Polyurethane Pouring Tutorial

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Wax block and Vacuum Chamber
The use of a vacuum chamber is required during the pouring of the polyurethanes we are using.  The entire palette must be able to be placed in the chamber.  Shown below is a 12"x12"x3" blue wax pallet that we are currently using.  The vacuum chamber we are using (shown in figures 10 and 12) has a 17" ID, and is about 10" deep, just large enough for the wax blocks we are using.

Figure 1

 

Place Embedded Components
In the example shown in this tutorial, we are embedding a piston and a servo-motor connector in a bi-material shoulder joint.  Before pouring the urethane, the cavity or mold should already be prepared, and components placed. Placement of the imbedded components can be critical for functionality, so appropriate means should be taken to secure and fixture them.  Here both parts have been press fitted into the blue wax.
Figure2

Prepare Equipment
Collect 2 sealable plastic bags, a glass stirring rod, an eye dropper, paper towels, urethane resin and hardener, gloves and cups.  Latex gloves should be worn at all time, and an outer (dirty) pair of green gloves (shown below) should also be worn when handling the un-mixed urethanes or their bottles.  2 cups are required (only one is shown in the picture!).  A small one (shown) is used with the eyedropper for precise measurements, and a larger one is used for mixing.  The size of the larger one depends on how much material you are pouring.  In any case it should be at least 3x as large as the volume of plastic you plan to use.
Figure3

Build a dam
Make a dam using the clay that is *higher* than the thickness of urethane you need to pour.  We commonly use modeling clay to build the dam walls, but other materials can be used.  In any case a water-tight structure should be created.  The yellow island in the middle was made as high as the walls, so that all of they clay could be removed before the urethane was machined--since the clay gums up the cutting tools.  Note that due to pressure difference caused by the vacuum, any and all small cracks will be filled with the urethane.

To avoid the formation of air bubbles in the urethane during curing, it is important to place the wax and clay in the vacuum chamber for a few minutes before the pouring begins.

Figure4

 

Dress to pour
In their liquid states, both the resin and the hardener are unhealthy, and therefore avoiding contact and inhalation are recommended.  Filtered masks, lab coats, and two pairs of gloves are recommended.  Excellent!

Figure5

 
 

Use of green gloves
The green rubber gloves should be worn on top of the (purple) latex gloves.  These should be considered "dirty" and used whenever the bottles or liquid resin or hardener are handled.  Also shown on the left side of Figure 6 is the scale which is used to measure the amount of liquid resin & harndener.  Make sure to have paper towel or the equivalent between it and the plastic cup.
Figure6

 
 

Pouring Hardener
Make sure the scale is zeroed with the empty cup and paper towel.
Listed on the materials page is the proportion of resin to hardener that is to be used.  Ratio of resin to hardener range from 5:1 to 1:1.
Make sure to pour the larger portion first.  (Usually Hardener)
Shake hardener for a while and then pour the appropriate percentage of the  volume you are going to need into the cup.
 

Figure7

 
 

Second Pour
Remember the scale reading, quickly calculate the amount of (resin) needed and zero the scale.
Shake the resin and pour it in until you are close to the required (use the eye dropper when you get close - the exact ratio is very important).  Pour a small amount of the resin into the second (small) cup to make using the eye dropper easier.  Be precise, but don't take too long.
Figure8

Mixing
Use the glass rod to quickly and thoroughly mix the urethane
Figure 9

 

Place the palette in the vacuum chamber (you may have to cross your arms backwards to get it in)
Put the cover on the top and start the vacuum pump.
Bring the pressure down to about 27mmHg below atmospheric.  The urethane my briefly boil and increase it's volume to 2-3 times its normal size, then decrease again.
Let the urethane bubble for about 30 seconds and then turn off the pump.
Slowly decrease the pressure differential.
Figure10

 

Pour the urethane
Pour onto the palette.  Make sure to pour at least 1/4" of urethane above the top of the actual part.  This is easily removed, and helps reduce the chance of getting surface irregulariites due to air pockets or bubbles in the actual part.
Figure11

 

Vacuum
Immeaditely after pouring place the palette and liquid polyurethane in the vacuum chamber and once again raise the relative pressure to about 27mmHg, and allow it to sit for a minute or so.  Air bubbles should rise slowly from the bottom and corners of the part.  After about a minute the number of bubbles should decrease.  As you slowly raise the pressure, all of the remaining bubbles should dissipate.
Let the urethane cure for 2 hours before proceeding.  You may opt to leave the part in the vacuum chamber at pressure differential of 5mmHG for some or all of this time.
Figure12

 

Waste Removal
Place all contaminated cups, eye droppers, paper towels, etc. inside both plastic bags.  Double seal the bags and dispose of in the hazardous waste material bins, which should be properly labeled.

Figure13

 
 

Final Part
Below is shown our bi-material part with imbedded pneumatic piston an servo connector in its final state.

Figure14