A well-designed mold exhaust prevents the silicone keypad from producing a bubble defect during injection production
May 14, 2020
What’s the Air traps?
Air Traps are also called bubble Traps.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,linke voids and bubbles, short shoot also known as incomplete fill.
Silicone keyboard includes two typed, 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.
Silicone keyboard button webbing thickness in about 0.3~0.8mm,but silicone pad and button thickness is much thicker than the key webbing,which make the rebber 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,air is trapped, these elements will result in thin area can not fill cmpletely.
If flow path in cavity is too lang,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 in 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.
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 flow bending, Because of larger resistance in thin area and insufficient injection molding pressure,rubber keyboard may have short shot problem.
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 silicong 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.
Mold temperature, injection speed,injection pressure, and vaccum condictions will affect mold ventiong 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 to try what is the best for mold venting and not cause incomplete fill products.
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 into cavity. Only one needle valve a large keyboard will lead to long flow path canse mold traps and short shorts.More needle valves allow material to flows more evenly to cavity corner an reduce flow path.
