Overview
This article provides a comprehensive guide to understand the proper techniques for storing, mixing, degassing, pouring, and curing Sil-Craft platinum liquid silicone.
Storing Platinum Liquid Silicone
Sil-Craft Platinum Liquid silicone has a shelf life of about 12 months provided that it is stored between 50°F and 77°F (10°C and 25°C).
Platinum liquid silicone is sensitive to temperature and humidity, so proper storage conditions are important to maintaining its quality.
Preparation
- Ready a gram scale to measure the Part A and B ingredients by weight.
- Use vinyl gloves (not latex) to handle the ingredients. Latex will cause cure inhibition. We do not recommend nitrile gloves. Do not mix with bare hands.
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Mixing containers should be either plastic or metal and should have a flat bottom and straight sides. Mixing sticks should be flat on the bottom and sides for scraping the sides and bottom of the mixing container.
For larger projects, you can use a mixing attachment on a power drill, but be sure to alternate between the power mixer and scraping the sides and the bottom of the container using a mixing stick, since the power mixer cannot easily reach those areas.
- Apply any release agents. If you are pouring against a surface that requires a release agent, apply it now and continue once it's dry.
Mixing
Sil-Craft platinum liquid silicone is mixed using a 1:1 ratio by weight and has a pot life (working time) of 30-40 minutes, which should give you plenty of time to completely mix and vacuum degas the material for best results.
If you are using a different product, make sure the working time is sufficient before proceeding.
Avoiding Cure Inhibition
- The number one reason why silicone doesn't cure properly is inadequate mixing – the two components were not mixed together sufficiently.
- The second most common reason silicone doesn't cure properly is inaccurate mixing. Too much (or too little) of Part A or Part B.
- The wrong temperature. Mix silicone between 69°F and 73°F (20°C-23°C). Too cold and the mixture will not cure or will be "lumpy". Too hot and it will cure too quickly.
How Much Silicone?
You should have already prepared your mold (including applying any release agents) and have calculated how much silicone will be required.
For example, if your mold requires 1000 grams of silicone, you will need 500 grams of Part A, and 500 grams of Part B.
1000 gram mold = 500 grams Part A + 500 grams Part B
Now add about 15% - 20% more material to your estimate. You don't want to run out of material in the middle of pouring your mold or casting. If we add 20% more to our example, we now need 1200 grams.
1200 grams = 600 grams Part A + 600 grams Part B
Prepare Part A and Part B Containers
- Gather two new mixing containers with sufficient capacity (800 milliliters each should be plenty).
- Mark one with an "A" for Part A, and the second with a "B" for Part B.
Dispense and Mix Part B
- Place the B container on the gram scale and zero out the scale.
- Pour 600 grams of Part B into the B container.
- Add any pigments to the Part B liquid if desired (you can also add them later when mixing the two components together).
- With a new, clean mixing stick, mix the Part B liquid in in the container for 3-4 minutes, carefully scraping the sides and the bottom of the container.
- Place the B container to the side.
- Close the Part B source drum tightly.
Dispense and Mix Part A
- Place the A container on the gram scale and zero out the scale.
- Pour 600 grams of Part A into the container.
- With a new, clean mixing stick, mix the Part A liquid in in the container for 3-4 minutes, carefully scraping the sides and the bottom of the container.
- Place the A container to the side.
- Close the Part A source drum tightly.
Combine Parts A and B
- Grab a new container that is at least twice the capacity of the total material you will be using. In our example we plan on using 1200 grams, so find a new container at least twice that size (about 2500 milliliters).
- Place the container on the gram scale and zero out the scale.
- Slowly Pour the Part B liquid into the container.
- Record the number of grams you poured into the container. It's not likely to be exactly 600 grams – some of the liquid will have stuck to the sides of the B container. Let's say we ended up pouring 590 grams of Part B into the container.
- Zero out the scale again.
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Pour the Part A liquid you just mixed into the container.
Critical: If you poured 590 grams of Part B into the container, be sure to also pour 590 grams of Part A into the container (and no more) so the ratio of Part A to Part B is exactly 1:1. This will ensure you have combined the components equally by weight.
Mix Parts A and B
Sil-Craft 720/800/820 Platinum Liquid Silicone has a "pot life" or mixing time of 30-40 minutes, which should be plenty of time to perform the following steps.
- With a new, clean mixing stick, mix the Part A and Part B liquid in in the container for 3-4 minutes, carefully scraping the sides and the bottom of the container. It's important to mix thoroughly but do not froth the mixture.
If you have a power mixing attachment for your drill, use it but start slowly so you don't splatter the liquid, and alternate between hand mixing and power mixing at least three times during the 3-4 minute time frame.
- Add any pigments or other additives (deadener, thickener, fillers, etc.) to the silicone. Carefully follow the manufacturer's instructions (which are outside the scope of this document).
Please be aware that additives will change the properties of the silicone. For example, Sil-Craft 720 Series Food-Grade Platinum Liquid Silicone will no longer meet food safety requirements once pigment has been added – although it can be considered skin safe since pigments are heavily used in lifecasting and prosthetics and are tested for skin safety.
- Double Mix (optional). Take another clean container and pour the mixed silicone into the container and mix again for 2-3 minutes. This will "flip" the silicone so the liquid at the bottom is now at the top.
Vacuum Degassing
Once the material has been thoroughly mixed, it will probably be full of bubbles. Removing those trapped bubbles has several advantages:
- By removing air bubbles, the risk of surface defects is minimized. This is important when a smooth, flawless surface is required – especially when casting objects with intricate details.
