Injection Molding vs Transfer Molding: Key Differences Explained

Plastic and rubber parts are a common sight in the industry. Most of these parts are made through molding techniques, with injection and transfer molding being the key examples. But in Injection molding vs Transfer molding, which technique is better? That’s the question most manufacturers and product developers raise.

This article aims to provide that answer by thoroughly comparing these two molding methods. It will detail their strengths and weaknesses, and discuss suitable applications for each technique.

What’s Transfer Molding?

Transfer molding is a plastic molding technique in which a preform thermosetting polymer is placed in the pot, where it’s heated and pressurized to flow into a closed cavity. It cures and attains the desired shape.

It has two variants. In one case, the preform plastic is in a ‘pot’ and it flows into the mold cavity via vertical runners. Silicone rubber molding is an example. In the second case, a plunger forces the heated material to move through side channels into the mold. Resin transfer molding is an example of this variant.

The transfer molding technique is best for thermosetting materials but can also work for some thermoplastics. Compatible materials are silicone rubber, unsaturated polyester, epoxy, and PF plastic.  

Advantages and Disadvantages of Transfer Molding

It is a simpler process and offers multiple advantages over other methods.

• Low Setup and Maintenance: The setup is simple to make and easy to maintain since there is no complex machinery.
• Best for Metal Inserts: Molding is better suited for adding metal inserts to molded products.
• Sharper and Thicker Edges: Higher pressures enable sharper, more defined edges.
• Part Design Flexibility: Accommodates complex shapes and intricate designs with ease.

However, it does fall short in some aspects such as:

• Slower Production Speed: The transfer process molding is slower compared to its counterpart, primarily due to the time needed to preheat materials before they enter the mold.
• More Material Waste: Material gets waste during mold transfer as some of it solidifies in sprue and overflow grooves.
• Small Volume Production: Transfer molding machines are usually designed for fewer cavities, meaning they produce less output per cycle. They are best for low to medium production.

What’s Injection Molding?

Injection molding is an advanced form of transfer molding in which a thermoplastic, granular form, is fed into the machine. It is heated to change it into highly plastic form and then injected via screw-ram directly into the mold. The whole process is quicker and takes 30 seconds to a minute to create a part.

Plastic injection molding is ideal for making complex and intricate shapes. But with more cavities and complex designs, it becomes slightly expensive to make the mold. However, in the longer term, or bulk production of quantities of larger thin-walled parts, the process becomes economical.

The part weight in injection molding can be as light as 50 g (simple casings) and as high as 25 kg when making large parts such as refrigerator doors and car bumpers.

The injection molding process primarily involves thermoplastics. However, one can also mold thermosets and elastomer but you’ll have to make some modifications in equipment. The common materials best suited for the process are liquid rubber, polyethylene, polyamide, PVC, ABS, and PP.

Advantages and Disadvantages of Injection Molding

It is a relatively modern version of the rubber transfer molding process, so it performs better in certain areas. Here are some strengths of the process:

• Quicker Process: The process is relatively quick, with production cycles between 30 seconds to a minute. For larger parts, the cycle might extend slightly.
• Complex and Intricate Parts: This method allows for the creation of complex and intricate shapes. It can handle parts ranging from 50 grams to 25 kilograms.
• Thin-Walled Casings: Particularly effective for manufacturing thin-walled casings for equipment. Ensures precision in molding thinner structures.
• Economical for High Volumes: The molds are expensive to make, which makes the process more economical for high-volume production.

However, the process does fall short in certain aspects:

• High Setup Costs: Injection molding equipment is relatively more expensive than transfer molding machines. Their setup process is also more complex.
• Rounded Corners: Injection molds produce parts with rounded corners. They are less capable of creating sharp corners compared to other methods.
• Thin Walls Requirement: The process necessitates uniform thin walls between 0.4 mm and 12.5 mm in thickness. Parts that are too thick or molds that are non-uniform can lead to defects in the molding process.

Key Differences of Injection Molding and Transfer Molding

Let me move to the key battle, transfer molding vs injection molding, and explore how the process differs in terms of equipment, speed, applications, and cost.

Comparison of Working Process

The principle behind the processes is similar, both involve heating and pressurizing the polymer into the mold. However, there’s a slight difference in their working process.

In injection molding, the polymer is fed in the form of pallets into the machine, transforming into a molten state and then getting injected under pressure. Typically, the assembly and injection are horizontally aligned.

Whereas, in transfer molding, a pre-heated preform material is pushed by a plunger into the mold. The force or push is in this case in the vertical direction.  

Production Speed and Efficiency

In injection molding, the material moves directly into the mold cavity. And in most cases, it hardly takes a couple of seconds to a minute to make one mold product.

However, in transfer molding, the charge moves from the pot into the sprue and then cures inside the mold, significantly adding more time.  Also, in this case, we preheat the material before placing it in the pot, this further adds more time to the production cycle.

