Rubber Injection Molding: Basic Guide To Molding Rubber

overmolding, rubber injection molding

Rubber injection molding is a common method of producing rubber and rubber accessory products, such as seals, gaskets, O-rings, and other rubber components with complex shapes. The process involves using a metal mold to transform raw rubber into end-use manufacturing plastic goods. With the help of expert workers and engineers, the rubber is placed within the mold cavity and heated and compressed to take the desired shape. This process is particularly useful when high precision, repeatability, and efficiency are required in the production of rubber parts.

Rubber injection molding manufacturers utilize this process to create high-quality products with good precision and mechanical qualities, making it a time and money-effective solution for industrialists in need of rubber products. By using this method, rubber injection molding services can create parts with intricate designs and shapes that would be difficult or impossible to achieve using other molding techniques. Additionally, the mold can be designed to produce multiple parts simultaneously, further increasing efficiency and reducing costs.
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Rubber Injection Molding

Rubber injection molding appears in many aspects of our lives. Let us go on knowing more about it: advantages, disadvantages, rubber injection moulding process, etc.

Advantages Of Rubber Injection Molding

  • 1. Precision and Consistency: Rubber injection molding services enables the production of high-precision parts with tight tolerances and consistent quality. The use of advanced molds and controlled injection parameters ensures uniform material distribution, resulting in accurate and repeatable part dimensions.
  • 2. Complex Part Design: With rubber injection molding, it is possible to create intricate and complex part designs that may not be feasible with other rubber processing methods. The ability to produce multi-component parts and overmolded components opens up new possibilities for engineering innovative products.
  • 3. Reduced Material Waste: Compared to other rubber processing techniques, such as compression molding, rubber injection molding generates less material waste. The automated and controlled process allows for precise material dosing, reducing the amount of excess rubber and minimizing overall production costs.
  • 4. Shorter Cycle Times: Rubber injection molding typically offers shorter cycle times compared to other methods. The automated and efficient nature of the process, along with precise control over parameters like curing time, contributes to faster production rates and increased overall productivity.
  • 5. Improved Surface Finish: Rubber injection molding results in parts with a superior surface finish. The closed mold system and reduced material handling minimize surface defects and blemishes, leading to smoother and more aesthetically pleasing finished products.
  • 6. Suitable for High-volume Production: Rubber injection molding is an ideal choice for high-volume production runs. Once the mold is designed and set up, the process can be efficiently repeated, resulting in cost-effective production of large quantities of rubber parts.
  • 7. Versatility: Rubber injection molding is versatile and can be used for a wide range of rubber materials, including natural rubber, synthetic rubber, and silicone. This versatility makes it suitable for various industries, such as automotive, medical, electronics, and consumer goods.

Overall, the advantages of rubber injection molding make it an attractive option for manufacturers seeking to produce high-quality, complex rubber parts efficiently and cost-effectively. The process’s precision, speed, and versatility contribute to improved product performance and reduced time-to-market for innovative solutions.

Disadvantages Of Rubber Injection Molding

  • 1. High Initial Investment: Setting up a rubber injection molding operation requires a significant initial investment in specialized machinery, molds, and equipment. This can be a barrier for small-scale manufacturers or businesses with limited capital.
  • 2. Longer Setup Time: Preparing the molds and machines for production can be time-consuming, especially when compared to some other rubber molding processes. Frequent mold changes for different products can lead to additional downtime.
  • 3. High Production Volume Required: The process is most cost-effective when producing large quantities of rubber parts. If the production volume is low, the unit cost per part can be higher compared to other rubber molding methods.
  • 4. Mold Maintenance: Injection molds are subject to wear and tear, especially with high-volume production. Regular maintenance and occasional mold replacements may be necessary to maintain product quality and consistency.

Despite these disadvantages, rubber injection molding services remains a valuable manufacturing process for industries that require high precision, complex geometries, and consistent quality in their rubber products. Manufacturers must carefully weigh the advantages and disadvantages based on their specific production needs and resources.

