WillHam

Mold question?

8 posts in this topic

Ok, so I have a prototype bait carved out and now I'm trying to figure out how to make more without having to carve them out over and over again. So my question is, what type of polyurethane type plastic should I use to make a much easier molded plastic bait. Also, if I have the prototype swimming correctly will the action change when I switch from wood(balsa) to the polyurethane plastic. Finally, how will I add weights, hook eyes, and rattles to the plastic bait. Sorry if these questions sound ignorant but I have no experience with molding baits.

Thanks, WillHam

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WillHam, first off, the action will change, sometimes dramatically. Your master of balsa probably has a specific gravity of 0.12 if you bought it from a hobby outlet, or it could be as much as 0.2 for the heavier balsas (water has a specifig gravity of 1.00). The best you are going to achieve with resin is 0.65. This is with feather lite resin, or regular resin with silicon microspheres.

This large difference in density changes everything. Basically, you start from scratch, to determine the amount of ballast required and where to install it. Also the weight of the lure is more evenly distributed, whereas the weight in the balsa lure was concentrated around the ballast location, therefore the balsa lure could waggle much easier than the heavier lure, so you should expect to have a less pronounced action.

Depressing, I know, I've been there.

As for molds, a general rule of thumb is 'hard master, soft mold'. So you really need to be looking at RTV rubber for your mold material (Room Temperature Vulcanization). It is fairly easy to work with, but a little pricey. Do a search and read up on RTV and 2pc molds.

Smooth-on sell both RTV and the resins, here is a link. Feather Lite

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WillHam, first off, the action will change, sometimes dramatically. Your master of balsa probably has a specific gravity of 0.12 if you bought it from a hobby outlet, or it could be as much as 0.2 for the heavier balsas (water has a specifig gravity of 1.00). The best you are going to achieve with resin is 0.65. This is with feather lite resin, or regular resin with silicon microspheres.

This large difference in density changes everything. Basically, you start from scratch, to determine the amount of ballast required and where to install it. Also the weight of the lure is more evenly distributed, whereas the weight in the balsa lure was concentrated around the ballast location, therefore the balsa lure could waggle much easier than the heavier lure, so you should expect to have a less pronounced action.

Depressing, I know, I've been there.

One way to seriously reduce the weight is to cast a wood core into the lure. First mix enough resin to mold an outside shell on both halves of the mold, After about 3 minutes, place a Balsa core into the mold and finish the fill.

Another method to lighten the casting is to cast the "shell" as described and then use Expanding foam for the core.

Here's some specs on the Foam. Which density needed to mimic Balsa is for you to find out.:wink:

What the above will do is give a sharp outer shell with a light core.

How you'll a fix hook hangers etc is something else. What do you think?

As for molds, a general rule of thumb is 'hard master, soft mold'. So you really need to be looking at RTV rubber for your mold material (Room Temperature Vulcanization). It is fairly easy to work with, but a little pricey. Do a search and read up on RTV and 2pc molds.

Smooth-on sell both RTV and the resins, here is a link. Feather Lite

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Husky. A solution indeed. I have experimented with a balsa core, it did work, but labor intensive. What are you working on now. You are one of the most progressive members of TU. Just interested about your current thinking is.

Secrecy respected as always.

Dave

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WOW! Thanks for the in-depth information guys. I never thought about the implications specific gravity could have on my baits. So basically vodkaman, you are saying that the more dense(higher specific gravity) resin will take less ballast than the balsa bait will in order to make the bait sink. Also, when you said "hard master, solf mold" what exactly do you mean, sorry in advance if this is a ignorant question.

Finally, I'm pretty sure that my baits are too small to add a balsa core, but I will for sure keep this information in mind when I start working on larger baits.

Thanks, WillHam

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Will, yes you got it, not as much lead.

'hard master, soft mold'. If you mold a hard master in a hard mold, you may have trouble retrieving your master, it could get locked in to the mold. An RTV mold is flexible, Allowing the hard master to be retrieved and of course the subsequent hard castings.

Dave

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Husky. A solution indeed. I have experimented with a balsa core, it did work, but labor intensive. What are you working on now. You are one of the most progressive members of TU. Just interested about your current thinking is.

Secrecy respected as always.

Dave

Hey Dave,

I've been casting plugs and exploring new TC's. Lately, I've fell in love with G-10 for lips.

As you had experienced, some of my castings came in too heavy, even with a near 50% mix of resin and MB's. I have had some success "coring the lures with Balsa and even foam insulation board. Using them lightens the lure a lot BUT it is a tad tedious and they don't hold screw eyes well. That's why I'm looking into using a foam core. The Resin will give a nice hard surface to finish and the foam will make a core that is water resistent and will hold hardware well. Plus all the finishing problems associated with foam, cease, as the resin is what takes the finish. It adds a step or two, but the end result should be good! You can cast your harness into the lures if you desire and the core will fit perfectly and totally fill the inside the lure.

If memory serves 8lb wt foam is pretty close to Balsa in density. Can you verify that please? The link is in my other post. 8 lb will hold screw eyes if they're epoxied in. :twocents:

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Progressive as I suspected Husky.

Your figures are correct, 8lb/cuft = Specific gravity of 0.13 which is just a tad heavier than modelling balsa.

Foam plus resin, good solution. Saves carving the wood.

I've gone back to wood, gave up on the resin. Nice to work with though. I used a cake icing syringe to inject the resin/MB mix, as it was too thick to pour. It was easy to clean, just draw and eject some acetone a few times, wipe over the plunger with tissue and it is ready for next time.

Dave

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