Establish Mold, Core Coating Control Points to Ensure quality.

This common-sense approach to quality control covers the best process monitoring points and most important considerations in a mold and core coating operation.

Because coatings almost always improve the quality of the casting surface and typically reduce the cost of making a quality casting, they make good economic sense. However, they must be selected, prepared and used properly to gain the best advantage.

The following information excerpted from the AFS Mold & Core Coatings Manual, 2nd Edition, covers quality control of your coating and application system to ensure defect-free castings and production benefits.

PROCESS CONTROL PLAN

To successfully control the coating application process, a foundry must develop a coating control plan that identifies the coating application process, the cores or molds to be coated, the coating type, and the operating range and aim points for the material. If the coating must be reduced or undergo additional processing before use, this information should be listed. The procedure also should identify special equipment needed and the operating specifications for each.

Work instructions should be established for pertinent application procedures such as dip immersion time, coating pump pressure and other critical process parameters. The use of photographs on the work instructions is particularly helpful.

If the coating process requires more than one step, each should be thoroughly identified to establish a clear repeatable process. Drying time and temperature ranges also must be addressed. Once a plan is established, the process must be monitored.

SELECTING CONTROL POINTS

The 10 best coating control points are manufacturing, delivery, receiving, storing, mixing, distribution, application, coated core/mold storage, closing and pouring and casting inspection.

Activities at selected control points may be classified as routine or audit activities. Routine activities are observations made with respect to time or units. Audit activities are carried out to spot-check the process, or as needed to answer a particular question, usually for problem-solving activities.

1. Manufacturing

This control point exists at the vendor manufacturing plant (or in the foundry, if it produces its own coating). To produce an acceptable product, ingredients must be added in the right amount in the proper sequence and be blended in a controlled, uniform method. Inspection tests at this stage are used primarily to ascertain that the coating has been produced as it was designed.

Typical testing includes an evaluation of the solids content of the final product, an assessment of the rheology as produced and specialized testing to evaluate unique characteristics of a given product such as settling or foaming. Also, an evaluation of the product in the diluted state is usually completed to assess the material at its designed operating range.

Typically, a solids test is completed to ascertain that the appropriate amounts of carrier and solids were used in preparing the batch. Since coating is normally purchased by the pound, the foundry is interested in tracking the amount of carrier present in the product as-shipped.

When determining percent solids, the foundry must verify the final weight to ensure that the test sample is completely dry. Also, the correct drying temperature must be maintained to ensure that the carrier is driven off and not other liquid additives present in the coating.

If the coating is manufactured in a premix form, several other tests may be used to ensure that the product has been properly manufactured, including:

* viscosity (single point or multiple point);

* pH;

* liquid density (weight per gallon);

* temperature.

The coating supplier should be willing to provide you with a summary of the results of these inspection tests in the form of a certification letter. In some cases, a statistical summary of the supplier's process performance also should be considered.

2. Delivery

Shipments sometimes are ruined during transport to the foundry. Delivery quality should be monitored initially and until the technical requirements can be met. Typical problems include container breakage, product freezing, prolonged warehouse storage, outdoor storage and loss of relevant paperwork. Special shipping requirements such as "do not freeze" should be identified. The transporter's responsibility should be specified and contractually controlled.

3. Receiving Inspection

If a bad product is accepted and used, then unnecessary problems and increased costs will be incurred. A primary function of receiving inspection is to ensure that the material has not been contaminated or altered during shipment or storage. Typical problems encountered include freezing of the material or exposure to excessive temperature.

Depending on the volume of material used, this control point may be handled as a receiving-inspection function. Other facilities that process a smaller volume of material may combine the pre-inspection with on-site mixing and preparation.

The following activities typically constitute receiving inspection:

* verify label requirement compliance;

* secure supplier quality assurance certification sheets and send them to the quality-control laboratory or department;

* collect and mark representative samples with remarks covering any unusual conditions noticed while collecting samples;

* place the shipment on a "hold" status until quality control has reported that the shipment is in specification, then release for foundry use.

4. Storing

A supply system will ensure that the coating is used in a timely manner. Most coatings do have a specific shelf life. A first-in, first-out (FIFO) system is recommended whenever possible. If coatings contain toxic, flammable or otherwise hazardous ingredients, the storage area should be safety-monitored. Coatings should be stored indoors, avoiding exposure to extreme heat or cold.

5. Mixing

Most coatings are designed to be used within a specific solids range. If the material is not diluted to the proper level and/or mixed properly, it will not perform as designed. Viscosity can be drastically altered if the mixture is not properly prepared. Also, it is important that the material be homogeneous and free of entrapped air. These requirements are critical, especially for operations that prepare material from paste or powder bases.

Additional attention must be given to the handling and storage of the material after mixing is completed. Coatings will tend to settle if left unagitated for long periods of time, making them unfit for use. Testing and quality-control functions used at this point will ensure that the material has been properly rehomogenized and adjusted for use.

Before beginning the preparation of any coating material, the mixing equipment must be checked. Routine maintenance of the mixers and blades will ensure that the properties of the coating are developed to the desired level. The mixing equipment also should be kept clean and free of foreign materials that could adversely affect the performance of the coating. In addition, the equipment occasionally should be treated with a disinfectant to eliminate the potential for biological contamination of the coating.

Mixing, preparation and production testing are covered in detail in the book.