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Rubber Over Molding

Rubber over molding involves molding rubber over an existing injection molded rubber parts or substrate, such as encapsulating electronics with a rubber protective layer. This process allows for the creation of complex hybrid components. Rubber over molding offers several benefits that make it a popular and effective manufacturing process.
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Main Advantages Of Rubber Over Molding

  • 1. Enhanced Product Performance: Rubber over molding services allows for the creation of composite parts that combine the benefits of rubber with other materials. By encapsulating or bonding rubber to a substrate (usually metal, plastic, or another rubber part), the resulting component gains improved performance characteristics, such as enhanced grip, shock absorption, vibration dampening, and sealing capabilities.
  • 2. Seamless Bonding: The overmolding process creates a seamless bond between the rubber and the substrate. This seamless integration eliminates potential points of weakness, preventing delamination or separation between the materials during use.
  • 3. Protection and Encapsulation: Rubber overmolding provides effective protection and encapsulation for sensitive components. By encasing electronic parts or fragile elements with rubber, manufacturers can safeguard them from environmental factors, moisture, and impact.
  • 4. Chemical and Corrosion Resistance: Rubber materials, such as silicone or fluorosilicone, offer excellent resistance to chemicals, oils, and harsh environmental conditions. Overmolding can add a layer of protection, increasing the component’s longevity and durability.
  • 5. Economical Production: Overmolding can simplify the manufacturing process by combining multiple steps into a single operation. This can lead to reduced assembly costs and shorter production cycles.

Relationship Between Rubber Insert Molding And Rubber Over Molding

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Rubber insert molding, and rubber over molding are rubber molding processes, involving combining rubber and other materials to create a single part. However, there are some key differences in these processes:

  • 1. Rubber insert molding: Rubber insert molding involves placing a preformed metal or plastic insert into a mold cavity and injecting molten rubber material around the insert. The rubber material then bonds to the insert during the curing process to form a single part.
  • 2. Rubber over molding: Rubber over molding involves placing a preformed plastic or metal part into a mold cavity and injecting molten rubber material over the part. The rubber material then bonds to the part during the curing process to form a single part.

The main differences between these processes are the order of the injection of rubber and other materials, and whether or not the other material is already present in the mold cavity before injecting the rubber material.
In terms of applications, rubber insert molding is used to produce parts such as electrical connectors, knobs, and buttons. Rubber over molding is used to produce parts such as toothbrushes, razor handles, and power tool grips.

Rubber Compression Molding

In compression molding, the rubber material is placed directly into an open mold cavity. The mold is then closed, and pressure is applied to the material to fill the cavity and shape the rubber product. Compression molding is suitable for both solid rubber and liquid silicone rubber (LSR) applications.

Advantages and Benefits of Rubber Compression Molding

  • 1. Cost-Effectiveness: Rubber compression molding services is cost-effective, especially for low to medium volume production runs. The simplicity of the process and the absence of complex equipment result in reduced production costs, making it economical for smaller batch sizes.
  • 2. Versatility in Rubber Compounds: The process is compatible with a wide range of rubber compounds, including natural rubber, synthetic rubbers (such as NBR, EPDM, SBR), and specialty rubber materials. This versatility allows manufacturers to choose the most suitable material for specific application requirements.
  • 3. Suitability for Large Parts and Thick Cross-Sections: The process is well-suited for molding large parts and components with thick cross-sections. It can handle a wide range of part sizes, making it suitable for various industrial and automotive applications.
  • 4. Mold Simplicity and Durability: The molds used in rubber compression molding are relatively simple in design, resulting in reduced tooling costs. Additionally, the molds are durable and can withstand numerous molding cycles, leading to a longer tool life.
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Rubber Transfer Molding

Rubber Transfer molding is similar to compression molding but differs in how the rubber material is delivered into the mold. The material is first preheated in a chamber (transfer pot) before being forced into the mold cavity using a plunger. This method is commonly used for molding intricate and large rubber parts.

