How To Cast Urethane Using A Pressure Pot

cyberpred

Hunter
I was talking to Carl this evening and was thinking that some of you would like to know how to do this also.

Urethane molding is a process where a master pattern, typically created by stereolithography, is used to cast a silicone mold that can then be injected with various urethane materials.

Step-by-step Process
1. Create and prepare master pattern. Usually the best choice for master pattern is a model. However, existing parts or machined parts can also be used with success. If an model is used, it can be polished for a smooth finish or textured for a custom look. The finish of the pattern will transfer to the mold and cast urethane pieces.

2. Create the silicone mold. The mold parting line and vent and fill holes are determined based upon the part geometry. There are various methods for creating a parting line; two of the methods used are clay for a two pour mold or tape for a single-pour mold. To create a two-part mold, the parting line is clayed and the part is enclosed in a box. Silicone is poured around the model to create its negative. The part is then flipped, the clay is removed and the second half of the mold is poured. Once the silicone has cured, the master is removed and the mold is cleaned and prepared for injection.

3. Inject and pressurize mold. Urethane material is mixed and/or colorized and injected into the mold through fill holes using low-pressure pneumatic gun injection method. The mold is placed into a pressure pot to compress air bubbles that may have been trapped during injection.

4. Remove parts from mold. After the allotted cure time has passed, the molds are removed from the pressure pot, opened, and the part is then carefully removed.

5. Post-finish urethane reproductions. Gates and flashing are removed from urethane parts. Straightening is sometimes required. The parts may also be painted, polished, or functionally assembled. Threaded inserts, if not added during molding, can be added at this time.




I hope this makes sense to all of you.

Cary :)
 

ScottAS

Hunter
I just wanted to add Cary,that if you are going to pressurize urethane resin castings you have to degass your silicone before you pour mold or place entire uncured mold in a casting pot to set up completely. If you don't you will have bubbles sticking out all over piece.
 
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drgnfyre4

Hunter
I remember the first time I put a silicone mold into a pressure pot and the silicone mold was one that was made without degassing the silicone...the casting I got out afterwards was lumpy and spiky since the pressure pot forced the resin into all of the microscopic voids in the un degassed silcone. Pressure casting with a good mold produces the nicest cleanest castings possible and when you are doing a production run is the most economical as you ideally want the least amount of clean up on your pieces before they go to paint or ship to your customers.
 

Don Jarr

Hunter
Hi all

I will also add, thinning silicon with silicon oil, (To ratios on the container) helps with degassing, and also de-gas the resin after mixing, and after filling your chosen tool for injecting, turn it nozzle up and tap it to expel the air, a bit like a syringe.

I have been making loads of pressure cast vent bits the last few weeks, and have found smooth-on 310 to be best as it’s a 30 min cure time, and allows for filling the mould, placing in pressure pot and pressurising. Last of bring the pressure up slowly, don’t just open the valve fully.

I must say once you start pressure casting, you tend to stick with it, as the results are far better.

:) Don
 

cyberpred

Hunter
Its amazing how many people come out of the woodwork when there is an intresting subject. Good points all.

Cary
 

ScottAS

Hunter
Here's pics of some degassed silicone and pressurized parts that I make.
IMG_0151.jpg

IMG_0116.jpg
 
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