6. Distribution

While many quality-control procedures performed at the coating mixing station are similar for large or small foundries, distribution system controls can be different for the many types of distribution systems possible. On one end of the scale, individual l-gal buckets may be taken from the mixing tank for use by individuals. On the other end, a sophisticated system of pipes and pumps may be used to distribute coating to application tanks in the foundry. Many problems can occur during the distribution process. For example:

* evaporation of solvent from open pails or tanks;

* thickening or thinning of the coating by pumps;

* contamination from debris or recycled coating;

* additional dilution, either deliberately or unknowingly.

Quality control should determine that effective guidelines for coating distribution exist and are being followed.

7. Application

For any coating program to be successful, it is essential that the appropriate amount of coating be applied to the chosen core or mold at the right time. This application must be successful to allow the production of a high-quality, defect-free casting.

Each of the various application methods available has advantages and disadvantages. (Application concerns are covered in detail in the book.)

Typically, the coating evaluation completed at this stage should include a visual inspection of the coated cores. This inspection will indicate whether the coating application is consistent and whether acceptable surface coverage has been obtained. The coated core should exhibit a smooth, even surface without evidence of runs, drips or excessive puddling. A core should be broken open periodically to check the depth of penetration into the core.

Besides the visual examination, another test that is frequently performed on a wet core is an estimate of the wet film thickness. These tests can be used as audit tools to verify that the expected coating thickness is being achieved for a given area, or several areas, on a core or mold surface. (All core coating evaluation tests are covered in detail in the book.)

Following actual application of the coating, the drying process must be monitored. Whether the cores are air-dried, lit off or oven-dried, it is essential to verify that all the solvent has been eliminated from the coating before the cores are placed into the mold.

For oven drying, the oven temperature and airflow rates should be monitored and recorded. Periodically, a finished core may be weighed, placed into a small oven and dried further. The core is reweighed to verify that all solvent has been removed in the drying operation. A similar approach can be taken for air-dried cores by setting selected cores aside and rechecking their weight after a period of time.

When evaluating dried cores, the most effective test is the dry film thickness test. This evaluation physically measures the dry thickness above the sand surface. For a more thorough evaluation of the entire coating, a core should be sectioned and examined under a microscope. This will provide information on the amount of dry coating above the sand surface and the penetration of coating material into the core. This gives a more representative view of the effectiveness of the coating application process.

8. Coated Core, Mold Storage

The desired effects of all the previous control efforts can be negated by inappropriate storage methods. Examples of frequently encountered problems are:

* storage in damp areas, where moisture can be picked up. Significant reductions in surface strength of coated sand surfaces also can occur with many binder systems;

* storage of coated cores in cold areas, then setting them in warm molds, resulting in condensation on the coated cores;

* rough handling while in storage, with subsequent damage to the coating layer.

9. Closing and Pouring

Improper closing and pouring can produce apparent coating-related defects that cannot be solved by coating-control activities. The resulting confusion can cause wasted time. Examples of some problems are:

* rubbing coating off of cores to obtain a fit, which can lead to metal penetration. Never break the skin of a coating;

* closing of molds well in advance of pouring, especially in green sand applications;

* setting cold cores in a hot mold, which can result in condensation and a likelihood of gas, erosion or spalling defects;

* pouring too fast or too hot.

10. Casting Inspection

Routine inspection of castings for coating-related defects must be done if the control program is to pay off. Many times, as a portion of the coating system is drifting out of control, coating-related defects start occurring to a minor degree. Routine inspection of the castings at shakeout will frequently reveal these minor breakdowns, and corrective actions can be employed before the problem gets out of hand and becomes costly.

Inspection further down the line, such as after shotblasting, may fail to reveal the developing problem. For example, crazing of the coating can sometimes be seen on the adhering coating before the casting is shotblasted. This may not be a problem at this stage, but the defect may worsen with burn-on/burn-in metal penetration, and result in spalling and inclusions.

Quality Control Structure

Suggested guidelines for establishing a quality control program are as follows:

1. Call in your supplier when the coating is performing well.

2. Walk through your process together, noting how the coating is used.

3. Together, select methods and control points for making various measurements. Select as many points as possible in terms of test equipment available. Control of quality cost comes later. Estimate the performance details and the test values expected from the coating. Be sure to write down any qualitative aspects. Supplier and user-should each have a copy to file for future reference.

4. Select the qualitative and routine quantitative tests to be performed. Set up a data sheet and test procedures. Begin running the tests and collecting the records in one book.

5. Select one nonroutine test and collect background information. When the background is understood and the data is seen to fit an expected pattern, select another test and repeat the data collection. Throughout this process, be sure the general coating performance and casting results are in-control and are acceptable. Discard any data associated with poor casting performance. Continue this process until the coating process parameters for success are fully defined.

6. Set up records to enable you to trace back any unacceptable casting results to a batch of coating, if possible. However, castings could be poor for some reason other than coating; be careful of this.

7. Establish and require a periodic report system prepared for management.

When problem performance is indicated, review the data for likely causes or shifts in properties. Involve the supplier as much as possible. Measure and observe suspect areas and compare to previous results.

Examples of areas to be covered in advance of problems are:

* Who pays for bad product?

* What defines bad product?

* How is bad material to be disposed of? By whom?

* If a coated core or mold is defective, who decides if it will be used? By what criteria?

* If poorly coated cores or molds are to be poured, who will follow them through? Who will pay for scrap castings generated?

These and similar decisions should be made quickly and handled by the group or department responsible for the coating quality control program.