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Rubber Molding: Three Ways to Make Rubber Molded Parts

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    Rubber products are ubiquitous in nearly every industry nowadays. They play a vital role in functions like sealing, abrasiveness, insulation, etc. As a result, they are critical in upholding high industry standards and development in today’s fast-paced world. There are several methods to manufacture rubber products, rubber molding being one of them.

    In this article, we will discuss three main types of rubber molding, common rubber materials, and industrial applications.

    Overview of Rubber Molding

    Rubber molding is a popular rubber manufacturing process. It converts raw rubber material into end-use production plastic products with a metal mold. The rubber is filled inside the mold cavity with the application of heat and pressure to form the desired shape, and of course, supervised by skilled labor and engineers.

    Besides, rubber molding is time-efficient and cost-effective and produces high-quality parts with good accuracy and mechanical properties, which is the reason why it is widely used by industrialists.

    Different Types of Rubber Molding Process

    Rubber molding is a diverse manufacturing method with lots of variants. Each type has its own specialized production procedure, advantages, and disadvantages. We will present a detailed summary of the most well-known ones: injection molding, compression molding, and transfer molding.  

    #1 Rubber Injection Molding

    Rubber injection molding is very similar to the more well-known plastic injection molding process. It starts with the uncured rubber strips that go into the machine via a hopper. Once through the hopper, it enters the screw chamber, where it is heated and compressed by external heaters and friction from the movement of the screw.

    Near the end of the screw, the rubber has heated to a very fluidic state. It is fed into the mold at high pressure and it fills up the cavity built inside the rubber mold. Due to its flow properties, the rubber material fills every nook and cranny of the cavity.

    The mold then gradually cools down so the rubber solidifies. Finally, the mold opens to eject the end product, after which it is ready for the next cycle.

    injection molding technique

    Advantages of Rubber Injection Molding

    High Accuracy: The application of pressurized rubber injection along with the good flow properties of rubber allows the raw material to fully fill up the mold and reach every corner. The result is thus very precise.

    Complex Geometry: This method can manufacture designs with special features like holes, curvature, and sharp contours. The mold cavity design has a lot of flexibility and it can accommodate elements like cores to allow for complex features in the rubber part.

    High Mechanical Strength: Due to pressure and ample curing time, the parts have high strength and durability.

    No Finishing: Injection molded rubber products have fewer defects or leftover material that needs after it comes out from the mold. Except for a small parting line and gate vestige, the output is a near-net shape.

    Quick Process: Modern control technologies ensure precise control of heat and pressure in injection molding machines. Optimization of these processes can lead to short curing cycles, giving a major advantage in terms of time.

    Economies of Scale: Apart from initial tooling costs, the molding cost per item is quite low. If mass production is the goal, it is perhaps the best way to maximize profitability.

    Disadvantages of Rubber Injection Molding

    Material Limitations: One downside to rubber injection molding is that it requires good flow properties. Not all rubbers have the necessary properties so the raw material options are limited to flexible, liquid-like rubbers like silicone rubber, polyurethane, nitrile rubber, etc. However, most rubber materials are capable of injection molding.

    High Initial Costs: The tooling cost and the molding machine are expensive. Hence, it is sometimes not financially viable for low volume injection molding.

    #2 Rubber Compression Molding

    compression molding

    Compression molding is similar to injection molding in the respect that it also utilizes heat and pressure. However, the application of heat and pressure are quite different, explained as follows.

    The mold has two halves, each carrying a portion of the cavity. At first, the uncured raw material, known as the charge, is pre-heated and placed in the lower half. The mold sections are also heated beforehand.

    Once the operator is content with the material’s placement, the plug (the movable half of the mold), lowers down into the fixed mold. Due to the pressure from the plug, the charge takes the mold cavity shape. The mold applies heat to cure the rubber in its new shape. Finally, the molded rubber comes out as the final product.

    More: You can learn some differences between compression and injection molding here.

    Advantages of Rubber Compression Molding

    Low Cost: The tooling is not very expensive owing to its simple design and the production costs also decrease as the volume of production increases.

