Success in the hobby home foundry depends to a large extent on your skills and the ability to produce simple patterns. Here simple patterns means patterns that can be easily mold, lift or separate from the sand mold after a gentle rap. A pattern with a faulty draft angle on vertical surfaces, badly finished corner fillets, or a pattern, which has not been made to an ultra smooth finish will be hard to work with. Pattern making is an art in itself, which requires a a lot of patience and hard work.
In metal casting, patterns are used to create the sand molds. They are placed inside one half of a flask and then they have the tempered sand packed around it. When the pattern is taken out from the sand, it leaves an exact impression. This impression in which the molten metal is poured through runners and gates provides us the finished product.Forms
Patters can take on various types of forms and can be made from almost anything from foam to timber, even an original casting can be reproduced with some little modifications.
In addition, “draft” and “surface finish” are another important points, which should be highlighted while making the patterns. While using the patterns that you have purchased from market, you will realize that some are completely unsatisfactory while some require heavy modification before they can be used.
This can be hard to bring up as sometimes the person designing the pattern has put relentless hours into its production. The more draft the better, however no less than 11/2 degrees for Co2 Sand and no less than 3 degrees for Green sand. For surface finish, make sure that the patterns are sealed, unsealed timber soaks up the moisture of sand and makes the sand attached to the pattern, not to itself. Make sure that all the surfaces are consistent and smooth. It means, no undercuts particularly around fillets, it will tear the sand when taken out.
The probability of making a successful casting is proportional to the care, which goes into forming the pattern that the sand is packed around. While designing, remember the fact that there can be no portions of it, which protrude or will interfere with the sand while lifting the pattern from the mold. Give all vertical surfaces a little taper or bevel, which will let the pattern to easily release from the sand. Merely one or two degrees is adequate. According to experts 1/16 inch per foot for shallow patterns and 1/8 inch per foot draft for deep ones. Avoid any abrupt angles in the design, which would make a sharp corner in the sand that would be prone to falling off or washing away during the pour. Vinyl spackling compounds have found to be suitable to produce fillets with good success. Simply smear it on then sand it down to give shape when dry. Wood glue can be used for smaller parts.Pattern Material:
Pattern MakingFor making patterns, common pine boards have found to be of great use. For thicker sections, 2 or more layers can be attached together. Common plywood can be used to make several patterns of large diameter, which would be otherwise expensive if produced from solid wood. Plywood requires slightly little extra care: voids should be filled, and the stuff does not easily sand smooth, but it is economical. Voids can be filled with wood putty produced from wood glue and sawdust mixed together. A sheet of wood glue smeared over the patterns of plywood will function to seal and smooth the surface when its is dry.
Design the pattern with a hard, smooth and shiny surface. Different patterns can be successfully made with a wood glue coating, however it has been found that a completely dried coat or two of good oil-based enamel makes a worthy improvement.