polishing process for aluminum and brass casting

Polishing or buffing is a value added service offered by zenith aluminum casting. Common polishing or buffing is used as a pretreatment for following operation such as painting or electroplating. Besides, special polishing such as mirror polishing is also used as a decoration means. Usually, after mirror polishing, the aluminum casting parts will be slightly lacquered to protect the shining surface appearance.

However, not all aluminum casting parts will apply for mirror polishing. Like aluminum sand casting part, because of its rough surface quality, even common polishing will become costly and bad function. General speaking, some decorative parts by aluminum die casting is likely to apply for mirror polishing, such as furniture parts, showing room products, or automobile handgrips.

Mirror polishing parts by zenith aluminum casting

Polishing process for aluminum casting parts by zenith aluminum casting

Degreasing and cleaning:

Before you start any polishing and/or buffing operation, you should be aware of a few things. All parts must be free of dirt, grease, oil, paint, rust and plating of any type. Soap and water or solvents remove dirt, grease and oil. Other finishes, such as paint, rust and plating need to be chemically stripped.

Polishing with buffing wheels and compound

After the aluminum casting parts are degreased and cleaned, the casting parts will be polished by hand on buffing wheel, which made from fine texture or wool. While polishing or buffing, some polishing oil is applied to increase the brightness.

Attention for Polishing as pretreatment for electroplating

Using buffing wheels and compound to bring the high luster back to currently plated parts is discouraged. If you were to buff a pair of chromium-plated aluminum die casting covers, you would actually be removing a portion of the chromium plating. This will naturally brighten the surface. However, the plating will also become thinner. More often than not, the chromium plating is penetrated and the nickel plating under the chromium appears. At this point, the pre-plated die casting part will most probably have to be stripped, polished, buffed and replated.

Basic buffing for aluminum casting parts

After parts have been thoroughly cleaned and/or stripped, you are ready to proceed. The buffer motor should be equipped with a wheel guard while in use. This will prevent you from trying to catch any flying debris with your teeth or body.

The rotation of the wheel should be down and away from the body when standing in front of the machine. This will prevent parts from being thrown back at your body.

You will find it helpful to slip a collar or spacer on the motor shaft before mounting your polishing and/or buffing wheels. This is to position the wheels as far away from the motor as possible and give you more clearance for working.

When applying compound, use small amounts often as opposed to a large amount at one time. One or two short jabs with the compound for a few seconds is all that is needed.

Buffing compounds, in the form of solid bars, melt by frictional heat when applied to the revolving wheel. When applying the compound, hold the part under the wheel to catch the excess, rather than wasting it on the floor. This extra compound will increase the cutting action.

You don't want to load the part with melted compound, only the excess from normal applications. Also, use some type of firm cushion to stand on. This lessens the strain on your feet, legs and back. No reason to make things harder than they have to be.

The contact point should be approximately 25 to 35 degrees below the center of the polishing or buffing wheel. The contact point is the area where the part touches the polishing or buffing wheel. This particular contact point is suggested until you become more familiar with the pulling action of different wheels.

Except for new buffs, all buffing wheels should be raked out before each use. To save a little expense compared to the cost of the industrial buffing rakes, simply use a grease-free long handled wire brush. This will provide a finer burnishing effect on the face of the wheel as well as saving money.

When the buffing wheel stands idle for a time, the compound will reharden on the surface. This does not mean that if you stop buffing for five minutes or so you have to rake the wheel again. Check to see if the compound has rehardened and rake if necessary. If hardened compound is not raked out, it may actually scratch the surface of the aluminum casting parts when first starting to buff. The compound may also be mixed with small metal particles from previous use that could create even deeper scratches in the aluminum casting parts. To prevent compound buildup on buffing wheels, especially spiral buffs, they should be periodically raked during use.

