Oh the pressure!

[brownsfn2]

I have been struggling with converting my on-tube blank making process to vertical casting using plastic tubes. Long story short I have had good success casting two blanks at a time with 5 drops of MEKP and 80 PSI.

I found when I used 3 drops of MEKP the plastic tube I am casting in would get soft and hazy. Which is not what I want if I need to show them off. Not sure why that happens but I am past that now.

The real problem I am having now has to do with pressure. When I have two blanks in the pot it works great and the tub compresses just like I need it to in order to keep the resin from shrinking away from the material.

When I add a plywood circle with 8 blanks on it to the pressure pot then the castings act like there is less pressure in the pot. Only half of the blanks come out right.

Has anyone seen this where the pressure is not the same with more items in the pot? My pressure gauge reads the same regardless.

Really has me perplexed. If anyone can offer some advice or insight I would love to hear it. Thanks for any help.

 

[Ed McDonnell]

Maybe it has to do with heat. 8 blanks will produce more heat as they cure than 2 will. Maybe that extra heat is enough to speed up the cure of the batch and give you the problems you are seeing.

Ed

 

[SteveG]

I was thinking that the added time it takes you to do 8 instead of 2 is having the effect of different results done at the same time in the same pot. Is that possible?

 

[Jim Burr]

I haven't done much pressure cast Ron...yet!! The first thing that jumped out at me is the 80psi. JMHO, but that seems kinda high. Again, not a lot of experience yet, but it just made me look twice.

 

[BSea]

Is there space between the plywood circle, and the tops of the tubes? If not, it MAY be preventing the tubes from being fully pressurized. This is just a guess, and frankly, not something I think is happening. I was just guessing.

I don't see the 80psi being an issue. I do think it might have something to do with heat. 5 drops seems excessive to me. I rarely use more than 4. And for clear casting, only 3.

 

[brownsfn2]

Thanks for all the suggestions so far.

@Jim - Whenever I go below 80PSI is seems that the PR pulls away from the material around the brass tube. Almost like there is not enough pressure around the outside of the plastic mold.

@Bob - There is nothing between the tops of the tubes and the top of the pressure pot. The plug I use for the bottom of the plastic tube mold is screwed to the plywood.

@Steve - I thought about that so when I tested just one blank I let the resin sit and cure for 10 minutes first. No issue.

@Ed - I think that the added heat would increase pressure though right? Not sure though.

I will let the trial and error continue.