Key Advantages Of Rubber Transfer Molding

  • 1. Consistency and Uniformity:Transfer molding provides consistent and uniform molding results, resulting in products with reliable dimensions and properties. The transfer of material to the cavity is controlled, leading to consistent part quality throughout the production run.
  • 2. Suitability for Insert Molding:Transfer molding allows for easy integration of metal inserts or other components during the molding process. This feature makes it suitable for producing overmolded products, combining rubber and metal parts in a single component.
  • 3. Less Flash and Defects:The transfer molding process typically produces less flash (excess material) and fewer defects compared to other molding methods. This results in reduced post-production trimming and finishing efforts.
  • 4. Improved Curing:Transfer molding allows for better control of the curing process. The material is preheated and then injected into the mold, ensuring that the curing temperature and time are precisely controlled, leading to uniform curing throughout the part.
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Other Categories To Mold And Apply Rubber

  • Thermoplastic rubber injection molding: Thermoplastic rubber injection molding, often abbreviated as TPR injection molding, is a specialized manufacturing process used to produce rubber-like products using thermoplastic elastomers (TPEs). Thermoplastic elastomers are a class of materials that combine the properties of both thermoplastics and elastomers, offering rubber-like elasticity and flexibility while being processable as thermoplastics. The thermoplastic rubber injection molding process shares similarities with traditional plastic injection molding but uses TPEs instead of rigid thermoplastics.
  • Calendering: Calendering is a process used to produce continuous sheets or films of rubber. The rubber material is passed between multiple rotating rolls (calenders) that compress and shape the rubber into the desired thickness and texture.
  • Coating and Spraying: Rubber can be applied as a coating or sprayed onto surfaces to provide a protective or non-slip layer. This method is commonly used in applications such as conveyor belts, industrial flooring, and non-slip grips.
  • Adhesive Bonding: Rubber can be bonded or adhered to various substrates using specialized adhesives or bonding techniques. This is often employed in applications such as bonding rubber to metal or rubber to rubber to create composite parts.

These categories cover a wide range of methods to mold and apply rubber, each offering unique benefits depending on the desired product and application requirements. You can choose the most appropriate method based on factors such as product complexity, production volume, material properties, and cost considerations.

Material Options for Rubber Molding

Rubber molding processes can accommodate a wide range of rubber materials, each with its unique properties and characteristics. The choice of rubber material depends on the specific application requirements, such as the intended use, environment, temperature range, chemical resistance, and mechanical properties needed for the final product. Here are some common rubber material options used in rubber molding:

  • Nitrile Rubber (NBR): Nitrile rubber is known for its excellent oil and fuel resistance, making it ideal for applications in the automotive and oil industries. It also offers good mechanical properties and is commonly used for seals, gaskets, and O-rings.
  • Ethylene Propylene Diene Monomer (EPDM): EPDM rubber provides excellent weather, ozone, and UV resistance. It is commonly used in outdoor applications, such as automotive weatherstripping, seals, and hoses.
  • Silicone Rubber (VMQ): Silicone rubber offers a wide temperature range and excellent flexibility. It is known for its high heat resistance, making it suitable for applications in the food industry, medical devices, and automotive components.
  • Fluoroelastomers (FKM/Viton): Fluoroelastomers are known for their exceptional chemical resistance, high-temperature resistance, and resistance to fuels and oils. They are commonly used in aerospace, automotive, and oil and gas applications.
  • Chloroprene Rubber (CR/Neoprene): Neoprene rubber provides good weather resistance, oil resistance, and flame retardant properties. It is often used in applications requiring durability and flexibility, such as hoses, gaskets, and weatherstripping.
  • Natural Rubber (NR): Natural rubber offers excellent resilience and tensile strength. It is commonly used for general-purpose applications, such as tires, mounts, and vibration isolators.
  • Styrene-Butadiene Rubber (SBR): SBR rubber combines good abrasion resistance with high resilience. It is commonly used for tires, conveyor belts, and other industrial applications.
  • Polyurethane (PU): Polyurethane rubber offers excellent wear resistance, high load-bearing capabilities, and good tear strength. It is often used in applications requiring high abrasion resistance, such as wheels, rollers, and seals.
  • Liquid Silicone Rubber (LSR): LSR is a two-part silicone elastomer that is injection molded. It offers excellent biocompatibility and is often used for medical devices, baby products, and electronics.
  • Ethylene Propylene Rubber (EPR): EPR rubber provides good resistance to weather, ozone, and UV exposure. It is commonly used for outdoor applications, electrical insulation, and cable jackets.

