Plastic Injection Mould Polishing Techniques - Technical article

The purpose of this article is to lay out a simple and foolproof method of polishing mould steel. With a few variations, it will work on any kind of steel and can be done by anyone.

The emphasis will be mainly on bringing up an adequate finish in the least possible amount of time.

We will also be concerned about doing this using the least costly and simplest number of tools and supplies.

There is a great deal of equipment available today to assist the mould polisher, but we will keep it as simple as possible. We will be concerned with bringing up an adequate finish in a commercially competitive time frame with basic equipment.

The Mechanical Equipment

The first thing you will need is a flexible shaft power unit such as the KFD or NSK Powered kit with foot control for accurate speed control.

You will need a reciprocating unit such as the Diprofil for lapping slots and stoning areas where only a very short stroke is possible. This can be fitted to the KFD.

You will need handpieces for the KFD. The first is a straight handpiece with collets. This is the most often used.

A telescoping right-angle head (the VTN) is also essential. It reaches places that cannot be reached with the straight heads and is very helpful in reducing operator fatigue.

There is also available a right-angle attachment (VTN-S) which accepts 1/4" collets.

A Diprofil Mini Filing Handpiece is available for very short stroke work, there are two versions, one has 0.5mm stroke, the other 1.8mm.

NOTE:

There are maximum speed restrictions on all Reciprocating Hand Tools:-

10,000 RPM for Mini Diprofil
15,000 RPM for Standard Diprofil

NSK Powered Hand Tools

NSK Powered Hand Tools are suitable for grinding, smoothing, polishing and deburring. A lightweight and compact motor with speed range of 1000-50,000 rpm with a wide range of attachments means that moulds can be polished quickly.

3 models are available:-

NSK Electer – speed range from 1000 to 35,000 rpm
NSK Espert 500 Torque System – speed range from 1000 to 50,000 rpm
NSK Sheenus Ultrasonic Polisher – ultrasonic polisher, with output of 45W and oscillation frequency of 24 KHz

nsk emax

Brushes and felts

Now you will need brushes and felts for the rotary tools. Cup brushes of varying shapes and sizes are available. The hard bristle are for better cutting. They are also made in soft bristle and nylon bristle and these also have their uses.

End brushes in 3/16" and 3/8" widths. Wheel brushes of various diameters and widths are needed for slots and radius areas.

You will need Felt Bobs with 1/8" shanks. Cylinder Bobs in 1/4" and 1/2" diameter and Felt Rectangulars for hand rubbing.

You will need some 600 or 1200 grit wet-or-dry. It is important that you get the best, as this and the use of stones are the most tedious parts of the whole polishing operation.

The Diamond Compound needed for bringing up a mirror finish are just three, Blue 1 Micron KD-C3, Green 3 Micron KD-C3 and Yellow 6 Micron KD-C3. Other grades of Diamond have their uses also, but the above three are all that are necessary for basic work.

A good bench light or headlamp with magnifier is important. Do not use a fluorescent lamp on the bench. They do not throw shadows and shadows are necessary to make the lines and swirl marks show up. The best light has a couple of arms and some springs. It moves easily and stays where you put it.

Stones

Gesswein Stones

The last of the essentials listed are the first thing that you will use and these are the stones.

There are two general types of grit used, silicon carbide used in the softer bond mouldmaker stones, and aluminium oxide used in the hard bond diemakers stones.

Grits range from a coarse 100 mesh to a fine 1200 mesh.

The Gesswein standard range supplied by Kemet uses a four number identification system for all their six inch stones. The first number identifies the family; mouldmaker stone, diemaker stone, EDM stone, etc. The second number is the grit, and the third and fourth number together identify the shape of the stone. Thus, the stone numbered 1601 means mouldmaker stone (silicon carbide grit) 600 grit, 1/4" X 1/8" shape. The 2401 stone means diemaker stone (aluminium oxide), 400 grit, 1/4" X 1/8". Number 6304 means EDM stone (aluminium oxide), 320 grit, 1/2" X 1/4" shape and so on. This is the simplest and best system that anyone has come up with so far. It is easy to learn and soon not only the polishers but all the tool-makers will know what stone number to reach for when a job comes up. It can mean a real saving in time and frustration.

