Which Rubber Molding Process Is Right for Your Project?

Rubber molding is a manufacturing process that uses pre-shaped molds to create parts and components from uncured rubber. Depending on the product and specifications, there are several different rubber molding method options—such as compression molding, transfer molding, and injection molding—each of which comes with unique considerations and best use cases. 

In the following blog post, we offer an overview of the above three methods, outlining their process steps, tooling considerations, design and material considerations, and applications. 

Overview of Liquid Injection Molding 

Liquid injection molding (LIM) is a highly versatile and cost-effective rubber fabrication method that involves injecting liquid silicone rubber (LSR) compounds into a high quality injection mold. Once injected into the heated mold and cured, the material maintains the shape of the mold, thereby producing a detailed component. The process readily accommodates automation, enabling it to be used to complete large production runs in a short period of time. 

Liquid Injection Molding

Tooling Considerations for LIM

Although LIM is quick and efficient, it is not ideal for all applications. For example, the process may not be suitable for producing prototypes for manufacturing operations.  The upfront cost of creating the mold, lead times, set-up cost, process validation cost and material costs can be limiting factors. 

Design and Material Considerations for LIM

Liquid injection molding operations utilize liquid silicone rubber. The material is particularly useful because it is consistently produced and has a useful range of temperature and mechanical properties. Its shrink rate is fairly predictable but as it cures after ejection may continue to shrink so it is important to consider shrinkage when designing a liquid injection molded part. 

Silicone likes to move into spaces in a mold so its flow characteristics can be of benefit when filling very thin walls and other complex geometry.  However, this same characteristic can create flash issues at parting lines of molds. It is also important to note that when variable geometry in a part is required it can create pressure variations as the liquid silicone moves through a mold.  It is important for LSR molding that gate locations, gate sizes and venting in molds be used to optimize the process.

There are many suppliers and distributors of liquid silicone rubber to be found and they all have many pre-designed and manufactured compounds that can be employed. Custom compounding of liquid silicone material is rare, but in the right application it can be done. However the cost is usually quite high and the lead time can be very long.

Be aware that the cost, availability and use of liquid silicone should be part of the research done by a designer or customer. Molders can be of great assistance in narrowing the field of available materials and suppliers and all have preferences based on experience but picking a material for a product and process is ultimately the customer’s decision. 

Applications and Uses of LIM

By altering injection pressure and speed, process temperatures, and clamp pressures, industry professionals can easily tailor the LIM process to suit a wide range of part and production needs. They can also integrate automation components to increase the level of precision and accuracy achievable and decrease the need for post-molding operations, saving manufacturers both time and money.

The LIM process can be a cost effective method of manufacturing high volumes of simple geometry and complex products. Typical parts produced include electronic components, isolators, O-rings, and seals for use in the aerospace, medical, consumer and automotive industries. 

Liquid silicone rubber is a versatile material that has low surface energy and a broad temperature range. This along with other properties makes it an excellent choice for medical device components. It can be formulated to be both conductive and non-conductive allowing it to be useful in electrical applications. Silicone can be colored and this along with its finish gives it aesthetic appeal.  Its elasticity provides an avenue for designing specific mechanical applications.  

Overview of Compression Molding

Compression molding involves the compression of specific compounds of catalyzed rubber between two halves of a heated mold while applying tremendous pressure. Once released from the mold by hand, the cured material holds the internal shape of the mold producing a specific part.

Tooling Considerations for Compression Molding

The tooling for compression molding typically consists of two pieces of steel or sometimes aluminum that, when put together, form an enclosed space that creates a part. One side of the mold may be machined to create the outside geometry of a desired part and the other side might contain the details of the inside of a part. 

The costs associated with building the mold are directly related to the amount of complexity required to be machined into the mold to make the desired design. The more complex or the larger the part, the number of cavities that are requested to be made that make individual parts in a mold, the time and type of material used to make the mold all go into the cost.

