Evaluating Permanent Mold Coating Removal Methods.

Detail the results of a survey of permanent mold foundries this article discusses the advantages and disadvantages of mold coating removal techniques

A foundry's choice of methods and materials to remove coating from its permanent molds is based on many factors, including production volume, mold cleaning frequency, casting complexity and environmental concerns. Since this choice has a direct effect on casting quality, including casting surface finish and dimensional accuracy, foundries often utilize multiple methods to determine which process or combination of processes works best for a specific job.

This article examines the results of an AFS Permanent Mold Practices Committee (2-E) survey to determine the most common coating removal methods used by aluminum permanent mold foundries in the U.S. and Canada. In addition to the results, the advantages and disadvantages of each method are discussed to provide a foundation for determining which method provides the most cost- and quality-effective way to clean molds.

Survey Sample

The survey asked permanent mold foundries to discuss:

* the material(s) they use to remove the coating from their permanent molds;

* which material is used the most;

* which material is used the least;

* their experiences with mold wear using their preferred material;

* if they used different materials for removing different coatings;

* if they used abrasive equipment such as grinding belts, cartridge rolls or hand pads for coating removal.

Sixty-three foundries responded to the survey. The results revealed that there are 12 materials used-silica sand (21 foundries reported), dry ice ([CO.sub.2]) (18), glass beads (15), non-silica sand (5), ultrasonics (3), aluminum chips (2), ceramic beads (1), plastic media (1), walnutshells (1), soda (1), crushed glass (1), steel shot (1) and a non-sand aggregate (1). Following is a look at each of these materials, including a specific look at the material's effect on mold wear.

Silica Sand

Silica sand is the material most used for the removal of permanent mold coatings. Foundries using silica sand included operations of all sizes--five had 25 or less employees and five had 250 or more.

Silica sand is an aggressive blast media that is inexpensive. Many of the 21 foundries using silica indicated either speed of coating removal or cost as their primary reason. Additionally, silica was favored based on historical perspective.

The sharp nature of sand grains and their aggressive cutting action have a distinct advantage in cleaning molds beyond the speed and cost. The sodium silicate used in most permanent mold coatings is tenacious and can leave a glaze on the mold that can inhibit adhesion of the next coating. Sand can clean the mold fully, while less aggressive media may not break that silicate glaze and fully remove the residue from the mold material. This aggressive cleaning also comes with a cost. Mold cleaning with silica sand can cause excessive mold wear and the breakdown of the silica has potential health risks for the workers (such as silicosis) and should not be used without engineering controls and proper ventilation.

Several of the foundries that reported using silica sand also cited the complete cleaning capability and the mold "texture" produced by sand blasting as essential for proper mold coating adhesion. One foundry uses dry ice for its primary removal, but still uses silica sand up to twice a month. Another foundry has replaced most of the silica with glass beads, but still uses sand on occasion to fully remove the sodium silicate. It reports that "none of the abrasives remove the hard water glass like silica sand. So we use it, but it can't be used all the time because it is too abrasive."

Mold Wear--Because silica sand is very aggressive, repeated use could cause excessive mold wear. Foundries in the survey that have changed from silica sand to less abrasive media report that excessive mold wear was one of the primary reasons for the change. One foundry stated, "We used to use silica sand and had severe wear problems. Over time the silica would remove material from around the grain boundaries of the mold material." However, finding an alternative, efficient media also is a problem. Said one foundry, "Mold wear is definitely a problem, but we have not yet found a more efficient way to get the job done." Another foundry reported that its non-silica (gamet) sand produces "severe wear on cast iron dies, (but) no real problem with H-13 (steel molds)."

The specific problems foundries experience with mold wear from silica sand include: "corner wear on die surfaces causing excess flashing," "dishing on pins, rounds the corners," and "wear in the areas of venting, ejector pins and hydraulics." The abrasive nature of sand grains also can create a wear problem when caught in moving mechanisms of the mold, such as one foundry, which stated its biggest problem is abrasive lodged behind the stripper pins. In addition, operator error plays a role in mold wear with silica sand, as one foundry stated "operators can cause premature wear if they blast too hard" and another encountered mold wear "when...our people would hold the nozzle too close to the mold."