You will need four covered containers in which to keep your stones. The different grits should be kept separate and covered. You will need containers for the 180, 300, 400 and 600 perhaps an extra one or two for finer grit stones. Many polishers use bread pans for this. We would fashion covers and fit the bottoms with coarse screen so that the stones do not sit in the sludge. It took a lot of time and work. It is now possible to buy containers that have been specifically moulded for holding stones. They have good covers, a moulded-in sludge grid so they are easy to clean, and are securely stackable.

You will need a stone lubricant in the containers to keep the stones wet. Kerosene or even EDM electrolyte can be used in a pinch but not for very long. It can cause serious skin problems. Use a good stoning oil. It evaporates slower, it is almost odourless, and is very easy on the skin. If you become a professional polisher and have your hands wet a large part of the time, you should consider using a protective cream that is formulated for use with petroleum products. One should also buy several "double ender" stone holders. Keep your stone containers covered when not in use and never mix grits.

gesswein stones

Xebec Stones

Kemet offer 3 types of Xebec stones for mould polishing:

a) The standard range of Xebec Ceramic Meister Finish stones feature Alumina Fibres very accurately graded. The abrasive is enclosed in a resin matrix for high strength. They are used by hand or with Power and Ultrasonic Polishers.

b) The Xebec Ceramic Heat-Resistant Range of stones offer resistance up to temperatures of 200°C. Having the same performance as the MEISTER range of ceramic stones, the are most suitable for removing EDM scales. The Heat-Resistant range has been developed for prolonged use with Ultrasonic Polishing Machines.

c) Xebec Diamond stones have diamonds embedded in layers enabling them to be used to the end without losing sharpness, and as the abrasive is granular these diamonds stones cut on the side as well as the tip. Suitable for hard materials such as hardened steels, Tungsten Carbide and Ceramics.

Stoning

Let's say that we have a flat 2" by 4" block of mould steel to polish to a mirror finish. We will talk about slots and holes and other details later. The simple, flat surface is all we need to learn the few steps necessary. We will say that this surface has been burned, because most of the polishing you will be doing will be done from an EDM finish. Assume also that it is a reasonably good burn, then we can start stoning with a #6302 Gesswein stone. If the EDM burn has been done with too much amperage and is deeply pitted, we would start with a 180 grit, #6182, to break through the surface crust.

Step 1. If the burn is fairly decent, start with the 320 grit 6302, in a double end stone holder. Stone in one direction until all of the colour from the burn is gone. You will still have a lot of pits left but all of the super hard crust left by the EDM machine is gone.

Step 2. Stone the pits out with a 5302 stone. This stone is the same 320 grit as the 6302, but the bond is a little softer and it breaks down and exposes sharp grit better. It is not necessary to cross-stone in this step because both stones are the same grit. Keep the stone wet and allow a slurry to build up on the surface. Rub with this stone until all the pits are gone. Wipe your work clean when changing grit.

Step 3. Cross-stone with a 2602 that is, rub at a 90° angle from your previous stone marks until all those stone marks are gone. This is quite a jump in grit size from the 320 but the 2 bond cuts pretty well. It's better to use a few extra strokes with this stone than to use a 400 grit stone in between.

Step 4. Cross-stone the 2602 with a 1602 mouldmakers stone. Do not use heavy stoning pressure in this step and keep your stone very wet. This will keep the stone from loading and make the next step easier.

Step 5. Cross the 1602 stone marks with 600 grit wet paper. This is the most tedious part of the whole process.

Here are some things you can do to make it a little easier. Cut some 1/4" Perspex or other bendable plastic into strips about an inch wide and six inches long. Drill a hole on one end so that it will accept the spindle of a 3/4" felt bob. Heat the perspex a little behind the holes and bend it up a few degrees so your knuckles will not scrape the workpiece. Make three or four of these with different degrees of bend in them for getting into deeper cavities, etc.

Cut the 600 or 1200 grit paper into squares of about 1-1/2", fold twice and use them under the felt bob. Turn the paper frequently, each piece gives four surfaces, keep it wet and rub until the stone marks are gone. (Some other time you can try using a 800 or 900 grit stone in between the #1602 and the paper but this time just do it this way). When you have finished this step the hard work is all behind you. From here on you can let the machine and the diamond compound do the work.

