Rubber Overmolding & Insert Molding
Rubber overmolding also known as silicone overmolding, silicone insert molding, two shot molding, which is a process to cover, bond or encapsulate a variety of high temperature resistant materials substrates including steel, stainless steel, brass, aluminum, and plastics with silicone rubber. Thus, it includes rubber metal bonding and plastic rubber molding. The insert molding process can produce a bond that is stronger than glued or assembled parts.
The main elements involved in the silicone overmolding processes are the substrate, the rubber, and the agents. Many kinds of rubber can be used for insert molding including Neoprene, EPDM, Nitrile, NBR, HNBR, FKM, Viton, Silicone, LSR.
Overmolding Silicone Material and Overmolding Ways
HCR(high-tempreture and low-tempreture silicone) and Liquid silicone rubber(LSR) are commom silicone material chosen to overmold with other parts by compression molding or LSR injection molding. Silicone cured by platinum vulcanizing agent need a lower tempreture than the silicone cured by peroxide vulcanizing agent in overmolding process. Platinum vulcanizing agent vulcanization rate is 20-30% faster than the ordinary vulcanizing agent.
Silicone Material | LSR | HCR(high tempreture cured) | HCR(low tempreture cured) |
Molding Tempreture | 120-150°C | 170-180°C | 135°C |
Overmolding Ways Comparison |
| LSR Injection Molding | Compression Molding | Transfer Injection Molding |
Advantages
| 1. Tight tolerance in size control | 1. Cheaper mold and raw material cost | 1. Tighter control of dimensional tolerance than compression molding |
2. Nearly without flash/burrs with small parting line |
2. Accommodate more cavities |
2. Smaller flash/burrs on the finished overmolded parts
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| 3. Lower curing tempreture and compatible to more substrates |
Disadvantages | 1. More expensive mold and material cost | 1. High rejection rate if substrates cannot be fixed | 1. Mold cost will be higher than compression mold because there is one more plate as material pot
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| 2. Mold cavities limitation | 2. Size control is not good as LSR molding and with more flash |
LSR Overmolding (Molded Rubber and Plastic) Manufacturing Process
Transfer Injection Overmolding (Bonding Rubber and Metal) Manufacturing Process
Rubber Overmolding Substrates
The selection of rubber overmolding substrates should take the the temperature required to cure the silicone and temperature resistance of substrates material into consideration.
Plastic Rubber Molding: Many plastic substrates have low softening and melting temperatures. The common reference point for thermoplastics is Heat Deflection Temperature, which may be an indicator that the material will not be rigid enough to overmold silicone using. Generally, GF can add to plastic to improve the application range of materials.
Plastic+GF(Glass Fiber)
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Advantages | 1. For GF is a kind of high tempreture resistance material, the plastic add with GF will have better high temperature resistance. 2.The impact resistance and tensile strength of plastics are greatly improved. 3.The shrinkage of plastics will decrease a lot. 4.The flammability of plastics decreases, and it can be used as a flame retardant material. |
Defects | 1.The toughness of plastics decreases, but the brittleness increases. 2.Plastic will change from transparent to opaque. 3.For the fluidity of the material decreased, GF is easy to float on the surface of plastic parts to form radial white traces. |
Common Plastic Heat Deflection Temperature |
| High Pressure | Low Pressure |
Plastic Type | Heat Deflection Temperature(°C) |
PC | 131 | 138 |
ABS | 84 | 89 |
PPSU | 207 | 210 |
PA66 | 75 | 220 |
PES | 195 | 208 |
PSU | 167 | 179 |
POM(Homo-)
| 124 | 170 |
PBT+20%GF | 192 | 217 |
PP+30%GF | 155 | 165 |
Rubber to Metal Bolding: Due to the temperature stability of most metals, bonding silicone to metals tends to be more straightforward and robust.
No matter for rubber to plastic overmolding or rubber to metal overmolding, the overmolding agent is needed to apply to the substrate prior to help in forming a tie layer between the rubber and the surface of the substrate being used. The selection of the overmolding agents depends on the component design, modulus of the rubber, and the rubber type to be overmolded. The mechanical bonding is achieved by mold overmolding while the chemical bonding is realized by applying glue primer, which makes the rubber and the substrates have stronger bonding.
What Will Lead to Flash in Overmolded Parts?&Why Silicone Flow to Substrates in Silicone Insert Molding Process?
1.Seclected substrate with low temperature resistance
2.Unreasonable sealing design between rubber and substrate
3.The mold used with insufficient precision, and improper production process control, including the amount of raw material placed in cavities, the pressure setting of molds, and the ways of applying glue on subtrates
4.Improper location for substrates
Our Rubber Metal Bonding Products
Silicone Overmolding Guide (Tips and Advice)
1.Additional Sleeve--Making additional metal caps to prevent the silicone rubber from flowing to the substrates when using compression overmolding. During compression overmolding process, the silicone material may runs to the substrate before the upper and lower plates are completely joined together. Thus, the metal caps are helpful to keep the silicone material from running to the substrates and keep the finished parts with clear and beautiful appearance. If using transfer injection overmolding and LSR overmolding, the situation doesn't exist because the silicone material will flow to cavities after the mold plates joined together. Generally, for the light and thin subtrates to be overmolded, transfer injection overmolding and LSR overmolding are recommended.
2.Substrates location--The substrates location design should be take into consideration before mold making. If the substrates cannot be fixed firmly when molding, the overmolded parts will be a failure. Here are some location design we use to ensure the finished parts with beautiful look.
a. Two holes designed in the stainless steel for fixing them in the mold cavities to produce silicone diaphragm/membrane
b. Two cylinder silicone caps are made to locate the buckles of the silicone bracelet in the cavities
c. The screws need to be overmolded can be located in mold directly. And a step added to screw, to prevent screw from rotating and to firmly fix in mold cavities
Rubber Overmolding Parts Features and Application
Advantages of overmolding rubber to metal or plastic and other materials include:
1. Rubber overmolding shortens the manufacturing time of the finished products. The process helps the product to come together quickly and simplifies assembly and improves sealing quality between components
2. Rubber overmolding creates bonds between the rubber and substrates that are difficult to break, even with heavy use. A product that is molded together will stay intact much longer than a product with two individual pieces combined using glue. This makes it ideal for demanding applications where the component may be exposed to chemicals or wear
3. Overmolding allows you to combine different materials in one product for improved functional or mechanical properties
Rubber molding parts are used widely used for a range of applications including medical, automotive, military, mechanical, electronic and more.
Plastic to rubber overmolding items are usually used for medical components, like silicone bonding to PC mask. They are also applied to some baby care products, daily necessities, such as silicone bracelet with PC bonding buckles, silicone baby toothbrush, baby bottles or silicone pacifiers bonding plastics, silicone keypads with plastic caps and so on.
Metal bonding to rubber parts can be used for the purpose of noise isolation, vibrations control in automobile, like rubber vibration bumpers, rubber overmolded threaded anti-vibration isolator mount, and other various engineering applications. Silicone gaskets, membrane bonding metal are also common applications.
