Embedded Files
Overview
Overview
In this step, you'll be making the gel pads for the GelSlim 4.0 sensor, which are highlighted in the diagram below. First, you'll use the gel mold that you created in the last step in order to cast the silicone gel pads. The mold doesn't have a top half to help with the degassing procedure. Instead, the silicone is leveled by gravity. Next, you'll extract the sheet of gel to paint one side with two layers of silicone ink: first gray, then black. Finally, you'll cut out individual gel pads. The painted side of the gel pad will be the contact interface on the final sensor. There may be some equipment in these instructions that is unavailable to you. Make sure to check out the Equipment Alternatives page to learn more about what you can do to make the best gel pads with the equipment you have available.
Step-by-Step Instructions
Step-by-Step Instructions
Filling the Gel Mold
Filling the Gel Mold
The silicone used in these instructions has a working time of 1.5 hours. It is important to move efficiently through these steps so that the silicone has enough time to degas in the vacuum chamber before it sets and air can no longer escape.
Don PPE
Put on nitrile gloves to protect the skin on your hands from irritation
Put on safety glasses to protect eyes from splatters
Put on optional lab coat to protect clothing from splatters
Mix silicone base and activator
Turn on digital scale
Place mixing cup on digital scale
Tare (zero) digital scale
Remove lid of silicone base
Pour 70g of silicone base into mixing cup
Record the actual weight of the silicone base you've poured, then take 10% of that weight as the target weight for the silicone activator
Example: If you actually poured 71.2g of silicone base, you should target 7.12g as the weight for your activator.Tare digital scale
Using a transfer pipette, gradually add silicone activator into mixing cup until target weight is reached
Take the mixing cup off the digital scale
Using a popsicle stick, mix the silicone base and activator together for 3-4 minutes until it is uniform in color and full of bubbles, as shown below
Tip: When mixing your silicone, make sure to incorporate the silicone from the sides and bottom of the cup as well.
Silicone mixture, uniform in color and full of bubbles after mixing
Fill syringes
Wipe the inside of the syringes to remove any dust or debris
Screw tip caps onto syringes so that silicone does not leak out
Pour about half of the silicone mixture into each syringe
Tip: When filling the syringes with silicone, they do not need to be filled to exactly the same level. Leave about 1cm of space at the top of each syringe.
Insert flow pistons
Using one finger, pinch in the side of the flow piston, as shown below
Slide the flow piston into the top of the syringe, still pinching in one side, until the flow piston is pressed against the silicone in the syringe, as shown below
Tip: When inserting the flow pistons into the syringes, pinching in one side of the flow piston allows air to escape the syringe as you press in the flow piston. If you let go of the flow piston, you will not be able to continue pushing it all the way up to the silicone. It's okay if this happens, or if there are still some bubbles at the interface between the silicone and the flow piston.
Pinch in one side of the flow piston with your thumb and forefinger before pressing it into the syringe
Press the flow piston into the syringe until the piston presses against the silicone
Remove air bubbles using centrifuge
Tip: Centrifuge speeds and times do not need to be exact. Experiment with your centrifuge to see which settings work best.Put each syringe into a slot in the centrifuge and twist it until it drops in
Close the lid of the centrifuge
Set the centrifuge to 4000 RPM
Set the time to 2 minutes
Press "Start"
After 2 minutes, open the centrifuge and rotate the syringes by 180 degrees
Set the centrifuge to 4000 RPM
Set the time to 2 minutes
Press "Start"
Prepare mold
Make sure acrylic is fully seated in mold frame
Use the cloth to wipe away any fingerprints or oils from the acrylic
Connect pressure regulator to compressed air source
Connect barrel adapter to pressure regulator
Check air lines for leaks
Make sure air is off
Make sure pressure regulator is closed
Turn on air supply
Listen for any loud hissing near connection points
If any leaks are found, turn off the air and tighten those connections as needed, then turn the air back on and check again
Dispense silicone into mold
Attach syringe to barrel adapter
Take off tip cap from the syringe and put on a dispensing tip
Position tip at an angle in the center of the gel mold, as shown below
Position the tip of the syringe at an angle in the center of the gel mold and slowly turn on the pressure until the silicone begins flowing
Extrude silicone bead until the flow piston is about a centimeter from the bottom of the syringe
Slowly turn on pressure until the silicone begins to flow from the syringe at a moderate rate and the pressure gauge reads about 20psi
Tip: When turning on the pressure regulator, it may take a few turns of the knob until the airflow begins. You may feel some resistance on the knob once pressure starts to increase.Hold the syringe at an angle inside the extruded silicone bead until the flow piston is about a centimeter from the bottom of the syringe, as shown above
Turn off the air using the pressure regulator
Take the syringe out of the gel mold and rest the tip on a paper towel
Tip: Silicone may continue to extrude after the air is off. Make sure to take the syringe out of the gel mold before the air bubble below the flow piston is pushed into the gel mold.Take the first syringe off the barrel adapter and put on the second syringe
Take off the tip cap and put on a dispensing tip
Insert the dispensing tip back into the center of the extruded silicone in the gel mold
Slowly turn on pressure until the silicone begins to flow from the syringe at a moderate rate and the pressure gauge reads about 20psi