A process that is worth trying once you have some experience is a 600grit OT stone followed by 1200 Sic wet or dry then 3mu diamond this method can be used on fine sparking without resulting to coarser stones.

Xebec Stones

Xebec stones are good for fine detail work and excellent with ultrasonic and reciprocating heads.

Cleanliness

Before we get into the diamond grits and how to apply them there is something that can't be stressed too much and that is cleanliness. From this point on everything must be kept scrupulously clean and free from foreign grit. The first thing you should do is to remove the shop coat you have been wearing while stoning, grinding your stones, resting your elbows and sleeves on not-so-clean surfaces, etc. Whatever you wear, be sure that it is clean. Clean hands are important also.

Try to do your polishing as far from grinders and people blowing compressed air as possible. Never put a brush or a felt down on anything but clean tissue or back in its own container. Toilet tissue works well for cleaning and wiping the part as you go along. The commercial grade of tissue used in most shops works better than the super soft types as it has less lint. Keep the tissue roll itself in clean container. A speck of foreign grit in the tissue can ruin hours of work if rubbed on the surface during the final polish. Keep the brushes and felts that are loaded with one particular grade of diamond on a glass or plastic jar when not being used. They can be used over and over again if they are kept clean.

Polishing

Step 6. Before starting this step be sure your part is washed clean. When you are working on a face with ejector pin holes, if you don't have them plugged, go through these holes with a brush to make sure all the grit is gone. The cup and end brushes you use for polishing will certainly pick up any grit left in there.

Now put 3 or 4 small dabs of Yellow Kemet (6 Micron) Diamond on the clean workpiece, mount a cup brush in the rotary head, add a drop or two of OS Fluid to the brush and start travelling slowly across the surface. Set the control to keep the brush revolving at a moderate speed. You don't want the slurry to be thrown off the surface.

Always keep the brush moving laterally when in contact with the workpiece. If you stop travelling, a cup brush can produce some nasty circular dwell marks. This is particularly true in corners where you must make either a 90° or 180° change of direction. It is best to finish out the corners with 3/16" or 3/8" end brush with which the dwell mark dangers are lessened. Stay a little way out from the corners with the cup brush so that the direction change will be smooth. Also, as you travel the cup brush, raise it frequently to trap the slurry under it. This is the secret to the fast cutting action of a cup brush. Most of the cutting action is done as the diamond compound works its way out from under the brush. If you don't trap the slurry the brush sort of pushes it around and does not cut as fast. Use a light to medium pressure, so that the brush flares out just a little. When you have worked the surface for perhaps five minutes, wipe a small area clean with tissue to see if all the 600 or 1200 grit paper lines are gone. When all you see are the swirl marks from your brushes it's time to switch over to a felt. Do not wipe the workpiece, keep the diamond slurry on there. The diamond will keep cutting practically forever or at least until it gets too heavy with sludge from the workpiece.

Step 7. Put the brushes in a clean jar or other container marked for Yellow Diamond. You will end up with quite a collection. Now use a felt with a straight taper on it, moisten it with Kemet OS Lubricating Fluid and run it over the workpiece to take up the diamond compound. Add a little new diamond to the work.

Run the felt across, or in other words, at a right angle to the direction in which you used the 600 paper. This way, if you still have some remnants of the 600 paper lines the felt will pick them up. Operate this felt at a slow to moderate speed so as not to get any orange peel started. The pressure should also be fairly light, so that not too much heat gets built up between the felt and the workpiece. Do not let the felt get dry as this produces more heat, and an orange peel condition will start to appear. Orange peel is the worst problem in the whole polishing operation. We will go into that in some detail later. The reason for using tapered felt instead of a round button felt is to keep the surface heat and therefore the surface distortion at a minimum.

When the swirl marks from the previous brush are gone you are done with the Yellow Diamond. Do not over-polish. It is better to wipe the block clean when you think it is done, then if necessary go back over just the spots you missed. This whole felt operation does not take long so keep a close watch on it. Put the felt in the container and wash off the workpiece.

