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injection mold problem

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Need some help guys. I'm still very new to this soft plastics bait making thing, and made some injection molds out of Durham's water putty. My crawtube baits turned out pretty well, but I am having trouble with a fat bodied ring worm. I Frankensteined a couple halves of a bait together for the body and put a curly tail on it. I am consistently getting air bubbles in the bait. At first I thought it was dimples from plastic shrinkage. But now I know by cutting into a bait, that it's actually some decent sized air bubbles that are collapsing once the bait cools. I vented the mold to get the tails to fill fully and that worked well, but I am thinking I probably need to vent the body cavity? 

I've tried shooting the mold fast, with the plastic at around 340 and down to about 310. Haven't tried much lower than that because it's a ring worm kind of thing and figured I need the plastic pretty hot to get a good fill. Like I said. It's pretty much collapsing air bubbles causing the problem. Below are pictures that I hope you can see what I am talking about. I am not happy with the mold so I am going to be remaking it, but my guess is I will probably have the same problem. Any help you guys can provide will be greatly appreciated.





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bryanmc, Jig Man, and McLuvin175, thank you all for your replies. they are much appreciated.

bryan, I was wondering about that. I wasn't aware of the siphoning affect of the cooling plastic drawing in hot plastic from the gate and runner. Think maybe that long gate is cooling too much and effectively shutting off the chance for the cooling bait to draw in more plastic from the runner? May even be closing any chance of air leaving the body of the bait. I do plan on remaking the mold, so I will shorten the gates when I do that. Just trying to make it work right now because I have a tournament Saturday and I pretty much made this bait to fish this lake. Anyhow ....

Jig Man, I have tried your suggestion. Still get the same results. I've even tried keeping pressure on for 30 seconds, and not just slight pressure.

McLuvin, I am thinking that I need to try adding a vent as well. As I said in my original post ... these are not plastic shrinkage dents, they are collapsed air bubbles. I do find it odd that on to of the baits it's always near the nose, and with the one bait, it's always in the mid portion of the body. I'm thinking that it being a ring worm type bait that it is catching even more air.

I'm going to add a vent at the nose and see if that helps. I'd like to get it figured out before I go making a new mold.

Once again, thank you guys VERY much for your replies. Got some ideas to work with now, and that's what I needed. Thanks again!!!

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Every rib is just another  opportunity to trap air . Plastic has to be hot to fill all of those little ribs but the hotter the plastic the more it shrinks as it cools so it's kind of a catch 22  . If the air is not displaced as the mold is filling with plastic it will have no way to get out once the mold is full resulting in trapped air pockets (bubbles in the bait ).  If possible you could try altering the mold plunger arrangement to pour with the cavities oriented vertically so the (thinner) air is forced upward and out towards the tail vent  by the ( denser ) plastic and pushed out ahead of the plastic as the mold fills . The vent at the tip of the tail should also be reoriented  pointing strait up from the tip with a loop to change direction .

Any air entrapment after that could be dealt with adding more venting where needed . 

This stuff is all just trial and error but persistence will usually pay off.

Edited by jigmeister
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jigmeister, thanks a lot for your response. I truly appreciate it. And yes, I definitely need to redesign the mold. Like you said, and as I also suspected, the ring worm characteristics are only adding to the problem. Since I was already planning on remaking the mold. I took the easy way out on venting the existing one and just simply drilled a hole near the nose of the bait. I figured I needed to be able to make the vent closable because I need pressure to be able to shoot the curly tail. The tail vent actually works perfectly, and I will probably use the same set up for the new mold.

I know that shooting the two color bait is causing me problems because that vent is shut off by leaving the tail in the mold when I shoot the second color, so there is no venting at all. But I still have the trapped air bubble problem even when shooting a single color bait. I'm thinking of going to a two bait mold and putting the sprue and gates near the curly tail junction. then venting the nose in the same fashion that I have the tail vent set up on my current mold. That way I will be pushing the air towards the end of the bait. I think I need those little channels because they add some resistance so the body cavity can get some pressure in it. That single hole I drilled in the current mold just took away any chance of building pressure in the mold. I got one good bait out of three, and even that one has a little trapped air. Anyhow ....

Also going to a two bait mold will leave me more room for adding venting channels with drilled holes on the ends of them, That seems to be working pretty well. Like you said though, it's going to take some experimenting until I get it right. I know that once I get it right it is going to be a great bait for me.

Thank you again for the reply. Much appreciated.

Oh and at least I have 5 baits that are usable for Saturday's tournament. Not the numbers I was hoping for, but that's what I get for putting off working with this mold. Live and learn

Edited by Bassbme
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That is a nice mold.  Before I scrap it I would try enlarging the gateway from the runner to the cavity.  Even if it looks ugly, it won't affect how the bait looks.  I would begin with enlarging the runner/sprue connection to the same size as the runner, and then taper it down to just before it meets the bait cavity.

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Mark, thank you much for the reply. I appreciate it. Definitely not going to scrap it until I get the venting process figured out, and until trying your suggestion of enlarging the gate. I know I have to vent the trapped air somehow, but with your suggestion are you thinking that the long skinny runner is cooling and not letting the body suck in more hot plastic from the runner?

Anyhow ..... I'll be starting a new thread in a day or two. I've got a few ideas for the new mold I'd like to bounce around with you guys. I really want to get this bait right and I'm sure that with the help of you guys and other helpful members of this site, I will be able to do  just that.

Thank you guys again for all your help. It is very much appreciated!


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