Need help! Molds

i have a friend who has asked me to make a repeatable mold to make fishing sinkers/ weights ( how ever you want to call it)
my question is what material should i use for this? is aluminum an ideal material to make a mold from? or any soft metal?

he wants several molds within a single block and several sizes/ weights.

Thanks in advanced for the Help, It is much appreciated

What will he be pouring into the molds? Will it be lead, or plastic with lead shot inserted, or something else?

i believe it will be lead

Might give you some ideas Sergio: X-Carve MDF Mold for Pewter Keychain | Inventables



I have some commercial molds that I bought years ago, they are made of aluminum.

I did this two weeks ago for my father. We used aluminum stock and it worked great. He is retired and fishes weekly out in the gulf and loves the new surf fishing weights he can now cast. Be sure you use a system to line up the mold halves and make SURE you do NOT get water near the molten aluminum as it is EXPLOSIVE if molten aluminum comes into contact with water.

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ok, now another question is, if i have the design of what im going to mold, how do i design the negative (i think thats what its called) or how do i make it into a mold in the cad software ( inventer pro 2016). to give more detail of what i have so far, the customer gave me a clay model (says its his own “design” patent pending) and i replicated the model into a cad file. so how do i make a mold having the model?

you need to program the CAM to carve the cavity in the mold stock. be sure to include the alignment pegs or you can mill an alignment bump and recess on each side of the mold. Pegs are far easier to do though. If you go look at Do-It molds you can get an idea of how it should look, or one way it can look.

The the thickness of your mold stock and then recess the CAD model into it. Make sure it is done in a way that you can mill it correctly without needing a 4th axis and make sure you design it so it will cast correctly and without air pockets.

It is not a fun process but overall it is not too hard. I will see if I can get my dad to take a few pics of his casting molds when he gets back in town. He is of course off fishing again!

the lead sinker molds i currently have are cast aluminum

There are 2 ways you can go about this.
Use your machine and get a copy of CAM software that will do 3D machining. Make up a set of wood patterns that have the right draft and shrink allowance. Then use those patterns to cast negative molds in aluminum. If you don’t have a foundry in your back yard you will need to build one. If you have never cast any metal you will have to study how to do that.
Or you take you CAD model. Add shrink allowance and make sure it has the correct draft so the finished sinkers will release from the mold. Now do a Boolean subtract in a block of material in you CAD program. This gives you a negative mold half. Do the same for the other side of the CAD model.
You now have models of both halves that you can add sprues, gates and runners to so you can pour the lead into a closed mold.
Now get aluminum blocks that are the correct size for your molds. Do all the 3D CAM work to machine the molds. Get some machinable wax and cut it to the same size as the alu blocks. mount the wax blocks in the machine and test cut the molds in wax. Why wax you ask. Because if you make a mistake or the Gcode crashes you run less chance of breaking a end mill and you don’t ruin a good piece of alu.
If the wax comes out the way you want it then and only then mount the alu. in the machine and machine the molds in alu.

If you are just getting started in CNC machining then ask lots of questions and see if you can find some one that will mentor you.

What you are asking is doable it just takes time to get it right.

Hope this helps.


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i have a friend that is interested in you making a part for him he can explain everything to you on why he needs it how much do you charge to make a part my email is

I’ve done my research on how to make a foundry and how to cast metal, its a delicate process for doing it the first time. I have some wood lying around I may just try with wood first until I get it right. Only problem is that the customer is pushing for the fastest process possible… so I’m kinda in a bind here… i know for a fact there really is no fast process for this unless your a very experienced professional at this kinda stuff.

Can you post a photo of the part you need a mold for. This would really help those trying to help to figure good ways for you to proceed with this project.


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I agree, if we have a better idea about the part we might be able to head in the right direction.

The best mold material would be gray cast iron known as Dura-Bar

What you make the mold out of totally depends on how many parts yo need that mold to make. If you are only looking to make a dozen or two dozen you can use plaster, clay, high temp rtv, or even wood. Aluminum molds tend to be good for any where from 20 thousand parts to a few hundred thousand parts. You can move up from there through the metals and end up with H13 tool steel. With lead I’ve seen H13 molds run for more then 20 years of 24/7 usage in production tools. I’m guessing aluminum would work well for what you are looking for.

You have a cad model of the part and need a mold model. The simple answer is that you construct two blocks in the cad program. Place them together. Place the part you wish to make right at the seam and do a Boolean subtraction. While you are at it and the blocks are together, put a couple dowel pin holes in to make sure they line up. If you want to make your life easier you can use a drill bushing and bullet nose pin to align the two halves. You can go a bit more old school and put a spherical pump extending from one side into the other (you’ll need two, one bump/hole should face one direction and the other one should face the other, so you can’t put it together backwards. You can do that even if you use pins. one pin on one side and the other pin on the other side)

Keep in mind this is not my design in anyway. and i have no idea the design came to be ( im saying so i dont get any why this and that questions)

im still trying to make the mold model. @StephenRBromley i will try that when i have a chance, thanks for the idea.

This will turn out easy for you. You will need to add draft to the model so it will draw from the mold. Parting line is easy because of the flat bottom of the part. The hole in the part can be drilled so you do not need a core for the hole. A simple tapered core pin could be added to the mold if you wanted but is not needed.
when you have draft added then you can just scale the model by a couple of percent for shrinkage.

Also add a small fillet where the round part with the hole in it joins the main body. You do not want any sharp corners any where on the part.

After you have all the changes do a save. Then scale the part up or down depending on if you want bigger or smaller sinkers. Do a save as and change the part name to reflect what the size is. Do this for as meany sizes as you need.

Now comes the fun part. Make a new part that is just a block that will accommodate all the sizes of sinkers you want to cast.
Next start doing boolean subtracts of the models of the sinkers. Space them out and make them flush with the surface of the block.

This will turn your black into a negative mold with the correct draft and shrink.

Then add a 2nd block hinged or pined to the first. Add sprues and vents as needed and machine it up and start casting.

hope this helps