How to make a tire mold?

[Fat Brian]

It looks like I'm going to need to cast some tires for some upcoming projects and I have some questions. 

1. Do resin and the "rubber" most aftermarket companies use need different mold materials?  I haven't decided which material to make the tires out of so if they need a specific mold material I need to know that first. 

2. Is there any specific thing either material needs in a mold? Is one material easier to construct a mold for?

3. Do I need a pressure pot to cast the "rubber" material like I do with resin?

4. Would it be easier to split the master in half and make a single sided mold instead of a two sided mold? Naturally I would only use this type mold for resin casting.

Here is one of the tires I want to cast, I need at least 14 or 16 of these.

[Rick L]

It’s easier to mold with silicon rubber. A silicon rubber mold will work for whatever material you are casting with. When casting, a release agent is recommended when using polyurethane but is required if casting with silicon rubber. A pressure pot is not needed when silicon molding but helps when casting any material. Splitting the tire in half would  make molding and casting easier because you would only need a one part mold if casting with urethane. The air bubbles would rise to the gluing surface. Glue the two halves together, clean up the parting line, and paint black. . A solid tire is a little trickier to mold and cast without having a parting line and sprue present. Try Smooth-On Corp for tutorials and material.

Edited June 5, 2022 by Rick L

[Mark]

If you are starting out, don't experiment right off the bat.  Get all your materials for both molds and castings from one source, that way you will know they are compatible.

For how to design the mold itself, you will want to examine tires cast by others in order to get ideas.  Most of the hard resin tires I have seen, have at least a bit of material in the center that needs to be trimmed out.  Putting something in the center (not a wheel, just a filler piece) would make the mold fill more quickly and easily.  

Most cast tires I have looked at have a feed tag (entry point for the resin) somewhere on the perimeter of the tire.  The feed tag isn't huge but needs to be big enough that you can get enough resin (hard or soft) into the mold before it begins curing.  The tag is clipped off and the area smoothed over after the resin cured.  That military tire is an ideal piece to learn on, as it has no detail in the area where you would put the feed tag.

I have cast thousands of parts but never got to soft tires.  I do know that I would make the mold one piece and slit it open only enough to get the original tire out.  I'd make the mold with the tire standing vertically, feed tag on the bottom to hold the tire in place while the mold material cures.

[Fat Brian]

1 hour ago, Mark said:

If you are starting out, don't experiment right off the bat.  Get all your materials for both molds and castings from one source, that way you will know they are compatible.

For how to design the mold itself, you will want to examine tires cast by others in order to get ideas.  Most of the hard resin tires I have seen, have at least a bit of material in the center that needs to be trimmed out.  Putting something in the center (not a wheel, just a filler piece) would make the mold fill more quickly and easily.  

Most cast tires I have looked at have a feed tag (entry point for the resin) somewhere on the perimeter of the tire.  The feed tag isn't huge but needs to be big enough that you can get enough resin (hard or soft) into the mold before it begins curing.  The tag is clipped off and the area smoothed over after the resin cured.  That military tire is an ideal piece to learn on, as it has no detail in the area where you would put the feed tag.

I have cast thousands of parts but never got to soft tires.  I do know that I would make the mold one piece and slit it open only enough to get the original tire out.  I'd make the mold with the tire standing vertically, feed tag on the bottom to hold the tire in place while the mold material cures.

The orientation of the tire was one thing I was thinking about. I had planned to fill the tire with clay and cover one side of it to the mid point of the tread and pour one half of the mold. Then remove the clay from the outside of the tire and add a few vents to the edge of the treads and a pour hole to that feeds to the center of the tire where it can be trimmed away and hidden by the rim. Basically, the mold would lay flat and I could inject the casting material down a tube into the center of the tire and it could vent to the edge. 

I would prefer to use rubber but I don't have a pressure pot so hard resin would be easier to repair any air bubbles on.

[Mark]

The military tire could be done in hard resin.  I was looking at doing a similar tire myself a few years ago.

I'd look at commercially available resin military tires in other scales.  Most leave the feed tags and other extraneous resin in place to save time, leaving removal to the buyer.  Looking at a few of those should give you a good idea of how they are doing it.  If several casters are doing things in one particular way, that should point the way for you.

[Chariots of Fire]

A three part rubber mold is the best way to go.  I've done several different tires this way.  

1.  Prepare a mold box and mix up enough rubber mold material to cover the bottom and let it set on a flat surface.  After the mold material has cured mix up a second batch, pour in enough to cover the top of the first cure and place the tire master flat making sure that there is uncured mold material all around the bottom.