There are a few precautions that should be pointed out to ensure safe operation while buffing. When buffing near holes or recessed areas, approach them from the top side. This will prevent the wheel from catching the edge or wall of the area due to the rotation of the wheel. In most cases, the part is simply turned around to reach the other half.

Keep a firm grasp on the part at all times. Keep your eyes on the contact point. Always have one hand near the contact point for better control.

When buffing especially softer aluminum castings, you may find that you are dragging the pits or scratches. This means that while buffing, you are actually creating small comet like tails from the pits and scratches. This causes the metal to lift upward from the surface, forming a bump. This is usually caused by excessive friction heat. This area will have to be polished smooth before continuing. Buffing is then repeated with less pressure and compound, creating less frictional heat that will not distort the area. This process may require a great deal of time to avoid reoccurrence.

Another problem that occurs occasionally is discoloration in the aluminum casting parts. This may be caused by the hardness of the aluminum part itself.

For example, a valve cover having what appears to be stains on the side may be a hard or soft spot in the metal, giving it a different color than the rest of the part. In most cases, this discoloration cannot be corrected because the varying degree of hardness extends through the full thickness of the aluminum.

When you see a cast aluminum intake manifold highly polished and buffed to a mirror finish sitting next to a billet aluminum water pump pulley, you will notice a definite difference between the two. The billet aluminum will be brighter, smoother, have better color and have more depth than the cast aluminum. Once again, this is due to the metal itself, not lack of experience of the person polishing.

Be sure to view the aluminum casting part being buffed from many different angles. This will help you locate any imperfections that may have been hidden by shadows.

When most imperfections have been removed from an area, make the last few buffing strokes downward using less pressure against the wheel. This will clear the excess compound from the part for viewing.

For example, while buffing, you are moving the part up and down against the wheel with continuous contact. What you want to do is lift the part away from the wheel each time you complete a downward stroke. This motion only applies when you want to clear your work for viewing or when you are nearly finished buffing with that wheel and compound. If after doing the downward strokes you still have a buildup of compounds, use a clean rag to remove the excess compound before color buffing.

The buffing procedure is complete when as many of the imperfections have been removed as the combination of buffing wheels and compounds will provide. When imperfections have been removed, we proceed to color buffing to bring out the high luster of the aluminum casting parts.

Buffing will not remove deep imperfections. Therefore, some type of polishing will be required if the buffing procedure did not remove them to the customer's satisfaction. A mirror finish may be obtained without removing all imperfections, provided they are not too deep. However, the compound will load up in the pits and when buffed to a mirror finish stand out.

Coloring. Although most coloring compounds are drier, they still have a tendency to load up on the buffing wheel face. This buildup may have small aluminum particles mixed in with the compound from previous use that can scratch the surface. Make sure you rake your color buff before each use and periodically during the color buffing procedure.

All buffing procedures should be performed with parts at room temperature. This is because the compounds are melted by frictional heat. If the aluminum casting part is cold, the compound will not work properly. The cold temperature of the aluminum casting part will cause the compound to immediately reharden on the surface.

The color buffing procedure is complete when you have achieved the desired finish. If you are unable to achieve what you feel to be a clear mirror finish, the following may be of assistance.

Be sure you have not mixed any other buffing compound on your color buffing wheel. Rake the buffing wheel out and rebuff the area using less compound. Make sure the last few buffing strokes are downward, using less pressure against the wheel.

If you have small scattered black spots on an area, this usually indicates the need for more compound. Using too much compound will cause a heavy buildup that is difficult to remove.

When working under fluorescent lights, you are more apt to see a light haze or fine scratch line patter on the aluminum casting part. This light haze will not be seen under normal conditions. If you want to make sure this haze or line pattern is from the last buffing operation, do the following test.

Rebuff an area of the part in a slightly different direction to see if this removes the pattern, and now shows it in the new direction. If so, this is your final finish. Or, if the line pattern is still preset, it is from a previous procedure. You will have to back up each step until you have removed the line pattern.

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