Why bubbles appear during liquid silicone injection molded process?

In Liquid Silicone Rubber Parts Molded Production, we find that bubbles appear occasionally, but many people do not know how the bubbles are produced? And how to solve bubbles problem in LSR Injection Molded Parts Production? After repeatedly test and analysis, our experienced engineers found the reason  and have solution to solve bubbles apperaing on the surface of liquid silicone products.

Totally there are 7 reasons that will cause bubbles in LSR Products. The followings are our analysis:

1. Vulcanization temperature is too low

Vulcanization temperature is an important parameter in the molding of silicone rubber. Common silicone products are set at 160~200 degrees Celsius, but sometimes the mold does not enter the machine in time due to the long time of the mold operation or other reasons, so the temperature of the silicone molding is low, which causes bubbles in the molded product.

How to solve bubbles problem in this case?

We need to increase the molding temperature, or warm the mold for a while.

2. Vulcanization temperature is too high

Low Vulcanization temperature  will cause bubbles to form in the silicone parts. But it will also casue bubbles if vulcanization temperature is too high. Why? Because if the molding temperature is too high, the surface of the silicone material has already begun to form during the mold clamping process.(before the clamping process is finished) And the air is trapped inside, it is difficult to release the air in the mold, so it will cause bubbles.

We can solve this problem by lowering the molding temperature appropriately.

3. Vulcanization time is too short

Like vulcanization temperature, vulcanization time is also one of the important parameters of silicone forming. The length of vulcanization time determines whether silicone can be fully vulcanized or not.If vulcanizaiton time is too short, silicone products are not cooked, and you can find bubbles in the products.

4. Mold Structure

If the structure of the mold is unreasonable, and the design of the silicone forming is not reasonable, which will also cause bubbles in the forming of the silicone.

The design of the exhaust structure is unreasonable, and the exhaust position should be designed at the end of the product forming, and if necessary, vacuum is designed in the mold.

5. Mold Cleaning

Mold is not clean. When cleaning the mold, need pay attention to the method and should gently scrub with a soft fabric to avoid deformation of the mold.

6. Design of Injection gate is unreasonable

For some large liquid silicone products, in order to improve the rubber filling,many engineers will design multiple injection gates/ports. Although the liquid silicone material looks very viscous, it actually has good fluidity and generally does not have to worry about filling. If it is necessary to design multiple injection gates, the position of the injection gate must be balanced and symmetrical, and the exhaust gate should be designed at the corresponding position.We dont’t recommend to design more than one injection gate.

7. Feeding system exhaust problem

LSR feeding system: When installing new materials or changing materials, if there is gas in the pump body, the pump should be exhausted. How to tell  whether there is gas in the pump or not: open the pump body exhaust ball valve, press the glue pump up or down to see if the material in the pump  exhaust ball valve has bubbles/sound or not, if there is bubble or sound then we can make sure there is gas.

The specific steps are as belows:

1. Unplug the quick plug

2. Open the barrel blow valve

3. Lower pressure plate

Air will be expelled from the blow-off balloon valve. When the air-free liquid silicone enters the transparent hose, it means that the gas has been exhausted and can close the blow-off balloon valve.

How to make sure there is not air in the AB Material?

Dosing cylinder exhaust:

1. Switch to the automatic state, the rubber will automatically enter the metering cylinder. The system automatically stops after the metering cylinder is full.

2. open the quantitative cylinder exhaust ball valve

3. Switch to the manual state, dial to the metering cylinder(upper) for 1 minute, then dial to the metering cylinder(down) for 1 minute, and the silicone oil and air will be discharged. Repeat the operation until the AB glue pressure is consistent.

4. After the silicone-free and air-free compound flows out, close the exhaust ball valve.