2. Weight down the tire so that only a small amount of mold material goes into the center portion of the tire.  Pour the rest of the mold material all around the outside, in your case, up to the center point of the tire and stop.  Let this second batch of the mold rubber cure.  Include some randomly placed plugs that just protrude out of the second pour.  This will insure alignment of the two halfs of the mold after it is broken apart.   After it has cured coat the entire inside of the mold box, the tire master and the surface of the cured mold with mold release.  Do it a couple of times to make sure all of the surfaces are covered.  

3. Now mix up the last batch of mold material and pour it into the mold box.  Cover the rest of the tire master including the inside.  (If the tire is hollow, fill it with modeling clay first to prevent mold material from going inside).  Pour the remaining mold material over the tire, the plugs and up to the top of the mold box or at least 1/2" above the surface of the tire.

After the last pour has cured, break apart the mold box and separate the top half of the mold from the bottom half and pull out the tire master.  You will have two perfectly aligned mold sections into which you can pour two part resin.  

I found that in almost all cases you can get a good casting with no air pockets by filling the bottom half of the mold first and them pouring the resin over the inside top half of the mold and let it begin to gel just a little.  Quickly place the top half of the mold over the bottom and squeeze out excess resin.  Put a weight on the top of the mold until the resin cures.  Here's a look at some of the tires I have cast using this process.  In the mold below I filled the bottom half of the mold to the edge of the tire so that any flash could easily be sanded away.  There was no flash at all in the middle of the casting.  The top half of the mold in in back showing the holes where the plugs align and the part of the mold that plunges into the center of the tire to keep it open.  In this case I also bored a small air release hole in the center of the top to make sure air pockets did not form.

Edited June 5, 2022 by Chariots of Fire

[NOBLNG]

I made a mold box from legos. First I filled the hollow tire with modelling clay. I set the tire master in the box and packed modeling clay around it and inside it to about 2/3 of the way up. I added an orientation dimple, a center locating dowel, a “wall” made from rectangular Evergreen and three vents made of Sprue. Then I poured the top half of the mold level to the top of the legos.

After curing I clipped off the sprue vents flush to the silicone, flipped it over and removed all the clay except the stuff inside the tire. I removed the dowel and evergreen wall. I added more legos to raise the height of the mold box. I sprayed it good with mold release and poured the bottom half.

The wall allows enough extra resin to be poured in to fill the top portion of the mold so that no air pockets remain when the top half is squished on.

Edited June 5, 2022 by NOBLNG

[Oldmopars]

I am in no way wanting to persuade you away from casting these tires. However, I have to wonder if it is the best way to obtain the final product you are after. There is a fair initial investment into all the stuff required to resin cast something, hard or soft, there is an investment. If you plan to do a lot of resin casting in the future, this is OK as you will get your money back in time from the many casts you create. 

However, if all you want is a run of 14-16 tires, 3D printing may be the most affordable and easiest way to go. Printing that many tires could be done in a few hours in one print as opposed to 14-16 different casts with only one mold. This could take many days. 

No matter what way you chose to go, there will be clean up, casting leaves the mold line and the fill hole/port. 3D printing will have small supports. 

I am only suggesting that you do the math and see what way is going to be the best for you. I assume you do not own a 3d printer and this would require you to have the tires printed by someone else. You can PM me if this seems like the better option and I can point you in the right direction. 

Either way, I hope you find a solution that works good for you. 

[Fat Brian]

2 hours ago, Oldmopars said:

I am in no way wanting to persuade you away from casting these tires. However, I have to wonder if it is the best way to obtain the final product you are after. There is a fair initial investment into all the stuff required to resin cast something, hard or soft, there is an investment. If you plan to do a lot of resin casting in the future, this is OK as you will get your money back in time from the many casts you create. 

However, if all you want is a run of 14-16 tires, 3D printing may be the most affordable and easiest way to go. Printing that many tires could be done in a few hours in one print as opposed to 14-16 different casts with only one mold. This could take many days. 

No matter what way you chose to go, there will be clean up, casting leaves the mold line and the fill hole/port. 3D printing will have small supports. 

I am only suggesting that you do the math and see what way is going to be the best for you. I assume you do not own a 3d printer and this would require you to have the tires printed by someone else. You can PM me if this seems like the better option and I can point you in the right direction. 

Either way, I hope you find a solution that works good for you. 

Believe me I would have rather just gotten them 3d printed but the set of 10 with rims was over $200, this single tire cost $25. I figure for that much I could at least learn a new skill.

[Fat Brian]

3 hours ago, Chariots of Fire said:

A three part rubber mold is the best way to go.  I've done several different tires this way.  

1.  Prepare a mold box and mix up enough rubber mold material to cover the bottom and let it set on a flat surface.  After the mold material has cured mix up a second batch, pour in enough to cover the top of the first cure and place the tire master flat making sure that there is uncured mold material all around the bottom.

