Silicone keypads are made using compression molds. Compression molds have a bottom tool and a top tool half. The molding process is very similar to how Belgian waffles are made.
First the mixed raw silicone material is cut into precisely cut slabs. Next the material is laid in the mold. Pressure, heat and time are used to transform the material to the shape of the cavity. The parts are removed from the tool, flash is hand torn and then the parts are post-cured to remove any residual catalyst material.
The mixed and pigmented raw silicone material must be carefully placed in the mold to ensure that enough material fills the cavity, and not too much material prevents the mold from overflowing.
Abatek achieves this by rolling the raw material to a specific thickness and width. Next an automated machine pulls and cuts the material into equal sizes. The cut pieces are weighed to ensure that their volume will correctly fill the cavity.
The cut pieces are laid into a pre-heated mold, and the mold is closed. Alternative molding methods requires a different process. Typical cycle time for silicone keypads is about 5-10 minutes - which is much longer than typical plastic molding. Multiple cavities are used to increase output (having 140 cavities is not uncommon). See also cavity identification
Inside the tool, heat and time transforms the silicone mixture. First the raw silicone liquefies, filling the cavity. Next the material hardens in what is called the vulcanization process. Here the molecules of the material cross-link, permanently transforming the part into its finished shape. After vulcanization, it is not possible to revert back to the original silicone material.
The parts are manually removed from the tool with the flash still attached.
Because the raw material liquefies, and must fill the entire cavity, the mold is design with overflow channels, which is why there is flash. All regions where the top of the tool meets the bottom, there will be flash. This includes the perimeter and any opening.
The flash is easily removed by hand. Note that on all parts, there will always be a flash line with a small amount of silicone flash (less than 0.3mm) remaining.
Some residual catalyst material from the raw silicone may not have fully vulcanized and can remain in the part. Overtime, this residual will leach out, depositing on the electrical contacts and potentially affecting keypad function.
To prevent this Abatek post-cures keypads after they are molded. During post curing, the parts are placed in an oven to heat and evaporate any non-vulcanized catalyst. Upon request Abatek can perform a weight loss test to ensure that the post-curing process is correctly performed.
Because of flashing, it is highly difficult to add inserts in compression tools. It is far easier to simply remove steel from the tool.
For this reasons the initial design of the part must be made with careful considerations to tool modifications. Existing molds can have steel cut away (adding material to the part), but steel shouldn't be added to the tool (remove material from the part). In the latter case, a new bottom and/or top half of the tool must usually be machined.
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If its to have multiple colors on a keypad or to even have harder keytops, Abatek provides two alternative molding techniques to help expand the color range of your keypad without needing a secondary painting process.
Many keypad designs require multiple colors. Compression molding allows for different colored silicones to be molded together at the same time. This is called flow molding.
The first step in the flow molding process is to make two or more silicone mixtures. The silicone mixtures are of different colors, but of the same durometer. The mixtures are cut and strategically placed in the mold. During the vulcanization process, the silicone materials will flow and bond together, molding into a single keypad.
The flow molding process is relatively low cost way of adding color to a keypad. Compared to a single color keypad, the process changes only include additional silicone mixtures and additional silicone placement in the main mold. There are no additional tooling costs
See also LED Pockets
However, it is difficult to control how the different materials mix and flow together. There is usually some color bleeding. Keys that are too close together cannot be easily flow molded in different colors due to bleeding.
Abatek can only guarantee that the upper 1/3 of the key will not have bleeding. The key should be 8mm or taller.
The silicone hardness needed for keypad webs is fairly soft (about 50 Shore A). For customer requiring keys with a harder Durometer, insert molding can be used. In addition, insert molded keys can be made in different colors.
First the "inserts" are molded in a separate tool. The inserts must be molded out of a harder durometer than the main part. Once the inserts are molded, they are placed in a special insertion jig. The insertion jig is used to push the inserts into the main tool. From there, normal molding takes place and the silicone material in the tool will bond and permanently vulcanize to the inserts. All conductive pills are insert molded.
The inserts will provide a clean line without any bleeding.
The inserts must not be able to move in the main tool otherwise the main silicone can flow under the insert ruining the process. The insertion jig essentially forces the inserts into the tool so they cannot more. Short keys do not have enough depth to hold the inserts. Only keys that are 6mm or taller can be insert molded. The insert must be at least 2.5mm thick.
Because multiple tools and molding steps are used, insert molding is more expensive compared to flow molding
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