Short Shot & Underfilled Issues in Liquid Silicone Injection Molding Production

Short Shot also called as Underfilled issue, Incomplete filling, lack of material, which is a common defect during Liquid Silicone Injection Molding Process, and is mainly caused by long flow distance, the incomplete area is far away from the nozzle outlet, or at a thin wall (such as the bone position). Poor mold venting will also cause short shot occuring.

So to design a good liquid silicone molding venting and reasonabel nozzles is very important to make sure a complete and qualified LSR molded part. Our experienced engineers will make a detailed manufacturability and moldability analysis on the product design structure before mold making, and then design reasonable mold injection gate and venting system. After mold test and continuous improvement on mold venting, mold temperature, injection speed, we can move forward to production.

Main Reasons to cause imcomplete filling/short shot:

1. Small injection volume

2. Venting problem/air trapping that prevents material flow successfullySome cross-sections are prematurely solidified (eg injection speed is too low or wrong mold temperature control)

3. Flow restrictions. Due to channels freezing or inadequate runner design.

4. Long or complex flow paths. If the flow path is too long, the material will not flow into cavities in time or the material will cured during flow. So the cavities lack of material, short shot occurs.
   

5. Inadequate venting. If venting is not enough, air traps easily, which will also cause a short shot.

6.  Low melt and/or mold temperatures.

7. Insufficient liquid silicone material entering the cavity. And if the Liquid Injection Molding Machine is not large enought, with low shot volume, or inadequate ram speed, also is the reason to cause short shot.
   

8.  Machine defects. Including an empty hopper, blocked feed throat, or a worn non-return (check) valve that causes loss of pressure or volume leakage.

How to solve Short Shot issue?

Followings are the common solutions:

• Increase shot size

• Inadequate cushion

• Increase fill speed, pack pressure, and injection time

• Increase melt/ mold temperature

• Plugged gates, runners or vents

• Inadequate melt flow rate(use higher MFR material)

• Undersized gates,runners, and vents

Let's slove the problems step by step

Firstly, confirm whether the screw is too forward, if so, we can:
1 Increase the amount of plasticization
2 Check the check valve
3 Check the safety valve

If the position of the screw is ok, the next step is to see if the curing speed is too fast, if it is, we need to:
1 Increase the flow channel
2 Increase the injection speed
3 Reduce the mold surface temperature
4 Increase injection pressure
5 Increase holding pressure and time
6 Reduce the screw speed
7 Increase back pressure

If the curing speed is no problem, we need to:
1 Improve exhaust
2 Check the feeding system
3 Check the nozzle hole