Using Polymers in SDM

Contents:

Jonathan Clark demonstrates respirator safety attire:
P5-SaftyGear.jpg

Mixing and Pouring Process and Safety

Note: The chemicals used for SDM are two-part castable urethanes, silicones and epoxies. These are not particularly hazardous, however they should be mixed and poured in a fume hood and care should be taken to avoid skin contact.

1. Contact MarkCutkosky about Safety Training requirements before using the RPL.

2. See Stanford Lab Safety Basics for a good introduction to handling polymers and other chemicals.

3. Before working with any of the polymers, read the data sheets that are stored with them regarding safety procedures. Most of the urethanes and silicones that we use are not particularly dangerous, however you should always work under a fume hood and wear proper attire. See

Safety attire

Safety glasses or goggles, gloves, lab coat. See Personal Protective Attire for general instructions on safety attire. In particular, for pouring or mixing any of the polymers:

  • Wear purple nitrile gloves (in the drawer below the fume hood)
  • If there is a danger of splashing (not the case with most of our polymers, which are quite viscous), use goggles
  • For some polymers it may be necessary to wear the green over-gloves.
  • For some polymers it may be necessary to use a respirator. Any persons working with such polymers must be trained in respirator use. See EH&S respirator FAQ sheet.
    • Respirator REQUIRED for the following:
      • Any "Crystal Clear" product (Smooth-On)
      • "Smash!" Plastics (Smooth-On)
      • Clear Flex 50 & 95 (Smooth-On)
      • Task 12 (Smooth-On)

Handling equipment:

  • See https://suehsaps2.stanford.edu/lcst/?q=node/13 for general information on handling equipment for chemicals
  • plastic cup or beaker for mixing
  • tongue depressor/popsicle stick (or other disposable mixing implement) Metal, glass or plastic stirrer to avoid moisture contamination
  • Ziploc bag for waste
  • Paper towels
  • (any items you will embed, see below)

Emergency procedures:

Mixing Procedure:

In some cases it is advisable to use green forearm gloves over the disposable nitrile gloves:
P6-GreenGloves.jpg

Pouring in the fume hood, on a digital scale:
P7-Pouring1.jpg

The process:

  1. Mask off the area around your part (or the whole block if your parts cover it) with tape. This will keep the polymer from spilling everywhere before it cures.
    • Some people recommend putting the block into the vacuum chamber and de-gassing at this point. This will help remove any air bubbles in/around the tape.
  2. Estimate how much polymer you will need; you'll want to cover your part by a layer of 1-2mm of polymer. This layer will trap any left over air bubbles so they don't stay in your part. It's best to over-estimate; we're always throwing away old/bad polymers.
  3. Shake both parts of the polymer thoroughly. Especially for urethane rubber Part B and both parts of Task plastic.
  4. When mixing, it is best to pour the thicker of the two parts first (Part B for urethane rubbers). The thinner is easier to control and you'll have better luck getting the exact proportions. There should be a sheet of mix ratios in the RPL, or look here
    • If mixing by volume, find a graduated beaker. They are scarce, so if possible mix by weight.
    • If mixing by weight, use a plain cup (don't waste a graduated beaker!).
    • Once you have the right proportions, mix the two parts of the polymer vigorously and thoroughly. You'll introduce bubbles, but we'll remove them later.
  5. Clean off the threads of the polymer bottles before you close them. The unmixed parts will harden and seal the bottles if you don't.
  6. Pour the polymer over your part, making sure to cover it completely.
  7. Move the wax block into the vacuum chamber and close the lid.
  8. Start the pump and de-gas for a few minutes. The polymer should bubble vigorously as the air escapes. Once the bubbles settle down you can stop the pump and slowly let the air back in.
  9. The next step depends on whether the polymer you poured is hard and can be planed after curing or is soft and cannot.
    • If the part is hard, skip to the next step.
    • If the part is soft, you want to remove the polymer over your part. You can try to squeegee it off using another tongue depressor, or (if the surface finish is important) stretch a piece of teflon film over it to squeeze the excess polymer out and create a flat surface. Try to keep this excess material from spilling all over the place.
  10. You can leave the part to cure in the vacuum chamber, or move it back into the fume hood. Moving it back into the fume hood is probably best, otherwise someone might have to move your block for you.
  11. CLEAN UP!!! Please keep the area as clean as possible. Wipe up any spilled and uncured polymer. Place everything (mixing implements, paper towels, and gloves) in the ziploc bag you have ready and put it in the blue hazardous waste bin.

