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Rubber Overmolding Introduction
Rubber Overmolding is the process to connect two or multi color rubber parts, different hardness silicone parts, or different rubber material parts together into a complete final rubber product by compression molding, or liquid silicone injection molding. Besides differernt rubber material combination, rubber overmolding can also be combination of rubber on metal, or rubber on plastic. But primer adhesive should be applied to surface of metal or other subsequent parts first, for a stronger combination of two different material.
In rubber buttons overmolding production, multi coloured buttons or dual hardness buttons are overmolded into a colorful and functional rubber keypad. Overmolding rubber buttons can be with beautiful appearance as the parting lines of colored buttons are clear without any color beeding. In addition, overmolding rubber keypad with dual hardness can have better tactile feel feedback to meet customers’ different actuation force requirements.
Overmolding Rubber to Metal Diaphragm
Rubber Overmolding Types
Rubber Overmolding includes three types, according to overmolding material classification.
Rubber on rubber overmolding
This rubber overmolding is by two set of compression molds, one for the colorful or dual-hardness rubber components, another to mold these separate components together into one product.
Dual coloured buttons or dual-hardness keypads are the typical products.
Overmolding injection of rubber parts with plastic
Plastic components are molded first, and then inserted into rubber parts or connect with silicone parts. Plastic with rubber overmolding process is by plastic injection molding and liquid silicone injection molding. Because most plastic material can not resist very high temperature, to prevent plastic parts melting in overmolding process, it requires low temperature curing. Thus, liquid silicone injection molding is chosen for plastic boned with rubber overmolding, as LSR injection molding curing temperature is about 90-120 °C. The most common product is liquid silicone baby bottle molded with PPSU ring opening, and plastic rubber keyboard for electronics.
LSR Overmolding Bottle (Plastic on liquid silicone)
Rubber parts with metal overmolding
This type of overmolding also can be called insert molding. Metal pieces are inserted into rubber parts. Metal with rubber overmolding can be by transfer injection molding, and compression molding. Different from rubber overmold with plastic, we don’t need to worry metal melting in overmolding process as it can resist high curing process. But we need to ensure metal pieces are firmly fixed in the cavities, and can not move their position. To ensure strong bonding between rubber and metal, generally metal pieces are prime coated first before they are positioned to the rubber mold cavities.
Rubber Buttons Overmolding procedures are more complicate than keysetting keypad production.
One of the prominent features of overmolding buttons is that parting line of different color areas is clear. While in keysetting rubber keypad, different color will bleed into different area.
Comparison of Overmolding Process of Rubber Keypad and Keysetting Silicone Buttons
Rubber Keyboard Overmolding requires two sets of compression tools, one for different color buttons, one for whole set keyboard overmolding, and key position tools. The purpose of key position tool is to fix different color buttons into mold cavities for overmolding with button pad. If it is the conductive silicone keypad with carbon pills, a set of carbon pills position tools is also required. Therefore, rubber overmolding process is two-shot molding.
Rubber keysetting keyboard is only one-shot molding, which means keysetting keypad only need a set of compression mold. In production, different color material is placed to keypad mold cavities at the same time, and then compressed at the same.
Silicone Keyboard by Overmold
Rubber Button Pad by Keysetting
Rubber Overmolding Process
Rubber Overmolding requires compression molding machine.
1. Different shapes of colorful keys should be compression molded independently. These silicone color keys can be different Pantone colors or RAL colors, or with different durometer.
2. After all the color keys are ready, place them corresponding to their position in the key position tools.
3. If silicone keyboard is with conductive carbon pills, carbon pills should be place into keyboard mold cavities first by carbon pills position tool.
4. Using the multi color button position tools to fix all rubber color keys into correct cavity positions of rubber keyboard molding.
5. Place rubber raw material to keyboard mold.
6. Close and clamp rubber keypad mold plates and go into curing. Overmolding temperature of rubber buttons is about 170°C. Curing temperature and curing time should accord to keyboard thickness and geometries. If the rubber keypad is thick, curing time should be longer to avoid unripeness of material.
7. When the rubber button pad is fully cured, mold come out. Take the rubber overmolding button pad from cavities and start next shot curing.
Advantages of Rubber Overmolding
Add flexibility to the rigid components
Make the appearance of rubber parts more beautiful
Reduce assembly time and production cost
Overmolding combination will be stronger than parts glued
Common Issues in Rubber Keypad Overmolding Production
If the material is too soft, it is not easy to fix the soft silicone buttons to the cavities. Once these soft rubber buttons are being cured with another material in overmolding, soft button keys may be squeezed and raw rubber material will flow to the void and cover these squeezed soft buttons. This result in color bleeding.
Where to divide parting line between two color in overmolding rubber button pad?
Button webbing is the boundary to separate rubber pad with colorful buttons. In rubber keyboard overmolding design, button webbing should be left to connect with the rubber pad, while keep only simple button keys for additional compression mold.