Hold the syringe at an angle inside the extruded silicone bead until the flow piston is about a centimeter from the bottom of the syringe, as shown above
Turn off the air using the pressure regulator
Take the syringe out of the gel mold and rest the tip on a paper towel
Tip: Silicone may continue to extrude after the air is off. Make sure to take the syringe out of the gel mold before the air bubble below the flow piston is pushed into the gel mold.Disconnect the barrel adapter
Turn off air supply
Let the silicone spread evenly in the mold as it rests on the table, as shown below
Tip: Don't shake the mold! Given a few minutes, the silicone will self-level.Throw away syringes, tip caps, dispensing tips, and flow pistons
Silicone will gravity-level to fill the mold corners
Degas silicone in vacuum chamber
Make sure the vacuum chamber is placed on a level surface
Make sure the vacuum pump is off
Connect the vacuum pump to the vacuum chamber
Make sure the outlet (vent) valve is closed
Make sure the inlet (hose) valve is open
Open the lid of the vacuum chamber
Gently place the filled mold into the vacuum chamber
Close the lid of the vacuum chamber
Turn on the vacuum pump
Pressure inside the vacuum chamber should slowly drop to approximately -29inHg
Tip: If pressure does not begin to drop, try pressing the lid of the vacuum chamber to make sure it's fully sealed.Turn off the vacuum pump once the pressure inside the vacuum chamber reaches approximately -29inHg
Leave the mold inside the vacuum chamber for at least 1 hour until air bubbles have come to the surface and popped, as shown below
Once silicone is free of bubbles or bubbles have ceased to shrink, open the outlet (vent) valve to release the seal on the vacuum chamber
Open the lid of the vacuum chamber
Gently remove the mold from the vacuum chamber
At the start of the degassing process, there will be lots of bubbles in the silicone
By the end of the degassing process, most of the bubbles should be gone from the silicone
Allow the silicone to cure
Place the mold on a level surface away from sources of dust or debris
Wait 16-18 hours for the silicone to fully cure
Remove the gel sheet from the mold
Press the exposed acrylic on the back of the mold to push the gel and acrylic out of the mold, as shown below
Tip: Press a corner and then rotate to the next until the gel begins to release. This will help prevent the gel from tearing.Gently peel the gel sheet from the acrylic, as shown below
Tip: Peel each corner up first, then rotate to a new corner. This will help prevent the gel from tearing.
Removing the gel and acrylic from the mold
Removing the gel from the acrylic
Use the X-acto knife to trim away any straggly pieces of silicone at the edges of the sheet
Painting the Gel Gray
Painting the Gel Gray
It is important to take proper safety precautions in this portion of the instructions. This paint is hazardous to respiratory function. Be sure to wear a mask or respirator and work in a well-ventilated area to protect yourself and others in the space. Additionally, for safety reasons, the oven used to cure this paint should not be used for food after the paint is cured in it.
Don PPE
Put on nitrile gloves to protect the skin on your hands from irritation
Put on safety glasses to protect your eyes from splatters
Put on lab coat to protect clothing from splatters of ink
Put on a mask or respirator to protect your respiratory system from inhaling aerosolized silicone ink
Assemble the airbrush
Mix the gray silicone ink
Place a mixing cup on the scale
Tare (zero) the scale
Using a transfer pipette, gradually add 0.3g of ink catalyst into mixing cup
Record the actual weight of the ink catalyst you've poured, then take 10x that weight as the target weight for the gray silicone ink and 30x that weight as the target weight for the ink thinner
Example: If you actually poured 0.34g of ink catalyst, you should target 3.4g as the weight for your gray silicone ink and 10.2g as the weight for your ink thinner.Tare digital scale
Pour black silicone ink into the mixing cup until you reach the target weight
Tare digital scale
Using a transfer pipette, gradually add ink thinner into the mixing cup until you reach the target weight
Take the mixing cup off the digital scale
Using a popsicle stick, mix the contents of the cup together for 3-4 minutes until it is uniform in color
Tip: When mixing your silicone, make sure to incorporate the silicone from the sides and bottom of the cup as well.
Pour ink mixture into the airbrush's fluid reservoir and cap the fluid reservoir
Spray isopropyl alcohol onto a microfiber cloth and clean the surface of the gel sheet thoroughly
Place the gel sheet onto a layer of paper towels such that it has 5-10cm of paper towel border around the gel sheet
Paint the gel sheet with gray silicone ink
Start the stream of paint to the side of the gel sheet in case of spatters at the beginning
Moving in slow and even strokes, completely cover the entire surface of the gel sheet with gray ink, as shown below
Gently lift the edge of the gel sheet and hold the gel up to a light source to see any areas that are patchy
Add more ink to any patchy areas
Gel sheet painted with gray silicone ink
Cure the silicone ink
Preheat the oven to 250F (121C)
Place the painted gel sheet on a clean oven tray with the unpainted side down
Put the painted gel sheet into the oven for 5 minutes
Remove the gel sheet from the oven and allow it to cool
Gently rub a finger on the edge of the painted surface to ensure that paint does not rub away
If the paint rubs away, place the gel sheet back into the oven for another 5 minutes
Clean out airbrush
Tip: Silicone ink will cure inside the airbrush and is difficult to clean out after it has cured. Make sure to thoroughly clean out the airbrush by dumping out the paint in the fluid reservoir then filling the fluid reservoir with ink thinner and spraying until the stream runs clear.
