Proper Mold Venting to Prevent Air Traps in LSR Injected Silicone Keyboard Production

What is Air Trap?

Air Traps occur easily in LSR injected silicone keyboard with button webbing. Air trap is that air get stuck in the mold cavity and can not run away from mold vents. Air traps always happen in the last-to-fill area and thick-to-slim area. Air traps in injection mold will cause defeects in molded parts, like voids and bubbles, short shoot also known as incomplete fill.

Why Mold Air Traps Happen in LSR injection Silicone Keyboard Production?

Silicone Keybaord includes two types, silicone buttons with key webbing, and keypad without key web. Silicone Keyboard is made of elastomer, which can be molded by compression molding, and LSR injection molding. While in LSR silicone injection molding, due to the race track effect, it is easy to lead to short shot in the keyboard webbing section.

Race Track Effect - Silicone Keyboard with large difference in the wall thickness

Silicone Keyboard button webbing thickness is about 0.3-0.8mm, but silicone pad and button thickness is much thicker than the key webbing, which make the rubber keyboard thick-thin-thick geometry. In liquid silicone injetcion molding, silicone material first flow to the thick section of chamber, because thick section resistance is less than than thin sections. In addition, when raw material flow to thin area, front part of thin section is cured quickly, then the cured part block the material flowing path to other thin section.  In these area, air is trapped. These elements will result in thin area can not fill completely.  

Long flow path can cause liquid silicone keyboard air traps, especially in the last-to-fill area.

If flow path in cavity is too long, air traps may occurs in the last-to-fill section and cause incompleter fill.

There are different flow paths in cavity, and their flow distance is different. For example, 3 flow paths in the following part. Way 3 is longer than the other two ways. Flow way 2 is the last to fill after flow way 1 and way 3. Mold venting problem may happen in flow path 2, causing air trapping and short shot.

Large flow turning angle may cause Liquid Injection Silicone Keyboard Short Shot.

In liquid silicone keyboard molding, if there are larger flow turning in cavity, it is not easy for liquid silicone to flow to turning area, especially the thin part after large flow bending. Because of larger resistance in thin area and insufficient injection molding pressure, rubber keyboard may have short shot problem.

How to Solve Air Traps in Rubber Keyboard Injection Production?

Improving Mold Venting can reduce the possibility of air traps in LSR injection molded keyboard production.

When we add more mold ventings to the air-trapping area, short shot can be improved largely. Like the the silicone keyboard in picture, we add three mold venting pins near the button webbing, keypad short short was solved. These ventings allow air to escape and smoothly flowing of silicone.

Change liquid silicone injection molding machine parameters

Mold temperature, injection speed, injection pressure, and vaccum condictions will affect mold venting as well.

If injection speed  is slow, there is more time for mold venting. But slow injection speed will result in lower temperature as well and the silicone flows slow too. Material will harden before other cavity area fill, and cause silicone keyboard short shot too. 

So adjust the LSR injection machine parameters should take time to try what is the best for mold venting and not cause incomplete fill products.

Add more needle injection valves to reduce flow path distance

If rubber keypad is large, one needle injection valve is not enough for one cavity is not enough. LSR injection needle valve provide the injection gate to allow silicone inject inro cavity. Only one needle valve a large keyboard will lead to long flow path canse mold trapps and short shorts. More needle valves allow material to flows more evenly to cavity corner and reduce flow path.