    Low Maintenance: The simple design and absence of complex spruce networks mean that cleaning and repairing/polishing the mold is easy and inexpensive. Also, it takes less time to maintain than more complex mold designs.

    Variety of Materials: Since compression molding does not rely on material flow, it can handle a wide variety of rubbers.

    No Gate Vestige: The mold does not have a gate so the final product does not have the gate vestige left behind. Also, this improves the product’s aesthetics and decreases the finishing cost.  

    Disadvantages of Rubber Compression Molding

    Dimensional Tolerance: Manufacturing accuracy is good in the case of compression molding but not as high as injection molding. There can be slight deviations from the mold cavity in places like high curvatures and pockets.

    Finishing: While there may be no gate vestige, finishing operations are still necessary to remove flashes and parting lines. Grinding or cutting takes time and money.

    #3 Rubber Transfer Molding

    Transfer molding utilizes a distinct rubber mold design. The die mold has two halves like any other mold but the upper half has an additional open-ended cavity known as the pot. A spruce network connects the pot and the main cavity. The preform is placed into this pot and ‘transferred’ into the cavity by a plunger through high pressure and heat.

    The raw material then cures and solidifies. Finally, the mold ejects the final product once it has cooled down and vulcanized.

    transfer molding

    Advantages of Rubber Transfer Molding

    Tight Tolerance: This technique can provide rubber molded parts with dimensional tolerance and is very suitable manufacture accurate part geometries.

    Metal Bonding: Rubber-to-metal bonding is a strong point of transfer molding. It is a convenient method to manufacture parts like combined metal-rubber mounts.

    Large Parts: The mold design can include large cavities with ease.

    High Cavity Count: The pre-form is a very simple shape and does not require much preparation. As a result, the same preform is reusable for many cycles without much hassle, saving time and effort.

    Disadvantages of Rubber Transfer Molding

    High Tooling Cost: The mold has additional features like the pot and spruce. These make it more complex to design and manufacture. But the complexity also adds cost to the initial setup.

    Finishing: Deflashing is always necessary when using transfer molding due to the spruce.

    High Cycle Time: Unlike other processes, the curing time is high for this technique, which is an important consideration in many processes.

    Rubber Material Options for Rubber Molding

    The rubber industry is much more diversified when it comes to materials. Each rubber type has specific qualities that suit a distinctive set of applications. This section will briefly discuss some of the most popular types of rubbers.

    rubber material

    Natural Rubber (NR): Extracted from rubber trees in a process known as lapping, Natural Rubber has a well-balanced set of properties. It has high strength and resilience, a high stretch ratio, and is water-proof. Its raw form, best known as Latex, is used widely in the Rubber industry, either in pure form or mixed with other compounds.

    Styrene-Butadiene Rubber (SBR): SBR is a synthetic rubber characterized by its elite abrasion resistance. Its durability, compression set, and water swell resistance make it suitable for demanding applications.

    Neoprene Rubber: Another synthetic rubber. Neoprene Rubber is famous for its ability to hold its mechanical properties over a wide temperature range, which is very desirable for applications like tubing for engine fluids and friction belts. It has high chemical stability and is corrosion-resistant as well.

    Nitrile Rubber (NBR): NBR is a high-performance rubber with a remarkable compression set and mechanical integrity. It also resists chemical degradation against common machinery fluids like oil and gas, enabling its use in sealing components.

    Butyl (IIR): Butyl Rubber is well-known for its flexibility and gas impermeability. This is a rare combination in rubber products, making this brand of rubber highly sought after by sealant companies.

    Fluoroelastomer Rubber: This rubber type has high resistance to permeation by fluids like water, oxygen, ozone, etc.

    Silicone Rubber: Silicone rubber possesses many favorable properties. Some of these are biocompatibility, fire resistance, impermeability against ozone, and availability in many colors.

