INRF application note
Mark Bachman and Joy Lau, Spring 2002.
Silicone rubber (PDMS) is a stable, high resolution elastomer which can be used make very accurate impressions of micro structures. It can be used as a final device or as a mold for making devices out of other polymers. Since it is somewhat compliant and very inert, it makes an ideal molding compound. Silicone can handle temperatures in excess of 300 °C, so it can even be used as a mold for low melting point metals such as tin. This is considered a safe cleanroom process and requires basic wetbench and equipment qualifications. At the moment, most PDMS molds fabrication is done outside the ANF as only the SU-8 (and other resists) molds are made in the ANF cleanrooms.
The silicone process takes about 60 minutes, about half of which is prep time.
- Silicone elastomer raw material and catalyst. Recommended: Dow Sylgard.
- Micromold (silicon, SU-8, or other)
- Plastic pouches (bottle liners)
- Syringe applicators and holders
- Aluminum foil
- Applicator sticks
- Razor blades
- Electronic scale
Since the silicone raw material is very sticky, it is a good idea to prepare a work area which can be easily cleaned up afterwards. Use a sheet of aluminum foil and tape to the table to create a disposable works space. Put a plastic pouch in pouch holder. Place on scales and tare the scale. Carefully pour 15–20 g of silicone material into the plastic pouch. Use the applicator sticks to help with this since the PDMS is viscous and difficult to pour. Avoid creating bubbles in the material or making a mess.
Add 10% that weight of catalyst by dispensing from dropper bottle. A little more makes a slightly firmer rubber. Remove plastic pouch from holder. Carefully press to remove trapped air. Seal bag in sealing unit providing enough room for expansion. Make a double seal to prevent possible bursting later. Knead the pouch continuously for 10 minutes. Attach bag to a vertical holder. Place in vacuum, and leave in vacuum for 15 minutes to degas the material. The mixture will bubble and foam, and the bag will expand as the trapped air is released from the material. Remove
from vacuum when finished.
Label the applicator syringes with “DOW Sylgard Silicone”, and then add your name and the date (if another silicone is used, label appropriately). Remove plungers from applicator syringes. Cut a corner from the silicone pouch and carefully pour into applicator syringes. Replace plungers, then let the silicone settle in the applicator syringes. The silicone material will remain in usable liquid form for approximately 6 hours. If it is stored in a cold environment or freezer, it will last longer. Discard the silicone pouch when empty. It is non-toxic.
Using the syringe applicator, carefully apply silicone material to the surface of the mold. Apply slowly to prevent air from getting trapped. If the applicator nozzle is too small, cut it with a razor blade to make a larger opening. If air bubbles occur, allow the silicone to settle for 15 minutes; most bubbles will rise to the surface. Optional: attach a top mold to the silicone to make a smooth or patterned topside. Do this carefully as it is easy to trap gas bubbles. Cure the silicone by one of the following three methods:
- 10 minutes in 90°C oven
- 1 hour in 50°C oven
- 24 hours at room temperature
Carefully separate the silicone material from the substrate(s). You may need to use some tweezers or a dull knife to assist in this process. Use a razor blade to roughly trim the cured silicone. Touch up torn edges with fresh silicone material if necessary.
Wrap razor blades in protective cardboard or paper. Remove aluminum foil and wrap up all used materials and scraps. Discard waste in the appropriate containers. Most of the PDMS work goes in the regular trash; the razor blades, broken glass or broken silicon goes in the “sharps” waste.
Safety & Emergency:
All UBC safety and procedural regulations must be followed. Safety glasses and clean gloves should be worn at all times. Review the UBC standard operating procedures for fire, chemical spill, and chemical exposure available at riskmanagement.ubc.ca.
Silicone elastomer base is not a hazardous material, but the curing agent contains some hazardous components. Use both chemicals with adequate ventilation and avoid eye contact. Store the base away from oxidizing materials, and store the curing agent away from water and moisture. The curing agent evolves minute quantities of flammable hydrogen gas that can accumulate. Adequate ventilation is needed to maintain vapors well below flammability limits and exposure guidelines. Keep containers closed when not in use. Clean area as appropriate because some silicone materials, even in small quantities, may present a slip hazard. Direct contact to eyes may cause temporary redness and discomfort, although there is no significant irritation expected from a single short-term exposure to skin or inhalation. In case of exposure to eyes, flush immediately with water for 15 minutes. Any small spills should be wiped up immediately with wipes. Dispose the wipes in the waste container. In case of large spill, follow the UBC Standard Operating Procedure for chemical spills. Report to your lab supervisor or report to riskmanagement.
- Dow Sylgard product literature
The following checklist is designed to aid the researcher when performing this process.
- Prepare workspace. Prepare plastic pouch.
- Pour ~15 g silicone in plastic pouch. Add ~1.5 g catalyst (10:1 wt.)
- Seal plastic pouch (twice). Leave room for air.
- Knead continuously for 10 minutes.
- Vacuum degas for 15 minutes.
- Prepare syringe applicators. Label appropriately.
- Pour silicone into syringe applicators.
- Let silicone settle.
- Carefully apply silicone to mold surface.
- Topside mold?
- Cure silicone rubber
- 10 minutes in 90 °C oven
- 5 hours in 50 °C oven
- 24 hours at room temperature
- Trim silicone for release.
- Carefully remove molds from silicone.
- Final silicone trim and cleanup. Clean up, dispose wastes. Unused silicone can be stored in refrigerator.