Adding Shear-Tracking Markers
Adding Shear-Tracking Markers
If you do not want to add shear-tracking markers to you GelSlim 4.0 sensor, skip this section of the instructions. Resume at "Painting the Gel Black".
Place the gel sheet onto the bed of a laser cutter with its painted side up
Download and unzip dot_patterns.zip from the GelSlim 4.0 hardware repo on GitHub.
Note: We use dot distances of 1.5mm and dot diameters of 0.2mm. You can change the dot pattern to suit your needs.Load compatible dot_pattern file into laser cutter software
Note: In the files provided, raster paths are filled black (R: 0, G: 0, B: 0) with no outline. If your laser cutter software uses a different fill color for raster paths, there are several ways to change the files provided to fit your setup. For example, you can load dot_pattern.ai into Adobe Illustrator and change the fill color of the raster paths to match the settings of your setup.In laser cutter software, set material to silicone with 3mm thickness
In laser cutter software, align the dot pattern such that it covers the entire gel sheet
Note: The dot pattern is larger than the footprint of the gel sheet, so there will be a border of dot pattern cut around the gel sheet.Execute the cut
Resulting gel sheet with rastered dot pattern
Painting the Gel Black
Painting the Gel Black
It is important to take proper safety precautions in this portion of the instructions. This paint is hazardous to respiratory function. Be sure to wear a mask or respirator and work in a well-ventilated area to protect yourself and others in the space. Additionally, for safety reasons, the oven used to cure this paint should not be used for food after the paint is cured in it.
Don PPE
Put on nitrile gloves to protect the skin on your hands from irritation
Put on safety glasses to protect your eyes from splatters
Put on lab coat to protect clothing from splatters of ink
Put on a mask or respirator to protect your respiratory system from inhaling aerosolized silicone ink
Assemble the airbrush
Mix the black silicone ink
Place a mixing cup on the scale
Tare (zero) the scale
Using a transfer pipette, gradually add 0.3g of ink catalyst into mixing cup
Record the actual weight of the ink catalyst you've poured, then take 10x that weight as the target weight for the black silicone ink and 30x that weight as the target weight for the ink thinner
Example: If you actually poured 0.31g of ink catalyst, you should target 3.1g as the weight for your black silicone ink and 9.3g as the weight for your ink thinner.Tare digital scale
Pour black silicone ink into the mixing cup until you reach the target weight
Tare digital scale
Using a transfer pipette, gradually add ink thinner into the mixing cup until you reach the target weight
Take the mixing cup off the digital scale
Using a popsicle stick, mix the contents of the cup together for 3-4 minutes until it is uniform in color
Tip: When mixing your silicone, make sure to incorporate the silicone from the sides and bottom of the cup as well.
Pour ink mixture into the airbrush's fluid reservoir and cap the fluid reservoir
Spray isopropyl alcohol onto a microfiber cloth and clean the surface of the gel sheet thoroughly
Place the gel sheet onto a layer of paper towels with the gray side facing up such that it has 5-10cm of paper towel border around the gel sheet
Paint the gel sheet with black silicone ink
Start the stream of paint to the side of the gel sheet in case of spatters at the beginning
Moving in slow and even strokes, completely cover the entire surface of the gel sheet with black ink
Add more ink to any patchy areas as necessary
Cure the silicone ink
Preheat the oven to 250F (121C)
Place the painted gel sheet on a clean oven tray with the unpainted side down
Put the painted gel sheet into the oven for 5 minutes
Remove the gel sheet from the oven and allow it to cool
Gently rub a finger on the edge of the painted surface to ensure that paint does not rub away
If the paint rubs away, place the gel sheet back into the oven for another 5 minutes
Clean out airbrush
Tip: Silicone ink will cure inside the airbrush and is difficult to clean out after it has cured. Make sure to thoroughly clean out the airbrush by dumping out the paint in the fluid reservoir then filling the fluid reservoir with ink thinner and spraying until the stream runs clear.
Cutting the Gel Pads
Cutting the Gel Pads
Place the painted gel sheet onto the cutting mat with the painted side down
Align the gel pad template on top of the gel sheet
Use the fine-point marker to trace out the circular gel pads
Remove the gel pad template
Use the X-acto knife to carefully cut along the lines
Throw away the excess gel
Use isopropyl alcohol and a paper towels to remove any residual ink from the unpainted side of the gel pads
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