Techniques for Polishing Molds
The objective of this article is to present a straightforward and reliable method for polishing mold steel that can be applied to any type of steel and carried out by anyone. The focus will primarily be on achieving an adequate finish in the most efficient way possible. Moreover, we will also consider accomplishing this with the most cost-effective and uncomplicated set of tools and supplies. Despite the array of equipment available to aid mold polishers, we will maintain a simplistic approach. Our goal is to achieve a commercially competitive finish within a standard time frame using fundamental equipment.
The Mechanical Equipment
To perform various tasks in the machining process, you will require specific tools. One of the essential tools is a flexible shaft power unit, such as the KFD or NSK Powered kit, which comes with a foot control to regulate the speed accurately. Additionally, a reciprocating unit like the Diprofil is needed for lapping slots and stoning areas with a short stroke. This unit can be fitted onto the KFD. The KFD also requires specific handpieces such as a straight handpiece with collets and a telescoping right-angle head called the VTN. The VTN is necessary as it allows the operator to reach areas that cannot be accessed with the straight heads, which, in turn, reduces operator fatigue. Another useful attachment is the VTN-S, which accepts 1/4" collets. The Diprofil Mini Filing Handpiece is also available for very short stroke work, and there are two versions with 0.5mm and 1.8mm stroke respectively. It is important to note that all reciprocating hand tools have maximum speed restrictions, with the Mini Diprofil having a limit of 10,000 RPM and the Standard Diprofil having a limit of 15,000 RPM.
NSK Powered Hand Tools
NSK Powered Hand Tools are proficient in performing several functions such as grinding, smoothing, polishing, and deburring. These tools consist of a lightweight and compact motor that offers a wide range of attachments and speed settings ranging from 1000 to 50,000 rpm, making it possible to polish molds quickly. Two examples of these tools are the NSK Electer with a speed range of 1000 to 35,000 rpm and the NSK Espert 500 Torque System, which offers a speed range of 1000 to 50,000 rpm.
Brushes and felts
In order to perform rotary tool polishing, various types of brushes and felts are required. Cup brushes of different shapes and sizes are available, with hard bristles preferred for better cutting, although those with soft and nylon bristles also have their specific uses. End brushes with 3/16" and 3/8" widths, along with wheel brushes of varying diameters and widths, are necessary for slots and radius areas. Additionally, Felt Bobs with 1/8" shanks, Cylinder Bobs in 1/4" and 1/2" diameter, and Felt Rectangulars for hand rubbing are needed.
It is essential to select high-quality 600 or 1200 grit wet-or-dry sandpaper, as this and the use of stones constitute the most tedious parts of the entire polishing process. Three Diamond Pastes - Blue 1 Micron KD-C3, Green 3 Micron KD-C3, and Yellow 6 Micron KD-C3 - are necessary for achieving a mirror finish, and while other grades of diamond may be useful, the above three are all that is needed for basic polishing work. To properly see the lines and swirl marks, a good bench light or headlamp with magnifier is important, but fluorescent lamps should be avoided as they do not throw shadows. The best light has a couple of arms and springs that can be moved easily and remain in place.
Mold Polishing Stones
Gesswein Stones
The use of stones is a fundamental step in the polishing process, and it is important to choose the appropriate grit based on the type of stone and material being polished. Typically, two types of grit are used: silicon carbide for the softer bond moldmaker stones and aluminum oxide for the hard bond diemakers stones. The range of grits varies from coarse 100 mesh to 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 of stone, the second is the grit, and the third and fourth numbers identify the shape of the stone. For example, stone number 1601 refers to moldmaker stone with 600 grit silicon carbide and a 1/4" X 1/8" shape, while stone number 2401 refers to diemaker stone with 400 grit aluminum oxide and the same shape.
It is important to keep the stones in separate covered containers to avoid mixing the different grits. Four containers are recommended for the 180, 300, 400, and 600 grits, with perhaps an additional one or two for finer grit stones. Specialized containers are now available with good covers and a molded-in sludge grid that makes them easy to clean and stackable. To keep the stones wet, a good stoning oil should be used in the containers, as kerosene or EDM electrolyte can cause skin problems. A protective cream formulated for use with petroleum products is recommended for professional polishers who have their hands wet a large part of the time. Finally, several "double ender" stone holders should be purchased, and the stone containers should always be covered when not in use to keep them clean and dry.
Xebec Stones
Kemet provides three different types of Xebec stones designed for mold polishing.
The first type is the standard range of Xebec Ceramic Meister Finish stones. These stones contain alumina fibers that are precisely graded and enclosed in a resin matrix for high strength. They are suitable for use by hand or with power and ultrasonic polishers.