Step 8. Now you are almost finished. Mount a tapered felt in the handpiece, put 3-4 dabs of Green (3-KD-C3) Diamond on the workpiece, moisten the felt with OS Fluid and start travelling the felt bob. This time have the rotary motion going in the same direction that the paper travelled. The reason for this is that if some small lines remain, the Green felt will polish down into them and make them much less obvious than if you crossed them. Travel this felt slowly and with not too much pressure. When the fuzzy finish from the Yellow felt is gone, you are done with the power felts.

Step 9. Use a 3/4" Felt Bob in one of the hand holders and rub in the same direction you travelled with the yellow power felt . Keep the bob moist and add a little Green Diamond. This step takes very little time and will remove the light fuzz left by the previous felt.

Step 10. Fold about four sections of clean toilet tissue into a two inch square and rub the block. Let the paper pick up what is left of the diamond slurry on the workpiece and if necessary add a little dab of Green Diamond. Rub it up to a nice mirror finish. If everything has gone right you should now be looking a pretty decent mirror finish. Later on you might want to experiment with cotton balls and Blue 1 Micron Diamond to improve the finish.

Troubleshooting

If the block is not quite perfect, let's troubleshoot it a bit and see where we went wrong. If you have some lines running in the direction of your last stone, you know that you didn't quite finish the job you set out to do with the 600 paper. if you have a line running in the direction of the paper, you know that you didn't quite finish the job you were supposed to with the green diamond brush.

If you have a swirl mark here and there you know you should have done a little better with the Yellow Diamond/felt. If you have any marks left from the Yellow/ felt, simply go back over them with the Green Kemet Diamond.

One thing I think you will have is a lot of lines and a rather poor polish close to the edges of the block. That's perfectly O.K. for this test block. If the block was a part of an actual mould you would have to clamp a piece of steel to the edge. This would allow you to travel your felts and brushes a little beyond the edge and end up with a polish that is uniform across the workpiece.

There you have a system for bringing up a fine finish, with a minimum of equipment outlay, and the least confusing array of supplies. This technique will do it every time, for everyone. There is no mystery to it and no depending on luck. If you follow the steps and keep everything squeaky clean the results are predictable. There is a lot of information about special tools that will make your job easier, but basically that's all there is to it. As you practice the trade you might want to experiment with a softer stone before the paper. Gesswein makes a softer one like the 7002. Just remember that the short cuts are very few and they usually bring with them more problems than benefits. The problems may be just a bad finish with lines in it or worse, typical surface finishing problems and how to fix them.

Felts

The type of felts that you use can be a big factor on orange peel. Never use button felts as power tools, as the amount of heat and stress that can be built under these is tremendous. Also, in working straight down on the workpiece, there is a strong tendency to apply too much pressure. The problem of heat and stress due to high pressure and overworking the surface can be a problem with high chrome content tool steels. so be very careful to avoid overs-tressing the surface.

Use mounted tapered felts almost exclusively on flat work, and use the bobs to rub out the power marks by hand. Even deeper cavities can be gotten into by using tapered felts in a right angle handpiece. When moving to Step 6, going from the 600 wet paper to Yellow Diamond Compound, always use a brush to remove the paper lines. Brushes are much less stressful to the surface than felts.

Lapping slots

Slots that are more than an inch deep and less than 1/16" wide are a special problem and require a lot of ingenuity. You can do them with the long reach diamond files. This is probably the fastest way. These can be used with the Diprofil Mini Filing Unit.

Another way is to take advantage of the diamond sheets and diamond foils that are available. These can be cut to shape and glued to thin metal strips. Some of the foils have Pressure Sensitive Adhesive (PSA) backing, eliminating the need for extra glue. These home made tools can be made to reach the bottom of any slot. in a reciprocating action machine. The small pencil type reciprocators are easy on the operator. Another good tool is the Diprofil transverse motion handpiece which has a side to side motion rather than reciprocating this helps when polishing up to the bottom of slots. If you don't have any of the fancy stuff available, your old cable Diprofil and a Copper Lap with Brown 14 Micron Diamond Compound followed by Yellow 6 Micron Diamond Compound will do the job.

Xebec Diamond Ceramic stones are excellent for small slots and are good for removing sparking. They can be used by hand or with reciprocating handpieces as they resist snapping. Use the NSK Sheenus ultrasonic polisher with xebec heat resistant and diamond stones. A brass lap with diamond paste can be used if the application requires a lap with less flexibility

Parting lines and ejector pin holes

Some parting lines can be finished with the core and cavity together. This makes for an almost invisible parting line and should be done this way whenever possible. It must be admitted that this ideal situation does not happen often.

