January 28, 2019

Over the years, silicon rubber have gained industrial popularity mainly due to its outstanding electrical and mechanical properties at varying temperatures. It is considered a natural choice for many applications ranging from medical to aerospace devices. It is therefore important to understand how this material is processed and fabricated.

The methods of processing silicon rubbers are classified as high-temperature curing process (HTV) and low-temperature curing process (RTV) depending on the type of polymer utilized. Curing is a process used in the formation of crosslinks between silicon and rubber before fabrication into actual products. Curing in silicon rubber is achieved through the use of platinum catalyzed system which the most commonly used, peroxide cure system, and condensation cure systems.

Silicone Rubber Block
Photo : Block of Silicone Rubber before processing

The fabrication of silicon rubber is achieved in various ways depending on the requirements of the final product. Each fabrication method is associated with its advantages and drawbacks. Some of this fabrication methods includes; injection molding, extrusion, and compression molding.

Extrusion is a fabrication method is designed to form a silicon rubber product through the use of a die. During the process, the raw materials (silicon, rubber, catalyst, binders among other additives) are fed into an extruder at high temperatures coupled with rotatory motion of the screw. The material is then forced through a die of a given shape before post-curing. The final product is then washed, cooled and cut into the specified sizes. This method is more efficient and suitable for high production. However, it requires skilled labour and high quality machinery to achieve good dimensional stability.

silicone tube made using old die
Photo : Silicone Tube extruded from an old and worn-out die

Injection molding is employed for creating complex shapes. In this process, cured raw materials the injected into a mold ranging from simple to complex shapes. The material is allowed to cool and then removed from the mold as a finished product. Injection molding is often optimized through regulation of vital factors such as sprue size. This method can accommodate high output production. However warping, sinks bubbles and flash are some common problems. 

Compression molding involves creation of products through placing a pre-cured silicone rubber into a heated mold. The mold cavity is then closed by a heated plunger that applies high pressure. The material melts and is forced through the mold cavities. The high pressure and heat is maintained until fully cured. The mold is opened to remove the formed part. This method is considered economical and suitable for high production rate. The limitations associated with this method includes flash problems and dimensional accuracy of the final product. These problems can be resolved by replacing the molds regularly to ensure consistency in the final product. 

In conclusion, processing and fabrication of silicon rubber is dependent on various factors. As such, cost, design criteria and the production rate should all be considered in deciding the production method to use. Based on the pros and cons of each fabrication method, it’s critical to fathom the sensitivity of the methods towards creating a profitable and a functional silicon rubber product.

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