dramone Posted December 30, 2008 Report Share Posted December 30, 2008 (edited) why are the air vents for softplastic lure molds not at the highest spot of the mold? i.e. Tackleunderground i made over 2 dozen rtv molds for hard baits with great results, all the airvents (if some) are on the highest spots in the mold (or pockets), to get rid of rising air and bubbles.... but when i see the softplatik molds all the airvents are on the opposite site of the pour hole... i dont get it.... Edited December 30, 2008 by dramone Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted December 30, 2008 Report Share Posted December 30, 2008 (edited) Gravity pouring soft plastics into two piece molds generally only requires vents at the extremities, where a bubble can get trapped. Because the pour is done down the centre of the pour hole, without touching the sides, the air rises with the plastic and gets pushed out of the pour hole. If a bubble constantly forms for what ever reason in the mold design, then a vent at the pour end could solve the problem. For vacu-venting, the plastic is dumped into the mold at high speed. Bubbles are inevitable and the venting strategy has to predict where to collect these pockets of air. One each side of the pour hole is a good strategy in this case. The same would probably be true for injection pouring. Ideally zero vents is the target. Reducing the number of vents by one saves another operation after the demold. So the design should be such that you start off with zero or the obvious vents only, then the slick design would allow extra vents to be cut easily, as required. This is done by pre-cutting the vents, but stopping short of the mold cavity. As you make more molds, you will become more skilled at predicting where the vents will be required. This is the interesting part, you have to think like the plastic. Dave Edited December 30, 2008 by Vodkaman Quote Link to comment Share on other sites More sharing options...
Travis Posted December 30, 2008 Report Share Posted December 30, 2008 The difference in the two materials plays an important role. The plastic flows into areas to fill and will trap air in pockets. With the nature of plasitc it starts to cool and viscosity increases and the bubble won't be able to rise up the bait and can get trapped. Also plastic is limited in the force it can exert to push the bubble through the small space between mold faces where as urethane foams expand and will drive air out. I can recall several times I left the urethane to cure too long and it had swelled so much in the cavity I destroyed the mold trying to get it out. Quote Link to comment Share on other sites More sharing options...
dramone Posted December 30, 2008 Author Report Share Posted December 30, 2008 (edited) thx Dave Gravity pouring soft plastics into two piece molds generally only requires vents at the extremities, where a bubble can get trapped. Because the pour is done down the centre of the pour hole, without touching the sides, the air rises with the plastic and gets pushed out of the pour hole. right but why an airvent at the bottom of the mold, opposite side from the pour hole? how can air escape trough the bottem (vent) of the mold if the mold stands vertikal? or are the differences beetwen the materials (plastisol vs. PUR) the big issue? when casting PUR, the mold is filled with material an stays "liqiud" for some time, so air has a chanche to rise and/or escape trough the vents and/or pour hole. i still dont understand the reason for a bottem vent if the mold is horizontal... btw: i dont only cast baits... Edited December 30, 2008 by dramone Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted December 30, 2008 Report Share Posted December 30, 2008 I agree, if the mold is a one piece, I cannot think of a reason other than something to extract the worm by. Just does not make sense. When pouring a two piece mold, if the pour does not hit the bottom, it may splash and trap a bubble or air pocket. The low vent will allow this air pocket to escape. Sometimes angling the mold improves the pour. I think this is to prevent the splash at the bottom of the mold. Dave Quote Link to comment Share on other sites More sharing options...
dramone Posted December 30, 2008 Author Report Share Posted December 30, 2008 Dave i forgot, the question is all about a two piece mold When pouring a two piece mold, if the pour does not hit the bottom, it may splash and trap a bubble or air pocket. The low vent will allow this air pocket to escape. do you mean that air is "flowing" out together with the hot plastic trough the bottom vent? Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted December 30, 2008 Report Share Posted December 30, 2008 Yes, the air escapes first, driven by the weight of plastic above it, then the plastic plugs the hole. Dave Quote Link to comment Share on other sites More sharing options...
dramone Posted December 30, 2008 Author Report Share Posted December 30, 2008 thx Dave makes sense, due to the short "curing time" of hot plastic Quote Link to comment Share on other sites More sharing options...
Delw Posted December 30, 2008 Report Share Posted December 30, 2008 Dramone air vents on the bottom of the mold will not work if and only if the bottom of the mold is perfectly flat( key word) and the surface in which it is standing on is perfectly flat. even a .002 gap (a hair thickness) between bottom of the mold and the surface in which it is sitting on will allow air to escape Quote Link to comment Share on other sites More sharing options...
HJS Posted December 30, 2008 Report Share Posted December 30, 2008 dramone - What others said. Vent holes at the bottom are crucial for many molds, especially those that make swim baits and paddletail lures (see pic). These molds neck down near the tail before expanding for the actual paddle part. If there were no vents at the tippy end of the tail then the plastic would never make it there. It would plug-up at the narrow part. So, the molten plastic will actually push the air out the bottom vent hole as it fills the paddle portion of the mold. Once you start pouring, plastic start setting up almost immediately. The mold pictured is stood up on its end when pouring. I'll also slightly cock the mold to the side so the vent holes don't seal to the table top. Quote Link to comment Share on other sites More sharing options...
dramone Posted December 30, 2008 Author Report Share Posted December 30, 2008 ahhh! many thanks for the answers! Quote Link to comment Share on other sites More sharing options...
HJS Posted December 31, 2008 Report Share Posted December 31, 2008 Oh, I forgot to mention... or I should say I was reluctant to mention, one thing I do to help the mold fill out completely when pouring the 4" paddletails pictured above. In quick succession, I'll pour the first cavity 3/4 full then lean over and blow in the pour hole like I'm trying to blow out a clogged trumpet... just a short 1/2 second toot. Then do the same to the next 2 cavities. Then go back and top them off. Do all these steps as quickly as possible. Fills out the tails every time with no blistered lips. Keep in mind that I've been cheating death and dismemberment my entire life... always pushing the envelope... whether it be running with scissors, riding bikes without a helmet, sticking forks in toasters or crawling thru the jungles of Vietnam... and have lived to tell about it. I guess blowing in hot molds is no different. Quote Link to comment Share on other sites More sharing options...
Richard Prager Posted December 31, 2008 Report Share Posted December 31, 2008 That's closer to insanity than pushing the envelope:eek: Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted December 31, 2008 Report Share Posted December 31, 2008 Good idea, but MAN, you gotta find a better way to do it. Maybe a flat plate, rubber coated to improve the seal, with a LONG flexible pipe through the centre. That should work a tad safer. Dave Quote Link to comment Share on other sites More sharing options...
