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Examining cleaning room requirements, part 2: abrasives.

Examining Cleaning Room Requirements, Part 2: Abrasives

This series is intended to provide information to permit examination of the foundry's cleaning room requirements. This month's column focuses on abrasives.

Looking at the steel shot used to clean castings, the first consideration is size and type of abrasive. A blast machine cleans best when there is a range of abrasive sizes.

Shot cleans by impact, like tiny ballpeen hammers. It breaks up heat-treat, forging and other scales and wears away the sand from castings. The larger particles are used to impact the surface of the work, loosening sand and other contaminants. They provide the "big hit" and greatly speed up the cleaning process. The smaller particles clean small irregularities in the work and provide additional coverage.

However, the smallest size possible that will clean the work should always be used. Small shot is denser than larger shot and less likely to be hollow. Small shot is needed to completely scour the surface, removing loosened components so that the work is thoroughly and uniformly clean.

Because it is the impact of each particle upon the work that actually cleans the work, it is important to realize that each pound of the smaller sizes gives many more impacts per pound than do the larger size abrasives. Reducing the abrasive diameter size by one-half, for example, increases the number of particles per pound by a multiple of eight.

Steel grit also is used in blast cleaning. Grit cleans by cutting, like tiny chisels. It undercuts the surface to provide "peaks and valleys" or a "tooth" for proper bonding of applied finishes. It is recommended that steel grit be used only as a "last resort."

The size of the abrasive in the machine should be adjusted according to the type of contaminants to be removed. Since there is always a range of contaminants, from tightly bonded to easy-to-remove materials, there should always be a range of sizes of abrasive to do the job.

Abrasive Operating Mix

The objective is to maintain a well balanced operating mix of various sizes in the machine for the following reasons: * consistency of finish on parts to be cleaned; * uniform abrasive coverage of the work; * conditioning of the abrasive for optimum cleaning; * lowest abrasive and machine-part wear and minimum downtime for maintenance.

Too coarse or too fine an operating mix will lengthen cleaning cycles. Also, sand retained in the shot hopper should be kept to a minimum as it increases wear and results in dirty, dusty castings.

Maintaining a consistent operating mix requires some diligence, but it pays off in terms of shorter cleaning cycles, longer life of equipment component parts and better utilization of the shot.

Also, the centrifugal wheel was targeted using the recommended operating mix. If the relative coarseness or fineness of the abrasive is allowed to change, so will the targeting. An improperly targeted wheel will not direct the abrasive at the work being cleaned, but rather will impact the wheel box liner and cabinet liners.

To maintain the operating mix, frequent small additions of shot to the machine should be made, either manually or through an automatic abrasive replenisher. In fact, a daily addition of the nominal size abrasive is recommended.

To make maintaining an operating mix simpler, a continuous type dust collector should be connected to the airwash separator to avoid the varying air flows produced by intermittent or shaker type collectors. A small continuous collector that would be dedicated to the airwash separator can be added to an existing machine.

Also, a magnet should be used to separate the sand from the fine metallics found in the pan of the screen stack. This will determine the amount of sand retained in the storage hopper. Remember, 5% sand mixed with the shot will double the wear to the machine component parts.

Screen Test

The abrasive should be screened at regular intervals. Screen kits are available to measure the exact range of sizes in the mix. This will indicate if more or less frequent replenishments are required to maintain the operating mix.

The test also indicates whether adjustments are required in the abrasive separator blast gate to the ventilating system, or to the metering system in the automatic abrasive replenisher. This important adjustment affects the efficiency of the blast machine, the degree and speed of cleaning, wear on the abrasive and machine parts and overall cost of operation.

Test samples should be taken from the abrasive storage bin. Take samples at three locations across the bin and blend before screening. If it is not possible to get a sample from the storage bin, take a sample from the abrasive curtain in the separator.

To check the fine refuse, catch the samples as it comes out of the fine refuse pipe. Never take a sample from the fine refuse receptacle, since this abrasive could be contaminated. Also, the dry dust collector can be checked for good abrasive by sampling the dust.

Thomas A. Briere Pangborn Corp Hagerstown, MD
COPYRIGHT 1989 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Title Annotation:Cast Facts
Author:Briere, Thomas A.
Publication:Modern Casting
Date:Aug 1, 1989
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