Disposal of used containers, tools:
P13-Waste.jpg

Waste Disposal

To get the hazardous waste bucket emptied, fill out the form found here:
http://www.stanford.edu/dept/EHS/prod/enviro/waste/pickup/WastePickup_form.htm
Make sure there is a Hazardous Waste tag on the bin when they come pick it up. The table below provides most of the info needed to fill out the online form and the tag.

Building 02-530, Room 129A: just estimate the volume of waste in the garbage can.

Description Ingredients Toxic? Reactive? Flammable?
10A Resin modified diphenylmethane diisocyanate
contains MDI
X   X
10A Hardener none X    
35A Resin modified diphenylmethane diisocyanate
contains MDI
X   X
35A Hardener none X   X
90A Resin modified diphenylmethane diisocyanate
contains MDI
X   X
90A Hardener none X   X
72DC Resin        
72DC Hardener        
Quik-Cast A modified diphenylmethane diisocyanate, 12-19%
With MDI, 44-50%
methylene bisphenylisocyanate, 33-38% concentration
polymeric plasticizer, 35-40%
triethyl phosphate, <2%
X X  
Quik-Cast B Polyether polyols amine based, 50%
polymeric plasticizer, 50%
X    

Polymer Properties

The following tables should be located in the RPL as well for quick reference. Please update these with as much data as is available (and add the polymer to the all tables) when we get new polymers.

The tables include:

  • Basics - Hardness, Color, Manufacturer, Type
  • Mixing and Curing - Working Time, Cure Time, Mix by Weight, Mix by Volume
  • Material Properties - Tensile Strength, Elongation, Young's Modulus
  • Curing and Bonding - Which polymers cure/bond to which other polymers

Basics

Organized by Manufacturer

Name Hardness Color Manufacturer Type Tech Data
Dragon Skin 10A Translucent White SmoothOn Silicone (Platinum Cure) http://www.smooth-on.com/tb/files/DRAGON_SKIN_SERIES_TB.pdf
EcoFlex 0010 00-10 Clear SmoothOn Silicone (Platinum Cure) http://www.smooth-on.com/tb/files/Ecoflex_Series.pdf
Vytaflex 10 10A Translucent-Amber SmoothOn Urethane
Vytaflex 20 20A Translucent-Amber SmoothOn Urethane
Vytaflex 40 40A Off-White SmoothOn Silicone
SmoothCast 60D 60D Amber SmoothOn Silicone
SmoothCast 61D 61D Amber SmoothOn Silicone
SmoothCast 305 70D White SmoothOn Silicone
SmoothCast 326 72D Clear-Amber SmoothOn Silicone
SmoothCast 327 72D Clear-Amber SmoothOn Silicone
Task 3 80D White SmoothOn Urethane
Task 9 85D Clear SmoothOn Urethane
IE-10AH 10A White Innovative Polymers Urethane
IE-20AH 20A White Innovative Polymers Urethane
IE-50AC 55A Clear Innovative Polymers Urethane
IE-60A 55A Cream Innovative Polymers Urethane
IE-72DC 80D Clear Innovative Polymers Urethane
P-20 20A Translucent Silicones Inc/Innovative Polymers Silicone http://www.silicones-inc.com/p20.pdf
P-44 42A Translucent Silicones Inc/Innovative Polymers Silicone
P-70 58A (70A, 1 week+) Light Green Silicones Inc/Innovative Polymers Silicone
P-100 80A +/- 10A Tan/Beige Silicones Inc/Innovative Polymers Silicone
GI 1040 30A (40A, 1 week+) Light Blue Silicones Inc Silicone (Tin Cure)
Silastic S 26A Green DowCorning Silicone http://www1.dowcorning.com/DataFiles/090007c880002438.pdf
Sylgard 170 40A Dark Gray/Black DowCorning Silicone
TAP Blue 26A Light Blue TAP Plastics Silicone (Tin Cure) http://www.tapplastics.com/uploads/pdf/Product%20Bulletin%207BRev.pdf
EPM-2492 75A White NuSil Silicone
FXRite 12 12A Translucent ArtMolds Silicone

Mixing and Curing

Working time is how long you will have to pour the part, while cure time is how long you need to wait before you remove it.