Steps Of Rubber Injection Moulding Process

Material Preparation

Rubber is prepared in a raw form, usually as granules or pellets, and is mixed with various additives, such as curing agents, accelerators, and fillers, to achieve the desired properties for the final product.

Melt Unit

The rubber material is loaded into a melt unit of rubber injection press. The unit melts the rubber and maintains it in a viscous state, making it ready for injection into the mold of rubber injection press.

Mold Setup

The mold, typically made of metal, is designed to form the desired shape of the rubber product. It consists of two halves, the cavity, and the core. The mold is clamped shut, creating a closed cavity for the rubber injection.


The heated and viscous rubber material is injected into the mold cavity under high pressure in rubber injection press. The rubber flows and fills the entire cavity, taking the shape of the mold.

Vulcanization (Curing)

Once the mold cavity is filled, the rubber material is allowed to cure or vulcanize. The curing process involves applying heat and pressure to initiate the cross-linking of polymer chains in the rubber, resulting in the formation of a solid and durable rubber product.

Cooling and Ejection

After the curing process is complete, the mold is cooled to a temperature at which the rubber solidifies further. Once the product has achieved the desired strength, it is ejected from the mold of rubber injection press.


Deflashing is a critical post-molding process used in rubber molding to remove excess flash or flash lines from the molded rubber parts.

Various methods are used for deflashing in rubber molding:

  • 1. Trimming: By using cutting tools to manual or automated trimming, suitable for parts with simple geometries and minimal flash.
  • 2. Cryogenic Deflashing: In this method, the molded rubber parts are tumbled in a cryogenic chamber, where they are exposed to liquid nitrogen or other cryogenic agents. The extreme cold causes the flash to become brittle, making it easier to remove using mechanical means.
  • 3. Blasting: Abrasive blasting techniques, such as bead blasting or shot blasting, effective for parts with intricate geometries and hard-to-reach areas.
  • 4. Deflashing Machine: Specialized deflashing machines use blades or rotating brushes to remove flash from rubber parts. These machines can handle high volumes of parts efficiently.

Troubleshooting Common Issues in Rubber Injection Molding

Flash and Burr Formation

Flash and burrs are excess rubber material that extends beyond the intended part geometry during the molding process. This issue can arise due to various factors such as excessive injection pressure, inadequate clamping force, worn-out or damaged mold components, or improper venting. Troubleshooting flash and burr formation involves adjusting the injection parameters, ensuring proper mold maintenance, and optimizing venting to allow trapped air to escape.

Incomplete Filling

Incomplete filling occurs when the molten rubber fails to reach all areas of the mold cavity, leading to unfilled or underfilled sections in the final part. This issue may be caused by insufficient injection pressure, inadequate mold temperature, poorly designed runners, or blocked vents. To address incomplete filling, it is essential to optimize injection pressure and temperature settings, reevaluate runner and gate designs, and ensure proper venting to facilitate complete material flow.

Curing Irregularities

Curing irregularities refer to inconsistencies in the curing process, which can lead to variations in part dimensions, hardness, or other material properties. Common causes of curing irregularities include improper mold temperature control, inaccurate cure time settings, or non-uniform heating. Troubleshooting curing irregularities involves fine-tuning the mold temperature and cure time, ensuring uniform heat distribution within the mold, and using appropriate curing agents for the specific rubber material.

