What Is a Rubber Molding?
Rubber molding is the processing of rubber into a specific shape using a mold or other means.
Rubber molding includes compression molding, transfer molding, and injection molding.
Uses of Rubber Moldings
Rubber molding is a technique used in many industries. Depending on the molding method, it is used for the following:
1. Compression Molding
This is a general rubber molding method. It is widely used for parts of automobiles, medical equipment, industrial machinery, etc.
2. Transfer Molding
This method is used for molding complex shapes because of its higher dimensional accuracy than compression molding.
3. Injection Molding
Injection molding is suitable for mass production because it requires less vulcanization time than compression molding or transfer molding and can significantly increase production volume. It is also possible to mold complex and thick-walled shapes with injection molding.
Principles of Rubber Moldings
The principle of rubber molding differs depending on the method of rubber molding.
1. Compression Molding
Rubber material is placed in a mold for molding, and pressure is applied and held for the required time. It is possible to mold with lower pressure than other methods.
2. Transfer Molding
The rubber material that has been heated and softened is placed in a portion known as a pot, and when pressure is applied, the rubber material flows into the mold through the inlet port. The mold is then held for the required time to mold.
More complex shapes can be molded than with compression molding. However, it is necessary to remove the remaining material from the pot. Mold cost is lower than injection molding.
3. Injection Molding
Injection molding is performed by injecting heated liquefied rubber material into a mold. Small and complex shapes can be molded. It is also suitable for mass production. However, the equipment is large and the mold costs are high.
Other Information on Rubber Moldings
1. Temperature of Rubber Moldings
Nozzle temperature and mold temperature must be controlled when molding rubber.
Nozzle Temperature
For natural rubber, it is recommended to set the nozzle temperature at 70°C. For ethylene and propylene rubber, it is recommended to set the nozzle temperature at 70°C. For ethylene, propylene rubber (EPDM), nitrile rubber (NBR), acrylic rubber (ACR), and chloroprene rubber (CR), the recommended temperature setting is 80°C.
Mold Temperature
For natural rubber, the recommended setting temperature is 160°C. For EPDM and NBR, the recommended setting temperature is 175°C. For ACR, the recommended setting temperature is 180°C. The recommended temperature setting for CR is 170°C.
2. Defective Rubber Moldings
Inadequate molding conditions in rubber molding will result in defects. The following seven factors are the main causes of molding defects.
- Insufficient Vulcanization
This occurs when the chemical reaction during molding is not completed. This occurs when the temperature is low or the vulcanization time is short. - Air Generation
Occurs when vulcanization is performed with air trapped in the material when it is poured. - Hiccups
This is a condition where dents are formed in the molded product. It is caused by material shrinkage during vulcanization. - Part-Line Cracking
This is a phenomenon in which the rubber breaks at the split surface (part line) of the mold. It is caused by the same reason as sink marks. - Burr Bite
This phenomenon occurs when burrs from the molding shot remain in the mold and enter the next lot of product. It is more likely to occur in the final filling section. - Burning
This is a phenomenon in which vulcanized rubber material mixes with the product. This phenomenon occurs when the injection time is long. - Occurrence of Pinholes
Pinholes are small holes that appear when the molded product is stretched during demolding. Pinholes are caused by poor carbon dispersion, foreign matter in the material, etc.
3. Rubber Moldings of Elastomer
Elastomer, a rubber material, is a thermoplastic resin. It has the physical properties of rubber at room temperature and plastic at high temperatures. Elastomers can be molded by combining different materials or by inserting molding, in which metal or other parts are placed inside the mold.