3" Chub Grub 2 cavity

[weltzing63]

Anyone else having problems with this chub grub in the 2 cavity? The body of the grub has shrinkage after the plastic cures. I've heated the mold slightly with a hair dryer and also kept pressure on the injector for a few seconds after the mold fills. Any help would be appreciated!
Thanks

[ctom]

I don't have the chub grub, but I do have a bait with a fairly thick body that will dent once in a while. I went thru all sorts of heck trying to figure it out and finally figured that I was clamping in the exact spot, shot after shot, and the clamp and pad itself were acting as a heat sink, drawing heat out of the cavity inside pre-maturely. I started to clamp at a different spot on the mold each time and that solved the problem for me.

Plastic temperature can be an issue in some molds too. Try shooting a cooler plastic, try shooting a warmer plastic.

[andrewlamberson]

I have that mold...and CTom's suggestions are exactly right. Thick bodied baits have a bad habit of denting (aka "sink marks") if you shoot them too hot. The skin has cooled way before the center of the bait, and the plastic in the center shrinks as it cools pulling in the skin (the dent"). It's called "differential shrinkage" and if we were making a plastic part like a tray we would utilize some ribs or change the gate (where we fill) to minimize the shrinkage issue. But we are kind of stuck with our bait molds to injecting at one point and at the head of the bait. Holding some "back pressure" can help but it's not safe to add the kind of pressure you really need to eliminate the problem. A foaming agent would also help...but not practical in our situation.

You need to find the lowest temperature that you can shoot the mold and still have enough flow (plastic viscosity) to fill the tail. The actually temperature will depend on how warm the mold is. Heat your plastic to 350 to make sure it has fully changed state (and won't make a sticky or cloudy bait) and then let it cool to the shooting temperature. According to my notes I shoot it at 330 to start....and once the mold gets hot I'm shooting at 320. If your tail is not filling ...you need to shoot a little hotter!

Don't clamp to hard and restrict flow to the tail.

But...I'm assuming that you are getting "dents" in the side of the bait...normally at the same spot each time? Or do you think the bait is shrinking because it's flat on two sides? The mold is actually semi flat...not fully round and the flat sides are by design (you wouldn't be the first person that thought they had an injection problem because they didn't know the baits was slightly flat on the top and bottom!)

[weltzing63]

Thanks for all the info! These are indeed dents (shrinkage) at the top/head of the bait. It's definitely not because of clamping it too tight in spots because I clamp the ends on the mold near the tails. The dents are on the head of the bait. Also the best results I've had so far is when I shoot one color let it cool take the bait out cut the tails off put them back in to make a multi-color bait, but this resulted in one decent bait and the other with the dent. Any other suggestions??
Thanks!

[Mic]

I have the 3.5" K grub, and when I first started I was constantly frustrated. Now I'm mad that I dont have more cavities. What I do is clamp really lightly with two small bar clamps at the tail and towards the very top of the mold. Then I inject as normal, maybe a little slower, and make sure to top it off. I do still get some dents or bubbles but those are usually my fault. I can also demold very quickly (with gloves on of course) and toss it in water and that leaves very little time for shrinkage and also help a lot.
I hope this helps

[weltzing63]

Holy smokes, it WORKS!! Clamping it near the top on both ends, shooting the plastic slow, and throwing it into water right away WORKS! Now I feel much better about spending my money on it! Thanks for all the help!

[Mic]

No problem glad I could help, and glad to hear you are enjoying it now. Now you just need to fish it

[pjmcla]

weltzing63  -  Ain't it good when it all comes together.   And; why the forum is working as it should --  Folks helping folks. 

[weltzing63]

Yes, the form is a great place to solve problems. lol....We still have ice up on the lakes up here in WI so trying out my plastics have been delayed :/. I still get to ice fish though! Last year at this time we had 80 degree weather already, typical WI weather lol