Vulcanized rubber molds

Greetings!

I'm having difficulty in determining what kind of material used for 3D printing that will not melt when making rubber molds. All attempts to make molds have melted the master and collapsed the hollow object I'm trying to develop a mold for lost wax casting.

If you can cc your reply to iss@netacc.net and this forum, it would help as I'm not on this site much.

Thanks!

Tim

Steel.

Steel can be printed? Wow, that is news. Why bother with molds?

Speed, surface quality, choice of metal ? The material page for the "Steel" Bathsheba suggests is here - this is actually a steel-bronze composite, where the steel component is "printed" as layers of powder held together by a binder, and the resulting "green state" model is sintered while liquid bronze is infused to fill any voids. (There is also a "DMLS" - direct metal laser sintering - process for steel (not offered by shapeways) but it is still very expensive and has its own drawbacks regarding support for overhanging parts)

Thanks for a short tutorial on the use of steel. What I need is a printing process that will stand up to the vulcanizing process of rubber mold making. Has anyone had experience with this and what material and process was used. Please understand I'm a novice at this and all my molds form 3D work have been done with silicon. I'm trying to have a mold made that is hollow and the material keeps collapsing when the mold is made. I've included the file for reference.

If your items are hollow perhaps you could fill them with something and make them solid. Plaster or concrete perhaps.

Humm, I'm not sure there is understanding about the part. It is a scale replica of a railroad semaphore signal base. The mold is for a lost wax process that results in a brass casting. A tube need to be inserted into the top and runs through the part. Hollow make it easier to drill the hole for the brass tube and a solid part will cost more in brass material.

There are some plastics used for Stereo Lithography Apparatus (SLA) in the form of a photocureable resin that can stand up to the heat of making a mold via vulcanizing, however, Shapeways doesn't have it. What scale is it? Shapeways has polished brass that would give great detail and stand up to vulcanizing. If you're making a mold for doing wax injections and you'll be wanting to manufacture these to sell then the extra cost of making it in brass is not that bad, that is, if it's a miniature.

By the way, here is the page the shows all of Shapeways' materials.

James,

Thanks for responding. The Scale is HO, 1:87.1. Brass would be ideal, but I would be concerned about the binders. I would like to hear from someone who has done this before I commit to this again. I've spent too much trying to get a silicon mold make correctly. The master I have is perfect, but the company that does my brass casting ruined my sample I sent to them to make a compatible mold for their process.

Tim

Oh ok, I just saw you your Solidworks model. Yep, that's small enough that you could make it in polished brass without too much cost.

Another way of doing it is to print in Frosted Extreme Detail and then instead of using vulcanizing to make your molds make the molds in silicone. To make the molds in silicone you will need a vacuum degassing camber though to get rid of the bubbles.

I see I see. We posted at the same time.

If you shop around you'll find that there are lots of lost wax casting companies that can make your mold WITHOUT vulcanizing. Do you have a wax injector? Are you planning on casting these yourself in brass?

Oh, by the way, whomever ruined your master by melting it has no idea what they're doing.

Premium silver may get more glide surfaces for creating a master-model for mold (if you have no plans to polish item manually).

Thanks to all who are responding.

Silver may be a little too expensive to work with. I'm looking into brass but waiting for an answer of whether or not the binder can take heat. I have tried with silicon but the company was not able to produce a mold with a hollow interior. I don't know if this was inexperience or incompetence. They claim they will do another mold for me but want changes to the master I'm not willing to make.

I don not have the ability to produce wax parts on my own yet, and the only molds I have made have been flat on one side so it was not hard for me to make silicon molds for my own use, which is casting with Hydrocal.

I'll be looking to see what other suggestions arise from my inquiry.

Tim

There is no binder in brass. There is no way the heat and pressure from vulcanizing can do anything to harm the brass.

Regardless of how it's done what you want is these relay cabinets made in brass, correct? And so how many do you want? Do you want something like 10 to 20 or do you want thousands?

James,

I was instructed by someone at shapeways that they make a wax master, then a plaster mold to do the brass. I though they might print with something that has brass particals in it. I may have my usual source of parts do that for me if they can handle the hollow piece.

My first orders would be in the 25 piece range, I don't sell lots of these parts, but over a few years they go piecemeal. It will become part of my inventory of railroad detail parts. I originally needed them for a large order, but gave up and found a substitute. I now have the time to investigate the process a little better and see where I can correct assumptions and learn some new things.

