should note that only when there is uniform heating across the mold, a consistent curing can take place.

The heat and pressure force the material to flow across the cavities to fill the mold. The heated and pressurized material undergoes a chemical transformation to take the shape of cavities. Some of the essential parameters for curing the material are time, pressure, and temperature control.

Mold Opening

Once the material is cured, the plunger returns to its initial position due to hydraulic control, which further releases the pressure upon the mold. A careful opening needs to be considered to ensure a defect-free or damage-free part.

Ejecting Finished Part

In this stage, the part is ejected from the mold through ejector pins which are placed in such a way that they push the part out of the mold without causing any damage. 

Benefits and Challenges of Transfer Molding Process

Why should you choose the Transfer Molding Process?

Transfer molding offers several advantages to manufacturers. Let’s understand the several advantages of the process:

transfer molding advantages
  1. Transfer molding process enables greater design flexibility compared to other molding methods. For instance: This method enables intricate designs with sharper edges because material transformation can fill the cavities of most complex features. Thus, it is possible to manufacture complex geometries with high precision and tight tolerances.
  2. The high pressure and temperature used in this process produce products of excellent durability and superior mechanical strength. Thus, making them ideal for demanding environments.
  3. It allows easy integration of inserts. For instance: Metal components in electronic parts can be easily inserted. 
  4. This process enables a high cavity count. Transfer molded products typically require few pre-forms and one pre-form can fill hundreds of cavities. 
  5. Transfer molding eliminates deflashing as it manufactures products with minimal or no flash. The micro-grind vents greatly reduce overflows thereby, reducing deflash and allowing for flash-less parts. 
  6. This process also helps to yield parts with a smooth and finished surface which further reduces the requirement of finishing steps. 
  7. Transfer molding reduces waste as there is precise control over material distribution. In this process, excess material stays in the chamber instead of the mold which can be reused, thereby reducing overall material waste. 
  8. Transfer molding has a simple pot and plunger setup which typically lowers the tooling and equipment cost when compared to other molding techniques. Since there is no complex machinery, the setup is simple and easy to maintain. 

Transfer Molding Process Challenges

While transfer molding has various advantages, there are some challenges to the process as well. Let’s understand these challenges further: 

  1. Transfer molded parts are prone to air pockets as air can get trapped easily in the mold during the material transformation stage thus, additional measures are required to ensure that air is evacuated correctly. 
  2. Comparatively, it has a slower production rate due to the additional step in material preparation and transfer.  The transfer molding process requires that the materials are preheated before they enter the mold.
  3. Material wastage is a concern during the preparation and transfer phases. This is when the sprue and overflow grooves are used which can further lead to the discarding of excess material. You should note that since the polymers are generally thermosets, the scraps are not reusable thus, the material gets wasted during mold transfer as some quantity of the material solidifies in sprue and overflow grooves.
Contact Us

The transfer molding process requires expertise and tooling experience to produce high-quality parts. It is crucial to understand the specific requirements of a project to determine whether transfer molding is suitable.

While we are more focused on injection molding and tooling, our wide range of services also include prototyping, retail assembly and more.