2. Weight down the tire so that only a small amount of mold material goes into the center portion of the tire.  Pour the rest of the mold material all around the outside, in your case, up to the center point of the tire and stop.  Let this second batch of the mold rubber cure.  Include some randomly placed plugs that just protrude out of the second pour.  This will insure alignment of the two halfs of the mold after it is broken apart.   After it has cured coat the entire inside of the mold box, the tire master and the surface of the cured mold with mold release.  Do it a couple of times to make sure all of the surfaces are covered.  

3. Now mix up the last batch of mold material and pour it into the mold box.  Cover the rest of the tire master including the inside.  (If the tire is hollow, fill it with modeling clay first to prevent mold material from going inside).  Pour the remaining mold material over the tire, the plugs and up to the top of the mold box or at least 1/2" above the surface of the tire.

After the last pour has cured, break apart the mold box and separate the top half of the mold from the bottom half and pull out the tire master.  You will have two perfectly aligned mold sections into which you can pour two part resin.  

I found that in almost all cases you can get a good casting with no air pockets by filling the bottom half of the mold first and them pouring the resin over the inside top half of the mold and let it begin to gel just a little.  Quickly place the top half of the mold over the bottom and squeeze out excess resin.  Put a weight on the top of the mold until the resin cures.  Here's a look at some of the tires I have cast using this process.  In the mold below I filled the bottom half of the mold to the edge of the tire so that any flash could easily be sanded away.  There was no flash at all in the middle of the casting.  The top half of the mold in in back showing the holes where the plugs align and the part of the mold that plunges into the center of the tire to keep it open.  In this case I also bored a small air release hole in the center of the top to make sure air pockets did not form.

 

2 hours ago, NOBLNG said:

I made a mold box from legos. First I filled the hollow tire with modelling clay. I set the tire master in the box and packed modeling clay around it and inside it to about 2/3 of the way up. I added an orientation dimple, a center locating dowel, a “wall” made from rectangular Evergreen and three vents made of Sprue. Then I poured the top half of the mold level to the top of the legos.

After curing I clipped off the sprue vents flush to the silicone, flipped it over and removed all the clay except the stuff inside the tire. I removed the dowel and evergreen wall. I added more legos to raise the height of the mold box. I sprayed it good with mold release and poured the bottom half.

The wall allows enough extra resin to be poured in to fill the top portion of the mold so that no air pockets remain when the top half is squished on.

Thank you both for the great information. What mold material and resin have you each had the most success with?

[NOBLNG]

I have only used Smooth On products. I have used the OOMOO 25 and 30 tin cure silicones. And the Smooth Cast 310 and 322 resins. I like these because they are mixed by volume not weight. The silicones make a much nicer bubble free mold if they are vacuum de-gassed. This is especially important if you use a pressure pot for the resin casting. Neither vacuum or pressure should really be necessary for doing those tires though.

This hood has a bunch of goosebumps on it because I made the mold under normal atmospheric pressure, but cast the resin in a pressure pot.

 

Edited June 6, 2022 by NOBLNG

[Chariots of Fire]

There is no way you can beat purchasing some mold material and casting resin for duplicating tires.  3D printed sets will cost $$$$$$$$$$$$!  It costs about $35 for RTV mold rubber that be used for many molds than just for the tires.  And casting resin costs about the same.  Again it can be used for many more types of castings.  Been doing it for a long time with good results.  Hoods, fenders, engine halves, tires, wheels, pump housings, and a lot of little stuff that otherwise would have to come out of a new kit.  And for making duplicates it can't be beat either.  One and two piece molds are not that difficult to do and are an additional tool in the modeling tool box!

[Fat Brian]

56 minutes ago, NOBLNG said:

I have only used Smooth On products. I have used the OOMOO 25 and 30 tin cure silicones. And the Smooth Cast 310 and 322 resins. I like these because they are mixed by volume not weight. The silicones make a much nicer bubble free mold if they are vacuum de-gassed. This is especially important if you use a pressure pot for the resin casting. Neither vacuum or pressure should really be necessary for doing those tires though.

This hood has a bunch of goosebumps on it because I made the mold under normal atmospheric pressure, but cast the resin in a pressure pot.

 

Alright, that's where I'll start. I don't have an air compressor so I'm glad I can do at least this project without a pressure pot.

[Fat Brian]

28 minutes ago, Chariots of Fire said:

There is no way you can beat purchasing some mold material and casting resin for duplicating tires.  3D printed sets will cost $$$$$$$$$$$$!  It costs about $35 for RTV mold rubber that be used for many molds than just for the tires.  And casting resin costs about the same.  Again it can be used for many more types of castings.  Been doing it for a long time with good results.  Hoods, fenders, engine halves, tires, wheels, pump housings, and a lot of little stuff that otherwise would have to come out of a new kit.  And for making duplicates it can't be beat either.  One and two piece molds are not that difficult to do and are an additional tool in the modeling tool box!