Aw ha! Now I understand what's going on here.

Let me give you a quick rundown on how it works. You've apparently hired a casting house to do the lost wax casting of the brass parts you want. They wanted a master to make a mold for wax injection. This is the first mold they make. Here's how it works, they take the master ( the actual tangible item that is usually 3D printed that you want duplicates of ultimately cast in brass) and make a mold from it in rubber. This usually ends up being a rubber mold that is composed of two halves. This mold is typically made two different ways, one being vulcanizing and the other being made via casting liquid silicone resin. The vulcanizing process uses two slabs of rubber and the master is placed between them and heat and pressure is applied to cause the rubber to soften enough that it flows into all the nooks and crannies of the master. This was how your master was destroyed, they melted it. They should have known that the vulcanizing would melt the master, that's why I say they didn't know what they were doing.

However, lets say your master was composed of a material that could stand up to the heat and pressure of the vulcanizing. In this case the mass of rubber with the master encased in it is cut into two parts and the master is removed. In the silicone rubber molding process they use a type of silicone called room temperature vulcanizing (RTV) compound that goes from a liquid to a solid without needing any heating. So what's done is the master is placed in the center of a mold and the RTV is pored into the mold. A huge problem occurs when one tries this and that is tiny air bubbles stick to the master causing the resulting hardened mold to be useless because wherever the air bubbles were ends up being holes in the mold surface, so to get around this the entire mold is placed in a vacuum chamber. This vacuum chamber, called a vacuum degassing chamber causes the tiny bubbles to expand until they pop leaving behind the pure liquid RTV silicone, which then solidifies into a nice imperfection free mass of solid silicone. From there the mass of solidified silicone with the master still inside is separated into two halves, as previously described.

Then, at this point it doesn't matter if the mold is made from the silicone or the vulcanized rubber, the two halves of the mold are connected together and via a machine called a wax injector liquid wax is injected under pressure into the mold and after the wax is cool the mold is opened relieving a perfect duplicate of the master in solid wax. Perfect except for one aspect and that is the sprue. The sprue does two things, it lets the liquid wax get into the rubber or silicone wax injection mold and it will also allow molten metal to enter the next mold. Here's an example of a wax injection mold with a few wax patterns made.


And so, here's how the next mold is made, this mold is called the investment mold. The wax pattern that is made of solid wax comprising the master and the sprue, which are connected together, is place onto a rubber lid and that is placed onto the end of a steel cylinder called a flask. Here is a ring wax pattern that has a wax sprue attached to it and the sprue is attached to the rubber lid.


The rubber lid is placed on one side of the cylinder flask and then a material similar to Plaster of Paris is pored into the area surrounding the wax pattern, this material is called the investment. Here's shot of a jeweler pouring investment into a flask that has the rubber lid attached to the bottom.

After this step the investment has that problem again of tiny air bubbles sticking to the wax so the whole thing is placed into a vacuum degassing chamber to remove the air bubbles. And so, at this point the investment is allowed to cure so that it becomes a solid and what one has is a steel cylinder with a rubber lid on one end. The rubber lid is removed from the flask and the end of the wax sprue can be seen protruding from the investment, like this.


And so, that is placed in a furnace and the wax is burned out, thus the term "lost wax casting", though it is more technically known as investment casting. Once the wax is burned out this leaves a cavity in the investment, which can then be filled with molten metal. Once filled with molten metal and the metal has cooled to a solid state, but still very hot, the flask is plunged into water and the shock from cooling so quickly causes the investment to shatter and it falls away revealing the metal casting, which is now an exact duplicate of what was once the initial wax injected pattern. From there the sprue is cut off and ground down and then the whole metal part is polished or other finishing processes are done.

The wax material that Shapeways sells is used in place of a wax injected pattern, but since there is no mold to make additional wax injection patterns each one must be 3D printed, which is not very cost effective unless one is making high end jewelry that can sell for a huge profit.

There's so much more I could have described, but I wanted to just give you a basic idea of what's going on behind the scenes so you can see better what's happening.

Making lapel pin molds with the pressure vulcanization process.
Now I see why the pressures are so high and why hollow items
would be a problem.
https://youtu.be/k_EH_eHnHzY