(1)
Casting Cold Cast Bronze

If the silicone apppears to be unexpectedly thick when decanting.
Some common causes.
a) Stored in very low temperatures.
b) Stood for a long time.
c) Old product getting close to shelf life.

Most of these problems can be rectified by bringing silicone up to room temperature, then mechanically mixed with a mixer drill and paint paddle for a period of time. This will re-blend the oils and fillers into the silicone base giving a consistant blend throughout the silicone and ensuring a consistent 'Shore A' value. As good practise all silicone base, should be thoroughly mixed prior to use to ensure a consistent result, as fillers will settle in time.

Silicone cures softer than expected.
Assuming that the silicone base is thoroughly mixed prior to use then the common cause is catalyst ratio. Common ratios of catalysts are 5% or 10% additives. 10% additives are normally 5% catalyst with added oil to give a 10% additive, this is done to give an east calculation when weighing.

Because 10% catalyst is a blend, the separation of oil and catalyst can result. Shaking the catalyst in all cases before use will ensure the catalyst is properly mixed.

Cured silicone has uncured or softer areas of cure.
The catalyst has not been thoroughly mixed into the silicone. Many catalysts come as clear or slightly yellow, but when added to silicone it is impossible to see how well the catalyst has been mixed into the silicone and can result in softer or uncured silicone. Catalysts can be pre-coloured allowing a visible measure of how consistently the catalyst is blended into the silicone.

(2)
Common problems with Condensation Silicones

If the silicone apppears to be unexpectedly thick when decanting.
Some common causes.
a) Stored in very low temperatures.
b) Stood for a long time.
c) Old product getting close to shelf life.

Most of these problems can be rectified by bringing silicone up to room temperature, then mechanically mixed with a mixer drill and paint paddle for a period of time. This will re-blend the oils and fillers into the silicone base giving a consistant blend throughout the silicone and ensuring a consistent 'Shore A' value. As good practise all silicone base, should be thoroughly mixed prior to use to ensure a consistent result, as fillers will settle in time.

Silicone cures softer than expected.
Assuming that the silicone base is thoroughly mixed prior to use then the common cause is catalyst ratio. Common ratios of catalysts are 5% or 10% additives. 10% additives are normally 5% catalyst with added oil to give a 10% additive, this is done to give an east calculation when weighing.

Because 10% catalyst is a blend, the separation of oil and catalyst can result. Shaking the catalyst in all cases before use will ensure the catalyst is properly mixed.

Cured silicone has uncured or softer areas of cure.
The catalyst has not been thoroughly mixed into the silicone. Many catalysts come as clear or slightly yellow, but when added to silicone it is impossible to see how well the catalyst has been mixed into the silicone and can result in softer or uncured silicone. Catalysts can be pre-coloured allowing a visible measure of how consistently the catalyst is blended into the silicone.

(3)
Common problems with Polyurethanes

Inconsistency of curing Polyurethanes and Moisture
Polyurethanes can be effected by atmospheric moisture. Common effects of moisture contamination can appear as excessive 'air bubbles to foaming' during cure. Moisture in polyurethanes will cause foaming.

To reduce the contamination moisture is to decant the polyurethane from container and replace the cap immediately. Ensure that temperature of polyurethane and working environment is as stated on the technical Data Sheet (TDS) do not allow any decanted polyurethane to stand for any length of time.

Polyurethane Weeping After Cure
Polyurethanes are a complex blend of chemicals. High and low temperatures and long time storage can cause separation of these chemicals. As cream in milk will migrate to the top so some chemicals in polyurethanes will do the same. The result of chemical separation can cause:
a) Sweating after cure
b) Reduced phase change (A change from transparent mix to opaque after cure)
c) Inconsistency of curing

To solve these problems a thorough shaking or stirring of the polyurethane will re-blend the chemistry together.

Rob Price
2017

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