Flash Prevention in a pour molding process

[MrTwenty6point2]

We have a molding process where a material is poured into a cavity, sealed with a lid and then heated so that it takes the shape of the mold. Similar to injection molding but not quite. The issue is that the material provides enough cavity pressure to force the lid open slightly leaving potential for flash. The mold is made of plastic (not perfectly flat) and the lid is made of a malleable silicon in order to conform around the circumference of the cavity opening when closed/clipped into a mold. The process requires the mold to be in motion to continuously be filled and emptied over and over again.

My initial thought is to measure the opposing forces to gauge how much force is coming from cavity pressure vs being applied downward by the lid. However, I'm not sure of the easiest/best way to do this. As the process is always in motion and enclosed I don't see a way to use a cavity pressure sensor to pull readings with. I thought about using force sensors but again you run into the issue of having to tie it into something to take readings. I also don't think that a force sensor would be accurate enough.

My thought is to have more downward force to counteract the cavity pressure and help the elasticity of the silicon lid. Any thoughts or suggestions on how to improve this and prevent the potential for flash would surely be appreciated.

 

[3DDave]

Introduce pre-heated air just before putting the lid on the mold; perhaps some degrees hotter than is used in the process. It will initially cool to lower the pressure and better seal the mold and then, as it warms, return to ambient pressure and avoid forcing the molding material out. This avoids the distortion, though some means to relieve the internal pressure decrease as the item cools should be added - something to puncture the molded item so it doesn't collapse if the change in pressure is too much.

Other than that, try Gay Lussac’s Law
According to Gay Lussac’s law, the pressure of a gas of definite quantity at constant volume is directly proportional to absolute temperature.

P œ T
P = KT
P/T =K

P1/T1 = P2/T2
P1/P2 = T1/T2

https://www.web-formulas.com/Formulas_of_Chemistry/Gases.aspx

 

[bobjustbob]

Why not just add a stronger backing material to the lid until you make it strong enough to eliminate the flash?

Bob

 

[3DDave]

Is this a closed mold - that is no vent and no air void? I thought there was a void. If not then the lid it trying to maintain pressure against thermal expansion of the liquid as it is heated - this could be 5-10 thousand PSI and require anywhere from a few tons to hundreds of tons to resist. That pressure would currently be relieved by the escaping flash material - adding a vent would prevent flash by allowing expanded material to exit at a low pressure.

 

[MrTwenty6point2]

bobjustbob- I am thinking of trying out a firmer silicon to see if it can better resist the pressure while maintaining an appropriate seal around the cavity. It is a bit of a delicate dance, if the silicon is too hard you will end up with(not a good enough seal/the lid begins to crack early in the process). The silicon lid is attached to a plastic arm that could potentially be changed in size/mass/rigidity. The arm comes down and clips onto the mold pressing the silicon lid against the cavity.

3DDave- Yes, there are three vent lines on either side of the cavity to allow air to void. The material for this particular item does not tend to leave discernable vent trails/whiskers, which I would expect to see on others. The material can flash parallel with the vents and/or perpendicular. I don't believe the vent lines setup to be of concern. Although, it could be worth a look.