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.
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 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.
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.
#2 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.
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.