Yeah, that's kind of what I was thinking, for as much as the printed set would cost I can learn a new skill because I've got other tires I'd like copies of as well.

[NOBLNG]

6 minutes ago, Fat Brian said:

Alright, that's where I'll start. I don't have an air compressor so I'm glad I can do at least this project without a pressure pot.

If I recall, the instructions say that a pressure pot is not needed…but I have always used one. Go to the Smooth On website. They have a lot of good info and videos there.

Edited June 6, 2022 by NOBLNG

[Oldmopars]

2 hours ago, Fat Brian said:

Believe me I would have rather just gotten them 3d printed but the set of 10 with rims was over $200, this single tire cost $25. I figure for that much I could at least learn a new skill.

Ok, I get it. I guess I'm thinking about from the stand point of a printer owner. For me it would just be the cost of the resin, about $10-15. 

[Fat Brian]

1 hour ago, Oldmopars said:

Ok, I get it. I guess I'm thinking about from the stand point of a printer owner. For me it would just be the cost of the resin, about $10-15. 

I bought this off Shapeways but this is expensive even compared to other things I've bought off there. Everything this person has is on the pricey side.

[Chariots of Fire]

Go ahead and get a tire (with an extra just in case) with your preferred tread design from Shapeways.  There are plenty to choose from.  Then use your RTV mold and some resin to get copies!  It works!

[robdebie]

You can also make a one-piece mold - I recently tried that, and it worked very well. I wanted to see whether resin tires would be an improvement over the vinyl (?) tires from the old Ertl IH Transtar truck. I made a mold using the set-up seen on the left; the tire itself was filled with Apoxie clay before, and fitted with an 0.5 mm disc. The resin copies came out fine. I use a vacuum casting set-up.

Rob

 

[Texas_3D_Customs]

18 hours ago, Oldmopars said:

Ok, I get it. I guess I'm thinking about from the stand point of a printer owner. For me it would just be the cost of the resin, about $10-15. 

Depends on what your printing in, I can say that printing in rubber resin is a bunch of not fun and expensive about 3X the cost of regular resin, but it makes really nice tires, more detailed than cast IMHO

[Texas_3D_Customs]

A completely different option would be to have a mold 3D printed then pour and go. The only issue there is you need a 3D model of a tire, or scan a physical tire, I am probably going to scan the tires from my 1:1 car for this purpose

[peteski]

Not sure what kind of a scanner you have, but as I understand 3D scanning (especially with consumer level scanners)  scanning process is not very accurate as far as exact detail dimensions go.  Often  a lot of manual cleanup of the scan is needed.

Since tire is a rather simple shape with a repeating tread pattern, wouldn't it be easier to draw a pie-slice-shaped slice of the tire, then just duplicate that shape around the center of the tire to get a nice sharp and accurate 3D file for printing?

[Oldmopars]

3 hours ago, peteski said:

Not sure what kind of a scanner you have, but as I understand 3D scanning (especially with consumer level scanners)  scanning process is not very accurate as far as exact detail dimensions go.  Often  a lot of manual cleanup of the scan is needed.

Since tire is a rather simple shape with a repeating tread pattern, wouldn't it be easier to draw a pie-slice-shaped slice of the tire, then just duplicate that shape around the center of the tire to get a nice sharp and accurate 3D file for printing?

Well, that depends. A high quality scan of a large object like a tire should work fine. You may miss some fine details, but once reduced to 4% of the original,  you won't notice. 

However, replication of a tread pattern and all the sidewall details is a fair amount of work, but will give very crisp high details.  

I guess it depends on if you have a high quality scanner,  or you have good design skills and time. 

I think both are a viable option depending on your skills, tools, time and desired outcome. 

Custom tires are easy, I don't have to copy any design, it's all up to me. So that one is easy.  

[peteski]

32 minutes ago, Oldmopars said:

However, replication of a tread pattern and all the sidewall details is a fair amount of work, but will give very crisp high details. 

Is it really all that much work?  As I mentioned, tire's sidewall is fairly featureless, and the tread pattern repeats around the circumference of the tire, so you could just draw a small section of the tire, then copy that "slice" all around until the tire is complete.  That doesn't seem like a lot of work.  But maybe I'm not thinking the way you are thinking.

[Oldmopars]

12 minutes ago, peteski said:

Is it really all that much work?  As I mentioned, tire's sidewall is fairly featureless, and the tread pattern repeats around the circumference of the tire, so you could just draw a small section of the tire, then copy that "slice" all around until the tire is complete.  That doesn't seem like a lot of work.  But maybe I'm not thinking the way you are thinking.

Sidewalls do not have a repeating pattern. Yes, the tread does repeat for the most part, but not the sidewalls. There is a lot of info listed and cool designs. This all depends on the tire style, brand, purpose, etc. but it is not a repeating pattern.