Ejector pin holes may be plugged with soft cold-rolled steel pins or with actual ejector pins cut to length, so that you can stone and polish right over them. This may be a necessity if the hole is in the bottom of a slot where it is impossible to protect the edges of the hole. Sometimes you may have to plug the holes that are out in the open if only perfection is acceptable. Usually, however, a little extra care in that area will give an acceptable product without plugging the hole.

Inside Radii

When working with an inside radius there are a couple of things you should not do. You should not gouge out the radius with too coarse a stone and put in lines that will have the radius sunken in by the time you get them out.

Secondly, do not try to stone right up to the radius with those little bitty strokes without stoning the radius first.

When you stone the radius, do it the easiest way, in one direction only. There is no need to cross stone. Also stone out about 1/4" to 1/2" parallel to the radius. Stone it up to 600 grit, then when you come stoning up the sides with your final stone you will have little trouble with "picky" stoning. Perhaps in the corners you can use an G-Flex wheel with power. Just remember not to go too far with the power, otherwise you may make more work for yourself than if you hadn't tried the short cut. Radii usually do not have to be worked out absolutely perfect either. The diamond polishing felts always make them look good anyway.

Burnishing tools

A little known polishing aid is the burnishing tool. This is a tool consisting of a various number of small metal wires or fingers bound to a hub and above that to a mandrel. This is similar to an end brush but the fingers are 1-1/4" long and slender. The number of fingers determine the diameter of the tool. Look for the in the Gesswein catalogue to see what I mean. These can be a tremendous aid in doing dead-end holes, deep radii and in certain cases picking out corners. A little diamond compound or lapping compound used with these tools will dig EDM pits out of some pretty impossible places.

Well, there you have it. There is an absolutely foolproof system for bringing up a fine finish and a few tips and techniques that will help you to do it a little faster and easier. It is not the only way, but you can feel confident that it will work. This may be all that is needed for the toolmaker who polishes only occasionally. He can force himself to follow those 10 steps very closely, get a decent finish, then he's done. He can then go back to doing something he's more likely to enjoy, like cutting steel or building electrodes. The person who needs a little extra motivation is the person who does mould polishing all the time, as a career. Most mould shops have recognised that it is very specialised operation and hire people specifically to train as polishers or they send their work out to a polishing shop.

Review of basic steps

Step #1Stone out the hardest and worst of the EDM burn with a 320 grit, 630x* type of stone.

Step #2 Finish stoning out the pits with a 320 grit, 530x* type stone.

Step #3Cross stone the 530x* with a 260x* type stone. If the steel you are working on has a ground finish you can probably skip the first two steps and start with this one, the 260x* stone.

Step #4 Cross stone the 260x* with a softer bond 600 grit, like the 160x* type. It makes the 600 paper process easier. The time consumed is pretty much the same.

Step #5Cross with 600 wet/dry paper. Use under felt bobs, in the holder I described or under shaped sticks for corners.

Step #6 Be sure your part is clean,then apply the Yellow Diamond Compound. Work it around with large or medium cup brushes on the flat areas. Use end, wheel, or even bottle brushes in special areas. Continue until the paper lines are gone.

Step #7Work out the yellow brush marks with a rotary felt. Tapered felts are the safest. Button or end felts may be used in special places, with care.

Step #8 Remove Yellow felt marks with Green diamond and tapered felts. Travel the rotary action of the felt in the same direction as the 600 paper.

Step #9Hand rub with a shoulder bob or felt stick in the same direction as the Green power felt.

Step #10 Hand rub with tissue and Green diamond in the same direction.

And once more we'll talk of cleanliness, because it is impossible to stress it too much. Before you start with the diamond compound, wash your hands. When you change diamond compounds, wash your hands. Before you start with a diamond, clean your rotary tool and the cable that drives them. Clean your lamp because it will be directly above your work and could easily drop grit on it. Cover your work area with shop towels, paper towels, anything clean. Threaten the life of anyone who even thinks about blowing compressed air in your area while you are using diamond compound!