Additionally there is a cost to set-up and test new molds that is usually folded into the mold cost.  This would include press time, material cost and any additional cost estimated by a molder.

Material and Design Considerations for Compression Molding

Compression molding can be used to manufacture a startlingly wide variety of products. The diversity of compounds within the types of material is nearly limitless allowing it to be designed to make parts that can perform with specific mechanical requirements, chemical resistance properties, and environmental qualifications.  

The compression molding process may not display the same level of accuracy and precision, which makes it less suitable for products where tight dimensional tolerance is critical and where flash and parting line shift may cause issues.  This makes trimming of parts, and nitrogen de-flashing of parts a go–to solution for some of these issues.

Applications and Uses of Compression Molding

The compression process is useful for the production of large or small parts with varying degrees of complexity usually lending itself better to smaller quantities of parts as it is slower and more labor intensive than injection molding of rubber. Designers and engineers often employ it for prototyping and sample production operations to test out new part or product designs.  However, this process can be used successfully to make high quantities of parts.  

Any industry can benefit in the use of compression molding rubber parts.  Medical, oil and gas, aerospace, electronics, automotive and other consumer products companies all use products made with compression molding.  

Overview of Transfer Molding

Similar to compression molding, the transfer molding process requires the use of pre-measured and positioned material. However, it offers greater precision and is less prone to flash on parting lines as the two halves of the mold that contain the part geometry are closed together before material enters the part detail in the mold. Compared to injection molding, it is much slower since the material is placed by hand in the mold and the part removed by hand forms the mold once molded.  

Tooling Considerations for Transfer Molding

While tooling is similar to compression molding it has some advantages for specific parts. The pot and piston design allows better efficiency and precise distribution of material in the mold.  This can be beneficial for parts where gate location may be critical to effectively fill a thin wall or in a larger part that might require a pressurized distribution of material to fill more effectively or a multiple cavity mold saving time in loading many individual cavities with material.

Choosing the right material can be very important. Aluminum molds can have problems with parting lines when molding some materials and the constant opening and closing of molds can create wear and tear. Steel is preferred for production tooling and different types of steel including pre-hardened alloy, hardened material, stainless steel and even more exotic material such as titanium can be employed in specific applications.  

Design and Material Considerations Transfer Molding

Compared to both compression molding and injection molding, transfer molding usually generates less material waste because overflows in the mold are usually not needed. It does produce waste in the form of leftover material in the transfer pot and sprues leading to the gates where the material enters the cavity in the mold. After the molded product solidifies, this material must be removed and discarded as it is a thermoset material and cannot be reused. 

When designing a transfer mold, it is important to consider the above. Ideally, the mold should produce the highest quality part in the shortest cycle time with the least amount of scrap. Partnering with an experienced mold maker can help achieve this goal. 

Material used in transfer molding is usually the same range of materials used in compression molding.

Applications and Uses of Transfer Molding

Transfer molding is faster than compression molding and less expensive than injection molding, qualities that make it ideal for use in the manufacture of parts in small production runs.  However, like compression molding it produces an amount of waste, and this has to be measured against the cost of the material being employed.

As with compression molding, any industry can benefit in the use of transfer molding of rubber parts. Medical, oil and gas, aerospace, electronics, automotive and other consumer products companies all use products made with compression molding.  

Molding Services at Elastomer Technologies, Inc.

Liquid injection molding, compression molding, and transfer molding are three commonly employed rubber production methods. Each offers unique benefits and limitations that suit them for different applications. 

At Elastomer Technologies, Inc. (ETI), we are well-versed in all three methods, enabling us to meet a wide range of rubber molding needs. Our customer base is highly diverse, ranging from aerospace and automotive to food processing and medical. By partnering with us, they benefit from our:  

  • Over 35 years of industry experience
  • Commitment to superior customer service
  • Continuous improvements to our molding techniques and technologies

To learn more about rubber molding or our services and capabilities, reach out to us today.

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