criggster Posted March 25, 2009 Report Share Posted March 25, 2009 I have recently started pouring some 4.5" swimbaits, but my POP molds are not withstanding the heat and are cracking after a few dozen baits. I am using a two piece mold fully cured with 50/50 elmers glue as a sealer. Each half is about 3/4'" in thickness. I have to heat each mold to get the tails to fill out. I am heating in the oven at 150 degrees for a couple minutes. There are no vent, but I need to install one near the head as they have some large dimples there. I thought about getting Durhams Water Putty as an alternative. Will it hold up to the heat better than POP? Any suggestions are welcomed. Quote Link to comment Share on other sites More sharing options...
nova Posted March 25, 2009 Report Share Posted March 25, 2009 That's a new one on me; I've never had that happen. How soon after making the mold are you using it. In others words, how much curing time are you giving the mold? Do you cure it in the oven and are you re-curing it after you seal it? www.novalures.com Quote Link to comment Share on other sites More sharing options...
finfever Posted March 25, 2009 Report Share Posted March 25, 2009 Make some vents in your mold, this will help you with pouring. If you heat plaster over and over again it will always crack(well it always did with my previous screw ups)if you have to heat it try some kind of torch so you just heat the cavity and not the whole mold. Hope this helps. Quote Link to comment Share on other sites More sharing options...
nova Posted March 25, 2009 Report Share Posted March 25, 2009 When I pre-heat my molds I use old plastic. Just heat it up and pour into the molds. Let this sit in the molds while you get the "real" plastic ready for pouring. By the time that plastic is ready your molds are nice and warm. www.novalures.com Quote Link to comment Share on other sites More sharing options...
criggster Posted March 25, 2009 Author Report Share Posted March 25, 2009 I'm letting them cure for about four days and sealing them two days after making them. I usually let them dry inside the house. With the heat running there is very little humidity. I just poured again a little while ago and after the initial oven heating I used the same mold repeaditly and did not have to reheat. It looks like this is the best way. Would the Water Putty be more durable for multiple oven heatings? Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted March 25, 2009 Report Share Posted March 25, 2009 First off, 3/4" inch is not thick enough. Fully cured PoP is brittle and needs more thickness. Some members recommend 1" minimum thickness around the bait. This would probably give a 5/4" overall thickness. Secondly, cure time. five days in a room is not long enough. After 2 days, I estimate that only 6 - 7% of water has been lost. Then by sealing the mold, you have dramatically slowed down the rest of the evaporation process. Do not seal until the mold is dry. To air dry your mold as it is now, down to 70% original weight (fully dry is 65%), I estimate 10 days. Low humidity will improve on this figure, but not that much. By pouring plastic at 350F, is creating steam locally, inside the mold. This, combined with the thin section of the mold, could be causing the cracking. To check your mold drying progress, weigh the PoP when you remove it from the box. After 2 days, weigh it again. Get your calculator, type the new weight and divide by the original weight. You are looking for 0.7 (70%), anything higher is not dry enough. Another indication of dryness is the feel. If you keep a fully dried piece of PoP in the drying room (un-sealed) as a comparison and feel both, the uncured PoP will feel cool. This is because the moisture still remaining is absorbing heat from your fingers, making it feel cold. Cured PoP is a very good insulator. Finally, be aware that PoP reverts back to plaster of Paris when heated to 150C (cured PoP is gypsum), so I hope your pre-heat is 150F and not 150C. If the mold is over heated, it becomes powdery and will break very easily. The reason that it can be used for plastics at 350F, is all down to the quality seal coat, which holds it all together. The plastic temp will not convert the whole mold back to PoP, just the surface, as again, it is a good insulator. Without the seal coat, the detail would soon erode away. This temperature thing is fact, Google plaster curing and read for yourself. I posted a thread in 'homebrew tools' on the subject last week. It might give you a few more ideas. Here's the link: http://www.tackleunderground.com/forum/homebrew-tools/16539-pop-dryer-re-visited.html You should stick with the PoP and just solve the problems, it is all part of the learning curve. Some members (Longhorn) Has molds still working after more years than he would care to admit. Dave Quote Link to comment Share on other sites More sharing options...
Pitbull Baits Posted March 26, 2009 Report Share Posted March 26, 2009 I'm letting them cure for about four days and sealing them two days after making them. I usually let them dry inside the house. With the heat running there is very little humidity. I just poured again a little while ago and after the initial oven heating I used the same mold repeaditly and did not have to reheat. It looks like this is the best way. Would the Water Putty be more durable for multiple oven heatings? Bake them in the oven at 250 for 30 to 40 mins to get them fully dry. I let them set on the edge for a few hours to day to let the moisture get out after they bake. If you set the large flat side down to cool it will hold moisture on the bottom of the mold I have noticed. Also try to pour your tail first before you put the mold together if the vents don't work. Quote Link to comment Share on other sites More sharing options...
