Key Factors for Shell Making Success
There are many aspects to ensuring success in the shell room: Materials, Equipment, Procedures, Training, and Management. One of the most important aspects is process control. I think the 80/20 rule holds for success in the shell area. Twenty percent of success is determined by Materials, Equipment, and Procedures and Eighty percent of success comes from being consistent. While an emphasis must be placed on process control, there are several key process items that are worth reviewing for possible adjustment of the shell process.
Wax Cleaning: The wax tree needs to be cleaned to remove oils, dirt and wax chips. The current norm is to use some type of Citrus based cleaner. Citrus cleaners havereplaced chlorinated hydrocarbon solvents due to environmental issues. Usually these cleaning and rinse tanks are not maintained well in terms of cleanliness or concentration of the active ingredient, d-limonene. D-limonene is also a VOC (volatile organic compound) that is potentially subject to State and Federal regulations. At a minimum, keeping the tanks clean by continuous filtering and flushing is good practice.
First Dip: The most common prime slurry consists of zircon flour with 10 – 20% fused silica flour and either large or small particle size colloidal silica and some type of polymer. The amount of fused silica used ranges from 0 to over 50%. At the higher levels of fused silica, the zircon is only present to improve the draining characteristics of the fused silica slurry. It is not present is sufficient quantity to have a significant effect on the casting. The main functions of prime slurry are:
- Compatibility with the metals to be cast. Most alloys are compatible with zircon and alumina. Some are not compatible with silica.
- Slurry draining characteristics should be able to produce a smooth uniform coating on the wax. Edges of patterns should be coated and wet enough to accept stucco. A non-uniform prime coating can result in casting defects, notably inclusions.
- Prime slurry needs to adhere to the wax and hold prime stucco tightly.
- The prime slurry should have usable life that is suitable for the size of tank and amount of product being dipped. Slurry life is a function of all the components in the slurry: binder, flour, polymer, and other additives. As an example, zircon flour generally drives the slurry pH down due to naturally occurring soluble contaminants. This can cause partial gelling of the binder leading to quality issues on castings and the entire slurry may need to be replaced. Work closely with your supplier to select appropriate binders, flours, and additives for your application.
- Green Permeability, especially of the early dips, is necessary for liquid wax to be able to soak into the shell to relieve increasing wax pressure on the shell during dewax. If green permeability is low, dewax cracking is very likely. See photo below of green permeability being run in our lab. Note that too much polymer in the slurry may decrease green permeability. See Figure 1.
- When dry, the prime slurry must hold the primary stucco tightly. Loose stucco on prime dip needs to be cleaned off. Loose stucco, especially in corners, can result in shell particles getting into the castings causing scrap and rework for inclusions. Polymer helps to hold stucco tightly.
Transition Dips: Their function is to gradually fill the detail of the wax pattern after first dip and before the backup layers. These slurries are usually…