Name Working Time Cure Time Mix by Weight Mix by Volume
Dragon Skin 20 minutes 5 hours 1A:1B 1A:1B
EcoFlex 0010 30 minutes 4 hours 1A:1B 1A:1B
Vytaflex 10 30 minutes 16 hours 1A:1B 1A:1B
Vytaflex 20 30 minutes 16 hours 1A:1B 1A:1B
Vytaflex 40 30 minutes 16 hours 1A:1B 1A:1B
SmoothCast 60D 5 minutes 20 minutes ? 1A:1B
SmoothCast 61D 7 minutes 60 minutes ? 1A:1B
SmoothCast 305 7 minutes 30 minutes 10A:9B 1A:1B
SmoothCast 326 8 minutes 60 minutes ? 1A:1B
SmoothCast 327 15 minutes 4 hours ? 1A:1B
Task 3 20 minutes 90 minutes 120A:100B 1A:1B
Task 9 7 minutes 4 hours 115A:100B 1A:1B
IE-10AH 10 minutes 10 hours ? ?
IE-20AH 12 minutes 10 hours 100R:100H ?
IE-50AC 12 minutes 4 hours 50R:100H 51R:100H
IE-60A 10 minutes 8 hours 25R:100H 22R:100H
IE-72DC 15 minutes 6 hours 100R:50H 100R:55H
P-20 60 minutes 18 hours 10A:100B ?
P-44 60 minutes 24 hours 10A:100B 10.9A:100B
P-70 60 minutes 24 hours 10A:100B 13.9A:100B
P-100 60 minutes 24 hours 10A:100B 13.2A:100B
GI 1040 2.5 hours 18 hours 10:1 100:11.3
Silastic S 45 minutes 7 hours ? 10A:1B
Sylgard 170 15 minutes 24h @ 25C°, 20min @ 70C° 1A:1B 1A:1B
TAP Blue 30 minutes 8 hours 10A:1B 9A:1B
EPM-2492 2 hours 6 hours 1A:10B ?
FXRite 12 60 minutes 12 hours 10:1 10:1

Moisture Sensitivity

Storage

Ambient moisture can greatly reduce the shelf life of the individual parts. In order to combat this we are purchasing a dry gas blanket can from Smooth-on (looks and works like the Endust cans for dusting computers). Basic procedure is to almost close the container and spray the gas for a couple seconds into the container displacing the ambient air and moisture from inside then immediately finishing sealing the container.

Note: please make sure to clean off the edges of the bottle lip and lid before trying to close it because if we don't then we will have to break the lid to open it.

During Casting

Ambient moisture can inhibit the cure of urethane rubbers and plastics and cause some bubbling. Apparently by using a wooden stirrer, we are guaranteeing that we introduce moisture during mixing. It is recommended to only use metal, glass and plastic containers and stirrers.

Note: We can try using the Dry Gas Blanket stuff to ensure molds have less moistures by blanketing the cavity. The gas used is both heavier than air and removes moisture.

Material Properties

Tensile Strength and Young's Modulus were measured experimentally in some cases, see YoungsModuliForUrethanes for details. If the measured value differed from the manufacturer's data, it is indicated in bold and parentheses.

Name Tensile Strength(measured) Elongation Young's Modulus
Dragon Skin 475 psi 1000% ?
EcoFlex 0010 200 psi 800% ?
Vytaflex 10 160 psi 1000% ?
Vytaflex 20 200 psi 1000% 195 kPa
Vytaflex 40 522 psi 660% ?
SmoothCast 60D 2200 psi 30% ?
SmoothCast 61D 1800 psi 20% ?
SmoothCast 305 3000 psi ? ?
SmoothCast 326 3170 psi ? 425 MPa
SmoothCast 327 3170 psi ? 425 MPa
Task 3 6650 psi 6% ?
Task 9 7800 psi 6% ?
IE-10AH 150 psi ? ?
IE-20AH 200(190) psi 175% 217 kPa
IE-50AC 1000 psi 470% ?
IE-60A 1000(200) psi 470% 560 kPa
IE-72DC 8000 psi 2% ?
P-20 525 psi 425% ?
P-44 600 psi 150% ?
P-70 775 psi 250% ?
P-100 470 psi 345% ?
GI 1040 525±25 psi ? 1550±175 kPa
Silastic S 1000 psi 900% ?
Sylgard 170 ? ? ?
TAP Blue 580 psi ? ?
EPM-2492 200 psi 250% 11 MPa
FXRite 12 ? ? ?

Multi-Polymer Curing and Bonding

This table quickly outlines which polymers will cure on other polymers, and how well they bond together. This does not indicate anything about how well small features will come out if the base polymer is used as a mold for the curing polymer. A color-coded, excel version of the table can be found here.

Cure options:

  • full cure - the polymer cures completely on top of the base
  • top cure - the portion of the polymer exposed to air cures, but the polymer in contact with the base does not.
  • no cure - the polymer does not cure.

