Perils of oil contamination: keeping a closer eye on how hydraulic oil enters your molding sands may be the answer to poor casting finish mysteries.

If you're experiencing poor casting finishes from your green sand molding systems, there may be something lurking in your sand. Leaving many clues behind, the source of the damage is elementary--it's hydraulic oil.

Green sand molding systems are the recipient of many undesirable materials. In many cases, problems associated with these materials have such a minor effect on final production that the cost of preventing or eliminating a particular contaminant doesn't justify corrective action. Yet in other cases, the effects of the contamination can be so severe that replacing all of the sand in the system may be necessary.

With the increased use of pressurized molding machines, one of the new problem contaminants of molding sand is hydraulic oil. Leaks in pressurized oil lines can occur, resulting in hydraulic oil contamination of the molding sand. In some foundries, molding sand is used to soak up spilled oil and is then added back into the sand system rather than being properly disposed. These kinds of situations result in sudden variations in the performance of the green sand.

The Clues

Two primary clues can tip foundrymen to the presence of hydraulic oil in green sand molding systems. As return sand enters the mulling area, it may have a light-grayish color and a shiny appearance. When contaminated molds are poured, excessive smoke is TABULAR DATA OMITTED usually present at pouring and shakeout.

In the muller, contaminated sand can have a waterproofing effect, resulting in a friable molding sand. In turn, the frequency of sticker-type defects on the mold pattern increases. This sand is more prone to developing friable mold edges that are easily destroyed when the mold is blown out.

At the stripping station, where the poured mold is removed from the flask or jacket, the mold cube is more likely to deteriorate and collapse. This results in downtime with loose sand or castings jamming up the conveying system because of premature mold breakdown. Castings poured with contaminated sand have unacceptable finishes due to increased gas-related defects and erosion of the green sand surface.

After the molds are poured, contamination in molding sand will be burned out to some degree. Generally, if the molds were not poured, the contaminated sand is diluted in the return sand system and has only a minimal effect on the second sand cycle.

Variations in Properties

A raw sand supplier investigated the effect of the sand properties when known amounts of hydraulic oil were added at different levels or concentrations (Table 1). Generally, both Dalton's in-house tests and this experiment show that hydraulic oil contamination increases moisture, density and LOI. Conversely, hydraulic oil contamination causes significant decreases in green compression, permeability, working bond and muller efficiency.

By observing the decrease in hot properties in these examples, premature mold breakdown is expected. Sand consultant George DiSylvestro explained this decrease in hot properties, writing that the presence of the oil makes molding sand feel and test wetter than it really is. The resultant lower moisture level causes lower dry and hot properties, which can lead to cuts, washes and inclusions. Scabbing may also occur, even at high active clay levels.

Reacting to the Problem

Monitoring hydraulic oil usage on a regular basis is recommended for identifying oil leak problems. An SPC histogram of oil usage can be an important variable to monitor to quickly pinpoint abnormal usage levels.

Here is a general checklist for controlling hydraulic oil contamination:

* watch for the indicative sand test results and excessive smoke at the pouring and shakeout stations;

* dilute the contaminated sand system with increased amounts of new sand;

* educate personnel in early detection and repairing hydraulic oil leaks;

* keep soaked cleanup sand from reentering the sand system;

* monitor your hydraulic oil inventory closely to determine normal usage rates, which helps pinpoint other potential problems at an early stage;

* conduct special tests such as infrared analysis for detecting contamination.

Redesigning portions of the system and establishing a preventive maintenance program will also help reduce leaks. Oil spillage can be better controlled by blowing a squeeze cylinder and purchasing more efficient O rings and hoses. By taking measures such as these, you can greatly reduce the damaging impact of hydraulic oil on the molding line performances.

Editor's Note: This research was presented at the International Green Sand Molding Conference, held March 5-6 in San Antonio, Texas.