Dry Ice

The second most popular material used by the reporting foundries was dry ice ([CO.sub.2]) blasting. Fifteen foundries reported it was their primary method to remove mold coating, and three others said it was one of several prime materials. A]though foundries of all sizes reported using dry ice, 87% employ more than 100 workers. The overwhelming reasons for using dry ice instead of silica for mold coating removal were reduced mold wear, ability to clean at the molding station, reduced environmental concerns and ease of cleanup (as dry ice evaporates).

One of the problems facing dry ice, as reported by several foundries, is that it isn't as aggressive a cleaning media and coating removal takes longer. However, as pointed out by several other foundries, although dry ice blasting is not the fastest method of mold cleaning, the reality is that, since they have started using it as their primary method, cleanup time has been greatly reduced and the molds are returned to production more quickly. In addition, said one foundry, "With the purchase of dry ice blast a couple of years ago, molds can be blasted and sprayed quickly in the casting machine while the molds are still hot."

Mold Wear--Ninety percent of the foundries using dry ice cited reduced mold wear as the primary reason for switching from silica sand. According to one foundry that still uses silica to clean hard-to-reach areas and ensure complete stripping of coatings, dry ice blasting has resulted in up to a 40% increase in die life.

Glass Beads

Fifteen foundries reported using glass beads as their primary coating removal material. Although foundries from all size categories reported using glass, 50% had fewer than 100 employees. The two primary reasons given for the use of glass beads were reduced tool wear and history.

Mold Wear--Wear is a problem with glass beads. The surveyed foundries recommended a maximum 40 psi air pressure with the glass beads for coating removal to reduce wear. One foundry stated that it now uses glass beads at 30 psi exclusively. Despite the pressure control, one foundry stated that "long term (glass bead blasting) is hard on the smaller, more precise areas of the tooling." In fact, another foundry recently switched to dry ice blasting from glass beads because "on our higher production molds, we've noticed a high degree of wear on the mold partings using glass beads. We have not seen any mold wear using [CO.sub.2]."

Ultrasonics

Three foundries reported using ultrasonics as their primary means of removing permanent mold coatings. Ultrasonic mold cleaning involves immersing the mold in a liquid bath and using sound as shock waves to break up the coating. The process involves four steps--a hot cleaning solution, a hot rinse, a cold rinse and a rust inhibitor.

Ultrasonic cleaning places equal energy on all mold surfaces regardless of mold geometry. As a result, the process excels at cleaning small detail and lettering that often require additional blasting during abrasive blast cleaning. A foundry stated "ultrasonic cleaning works especially well for cleaning out small crevices that would normally be extremely difficult to clean otherwise. Critical air vents as small as 0.4 mm, cooling channels, etc. are easily cleaned via ultrasonics." Based on the survey, foundries reported an increase in tooling life of 40% with a switch to the ultrasonic cleaning method from silica sand.

The spent mold coating, however, can accumulate in the tank and requires regular tank and chemical maintenance. This condition led a foundry using ultrasonics to begin removing the majority of the coating by a light steel shot blasting prior to the ultrasonic cleaning.

Mold Wear--The process doesn't have any measurable mold wear because there isn't an abrasive blasted at the mold.

Other Materials

Several other methods were mentioned by survey respondents:

* a foundry reported changing to soft steel shot in conjunction with ultrasonics and, with careful control, has seen no detriment to mold life and virtually no detectable dimensional or weight changes;"

* two foundries reported using wire brushes on die grinders as their primary method of removing coatings;

* a foundry praises olivine sand (70, 90, 120, and 180 grit) as it has experienced minimal wear on its molds as long as blasting "is done properly;"

* a foundry reported using aluminum chips as its primary material and says it has experienced minimal mold wear, but it says that pressure and distance from the mold surface in important.

In addition, 61% of the foundries reported using an abrasive operation other than blasting as part of their mold cleaning process. The most common abrasive products used were sandpaper, cartridge rolls, steel wool, wire brushes and abrasive hand pads. These products were used for spot cleaning mold sections while the mold is on-line and for removing aluminum that is soldered onto the mold that the blast cleaning did not remove.