Jig & Spinnerbait Mold Design questions

[Foo G]

I've been working on some jig and spinnerbait designs for a while with the intention of having some aluminum molds cut. However, I don't have a lot of knowledge about pouring lead. My biggest mold design concerns are in the areas of venting, sprue size, and gate size. I understand that the cavity size and geometry, pour temp, mold temperature, pour rate, etc. will all play a big part in it, so I'm really looking for some generalities. What works for you or what are the characteristics of your more successful molds? I'm basically looking for input on the following:

• Vent depth -- What depth will allow for air to escape but still shut off without allowing molten lead seep in?
  
• Gate shape -- Is a rectangular gate better or worse when pouring? I assume it would be better when it comes time to trim the runner after the baits cool, but I don't know if it will equate to fill problems or not.
  
• Gate size (area) -- Ideally I'd like to minimize this without creating fill problems. What gate dimensions (width x depth for rectangular, diameter for round) do you need for 1/4, 3/8, 1/2, 3/4, and 1 oz. baits? Do I need different gate sizes for different bait sizes or is it usually a 1-size-fit-all thing?
  
• Sprue size -- In plastic injection molding, the sprue size is usually around two times the size of the gate. Does the same ratio hold true for molding lead?
  
• Weedguard core pin diameter (for glued-in weedguards) -- How oversized should the core (base hole) pins be for the weedguard hole? In other words, how much will the hole shrink after the bait cools?
  

Thanks for your help and input!

(I also posted this thread in the Homebrew Tools section because I wasn't sure which category would yield the best or larger audience. I apologize to those who may be a little annoyed at seeing it twice.)

[sagacious]

Plastic injection molds are subject to constraints that gravity-fed lead molds aren't. Fortunately, all you really require are generalities to make an effective mold.

Vent depth You aren't likely to need vents for your mold. The sprue, combined with tolerances within the wire and hook channels should allow adequate venting. Otherwise, 0.05x0.001" is usually adequate.

Gate shape Rectangular gates are generally used for metal castings so that the castings can later be de-sprued en-masse in a vibratory tumbler. For a variety of reasons, and since you'll be manually-cutting the sprues, I'd suggest a round gate.

Gate size For gravity-fed lead molds of the cavity volume you specify, a 1/8" round gate should be adequate for all cavities. Gate length and width should be approximately equal or perhaps slightly shorter. You are unlikely to encounter fill problems with proper lead-pouring technique, and note (before you enlarge the gate or cut vents) that lead mold cavities may need to be 'conditioned' before they pour consistently.

Sprue size There are no strict gating or sprue ratios within the generalities of the gravity-fed mold cavity sizes you propose. With no injection, the sprue serves as a reservior that the cavity draws from as the lead solidifies. For your purposes, a conical 1/2"x1/2" sprue should work fine, but note that a larger volume sprue will not have any deleterious repercussions, and may allow for slightly faster pouring.

Core pin diameter For your purposes, the shrinkage within that small hole should essentially be zero. Other tolerances will be more influential, and a slight gap is necessary to accomodate paint and/or glue. The easiest thing to do would be to just buy some spare core pins for a mold that accomodates the weedguards you'll use, and cut accordingly.

Hope this helps, good luck.

[Foo G]

sagacious,

Thanks for the great information!

Is it safe to assume that the better pour achieved with round gates largely outweighs the easier removal of rectangular gates? I wasn't sure whether there would be a lot more difficulty involved in cutting a round gate as opposed to rectangular.

By mold "conditioning", do you mean that there is a "break-in" period associated with it? Or were you referring to smoking or treating the cavities?

Would sandblasting the cavities have a negative effect on the casting process? I would guess that it might be beneficial when it comes time to powdercoat.

[sagacious]

...

Is it safe to assume that the better pour achieved with round gates largely outweighs the easier removal of rectangular gates? I wasn't sure whether there would be a lot more difficulty involved in cutting a round gate as opposed to rectangular.

If you're cutting the sprues off with gate shears or nippers, there won't be any difference in cutting effort, or time spent removing the sprues. Round sprues will offer other beneficial attributes, though.

By mold "conditioning", do you mean that there is a "break-in" period associated with it? Or were you referring to smoking or treating the cavities?

Well, yes. Smoking the cavities shortens or eliminates the break-in period, and prevents a number of potential frustrations. Smoking the cavities takes little time, but can save a lot of time later while pouring. Moly disulfide surface treatments also work very well. "Break-in" not only refers to possible adjustments, but also to establishing the proper preparation, pouring, and de-molding techniques, etc necessary for that mold.

Would sandblasting the cavities have a negative effect on the casting process? I would guess that it might be beneficial when it comes time to powdercoat.

I wouldn't do it, as any surface roughness can detrimentally affect the de-molding process. Occasionally a random cavity will stubbornly refuse to drop the casting until it is pried free, and that slows production. Finding the cause can sometimes be tedious. Intentionally roughing the surface invites trouble. Most powder paints used on lures are designed to adhere tightly to bare lead, so the possible benefit seems vanishingly slim-- but the risk of damaging a perfectly good mold seems quite high.