A WORD ABOUT FAILURE

“If your personality is one that cannot run the risk of failure, you may be in for some psychological trauma. Frequently, your early work will be failures. If you are working with someone who has had previous experience with the materials, perhaps he can tell you why. Don’t look to your mentor expecting godlike qualities – he will have failures too. Usually the “old timer” will know enough to use an untried system, or process in private until he is comfortable with it. Then he can show it off without spoiling his reputation as an expert. Well, good for him! That technique is, in itself, one that is acquired as the result of experience. You must expect occasional unusual results. They happen. If the shelf life of some products is exceeded, they may not give the expected results. If some plastic chemicals become contaminated, they will not work as expected. If you work for a long time with resins, the time will surely come when you forget to use a mold release, or you will use the wrong catalyst, or you will forget again and use no catalyst at all. These mistakes will bring predictably unsuccessful results. You will be able to understand immediately why your casting failed as soon as you realize your mistake. But there will also be the times that you will run into freakish, unreasonable problems that defy explanation.” (THURSTON JAMES 1989)

• Ratios—Do I need to measure by weight or by volume?
• How Does Temperature Effect Material?
• What To Do If Material Freezes?
• How Should Materials Be Stored?
• What Can I Do If I Get Moisture Contamination In Urethane Resin?
• How Can I Warm Materials Or Molds?
• How Do I Know When The Material Is Too Aged To Use?
• How Can I Stop From Getting Bubbles In My Finished Part?
• How Should I Mix The Material To Avoid Bubbles?
• What Should I Do When The Material Does Not Cure Properly?
• What If The Material Is Curing To Fast?
• Can Silicone Be Used Without The Use Of A Vacuum Chamber?
  
  
  

Ratios—Do I need to measure by weight or by volume?

Silicone RTVs 
Our systems are formulated to achieve the best physical properties by weight. Silpak’s systems are flexible enough to mix by volume also, but you could run short of the catalyst (B side). **Exception – Platinum based R-2100 series must be mixed by weight to within 0.5%- high or low range.

Urethane Plastics & Urethane RTVs
The balanced ratio is by weight. Various systems can end with curing problems because the density of part A and part B are different. When mixing by volume, you will need to pour the A side into the B side without scraping the container to achieve a somewhat accurate measure. Otherwise you will be short on the part A side and will need to order extra.

Epoxy
Epoxies are very sensitive to ratios, and, because of the density of the base to hardener, it can be 20% or more off if working by volume.  Trying to work out volume ratios can be done but the weight must be worked out first.  An accurate scale is recommended.

Polyester Resin
This system is unique in that, typically, the catalyst is added at a minute amount, between .75 to 1.5% to the resin weight.  Adding more catalyst will accelerate gel and cure time.  Volumetric measurements have been generated for these types of resin.  See MEKP Catalyst Chart.

Silicones
Cold temperatures do affect the cure of silicones, more with the platinum series since it cures quicker with heat and stops curing with freezing temperatures. Tin based silicones are not heat curable and will cure in cooler temperatures. The first hour of cure (Tin based) will effect the final time the silicone cures. Very dry or low humidity will also slow the curing process.  Do not try to heat-cure the Tin based silicones. Adding more catalyst will cure Tin based silicones and some Platinum based silicones quicker; although, this will affect the physical properties and the library shelf life. Platinum silicones will cure faster with each degree of temperature over 90F (Not recommended to cure by elevated temperatures unless pre-tested). Heat saturation for depth of rubber thickness needs to be factored in for final cure of silicone—add 1 hour for each 1” thickness.

Urethane Plastics and Urethane RTVs
Urethane materials cure best when the temperature of the material is between 70F – 80F. As the material becomes warmer it will react and cure quicker, and when it is cooler the cure process is slowed—if too cold, a non-cure of material could result. The temperature of the mold and master tool can also affect the cure process.

Epoxy
The best temperature for a consistent cure is between 70F – 80F. If the epoxy material is quickly cured by using heat, it can boil creating bubbles if you are not cautious. If the epoxy does not cure properly you may see softening and/or dimensional distortions during heat exposure greater than 150F.  Post cures at 125F – 140F are used to assure the completion of cure before subjecting it to a higher temperature.

Polyester Resin
The temperature of the material and mold will control cure of plastic with sticky surfaces or a finished product with surface wrinkle marks.  Clear resins are the most sensitive.  Preheat molds to 100F. Use a lower percent of MEKP and heat (100F – 125F) to cure.

Urethane Plastics and Foams
Warm up before using in a hot water bath.

Urethane RTVs
Not normally, but you can heat up before using in a hot water bath.

Silicone RTVs
Heat up before using in a hot water bath.

Latex
DO NOT ALLOW TO FREEZE—it will be no good.

Epoxy
Will need to be heated and mixed or it can turn solid and separate. A hot water bath works best.

Do not store on cold cement floors or near direct heat. Room temperatures of 70F – 80F are best.

Add some BAYLITH L MB (moisture scavenger) to the B side or vacuum B side for 1 hour and test.

The safest way to heat up cold materials is in a hot water bath. Hot tap water works well. The material container can be placed directly in a plastic trash bag and sealed off to avoid moisture contamination. Large tools and molds can be heated by using an electric heating blanket.

The first noticeable change is the condition of the base--a gain in viscosity (thickness), formation of chunks or crystals, unusual color change.  A small test batch can be mixed and cured to establish conditions. Some materials can be salvaged while others should be turned into a solid waste and disposed of in the trash.

Urethane Plastic
Air can adhere to the surface. Dust the mold surface with Prolite FR 50 or a dry talc powder. Cast parts under pressure in a pressure pot between 60 – 80psi.

You need to mix material with shear (aggressively) or problems with cure could result. When mixing with a drill mixer care should be taken not to pull air into mixture. Keep mixer deep, scraping sides towards the middle. (Jiffy mixers work great)

After you’ve checked material age, mix ratio accuracy, accounted for ambient curing temperature of material, contact Silpak and ask for Technical Support.

Urethane Plastic
In warm summer heat, cool the A side to 50F – 60F in the refrigerator for 30 minutes before using.  Cooling both sides can result in poorly cured parts.

Silicone
In hot summer weather, cool B side in refrigerator. Reduce the catalyst ratio, i.e. a mix ratio of 10% down to 9%.  Mixing smaller batches and pouring several layers will allow the air to surface quicker.

Silicone can be used without the use of a vacuum chamber. Brush on the surface coat using forced air to work out any surface air.  Back fill 30 minutes later. When pouring rubber into the cavity, pour thin streams of mixed material from high above the surface into the corner of the mold, allowing the rubber to roll out, pushing air away as it fills the mold cavity. Curing a mold while under pressure at 50-60 psi also works well in providing strong, bubble free molds. After pouring mold, place it in a leak-free Pressure Pot, and allow it to remain until cured.  Air compressor should always be larger than the pressure pot used.