Surface Finishing - Typical Surface Finishing Problems & How to Fix
Let's say that you don't have any lines in the workpiece, and you have a very high finish on it, but the thing looks like a piece of skin from a chrome plated orange! You have just been introduced to the biggest problem. This orange peel condition is almost always caused by over-stressing the steel during polishing, that is to say over-polishing.
Many years ago most of the high polishes were applied by hand lapping with wooden sticks and felts. The danger of over stressing the steel was negligible. Today no shop can afford the time it takes to hand lap and so we must go to mechanical polishing. While using polishing machines it is very easy to apply pressure and too much speed and over stress the surface of the steel.
When a polisher comes up with a bad case of orange peel, one of the first reactions is to blame the steel. If he has some pitting also this reinforces the suspicion of bad steel. As a result of this, the steel companies have done a lot of research on the subject. From the results of some research and from actual experience with various mould steels, the following information about the causes of orange peel has been put together.
Almost any work you do to a block of steel stresses it. Conventional machining puts deep stresses in it. EDM burning puts stresses in it, grinding stresses it. Carbide and ceramic machining can put enormous stresses in it. Under actual working conditions that block is hardly ever stress relieved before you get it for polishing so you start out under a handicap.
The surface of each type of mould steel has a certain yield point. This surface moves like plastic under your polishing tools. If you do not move it beyond its yield point it will go back to its original position. If you stress it beyond the yield point, the steel will remain in that position and you have produced what amounts to little ripples on the surface. If you continue to work the steel you will tear the tops off these ripples and produce pits. Even if you do not see pits when you finish polishing they may appear while the mould is running, because of the stresses involved there and they will surely show up in repolishing after production. Of course, if the orange peel is not too serious, the pitting may not occur.
Now in actual production, it is almost impossible to mechanically polish a mould and not get some orange peel. Remember that it shows up worse when you get in a hurry and start using too much pressure and too much speed on your polishing tools. You will notice as you polish that the harder the steel you are polishing, the less chances are that it will orange peel. The harder the steel, the higher the yield point and the more polishing pressure it can take before rippling and tearing.
Mould steel with a Rockwell in the low thirties will not take as much polishing abuse as a steel with a Rockwell in the middle fifties. When polishing soft steel one must keep a close watch on ones brushes and felts to make sure the speed and pressure is kept down. The polisher must also keep close watch on the surface and move on to the next step as soon as possible so that the rotary traffic is kept to a minimum.
When stainless steels started gaining popularity in the mould industry, the steel companies had a lot of complaints about the steel pitting during polishing. The customers thought they were getting bad steel. It invariably turned out to be caused by over-polishing. Stainless steel is very sensitive to polishing pressures. The surface moves and pits easily. If real care is taken, however, both will polish up quickly and beautifully.
Plucking of minute particles from the surface, causing cavaties and 'comet tails'
Uneven flatness of between adjacent metals sometimes called 'releif'
Surface flow or smearing of metals leading to indistinct boundaries of adjacent metals
Another cause of orange peel
Another cause of orange peel that should be mentioned is over-heating during heat treating. The information that Crucible Steel Company puts out says that steel that is overheated in heat treating will retain some austenite in its structure instead of converting all to martensite. Austenite is softer than martensite so you have a surface that has hard and soft areas, ready-made to ripple or orange peel.
I think this explains why once in a while, as a polisher, you come across a piece of steel that will orange peel no matter how careful you are in polishing. Sometimes you can see it forming even in the stoning of the block. Apparently the soft areas wear away quicker under the stone and the ripples start to show early. You can hand-work this block and maybe salvage but, it will never be very good.
What to do about orange peel
There are a couple of things you can do to try and remedy the situation. You can go back to the 600 stone and work out the distortion. Just a superficial stoning will not do it. You will have to put enough time into the stoning to really get below the ripples. Then re-polish it, being more careful this time about pressure, speed and the actual amount of traffic put on the block. You may improve the surface, but you will hardly ever eliminate it. Sometimes it gets worse. The best thing you can do if you are really serious about eliminating the orange peel is to carefully re-stone it with the 600 stone, making very sure that no pits get overlooked, then stress relieve the steel. This should be done at about 100oF. below the tempering temperature of the steel so that the block does not anneal.
Then re-stone the block with the 600 grit stone, remove these lines with 600 wet paper, and polish with diamond compound as described earlier, this time being more careful to keep polishing pressure to a minimum. Do not skip any steps, and be careful not to spend any more time than necessary in each step, thereby reducing the overall amount of polishing stress on the block.
Surface Finish Chart
|Rt (μm)||Rz (μm)||CLA Ra (μm)||CLA Ra (μ”)||RMS||RMS|
Note: The equivalent values are only an approximation. Depending on the characteristics of the surface finish, there can be an error of about 25%