Make Your Own PDMS Window for Ember

When the PDMS window in the Ember's resin tray is used for a long time, it tends to get cloudy and damaged. Instead of getting a whole new tray, there is a way to make a new window and replace the damaged one.

At Pier 9, we have made our own PDMS windows when we really needed one, and they have been working pretty well. Here's how it's done.

Here is a whole list of ingredients and equipment used:

Equipment:

-For safety, always wear safety glasses and nitrile gloves

1. Oven with ventilation

2. Air tight box with an air pump that can pull a vacuum

3. Balance for weighing things

4. Large glass slab

5. Glass slides (75 mm X 50 mm X 1 mm)

6. Silicon rubber sheets (3 mm, 1/32 in, and 1/32 in with adhesive backing)

7. Plastic pipettes

8. Disposable stirring sticks

9. Delicate paper wipes

10. Squirt bottle

11. Metal Ruler

12. Razor blade

13. Disposable cups

14. Oven gloves

15. Heat gun

16. Double sided foam tape (2 in wide)

17. Microfiber cloth (something that doesn't scratch the PDMS)

Chemicals:
1. Acetone (put into squirt bottle, used for cleaning)

2. Glass primer (we use Wacker Primer G 790)

3. PDMS mix (we use Sylgard 184)

4. Rain-X (used to make glass non-stick)

While the window might be done for, the tray is most likely still fine. To remove the window, a heat gun was used to heat the bottom side of the window until it is almost unbearable to touch. Then, the window is slowly pushed out using gradual force.

The resin tray still had some sticky residue from the tape, it was cleaned with a razor blade, and paper wipes with a little acetone. Be careful, as the acetone can actually start to dissolve the tray. However, if applied only on the lip where the window is attached, it roughens the surface, and prepares it for adhesives later.

The glass slides are the right size to fit correctly in the resin tray. Using it as a guide, a rectangle is cut out that is larger than the glass slide, leaving a generous border on all sides with all three kinds of silicone sheets. All three types of silicone sheet are cut into roughly the same size.

The surfaces of the silicone sheets were cleaned with acetone and paper wipes until there are no dust or debris on the surface.

Then, the backing for the 1/32 in silicone sheet that has the adhesive is peeled off, and stuck together with the 3 mm sheet. Most of the trapped bubbles were pressed out.This new piece will be referred to as the thick silicone sheet

The thick silicone sheet gave the right total thickness to make the PDMS that is about 3 mm thick in the end.

The 1/32 in sheet without adhesive is saved for the next step.

The glass slide is positioned in the middle of the thick silicone sheet, and using the edges of the glass as a guide, a hole is cut in the silicone sheet that is the same size as the glass slide.

This step needs to be fairly precise. After the hole is cut, the glass slide should snugly fit into the hole.

Note:

The chemicals used here, including the PDMS (before curing) and primer, should not be poured down the drain or thrown out normally. They should be disposed of properly as hazardous waste.

Glass slide:

The glass slide is treated with the primer. 2-3 drops of the primer were dropped on one side of the glass slide, and spread thinly using the paper wipe. It was air dried for at least 1 hour. Don't wait too long before using the primed glass, as the primer's effect could lessen.

Note: Keep track of which side the primer is applied on, because after it dries, it can be hard to see. Both sides of the glass can be primed so there's no confusion, but the primer on one side needs to be rubbed off later, which is a bit tedious.

Large glass slab (highly recommended):

Eventually, the silicone mold will be peeled off of the glass slab. If it sticks to the glass, then the window could be damaged during removal. There were two methods found that prevented sticking.

Option 1:

Rain-X is something that is normally used on car windows to make the water roll off of it instead of sticking. Basically, it is used here to make the surface more slippery.

Rain-X is applied onto a paper wipe, and rubbed on both sides of the glass slab. The glass will look foggy, but after a few minutes of drying, and then vigorous wiping with paper wipes, the glass became clear again. The surface actually felt smoother.

