urethane molding

[MBG]

Does anybody here have experience with doing the hand casting urethane molding?

I can make the mold on the cnc mill but what about the casting process? How do I heat the plastic? What temperature?





This is just an example I found when sample. I can make those plugs on a cnc mill. It is the other techniques I need help with.

[Frantic Fab]

Hello,

I am Garrett Gordon, i work for a company that specializes in two part polyurethane casting. please check us out at www.masterpiecemodels.com

the process your about to embark on is somewhat tricky!

are you trying to establish the ability to use these processes frequently?

i am more than willing to help you get started but i can tell you there are many irritating obstacles when it comes to production.

we are one of the largest polyurethane casting facilities in the Pacific northwest. i can definately point you in the right direction.

what you will be dealing with is a two part urethane, these parts are usually mixed 50%-50% by weight or volume, depending on the material. now the media used for the mold is important as well. urethane master molds are used from time to time but silicone molds capture the most detail and release from complex parts much easier. temperature only effects the cure time, usually at room temperature after a thurough mix of side A and side B you have just under 3 mins working time to get the material into the molds before it begins to harden. next is important, the majority of the time the material will always have bubbles trapped inside. this is where a pressure chamber is needed. while the mixture is still liquid the mold needs to be pressurized to 40-80psi depending on the level of detail required. you dont always need a pressure chamber but believe me it helps! the process itself if going to be a learning curve of trial and error. each part needs to be poured and prepped a little differently.

if you need any advice or pointer along the way, or if you would like us to handle your production after you make the molds let me know!

contact me anytime at
garrett.gordon@masterpiecemodels.com

[MBG]

Originally Posted by Frantic Fab Hello, I am Garrett Gordon, i work for a company that specializes in two part polyurethane casting. please check us out at Masterpiece Models the process your about to embark on is somewhat tricky! are you trying to establish the ability to use these processes frequently? i am more than willing to help you get started but i can tell you there are many irritating obstacles when it comes to production. we are one of the largest polyurethane casting facilities in the Pacific northwest. i can definately point you in the right direction. what you will be dealing with is a two part urethane, these parts are usually mixed 50%-50% by weight or volume, depending on the material. now the media used for the mold is important as well. urethane master molds are used from time to time but silicone molds capture the most detail and release from complex parts much easier. temperature only effects the cure time, usually at room temperature after a thurough mix of side A and side B you have just under 3 mins working time to get the material into the molds before it begins to harden. next is important, the majority of the time the material will always have bubbles trapped inside. this is where a pressure chamber is needed. while the mixture is still liquid the mold needs to be pressurized to 40-80psi depending on the level of detail required. you dont always need a pressure chamber but believe me it helps! the process itself if going to be a learning curve of trial and error. each part needs to be poured and prepped a little differently. if you need any advice or pointer along the way, or if you would like us to handle your production after you make the molds let me know! contact me anytime at garrett.gordon@masterpiecemodels.com

What do you consider production numbers? over 100? I want to make about that number of complex shapes.

Also, the equipment needed for this is what? You already said pressure chamber.

Am I incorrect thinking these molds are made on a CNC mill first then cast?

I have some experience working in plastic injection molds so this is where my background is (I am mainly a cnc machinist/engineer now)


I came across this site. Is it any good the resin they are selling?

Prototyping and Inventing | Silicone Rubber, Urethane Rubber, Liquid Plastic, Casting Resin, Lifecasting, Epoxy Resin, Rigid and Flexible Foam from Smooth-On

[rowbare]

Check out some of the videos here: Mold Making Videos - Silicone Rubber, Urethane Casting Resin, & more

They are quite well done and will give you a good idea what to expect.

bob

[p38nut]

I have used in the past the Hysol brand. We used a vacuum bell jar instead of a pressure chamber. In a vacuum the mixed urethane bubbles and more than doubles in size. You MUST use proper ventalation or you will do harm to yourself. There are many variables to be considered such as durometer, color, etc. Hook up with a good local distributor and pick there brains.

Thanks. Mike

[awerby]

That's what you see in that illustration. Urethane is sticky stuff, and it will adhere to almost everything except silicone rubber.

The hardest thing I found about hand-casting it was the short pot-life. I barely had time to add part A to part B before it started setting up. You'd need something with extended pot life to do the de-airing recommended above - either the vacuum or the pressure. Both need some time to work.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[Frantic Fab]

MBG

yes the smooth-on brand has everything you need and we use them daily. we buy wholesale getting 55gallon drums of silicone rubber and all of our resins.

if you get a pressure chamber and a compressor your set for low low production. it will be slow. and the silicone molds depending on how many undercuts and finish required last anywhere from 20-100 pulls for really simple parts.

hydroseal is another good brand we use fairly frequently, so let me ask you what exactly your doing. the purpose for your parts, what conditions they will be exposed to and i will give you the exact material you need. something with a little longer pot life too.

and the molds you see are just plugs (awerby) they will make one master pattern, make a two half or flat cast mold, then take the individual halves and put them in a box and cast the negative. then they can take each half later on and re-make the mold. this saves time being able to pour each half at the same time. the first mold made always takes the longest. but this process will let the user make as many duplicates of the mold halves while assuring they will all be interchangeable.

depending on the application the only other equipment you will need is lab oven. or your home oven really. so your set!

a good trick for the fast setting urethane is to refridgerate it for about an hour. store the material in a fridge. whatever you do, haveing the material cold instead of room temperature will allow you an extra 2-5 mins working time with all two part urethanes.

note there are hundreds of different materials to get confused with. so let me know and i will give you the best suggestions i can.

talk to you soon!