Bond options (in decreasing strength):

  • full bond
  • bond
  • mild bond
  • no bond

Cured Polymer Base Polymer
  Vytaflex 10 Vytaflex 20 Vytaflex 40 IE 20 P-20 P-44 P-70 P-100 GI 1040 Sylgard 170 TAP Blue
Vytaflex 10 full cure
full bond
full cure
bond
full cure
bond
full cure
no bond
full cure
no bond
full cure
no bond
full cure
no bond
full cure
no bond
  full cure
no bond
full cure
no bond
Vytaflex 20 full cure
full bond
full cure
bond
full cure
bond
full cure
no bond
full cure
no bond
full cure
no bond
full cure
no bond
full cure
no bond
  full cure
no bond
full cure
no bond
Vytaflex 40 full cure
full bond
full cure
bond
full cure
bond
full cure
no bond
full cure
no bond
full cure
no bond
full cure
no bond
full cure
no bond
  full cure
no bond
full cure
no bond
IE 20 full cure
mild bond
full cure
mild bond
full cure
mild bond
full cure
full bond
full cure
no bond
full cure
no bond
full cure
no bond
full cure
no bond
  full cure
no bond
full cure
no bond
P-20 no cure
no bond
no cure
no bond
no cure
no bond
full cure
no bond
full cure
mild cure cure
full cure
no bond
full cure
no bond
full cure
mild cure
  full cure
no bond
no cure
no bond
P-44 top cure
no bond
top cure
no bond
top cure
no bond
full cure
no bond
full cure
full bond
full cure
full bond
full cure
full bond
full cure
full bond
  full cure
mild cure
no cure
no bond
P-70 no cure
no bond
top cure
no bond
top cure
no bond
full cure
mild cure
full cure
full bond
full cure
full bond
full cure
full bond
full cure
bond
no cure
no bond
full cure
full bond
no cure
no bond
P-100 top cure
no bond
top cure
no bond
top cure
no bond
full cure
no bond
full cure
full bond
full cure
full bond
full cure
mild cure
full cure
bond
  full cure
mild cure
no cure
no bond
GI 1040   full cure
no bond
                 
Sylgard 170 no cure
no bond
no cure
no bond
no cure
no bond
full cure
no bond
full cure
bond
full cure
full bond
full cure
full bond
full cure
full bond
  full cure
full bond
no cure
no bond
TAP Blue full cure
no bond
full cure
no bond
full cure
no bond
full cure
no bond
full cure
bond
full cure
mild cure
full cure
full bond
full cure
full bond
  full cure
mild cure
full cure
full bond

UV resistance enhancer (urethane rubbers)

Smooth-On makes a UV resistance enhancer called SunDevil. This should help with the problem of UV degradation in the flextures. We need to learn how much to add without inhibiting the cure. The basic usage is to mix SunDevil (1% of total weight) with Part B and then mix Part A with that combination.

Embedding Materials

These are some quick tips and trick for embedding simple materials/components. More advanced techniques can be found on the SDMAdvancedTechniques page.

Fabrics

Fabrics embedded in soft polymers can be used to keep the part from stretching (or from stretching in a particular direction, depending on the fabric) while maintaining the ability to flex. A great example of this are Rise.StickyBot's toes.

Embedding the fabric will usually result in trapped air bubbles, so extra steps must be taken. One approach that works fairly well is outlined below, but feel free to experiment with other combinations of de-gassing and pre-coating the fabric.

Approach 1:

  1. Pour the part as described above, including de-gassing it.
  2. At the same time, coat the piece of fabric with some mixed polymer, making sure the polymer soaks in. De-gas this with the main part (i.e. set it somewhere on the wax block when you de-gas). You probably want to have some tweezers to manipulate the soaked fabric. Make sure you CLEAN them off when you're done.
  3. Carefully place the fabric in the part, avoiding creating trapped bubbles as much as you can, and de-gas again. The first de-gas step should limit the trapped air in this step, which should keep the polymer from bubbling the fabric out of you part.
  4. Carefully adjust the location of the fabric with the tweezers if it moved significantly during the second de-gassing step.
  5. Cure and remove as usual.

Partially Embedded Components

Sometimes you might want to partially embed components so that portions of them are not covered in polymer. Some examples include LEDs or the nitinol flexures in the ZMan toes.

  1. Design your part to include the actual part you want, plus some space for the extruding components to sit (ex. room for LED leads).
  2. Before pouring the mixing or pouring the polymer, place the component how you would like it to be embedded in the part. You will need to fill the extra space you created with the green clay we have. Use this to help position the component as well.
  3. Make sure the clay fills only the extra space and does not spill into the area of your part.
  4. Pour the part as usual.
  5. When you remove the part, simply clean away the clay and you'll be left with your part and extruding components.

 
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