Option 2:

The other option is using wax paper. A piece of wax paper that is larger than the silicone mold was applied onto the glass slab. Using the heat gun, the wax paper was heated up along with the glass slab. This melted the wax on it and made it stick onto the glass slab. This provided a smooth, non-stick surface to put the silicone mold on. In this case, the silicone with the cured window was taken off just after it was removed from the oven so that the wax was still slippery.

The 1/32 in silicone sheet is put onto the glass slab. The glass slab helps stabilize the assembly, and evenly distributes the heat.

The silicone rubber, if clean, should lightly stick to the glass by itself. Again, trapped bubbles were pressed out.

Then, the thick silicone sheet is put on top. It stuck to the first silicone sheet.

Lastly, the glass slide is inserted, primed side up, into the hole in the thick silicone sheet, making sure it goes all the way to the bottom. It was visually obvious when the glass touched the bottom.

The two parts of PDMS is added in a disposable cup. For Sylgard 184, it is a 10:1 mixture. About 15 g of PDMS was weighed out, which was just a little more than necessary for one window.

The two parts were thoroughly mixed with the stirring stick. This introduced a lot of bubbles, which is unwanted. To get rid of it, the mixture is put into the vacuum, and stayed inside for about 30 minutes. This helped the bubbles rise to the surface, and pop. Not all of the bubbles were gone, but it didn't matter, because it was vacuumed again in the next step.

The mixed PDMS was poured into the mold, making sure that all of the corners are filled. PDMS was poured till it goes to the top of the mold. It was found that it is better to overfill than underfill, because with surface tension, an underfilled PDMS window has slightly higher edges than the middle, which can lead to jamming.

After pouring, the whole assembly was put into the vacuum, and vacuumed for another 30 minutes or so. After a long time, certain tenacious bubbles still won't pop. However, when the vacuum was released a lot of the bubbles popped, and the rest were popped or picked out by the razor blade.

All of the visible bubbles need to be gone, because bubbles left in the resin will expand in the oven and make a big hole in the PDMS.

The whole assembly is put into the oven at 150 degrees Celsius for 30 minutes (this is for Sylgard) to cure the PDMS.

It is advisable to make sure the oven is a level surface, because it will ensure that the window has a consistent thickness.

The assembly was taken out of the oven with oven gloves(careful, it's hot!), and left to cool off. The silicone sheets were peeled off of the glass slab. It came off fairly easily when Rain-X was used.

The 1/32 in silicone sheet was peeled off. This leaves the window surrounded by the thick silicone.

Using the ruler as a guide, it was positioned a few millimeters in from the PDMS - silicone interface, and using a single, steady stroke, the entire length of the PDMS was cut on all four sides. Cutting a few millimeters in is helpful because it is easier to extract the window, and it has a smaller chance of cutting up the silicone mold, so it can be reused.

Pressing down on each of the four corners, the window is carefully extracted from the silicone. The corners were pressed because they tend to be the points where the PDMS starts to peel off of the glass.

The window is now complete.

Afterwards:

Sometimes the corner will still start to peel. It is still reparable by cutting away a few more millimeters from the sides, because the printing area is actually smaller than the full glass slide.

The extra PDMS still stuck on the thick silicone is trimmed off, and the silicone can be used again.

The mold should be remade after 2-3 uses because the fit for the glass slide will be worse every time it's used.

The new window can now be put into the resin tray. Before installing the window, make sure the resin tray is free of resin and is clean. Pressure sensitive tape is used glue the window to the tray. The tape is thick, and is sticky on both sides. The tape should be the save width as the glass slide, so a piece that is the same size as the glass slide is cut out, and adhered onto the glass side of the window. Then, the backing is peeled off, and attached to the resin tray. Next, holding the window in place, the razor blade is used to cut away the extra tape on the bottom side of the tray. This exposes the window.

To make sure the adhesive is well attached, the glass side was heated up with the heat gun until it is just bearable to touch, and then a 2 kg weight is placed on the top side, cushioned by a microfiber cloth, to keep pressure on the window for about 5 minutes.

Now the tray should be back to functioning conditions.

One more thing that can be done is to fill the tray with some water to make sure that there are no leaks.