[Nails]

I am new at this. I recently successfully made my first molds out of silicone, and made some urethane parts from them. I used the Alumilite stuff. Bubbles in the urethane parts are definately a issue. I didn't use a pressure chamber.

The silicone mold turned out great. The urethane parts....useable. For my flat cast mold parts, there were lots of bubbles forming after pouring. Once it started to cure (quite quickly), the bubbles wouldn't pop anymore from the top side. The bottom portion of my parts were prefectly fine. This was ok, as I could "overfill" the mold, and sand off the flat section that had the bubbles trapped in it with a belt sander pretty easily.

This stuff is pretty cool to work with. A little pricy, though. But considering how quick it is to make a complex part, it is worth it.

Don't count on dimensional accuracy. The stuff shrinks a little as it cures (something like .005 per inch). You may be able to demold it in 10 minutes, but it hasn't really cured fully, and as you remove your part from the mold, if you have thin walls (1/8"), you might end up deforming it (as in my limited experience), and it will cure deformed.

The silicone is really stretchy and tear resistant, which is great for demolding your part, but also not real good at dimensional accuracy for the 1/8" walls I have that are 1" deep. The thickness of the wall is greater at the top face of the mold as the weight of the urethane pushes it outward. I think I may need a rigid, outer "mother" mold to get this better.

Overall, it's been a great experience for me.

[RomanLini]

Where's the photo's of all these cool molded things?

[Frantic Fab]

Nails-

bubbles always cause trouble! it sounds like you have the basics down pretty well.

as for the dimensional accuracy it is very easy and more than possible. what you will need to do i create a larger resivor around your small parts. build a box around your part and pour the silicone into this box. make the box in a manner that it will support all 4 outter walls and be easily and accurately reassembled. the best way to do this is to put "keys" in the sidewalls and if your mold is a two part mold the keys are essential. the more the better you will be in the long run. this will assure that the mold fits together exactly the same way each time.

one thing to watch out for is heat. you can put the molds into an oven at about 120F and it will help the part cure. but the urethane eventually dries out the part so the time left in the mold will effect the number of pulls you can achieve.

they also have the silicone rubber available in many different specs. the platinum series is much harder and ridgid. it is also necessary for casting crystal clear parts. regular silicone will inhibit a full cure for some of the WC-series urethanes.

again if you find yourself needing more pot life, just refridgerate side A and B. this adds minutes to your work time.

to see some examples of the cast parts we produce on a daily check out this link!

Latest Aquisitions ----LARGE CASTINGS

Casting ---- a few neat pieces

Dino Bones Models for Sale --- Our Kit Line- check out the catagories! sci-fi, military, space!!!

Masterpiece Models Our Blog Spot includes quite a few awesome projects, all of which use cast parts in one way or another!

Masterpiece Models: Mars Rovers This is our most impressive exibit in my opinion. JPL labratories acutally contacted us after sending them pictures and they replied "you do know its illegal to steal government property, even if it is on a martian planet... Right?" they were amazed we did it without plans!

[Nails]

I guess I got shamed into showing my work....

Top Piece:


Top Piece side angle:


Bottom Piece:


Bottom Piece side angle:


Covers for top and bottom pieces:


Actual first run parts:


Back side:


Explanation: This is a custom relay/fuse box that I am making for my 1993 RX7. I am putting in a LS1 motor, and need to relocate an existing relay box that interferes with the new radiator. Most people tuck the old relay box back and hide it (not allowing any access to it), and tie all power circuits to one fused "ignition-on" 12V supply. I wanted to be consistent with the GM fuse schematics, so I added the additional fuses, and also added capacity for additional relays needed for the swap. I was able to find a place close to the main fuse box along the frame rail that I could mount this, so I designed with those dimensions in mind.

As you can see, the outside of the mold is sprinkled with junk. This is because I made my original master out of layers of glued MDF. A mistake. Was very hard to clean the mold, not to mention digging it out of the mold. I think I need some hard modelling wax, or something else.

I haven't shown the top or bottom covers--not interesting.

The back side of the top piece was milled, and you can see chatter marks near the end of the retaining bar. Ugly, but usable. The bottom piece was sanded on a belt sander.

The top piece, I spent too much time mixing parts A and B together, therefore you see a lot of pitting from bubbles. The bottom piece is not so bad.

All the relays have a recessed indention to allign them. You can see this in the top piece that doesn't have a relay present. All holes that needed to be tapped were already in the part. Took about 1 minute to tap all the holes. The shelf that holds the power distribution bar had little dimples from the mold that allowed me to drill the holes without measuring anything, as did the cover holes.

I used a gallon of silicone for these molds.

To machine these parts would take a very long time. But this process is about 20 minutes of actual work for all 4 pieces. More actual time that that, as I should wait about an hour to cure better before I remove from the molds. I hope to sell some of these to other people doing these swaps after I finish my build and verify proper electrical functionality of my modified wiring. Thus, the molds.

[Nails]

Fantastic Fab - WOW! That is some pretty great results. I knew I had lots of room for improvement, but man... Like I said, I have something useable, and am very happy with the results for my first attempt. I will have to check out the smooth-on brand after seeing the results you get.

Thanks for the tip about needing to remove the part from the mold sooner to extend mold life. I didn't know that. I guess I will make something to place inside my parts to straighten the walls after I pull it. The mold is so deep, it is impossible to get it out without deforming the part.