Casting answers & advice.

Q we use silica sand to remove the mold coating from permanent molds. Due to environmental concerns, we are considering a system that uses a C[O.sub.2] dry ice material. Will this be aggressive enough to clean the molds?

Background: Silica sand is olden used for the removal of permanent mold coatings. The sodium silicate used in most permanent mold coatings is tenacious and can leave a glaze that can inhibit adhesion of the next coating.

Silica sand is an aggressive blast media that removes the mold coating quickly and completely and is a relatively inexpensive material. The sharp nature of sand grams and their aggressive cutting action can clean the mold fully, while less aggressive media may not break the silicate glaze and frilly remove the residue from the mold material to produce the mold "texture" that is essential for proper mold coating adhesion. How ever, excessive exposure and inhalation of free silica has potential health risks.

Although some facilities clean their molds in sealed blast cabinets, much of the blasting of permanent molds is performed in an open air environment. Engineering controls and proper ventilation should be an essential part of any mold cleaning operation using silica sand. Workers in the blast area should use respirators approved for dust containing quartz by the appropriate safety agency.

Another reason why many facilities move away from sand is the propensity for mold wear. Repeated use can cause excessive mold wear, especially on corners (causing excess flashing), fine features (loss of detail) and moving mold pieces such as pins (which can lead to mold misalignment).

Many aluminum permanent mold facilities are using C[O.sub.2] or dry ice blasting to eliminate the potential health concerns of silica and minimize the danger of mold wear. C[O.sub.2] pellets are used as blasting media, and coating removal can Ire performed at the molding machine while the molds are still hot. The spent material evaporates, leaving little cleanup.

Recommendation: Used properly, CO., can be an effective method of mold coating removal. C[O.sub.2] is not as aggressive a cleaning media as sand and may take longer to thoroughly clean the mold, but this is offset by the fact the mold does not need to be removed from the molding machine and cleanup time is reduced. C[O.sub.2] cleaning will reduce mold wear.

While C[O.sub.2] blasting is effective for routine mold cleaning, metalcasting facilities could find that some molds may need to be removed from the molding machine at regular intervals and taken for treatment with a more aggressive blast media to provide the proper substrate texture for coating adhesion or to ensure complete stripping of the mold coating in hard-to-clean mold areas.

Q we are using grinding wheels on our iron castings. Can we replace these wheels with abrasive belts?

Background: Grinding wheels are a "bonded abrasive" product (abrasive material that is "bonded" together, reinforced for safety and molded into shapes). Bonded abrasives such as wheels, cups, discs, cones and mounted points are a standard method for grinding ferrous castings.

Grinding belts are part of a family of products called "coated abrasives" that are manufactured by coating paper or cloth backing material with abrasive grams. Technological advances have al lowed coated abrasive products to be used in applications and on metals formerly dominated by wheels and stones. Stronger backing materials are more resistant to splitting, breaking and fraying so more pressure can be applied to the belt, resulting in faster metal removal.

Improved joints and the addition of abrasive materials, such as zirconia and ceramic to the abrasive mix, also have improved belt life. Coated abrasive belts have been used for grinding aluminum and other nonferrous castings, but these technical advancements have developed coated abrasives that can compete with grinding wheels in many applications.

Recommendation: Under the right circumstances, coated abrasives can be used effectively for grinding ferrous castings and can offer higher metal removal rates and less operator fatigue. Ferrous metalcasting facilities considering replacing bonded-abrasive grinding wheels with belts should test zirconia or ceramic abrasives and the heaviest polyester backing materials (Table 1).

Ceramic abrasives are premium products designed for hard-to-grind metals and perform best in applications that require heavy material removal. Ceramics thrive on pressure and wheel speed and are especially effective in machine-fixtured, high-pressure applications.

For those using grinding machines with moderately fixed speeds and where pressure depends on the method and strength of a human operator, ceramics may not perform effectively. Those considering converting to grinding belts should test a variety of materials and perform a cost analysis to determine if a conversion is worthwhile.

Table 1. Abrasive Belt Materials

                     Coated Abrasive Recommendations

                                Abrasive Type

Metal              Aluminum Oxide   Zirconia-Alumina   Ceramic-Alumina

Cast Iron                 3                1 *              2 *
Ductile Iron              3                1 *              2 *
Carbon Steel              3                2                1
Stainless                 3                1                2
Specialty Steels          3                2                1

1-Most Effective   2-Good   3-Not Recommended

* Products need to be tested to determine best application.