Double mixing involves mixing the A & B components thoroughly in one container and then transferring the mixture into a new, clean container. Double mixing leaves much of the unmixed material that hides in the corners and sides of the cup behind, ensuring a more thorough mixture. Unmixed material can show up in the part as soft spots or unsightly swirls/streaks.

Some materials can have chemical incompatibilities when poured into or up against other materials. For example, platinum silicones may show inhibition characteristics against natural rubbers, cyanoacrylates, tape residue, and sulfur based clays. Whenever in doubt, a small side test is always recommended.

Certain urethane systems may be affected by the type of silicone mold being used. Tin based silicones (condensation cured) are known to cause issues with Aliphatic urethanes. Platinum silicones (addition cured) are recommended for Aliphatic urethane. Silicone mold material must be carefully selected depending on the urethane system you plan to cast.

Individual components in a container need to be mixed and/or stirred prior to weighing them out on a scale. With storage time and ambient temperature changes, ingredients can separate out and will need to be remixed. Filled, pigmented or fire retardant materials need to be pre-mixed before every use or the heavier fillers could settle to the bottom and cause curing problems.

Pot Life is the time you have from the moment you begin mixing to the point you can still easily pour material into a mold. Gel Time is the measurement of time when the material actually gels. As materials get higher in viscosity, they do not have optimum flow characteristics and may not produce as good a part. Always be aware of the pot life and gel time rating on any data sheet as well as the mass it is calculated at. Adding more mass (e.g. 400g vs. 100g) will shorten the pot life. Mixing an appropriate batch size can assure you have time to vacuum and pour your material. Always choose an appropriate pot life based on how difficult your cast is and how much material will be mixed prior to casting.

Storage temperature can have a drastic affect on materials. Working time, demold time, viscosity, and physical properties of the material are all affected by temperature. In cold winter months working with thermal set systems (urethanes, epoxies, etc.) can be a problem. Materials tend to be much higher in viscosity. Higher viscosities can make mixing, degassing, and pouring as much of a challenge as brittleness can be when you demold. This is not the fault of the material; it is the nature of chemistry. Extra care needs to be taken to pre-warm materials prior to mixing as well as pre-warming of molds. Warm material poured into a cold mold can have undesired results, especially if the material has a long working time. Heat can work to your advantage if you need to process faster but it can also work against you. Pre-heating materials lowers viscosity, making it easier to mix and pour, but it also shortens your work time. Sometimes going with a longer pot life and heating the material can give you the benefit of lower viscosity without sacrificing valuable work time. Elevated temperatures can also increase the physical properties of a material. See Post Curing below for more details.

Pre-warming a mold and or material will help the cure on the front end of a job, but what about after it has initially set up? Post curing at elevated temperatures can increase the physical properties of a material; tear strength, flexural strength, and heat distortion can benefit from a proper post cure. Some rigid materials may appear brittle upon demold after the initial cure. Post curing can assist the chemical cross linking to maximize the material’s physical properties above and beyond what can be achieved at ambient temperature over several days. Post curing is best done in an accurate, temperature controlled oven (never a household oven). Always refer to the TDS (Technical Data Sheet) for proper post curing conditions. Some materials can have undesired results at elevated temperatures (swelling, bubbles, deformation), so always check the recommended procedures.

Aromatics are the most commonly used urethanes. They tend to be more economical to produce, have good physical properties, and range from low durometer elastomers to rigid plastics. Their down sides tends to be poor UV resistance and lower chemical resistance. The Aliphatic urethanes have outstanding UV stability, color stability, and chemical resistance. Their down sides tends to be longer gel and demold times, low heat distortion, low tolerance to colder casting temperatures, and higher material costs. Blends of Aliphatic and Aromatic urethanes can exhibit excellent UV resistance, good heat distortion, and good physical properties.

Aliphatic Water Clears are not tolerant of cold casting temperatures. Clarity and physical properties can be severely affected if cast below 70 degrees F. Molds must also be preconditioned to minimum temperature (75-85°F) or heat may be pulled from the curing material. Clear materials require a good vacuum system to pull the trapped air out of mixed material prior to pouring into a mold. Water Clears do not self-release entrapped air. Without pulling a vacuum on the mixed material, a cast part may end up looking more like a glass of carbonated soft-drink than a glass of crystal clear water. Casting the part in a pressure tank/vessel can also aid in reducing bubbles by compressing bubbles down to the point that they are not visible anymore (see Vacuum and Pressure). Casting in thin sections may also require additional heat to make up for the lack of internal exotherm (less mass equals less self- generated heat). Water Clears cure best if preconditioned to a minimum of 75-85°F range and a max of 100-110°F, keeping in mind that higher temperatures will shorten work life.

When you are hand mixing casting materials, air bubbles are inevitably going to be stirred in. Viscosity, temperature, and surface tension of the material will determine how well the air will or will not self release from the liquid. Mixed material should always be placed in a vacuum chamber to remove air bubbles prior to pouring material into the mold; this process is called de-airing or de-gassing. Vacuuming the material expands the trapped air, causing the bubbles to grow and rise to the surface of the material for release. After a period of time these bubbles decrease in quantity, meaning that the air has effectively been removed from the material. Air can cause voids and potential mechanical issues with a cast part. Cold materials will have higher viscosities and may be more difficult to degas. Raising the temperature of the material lowers the viscosity, which will aid in pulling air out but may also reduce working time if overdone. Pressure casting can be an alternative or a secondary process to vacuum ‘degassing’ material. Pulling air out of the material ensures the material isn’t latent with bubbles as it goes into the mold, but air can be re-introduced into the material as it is poured in from splashing or from mold features that trap air, e.g. sharp corners. In some applications, vacuum and pressure must be used in conjunction to produce air-free parts. Typical pressure values are in the 40-50 psi range. Higher pressures may be required if material is not degassed. Pressure does not remove air; it compresses air bubbles down to the point that they are not visible anymore. If pressure is released too soon (before material has had time to set), the bubble, could potentially reappear.