Flexil Hot Melt Compound is a castable material based on vinyl resin. Available in two different grades of hardness H.G. Blue, a hard grade and S.G. Natural, a soft grade. The two can be used separately or mixed together to give a range of intermediate hardnesses. It is designed for the manufacture of flexible moulds for casting polyester resin, as well as for glass fibre laminates. It has good tensile strength and is a resilient compound with a melting point of about 150°C. Moulds from Flexil give faithful reproductions of the model; rough surfaces will reproduce rough, while glossy or polished surfaces will leave the mould face with original degree of gloss or polish. Flexil moulds allow greater versatility in design, either functional or decorative, because the extraction problems associated with rigid moulds are not present.
Moulds can be washed with soapy water to remove dust or the remains of previous castings - the oily surface makes it unnecessary to treat moulds before casting since it acts as a mould release, but care should be taken to avoid the excessive use of strong detergents which may remove the oil film, Organic solvents should not be used to clean a mould since even if they do not dissolve the compound, they may be taken up in small quantities causing the mould surface to swell and distort. When Flexil moulds are fitted in plaster cases for storage, the inside of the case should be coated with a solution of shellac to prevent the plasticizer to the Flexil form being absorbed by the plaster case.
When a mould is damaged or worn out it is cut up and pieces inspected for cleanliness - dirt can be washed off without affecting the Flexil compound. After any necessary cleaning the material is ready for re-melting and re-use. Flexil can be re-melted 25 - 30 times before it loses its elasticity.
Methods of Melting Small Quantities
Small quantities of Flexil can be melted in an air bath similar to a glue pot or porringer but without water or oil in the other vessel, since boiling water is not hot enough to melt the compounds. When using this device the contents must be stirred frequently, and only quantities of up to 5lb (2.2kg) can be melted. The Flexil should be cut into small pieces and a small quantity placed in the tin, the compound being stirred while being heated. When the first charge begins to liquify, a further few pieces should be added with further stirring; this procedure should be repeated until all the hot melt compound has melted.
Melted Larger Quantities
A range of melting appliances designed for use with Flexil Hot Melt Compounds is available. All are electrically heated and thermostatically controlled, and are the safest and most efficient means of melting the material without danger from over-heating and burning. This controlled heating results in the life of Flexil being increased by up to 40%. Models of 5,12 and 20 gallons (22.7, 54.5 and 90.9ltr.) capacity (50, 120 and 200lb (23, 44 and 90kg) Flexil respectively designed for users of large quantities of Flexil, are fitted with a quick melting device making it possible to melt about 35lb (16kg) of Flexil an hour.
A model of almost any rigid material can be used provided it does not soften at the melting point of the compound - thermoplastic resins are therefore unsuitable. Porous materials such as dry plaster of paris and wood, should be sealed or otherwise treated to prevent entrapped air escaping and marring the mould by bubble formation. Bulky metal parts should be warmed to counteract any chilling of the Flexil.
China and glass objects should be carefully warmed beforehand to prevent breakages when the hot compound is poured over them. Cement or cast stone objects, which are usually large, are best used water wet. They should be soaked for several hours, dried superficially and used as masters without preparation.
Open or Flood Moulds
Open or flood moulds are very simple to make; the model, provided it is in low relief as a panel, can be moulded in the following manner. The panel is laid on a flat surface and a retaining wall of clay or wood built round it an inch higher than the highest point of the model. Flexil is then poured between the wall and the model in a continuous stream to avoid trapped air and splashes.
Pouring a Mould
When all the material has melted to a thin liquid, the Flexil should be poured quickly into the mould box, without stopping, until it is filled to the top. Care should be taken not to pour on to the model. The stream should flow down the side of the encasement to avoid splashing and trapping air. The danger of premature gelling during pouring is not very great, and slightly gelled Flexil will still give excellent reproduction. Checking the temperature with a thermometer reading to 200°C is recommended as this will help to prevent the compounds being poured above their recommended temperatures.
Treatment of the Mould
If the mould is faulty, owing to blemishes on the master, or air, which has come out of the model, it can be repaired by touching up with a hot knife. A tear in the mould can be repaired by the same method.