Part Sticking or Ejector Problems

Part sticking or ejector issues occur when the molded rubber part does not release smoothly from the mold after curing. This problem may arise due to inadequate draft angles, insufficient mold release agents, or ejector system malfunctions. To troubleshoot part sticking, it is essential to verify and adjust the draft angles, apply suitable mold release agents, and ensure the ejector system functions correctly to facilitate smooth part ejection.

Identifying and resolving these common issues in rubber injection molding is vital for maintaining consistent part quality and minimizing production downtime. Regular mold maintenance, thorough process monitoring, and adjustments based on troubleshooting results are key practices to achieve successful rubber injection molding operations.

Best Practices and Tips for Successful Rubber Injection Molding

Proper Material Handling and Storage

The proper handling and storage of rubber materials are essential for successful injection molding. Rubber compounds are sensitive to temperature, humidity, and exposure to contaminants. To ensure consistent material performance, store rubber compounds in a climate-controlled environment with controlled humidity levels. Follow the manufacturer’s guidelines for storage conditions, and use the oldest materials first (FIFO – First In, First Out). Additionally, handle the materials with care to avoid contamination and keep them sealed when not in use.

Regular Maintenance and Machine Calibration

Routine maintenance of injection molding machines and molds is critical to prevent downtime and ensure optimal performance. Create a maintenance schedule and follow it diligently, including tasks like cleaning, lubrication, and inspection of components. Regularly check for wear and tear, and replace any damaged or worn parts promptly. Calibration of the injection molding machine should also be performed regularly to maintain accurate control over injection parameters such as temperature, pressure, and curing time.

Process Optimization and Parameter Control

To achieve the best results in rubber injection molding, optimize the process parameters according to the specific rubber material and part design. Adjust injection pressure, temperature, curing time, and other variables to find the best settings that ensure complete filling and consistent part quality. Utilize process monitoring systems to gather data and make informed decisions for process improvements.

How to Choose A Rubber Injection Molding Manufacturer

Choosing a right rubber injection molding supplier is crucial to ensure the production of high-quality rubber products and a successful partnership. Consider some factors when selecting rubber injection molding companies, especially you need to achieve several vital projects.

  • 1. Experience and Expertise: With a proven track record in rubber injection molding, Sungplastic has many years of experience nd expertise in handling various rubber materials and complex molding projects.
  • 2. Capabilities and Equipment: Sungplastic has modern and well-maintained injection molding machines and equipment suitable for the type of rubber products you need and have the capacity to handle the volume and specifications of your project.
  • 3. Quality Standards:Welcome to inquire about Sungplastic’s quality management system and certifications. As a reputable rubber injection molding company, Sungplastics has ISO or other relevant certifications.
  • 4. Material Selection: Contact us and verify if Sungplastic can work with the specific rubber materials required for your products. We can provide different rubber compounds and the molding processes.
  • 5. Prototyping and Testing: Sungplastic offers prototyping and testing services. Prototyping allows you to verify the design and functionality of your rubber parts before moving to full-scale production.
  • 6. Design and Engineering Support: Sungplastic provides design and engineering assistance, can help optimize the design for manufacturability and identify potential issues early in the process.
  • 7. Production Volume: With a professional team, Sungplastic is able to handle your required production volume.
  • 8. Cost and Pricing: While cost is a crucial factor, it should not be the sole consideration. Sungplastic offers competitive pricing while maintaining quality and service standards.

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About Sungplastic

Sungplastic is a plastic product manufacturer with rich experience in injection molding. According to the different product development requirements, we flexibly adjust the manufacturing process to achieve high quality, high efficiency and more economical.

We offer a variety of manufacturing services: Rapid Prototyping, Tool Making, Injection Molding, Product Design and Development, CNC Machining and Metal Stamping. You can choose from a variety of plastics, silicone rubber, or metal for your product. Regardless of mass production or small batch customization, Sungplastic has always been committed to providing assured, efficient and more economical one-stop processing services for your projects.

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