    Considerations for Choosing Suitable Rubber Molding Material

    rubber injection molding parts

    How to choose a rubber material, one may ask? Just as the choice of rubber molding technique depends on the end product, so is the choice of rubber material. Some of the main considerations one must keep in mind are:

    • Material’s Properties: Strength, durability, abrasion resistance, impermeability to certain fluids, temperature-bearing qualities, etc. are all important factors to look into when finalizing the rubber material for a product.
    • Industrial Applications: Depending on the applications of the final product, the choice of material can change significantly. There are specific rubbers for environments with high temperature, high humidity (not only water but any fluid), chemical exposures, loading type, and total mechanical load.
    • Your Budget: The budget is an important factor in deciding upon a rubber material. Some cost more with added advantages while some are cheap but slightly compromise on quality. For example, EPDM and Silicone are two rubber materials with similar properties and applications. Generally, however, EPDM is slightly less costly than Silicone but also requires more frequent replacements, even when used in similar environments. So, there is always a trade-off between cost and quality.

    Samples of Rubber Molded Parts

    The rubber industry has its roots in various markets. Let’s discuss a small portion of its massive expanse.

    sealing gasket for pipe flange
    • Rubber O-Rings: O-rings are arguably one of the most common rubber products. Their primary application is in sealing fluids in mating components, like piston-cylinder assemblies. Depending on application requirements like strength, durability, and shore hardness, different rubber materials have the use for manufacturing O-rings.
    • Rubber Seals: Rubber seals have various applications. Typically preferred where the standard ring-like shape of O-rings is not applicable, they come in a range of shapes and sizes. Some of their applications include leak-proofing flange couplings, bellows, vent ducts, etc.
    • Rubber Gaskets: Anyone who loves cars will know how important gaskets are for good performance. They are used in high-pressure sealing applications and come in customized forms to suit the specific application, for example in engine blocks as mentioned before.
    • Rubber Tubing: Tubes are used to transfer fluids. Rubber tubing is used for the piping itself and for its coating as well. Not only does this prevent leaks, it also provides much-needed thermal insulation. A common application is rubber-coated tubing in domestic piping in cold regions.
    • Rubber Mounts: Rubber has excellent damping qualities and is extensively used in vibration suppression. Heavy machinery which is prone to damage from vibrations is oftentimes placed on top of rubber mounts to prolong its life and enhance performance. CNC machines, for example, come with rubber mounts, without which they are incapable of achieving tight tolerances.   
    rubber o rings

    RapidDirect excels in manufacturing prime-quality rubber products. Our top-of-the-line injection molding equipment and seasoned experts are highly experienced in providing impeccable R&D and manufacturing services to our esteemed clientele.

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    RapidDirect: The Best Partner of Custom Rubber Molding 

    A huge advantage of the processes of making rubber parts is their customizability. With the discussed technologies, engineers enjoy the liberty of designing rubber parts that are unique and best fitted to their products. This enhances user experience, promotes creativity, and boosts your sales.

    With custom rubber molded parts, there is an opportunity to develop your brand identity with products that target your users.

    When it comes to customized rubber molding, RapidDirect leads the industry with its cutting-edge compression molding and injection molding services. Our precision rubber molding equipment is capable of achieving superior manufacturing tolerances and unparalleled rubber curing.

    We boast a talented team of creative and technical professionals, who have spent years creating custom molded parts with complex designs for our valuable clients. We specialize in providing solutions specific to your needs. Based on your application, we conduct directed research and select the most well-suited rubber material from a pool of high-quality candidates.

    Our online quoting platform makes communication with our experts very convenient. Just upload your product designs via our online platform to get quotations and get a Design For Manufacturing Analysis, completely on the house!


    While the debate of which rubber molding method and rubber materials are the best continues on various forums, what it really boils down to is how well you use these tools.

    Rubber production is a highly diversified field with innovations and ideas frequently improving the market. As discussed throughout this article, each technique has its own pros and cons. So does every material. However, experience and creativity are irreplaceable!

    When it comes to molding rubber parts, design sense and manufacturing expertise are what makes the true difference. To get professional consultancy from our team, you are welcome to contact us.

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