- You don't want bubbles in food grade molds. Bubbles can trap bacteria, making your mold unsafe.
- The presence of air voids can weaken the material and reduce its strength, elasticity, and tear resistance. Vacuum degassing promotes a more uniform silicone structure, leading to improved mechanical performance.
- By removing air bubbles (which can act as barriers to curing agents) vacuum degassing allows for more efficient cross-linking of the silicone polymer chains.
Degassing Procedure
- Place the container into the vacuum chamber.
- Cover the chamber, open the input valve and close the output valve.
- Turn on the vacuum pump.
- Once the gauge reads 29 inches of mercury (depending on altitude) you can turn off the pump and close the input valve (don't leave the pump running unnecessarily or you will shorten its life).
- Degas the mixture until no more bubbles appear, which should take 5-15 minutes (depending on the amount of material).
- If the liquid silicone is about to overflow the container during the degassing process, release some air from the output valve of the vacuum chamber to collapse the mixture, close the valve, then slowly build up the vacuum pressure again.
Pouring Liquid Silicone
- From at least 18 inches above the mold, slowly pour the mixture in a thin stream into a single spot in lowest point of the mold.
- Let the liquid seek its own level.
- Look for and correct any mold leaks if found.
Curing
The demold time for Sil-Craft 720/800/820 Platinum Silicone is 4-6 hours at room temperature (73°F / 23°C). This is the minimum amount of time to wait before carefully removing the silicone from the mold.
Full curing depends on the size and thickness of the material, but generally takes about 24 hours at room temperature.
The offgassing process for platinum-cured liquid silicone involves the release of gases during curing or curing and post-curing stages. Platinum Silicone offgassing is generally considered to be minimal and safe, but do not cover or obstruct the flow of air around the material during the curing process or you could delay the curing process or cause curing problems.
Post Curing
Post curing is not required when using Sil-Craft Platinum Liquid Silicone, but it can be used to accelerate the curing process, enhance the mechanical properties of the silicone rubber, and ensure complete crosslinking.
Post Curing Platinum Silicone
Post-curing involves subjecting the silicone to elevated temperatures for a specified period of time. The exact temperature and time period depend on the size and amount of material being post cured, and the type and size of oven being used.
This information applies to Sil-Craft 720/800/820 platinum cure liquid silicone only, not tin (condensation) cure silicone.
For post curing, use either a laboratory oven or “hot box” with adequate ventilation (a convection type oven). Do not attempt to post cure in a household oven or with a heat gun.
Post Curing Examples
Testing is the best way to determine the correct temperature and time for post curing silicone. We generally use the following as a starting point, adjusting time and temperature as required:
- Preheat the oven to 122°F (50°C)
- Heat the silicone for 40 minutes
- Allow the silicone to cool to room temperature.
- Wash the silicone with dishwashing liquid and warm water.
As the amount of material increases (especially the thickness) the longer the post cure process will be.
Verify the Material is Cured
Silicone that is not completely cured will leave a "smudge" on a clear piece of glass or other high gloss finish, whereas cured silicone will leave a very slight impression on the glass, or none at all.
If your sample leaves a noticeable smudge, bake it again for another 20 minutes and see if it improves. Once you find the ideal the time and temperature, you should receive consistent results going forward.
Cure Inhibition
Everybody's worst fear – you spend a lot of money on Platinum Silicone Rubber and the material does not cure correctly. It's sticky, or clumpy, or has imperfections in the surface of the material.
The most common causes of sticky, clumpy or imperfections in the silicone include:
- Not mixing thoroughly
- Improper mixing ratio
- Temperature too high and the mixture cures before (or while) you are pouring the material into the mold. This can also happen when you exceed the mixing time or pot life of the material.
- Temperature too low and the material has "clumps" that you can see when pouring the material.
- Imperfections in the surface of the material are usually caused by excessive air bubbles. Be sure to vacuum degas the material before pouring.
- Contamination (see below)
Contamination
The following materials may contaminate the silicone, causing cure inhibition:
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Tin-Cure (Condensation-Cure) Liquid Silicone
Tin-cure and platinum-cure silicone do not work well together at all. Never use any mixing sticks or containers that once contained tin-cure silicone. -
Sulfur
Sulfur clays can cause cure inhibition, even those that have been coated with a sealant. - Phosphorus
- Enamel Coatings (Lacquer)
- Latex
- Some SLA 3D Printing Resins (test before using)
- Urethane Rubber
- Nitrogen
- Copper
- Water (and excessive humidity)
Testing for Cure Inhibition
Cure inhibition can be avoided by testing the silicone against the material(s) it will come into contact with, before committing to any project where a lot of expensive material will be used.
- Mix a small batch of liquid silicone. Keep the mixing container.
- To check for possible surface inhibition, brush or pour a small amount of silicone onto a noncritical area of the pattern.
- If the silicone is gummy or very sticky (uncured) after the recommended cure time has passed, inhibition has occurred.
- If the material left over in the mixing container is also gummy or sticky, your mixture is either incomplete (not mixed sufficiently) or inaccurately mixed (too much A or B Part). Mix a new batch and retry the test.
- If the material in your mixing cup is fine (cured well), then your pattern is causing the inhibition. Try a release agent or coating to eliminate the inhibition, or switch to a new material for the pattern.
Summary
Mastering platinum liquid silicone involves storing the material at the right temperature, accurate measuring, thorough mixing, effective degassing, proper pouring, and sufficient curing. It's crucial to be mindful of potential mixing and curing issues.
Please let us know if you have any questions or suggestions in the comments section below.
Revised: 08/20/2023
By: Stephen Brown