Over the investment casting processes, making wax pattern is the key progress. The quality of wax pattern will greatly decide the shape and surface of the final parts.
Wax based pattern mold material is actually paraffin-stearic acid pattern material, the commonly used formula is paraffin 50% + stearic acid 50%, ingredients requirements paraffin (CnH 2 n + 2) should choose melting point> =58℃, refined or semi-refined paraffin. Stearic acid (C17H35COOH) should be selected first level (three pressure) stearic acid (block).
Paraffin wax is the basic component of pattern, adding the effect of stearic acid is due to stearic acid molecules is polar. It is good to the coating, can improve the coating of the mold, and is beneficial to improve the mold thermal stability, liquid paraffin and stearic acid soluble. The pattern melting point is low. It’s convenient to prepared. Molding and dewaxing are easy. Wax recovery rate is high. So it is good to be reused too. If the ratio of paraffin and stearic acid is changed, the performance of the mold will be affected. When the content of paraffin is increased by 10%, the strength of the mold can be increased. When the content of paraffin is increased by more than 80%, the mold surface is easy to bubble, the surface quality of the melting mold is poor, and the coating and fluidity of the mold are reduced. In that case, it is not suit for precision casting. If the content of stearic acid is increased by 10%, the coating fluidity and thermal stability of the mold will be improved. When the content of stearic acid exceeds 80%, the strength of the material is very low and the toughness is very poor, so it can not be used.
The strength and thermal stability of paraffin-stearic acid pattern is not high (softening point is 31℃), and during processes, stearic acid and active metal cause replacement reaction, also with alkali or alkaline oxides, generate insoluble in water saponification (stearate). It makes the mold deteriorate. And the viscosity of saponification residue on the surface of the cavity, will affect the surface quality of the casting. Because the saponification reaction will consume part of the stearic acid in the mold, adding new stearic acid in the mold reuse is beneficial to stabilize the performance of the mold.
The performance of paraffin-stearic acid mold pattern is not only related to the ratio of the mold, but also affected by the melting point of paraffin. If more than 60℃ is used to replace 58℃ paraffin, the strength and thermal stability of the mold are significantly improved, and the shrinkage rate is reduced, so the performance of the mold can be improved.
Paraffin-stearic acid mold pattern is convenient, easy, re-usable, but its thermal stability is not high. This makes it easy to deform, especially in the production process of the saponification deterioration. The recycled wax must be processed before it can be reused. Reuse of residual acid is easy to pollute the environment, so, in recent years, no stearic acid wax base mold pattern is recommended.