The wall thickness does not have to be uniform because the material does not sink like thermoplastic. Very thin walls are possible with silicone molding even down to 0.005 in / 0.127mm.
Tolerances and Molded Part Shrinkage
Tolerances for molded parts are directly related to part shrinkage. The best practice is to build a test shrinkage mold of the part and use data to build prototype or production mold. The variations between lots of Liquid Silicone can be little, compared to thermoplastics, therefore, tolerances can be a lot closer. We typically apply RMA A3 “Commercial” tolerances, however, due to our precision machining capabilities and silicone’s ability to replicate microscopic features, tighter tolerances are possible, including tolerances as tight as +/- .002 in / .050 mm are attainable.
If a fit tolerance is required it is best to fit the silicone molded part to the fitted part, since measurement of a silicone molded part requires non contact measurement and measurements under .002 in / .05 mm are very difficult.
Source: Stockwell Elastomerics, http://www.stockwell.com/molding-tolerances.php
“A3” Commercial Drawing Designation
Dimensional Tolerance Table for Molded Rubber Products
– Occurs during molding and post curing or until fully cross-linked.
– Varies by the manufacture and product range from .025 per inch.
– Typical shrinkage is between 1% and 5% depending upon material
– Check with your material supplier regarding shrink percentage of the material you plan to work with
Coefficient of Thermal Expansion
Silicone has a high thermal expansion coefficient and can be used as a temperature activation method or sensor in the design. This expansion is an advantage for sealing applications. The coefficient of volumetric thermal expansion for all silicone rubber products is in the range of 5.9 to 7.9 x 10-4-4/°C. The linear coefficient of thermal expansion is roughly one-third of the volumetric coefficient of thermal expansion, and can be used to calculate the total linear thermal expansion of a rubber part over a temperature range.
Example: If the volumetric coefficient of thermal expansion is 5.9 x 10-4/°C and the temperature span is 150°C, the total linear expansion of a part one-inch long would be … (5.9 x 10-4/°C)/3 x 150 °C x 1 in. = 0.0295 in.
Thermal and Electrical Conductivity
Thermal values for any thermal insulating material usually range from 0.330 to 0.515 x 10-³ g-cal/sec/cm²/cm/°C. An electrical value for any electrical insulating material is the value of dielectric strength changes combined with the thickness of the test specimen. For a 10-mil sample silicone rubber, 1000 volts per mil is quite normal.
Silicone is a good insulator and can be compounded to be a good thermal conductor as well by adding certain fillers. A good example of this would be silver.
(See http://www.siliconesolutions.com/elec_therm_conductive.html for more details)
Silicone is now being used to replace plastic and glass optical lens. Some Silicone refractive index can equal that of glass (1.53), and are optically clear. It is even possible to have higher surface finishes than typical glass lens at a reduced cost.
The advantages for using Silicone over plastic are UV aging, ability to operate at high temperatures, and the wider spectrum of transmission compared to acrylic and polycarbonate.
Other products that take advantage of Silicone’s Dynamic Properties are:
Anti-vibration pad, Spring Hinges, Flexible connection, Pushbutton (that snaps back), Bumper
Radii and Fillets
Radii and fillets improve a product’s appearance and increase it’s tear strength.
The surface finish matches the mold texture down to the micron level. Surface finish can be dull, very shiny, or include part identification marks. Surface finish has a direct bearing on the coefficient of friction.
Note: Textured surfaces provide better part releases when molding than polished surfaces.
Draft angles are not required on silicone parts that can be deformed to remove from the mold. Solid parts should have draft angles.
Undercuts on de-formable parts are obtainable. The parts can be stretched to approximately four times their size to remove from cores without deforming the part. Isopropyl Alcohol can be used to assist in expanding the parts to remove from cores or to assemble. The parts will contract once the Isopropyl Alcohol evaporates.
Note: Under cuts do not normally require expensive actions (slides) for the mold.
Parts can be rolled off the cores as described in the diagram below.
Parting Lines and Gate Location
Silicone parts are flexible and can be molded over cores allowing the parting lines to be put at the most desirable locations. But as shown in the diagram below most of the parting lines are straight because silicone can flash at .0001 in/. Irregular parting lines are very difficult and should be avoided.
Note: Compression has no gate vestige.
Silicone rubber can be used to over-mold plastics, metals, electrical connections, electronics and heaters. It has good adhesion, is a good electrical insulator and will keep out moisture.
Bonding of the silicone can be achieved by any of the following methods:
– Mechanical bonding with under cuts on part to be over-molded
– Use of primes on part to be over-molded
– Use of self-bonding silicones
Post curing will assure molded parts are fully cross-linked.
Bonding silicone to metal can be seen in this handle used for medical imaging equipment.
Heat Aging Effects on Silicone’s Mechanical Properties:
Source: Dow Corning, http://www.dowcorning.com/content/rubber/rubberprop/thermal_aging.asp
As silicone rubber is heat aged at temperatures from 150 to 316°C, it gradually hardens and loses its elasticity. However, certain properties may improve. Resistance to compression set normally improves when the sample is heat aged before compression. Tensile strength shows various changes, depending on the rubber being tested. It may decline slowly, remain fairly stable, or may even increase somewhat with continued heat aging. Heat aging performance can be improved with special compounding.