How is silicone rubber molded?
What is silicone rubber molding?Before the silicone rubber product was formed(shaped), their raw material is in the form of a solid gum shape or liquid glue texture. No matter which forms it is, they all belong to "thermosetting" polymer. Therefore, it is not difficult to imagine that the silicone molding method is shaping it by temperature heating. Most products without post-process are completed after this stage. Some products will enter the oven for secondary baking (Post curing) as required to complete the final product shape.
What are the types of silicone rubber?
High-consistency silicone rubber (HCR)
Its raw material is solidified texture, like block gum, and the appearance is generally milky white. The conventional hardness is about 30-70 shore A, and some special products will require 80-90 shore A. These will be modulated during the raw material processing and mixing stage, and the products with special color requirements will also be mixed with color pigment at this stage. Most of the daily necessities and consumer electronics products on the market are made of this type of silicone molding. On the other hand, the material cost is more competitive.
Liquid silicone rubber(LSR)
The raw material is in the form of a liquid water paste, with a transparent appearance and texture, almost as clear as water. Commonly used hardness is 10-20 shore A, some special products will require 50-60 degrees, but it is not common, and not recommended, because the increase in hardness will slow down the fluidity of the raw materials and cause poor quality. Generally, silicone molding products using liquid silicone rubber as the raw material have no post-process and coloring requirements, so that the unique feature of the high transparency of the raw material won't be hidden. So it is easy to understand that the liquid silicone rubber somehow compensates for the specific limitations (transparency and softness) of the solid silicone rubber itself. Since liquid silicone rubber is more expensive than solid silicone rubber, it doesn't be used so commonly. At present, it is still used in products with higher precision and high-value products, such as the optical and medical categories, so its popularity is not as high as that of solid silicone.
Two common silicone rubber molding methods
Compression MoldingThe compression silicone molding process is only suitable for solid silicone rubber (HCR) of raw materials, so you can imagine the process is pretty much like making Belgian Liège waffle just by imagining it. When received the silicone raw material, will be mixed through the roller to improve the softness and processed by adding the curing agent, color pigment, and other auxiliary agents to complete the semi-finished material before going on the compression machine. Then according to the product design and structure, take the semi-finished product with the corresponding weight and place it flat on the opened tooling, and finally close it and heat up under setting temperature, and finally complete the final silicone molding product. The compression molding technique is a quite mature and traditional manufacturing process. Both raw material and tooling of silicone products are relatively cheap. If the appearance and accuracy of the product itself are not strict, then compression molding can meet almost all the requirements. Of course, it is not necessarily an upside for manufacturers, because the production process can only be operated manually, which is time-consuming and labor-intensive. The defective rate is also high that wastes the material more.
Liquid Injection MoldingInjection silicone molding is only suitable for liquid silicone rubber of raw materials. If linked the principle of plastic injection molding to this process, it will be easier to understand. After all, liquid silicone rubber injection molding is basically derived from the plastic injection process. The liquid raw material has been formulated before being delivered to the silicone molding manufacturer in the sealed barrel packages. No processing is required before mass production. After the raw material is installed on the machine and tested on the machine, it is automatically fed and injected into the closed tooling on the machine. The whole process does not require too much manual intervention, and because it is produced in a sealed environment, it has a good appearance and cleanliness and is highly used in high-sensitivity products related to medical treatment, baby, and skin contact. However, the development cost of raw materials and tooling are high, the budget will be quite tight if there is no demand for large quantities of products to support.
Comparison chart of two molding types
|Compression Molding||Liquid Injection Molding|
|Product feature||Has obvious parting line, without injection sprue.||No obvious parting line, with injection sprue.|
|Production techniques||Technically low with more manual operation.||Technically high, can be produced automatically.|
|Applications||Silicone rubber buttons/keypads for remote controls and other electronic devices, interior parts that do not require precise appearances, such as O-ring waterproof rings, silicone molding products with low precision or high tolerance, silicone parts with multi-color processes. The larger size of silicone parts such as jelly protective cover, dust cover.||Products that require high transparency, such as optical products, diving goggles, and oxygen masks. High-precision parts with fine appearance dimensions, such as automotive lamp silicone rings, medical equipment, and biocompatible implant products.|
|Cost||The development cost of raw materials and tooling is low, but due to the large labor demand and high defective rate, the unit price of the product won't be cheaper in the long run.||The development cost of raw materials and tooling is relatively high, but the automatic production increases the production efficiency and yield rate which causes the unit price of products cheaper in the end.|