Process control of mold coatings assures safety and efficiency.

In-plant measures toward consistent, correct usage help maximize coatings performance benefits.

While the benefits of using the fight mold and core coating are obvious and well documented, the proper handling and use of a coating cannot be stressed enough. If used correctly, a corn or mold coating will reduce or eliminate burn-in/burn-on/penetrations defects, improving overall as-cast quality and reducing cleaning department costs.

If mishandled, however, not only will those benefits be lost but other defects may occur. As in all foundry processes, consistent and correct procedures for coatings are crucial to the quality of the end-product. Hence, the need for proper in-plant process control (IPC).

Coatings IPC must occur in the functional areas of receiving/storage, mixing/dilution, core/mold surface applications, drying and disposal of supply containers. This article takes a brief look at the IPC steps necessary at each of these stages.

Receiving and Storage

Upon receipt, incoming containers of coatings should be inspected for damage, making certain that lids are properly affixed, the coating is the right type and weight, and all labeling and documentation are correct and in compliance with pertinent government regulations.

When storing coatings, supply containers should always be kept indoors and out of direct sunlight. If the coating is flammable, the storage area must be suitable for flammable materials.

Temperature control in the storage place is crucial. Since the flow characteristics of a coating are affected by temperature, low or high extremes must be avoided. Between 55-100F (13-37C) is the appropriate year-round range. Water-based coatings should never be exposed to freezing temperatures and in winter, should be delivered in heated trailers or boxcars.

Mixing and Dilution

As with the storage area, the locations of the mixing and dilution plants should conform to required ambient temperature ranges. If the materials are flammable, all equipment used in the process must be grounded and rated "explosion-proof."

Coatings supply containers must be opened slowly and carefully to ensure that any pressure buildup can be safely relieved. The contents should then be visually inspected for color and form to be certain the product is consistent with previous deliveries. While this IPC measure is subjective, it can provide useful insight in judging the results of later tests.

If the coating is a slurry, the IPC steps differ somewhat from those associated with paste or powder products. For slurries, the following steps apply:

* If the coating is to be used directly from the drum as supplied or only slightly diluted, the coating should be remixed thoroughly until homogenous from top to bottom, without "free liquid" at the surface. Mixing should be in a gentle rolling motion, minimizing splash and air entrainment.

* Before use or further dilution, coating control tests such as weight per gallon, baume density, viscosity, free liquid percent and solid percent must be conducted. All tests must be done consistently and with maximum reliability and reproducibility.

* Any further dilution should be done slowly and carefully, a little at a time, to minimize the possibility of overshooting the desired endpoint.

* All transfers of coatings must be carefully monitored and the correct equipment must be used. Before transferring only a part of the coating, it should be mixed with the rest and then, upon completion of the transfer, be tested separately.

If the coating is supplied as a paste or powder requiring extensive mixing/dilution at the foundry, the supplier's guidelines for handling must be followed exactly. Often, there is a required "aging" period to allow the coating's gel system to fully develop and stabilize before use.

When fully stabilized, generally the same IPC tests should be performed to assure compliance with in-plant specifications.

Whether slurry, paste or powder, once mixing and dilution are complete, the coating container must always be kept covered to minimize contamination by airborne foulants, evaporation losses and potential ignition of flammable vapors.

The coating should be mixed only as often as necessary, depending on the product's specifications. The IPC coatings tests should be repeated at least once per operational shift to be sure that application property ranges are maintained. Coating equipment and containers must be regularly cleaned, since coating solids and outside contaminants can build up in a vessel.

Application

Whether the coating is applied to the core or mold surface by brush, spray, swab, dip or overpour, the following IPC tests help ensure that the applied coating is uniform, of a proper wet thickness and free from drips or runs:

* Wet Thickness--a special "stepped" gauge measures the applied wet coating layer on the actual mold or core.

* Wet Coating Weight Pickup--best suited for smaller cores, the test compares the weights of the core immediately before and after coating application.

* Substrate Penetration--after coating, the core can be broken and the cross-section examined to determine the degree of penetration (wetting).

* Matte Time Test--indicates the coating's substrate wetting properties and a correlation to drying time by measuring the time it takes for the applied coating to lose its wet, glossy look and take on a matte finish.

In addition to testing, proper maintenance of application equipment is essential. If spraying, nozzles must be unobstructed and clean. If brushing or swabbing, the correct type of each must be used and regularly cleaned. When dipping, the coating vessel should be continuously and gently mixed.

Drying

It is critical that the applied coating layer be thoroughly dried before the mold or core is subjected to molten metal. Insufficient drying can lead to safety risks (boiling, expulsion of molten metal from mold, etc.) and casting defects such as burn-on, pinholing, layer-spalling and coating refractories inclusion.

Specific IPC tests that determine adequate drying include:

* Dried Coating Thickness--a dried coating layer is typically 50-80% of its wet thickness, the result of the removal of the liquid carrier. This change can be measured by several different tests.

* Percent Liquid Removal from the Coating--the applied coating has a known amount of carrier associated with it. When the percent of liquid content is determined, the degree of removal of this liquid upon drying can be monitored for an indication that the degree of drying is correct and in control. The percent of liquid removal from the coating (R) can be calculated from a specific equation.

With this test, it must be remembered that the value of "R" is only a guideline and subject to variation. Each foundry must establish its own acceptable range of values for "R" that will correlate to effective control of the drying process.

Again, it is important to supplement IPC testing with proper maintenance and calibration of the drying device. Depending on the method used (convection ovens, torch-drying, forced ambient air-convection), all pertinent mechanical and chemical variables must be monitored and adjusted accordingly.

Disposal/Return of Containers

Once a coating supply container is emptied, it must be discarded or returned to the supplier. As with other phases of the coatings handling and usage, certain IPC steps should be taken at this point, particularly in the case of reusable containers.

With disposable containers, all interior surfaces should be as clean as possible for several reasons. Since the material has already been paid for, leaving coatings inside is wasteful and costly. The foundry may wish to recycle the drums for a different use within the foundry or sell the vessels to an outside drum recycler, who would probably insist that they be free from coating residues.

If returnable containers are to be sent back to the supplier for refilling, cleaning is also important. While it can be argued that cleaning is the responsibility of the supplier, the astute foundry owner is quick to realize that any excessive cleaning done by the supplier will end up factored into the price of the coating. And again, coating left inside is wasted material, because it already has been purchased.

With the advice of the coating supplier, the foundry can recycle container "cleanout" batches in-plant to minimize waste and disposal problems. These recommendations should include storage container materials, allowable storage time and a procedure for reintroducing the recycled cleanout material into subsequent mixes.

Further Input

This brief view of key in-plant process control measures should allow the foundry to achieve the maximum performance benefit available from the proper use of core and mold coatings.

The points detailed are merely guidelines. Consistent with the concept of "total quality," foundry operating personnel, as well as their coating suppliers, can provide valuable input on ways to improve overall process control.