How will expanding foam fill this hypothetical mold?


Well-Known Member
Imagine you have a human scale mannequin mold, which fills from the feet. This is just an example shape, I'm not actually doing this in real life.

If you were to fill it with liquid resin (shown below), you would end up with voids because air gets trapped. You'd have to add sprues to allow proper venting of the trapped gas.


Now imagine an expanding foam was poured into the mold. Assume a back-pressure plate is added after filling, like what's shown in this Smooth-On video. Also assume it is poured down a single leg and pools in the head before kicking off.

Would the foam rise and fill the mold completely, or would trapped air again prevent the material from flowing into the hands? Could there be other complications as well, like the foam not flowing up both legs, etc?
There is spray foam used for construction...mostly with retrofitting windows. With some of the early stuff if you sprayed too much, the foam would expand so much it could warp the window frame. The new stuff doesn't do that as much and "should" only expand to the limits of the enclosure. I'm not certain how much pressure it actually applies to the walls though.

If filling from one foot then the other foot would need a vent and if each hand has individual fingers then each finger would need an open endened vent (small tube) up to the fill level of the foot you are pouring into.

The tubes could be small as you are only expelling trapped air. As soon as the expanding foam reaches the tubes it doesnt matter if the foam wont travel through them as the fingers will now be filled.
I agree that you need vents for this hypothetical mold. The foam will take the path of least resistance, which will send it up the pour spout before traveling other paths.
Thanks for the info everyone!

I did some more research, and discovered you also have to think about how the foam might expand as it travels and "drags" along edges in geometric shapes. This video shows a good example.

Your message may be considered spam for the following reasons:

If you wish to reply despite these issues, check the box below before replying.
Be aware that malicious compliance may result in more severe penalties.