Precision and Tolerances

The precision in both processes is dependent on the mold. If the mold is CNC machined, you can expect precision between 0.05 to 0.01mm in each case. However, the injection molding doesn’t make sharp edges. In most cases, you end up getting around the edges.

Waste and Flash

Since both are plastic molding techniques and when the material solidifies in between channels, waste is common. But the amount of it varies.

In injection molding, the polymer largely stays within the cavity, so you see very little flash (extended portion of the material). Whereas, in transfer molding, the amount of waste is significantly higher. This is because of large sprue channels, air holes, and overflow grooves. It is particularly more when working with thermosetting material, which we cannot reuse after they solidify.

Materials Used in Each Process

Transfer molding is more suitable for thermoset materials although they can work with some thermoplastic materials.  Examples include silicone rubber, unsaturated polyester, epoxy, and PF plastic.

Likewise, injection molding is more suitable for thermoplastics but can also mold a few thermosets. Liquid silicone rubber, polyethylene, polyamide, PVC, ABS, and PP are common materials.

Equipment Differences

Both processes utilize different equipment. The transfer molding machine is vertically designed with a charge pot and plunger mechanism on the top, and a mold cavity beneath it – a vertical configuration. The pre-heating equipment is separate.

In the case of injection molding, the machine itself contains two compartments, one for heating the thermoplastic pallets and one for the clamping mold cavity. The machine expands lengthwise, where the material is fed from one end and pushed horizontally through a screw into a mold on another end. However, this setup can vary based on applications.

Applications

The two plastic processes seem similar but considering their materials compatibility and machine configurations, the applications and use cases are quite different.

Transfer molding is suitable for small to medium-scale production of small rubber parts. Its common applications are seals, pins, connectors, and studs. Whereas, injection molding is for high-volume production of small to large parts, particularly thin-walled substrates.

Its applications spread across a range of industries including automotive, medical, food, electronics, and construction. Car bumpers, refrigerator doors, packing, electronic equipment casing, and overcaps – all are injection molded products.

Cost Comparison

Typically, the manufacturing processes have three types of costs: setup costs, operational costs (materials, power, and labor), and maintenance costs.

In terms of investment, an injection molding machine is quite costly, with prices ranging between $50,000 to $100,000. Transfer molding machines are relatively cheaper, available for $10,000 to $50,000.

Materials costs are relatively similar but more material gets wasted in transfer molding, affecting costs slightly. In some cases, the material properties demand a different machinery. For instance, materials requiring a high injection pressure also demand more powerful molding machinery, which is costly.

The larger share of operational costs goes to mold design. In both cases, if the mold design is complex both processes will incur high costs. Transfer molding may be easy to set up with low investment costs, but for large-scale productions, injection molding is more cost-effective.

How to Choose Between Injection Molding and Transfer Molding?

Transfer molding and injection molding both have their use cases and strengths. Overall, injection molding is an advanced technique with a large number of applications. However, choose based on your needs and requirements.  

Choose injection molding if you want to bulk-produce thin-walled castings or complex structure thermoplastic components. Select transfer molding if you want to design rubber seals.

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Our facility has machines from 30t to 1800t, perfect for both prototypes and full-scale production. Benefit from our complimentary design consultation with every order. We promise quick delivery, with lead times starting at just 15 days.  

Comparing Differences Between Injection Molding and Compression Molding

Compression molding stands as a time-tested, straightforward method for shaping thermoset materials. In this process, a pre-heated mass, or ‘charge,’ is placed into the lower section of a heated mold. The closing of the upper mold exerts pressure, sculpting the charge into the desired form. The heat within the mold not only shapes but also triggers polymerization, essential for curing the material.

On the other hand, injection molding takes a different approach. It begins with feeding thermoplastic pellets into a hopper. These pellets are then melted and injected under high pressure into a secured mold, where they take on the mold’s shape.

Firstly, both processes differ in the working process. Then, in material placement. In compression molding, the material is fed into the mold directly whereas in injection molding the material is fed to the feed hopper. The machine orientation also differs. In compression molding technique, it’s vertical while in injection molding it is usually horizontal.

The best materials for compression molding are melamine, urea-formaldehyde, epoxies, urethane, and elastomers. Whereas, injection molding works best with polyamide, polypropylene, ABS, PVC, and liquid silicone.

Conclusion

The debate, injection vs transfer molding, now concludes. Each method has unique strengths and limitations. Transfer molding excels with its low setup costs and proficiency in creating small rubber parts. Conversely, injection molding stands out as a more advanced method, ideal for crafting small to large plastic casings and thin-walled parts with high precision.

Interested in leveraging these molding techniques for your next project? At RapidDirect, we provide customized injection molding solutions. Contact us today and get a free design consultation.