The second type is the Xebec Ceramic Heat-Resistant Range of stones, which can withstand temperatures up to 200°C. These stones offer the same performance as the Meister range of ceramic stones, making them ideal for removing EDM scales. The Heat-Resistant range has been specifically developed for extended use with ultrasonic polishing machines.
The third type of Xebec stone is the Xebec Diamond stone. These stones have diamonds embedded in layers, which allows them to retain sharpness until the end of their lifespan. The granular abrasive allows for side cutting as well as tip cutting, making them ideal for use with hard materials such as hardened steels, tungsten carbide, and ceramics.
Stoning Techniques for Polishing Mold Steel to a Mirror Finish
To achieve a mirror finish on a flat 2" by 4" block of mold steel, stoning techniques are an effective method. Assuming the surface has been burned with an EDM finish, we recommend using a #6302 Gesswein stone to begin the polishing process. However, if the burn is deeply pitted due to excessive amperage, start with a 180 grit, #6182 stone to break through the surface crust.
Step 1: Using a double-end stone holder, begin with a 320 grit 6302 stone to remove the surface crust. Stone in one direction until all the color from the burn disappears.
Step 2: To remove the pits, use a 5302 stone, which has a slightly softer bond than the 320 grit 6302 stone. Keep the stone wet and rub until all the pits are gone. It is not necessary to cross-stone in this step.
Step 3: Cross-stone with a 2602 stone at a 90° angle from the previous stone marks until all those stone marks are gone.
Step 4: Cross-stone the 2602 with a 1602 moldmakers stone. Keep the stone wet and apply light stoning pressure to avoid loading.
Step 5: Cross the 1602 stone marks with 600 grit wet paper. To make it easier, cut 1/4" Perspex or other bendable plastic into strips with different degrees of bend. Drill a hole on one end to accept the spindle of a 3/4" felt bob. Cut 600 or 1200 grit paper into squares of about 1-1/2", fold twice and use them under the felt bob. Rub until the stone marks are gone.
For a finer finish, try using a 600-grit OT stone followed by 1200 Sic wet or dry, then 3mu diamond paste. This method can be used on fine sparking without resorting to coarser stones. With some experience, stoning techniques can produce an excellent mirror finish.
Xebec Stones
Xebec stones are abrasive tools that are particularly useful for tasks that require precise and intricate detailing. They are especially effective when used in conjunction with ultrasonic and reciprocating heads, as their small size and shape allow them to access hard-to-reach areas and provide greater control and precision during the cutting or polishing process. Overall, Xebec stones are a valuable tool for anyone who requires precise and high-quality finishing work, particularly in industries such as jewelry-making, dental work, and microelectronics.
Cleanliness: The Importance of a Clean Workspace for Effective Polishing
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 commencing the polishing process, ensure that the component is thoroughly cleaned. For components that have ejector pin holes, it is essential to brush the holes to remove any grit. The polishing brushes will effectively remove any leftover debris. Next, apply three or four small dabs of Yellow Kemet (6 Micron) Diamond compound on the cleaned workpiece. Attach a cup brush to the rotary head and add a drop or two of OS Fluid to the brush. Start moving the brush slowly across the surface while maintaining a moderate speed. Avoid throwing the slurry off the surface by controlling the brush's revolution rate. Always move the brush laterally when it comes into contact with the workpiece to avoid creating circular dwell marks, especially in corners where a 90° or 180° direction change is required. For corners, utilize a 3/16" or 3/8" end brush, as the risk of creating dwell marks is lower. Maintain a smooth direction change by moving the cup brush a little distance away from the corners. Periodically lift the brush as you move it to trap the slurry underneath. Trapping the slurry promotes the cup brush's fast cutting action as the diamond compound works its way out from under the brush. Apply light to medium pressure on the brush, allowing it to flare out slightly. After working on the surface for approximately five minutes, wipe a small area clean with a tissue to check if the 600 or 1200 grit paper lines are gone. When all that remains are the swirl marks from the brushes, switch to a felt.
Step 7: Place the brushes in a clean container labeled for Yellow Diamond. Using a felt with a straight taper, moisten it with Kemet OS Lubricating Fluid and run it over the workpiece to absorb the diamond compound. Add fresh diamond to the workpiece. Run the felt across the workpiece or perpendicular to the direction in which the 600 paper was used. This ensures that any remnants of the 600 paper lines are picked up by the felt. Operate the felt slowly to a moderate speed to avoid creating an orange peel condition. Maintain light pressure to prevent excessive heat build-up between the felt and workpiece. Avoid drying the felt, as it increases heat production and can result in an orange peel condition, which is the worst problem in the polishing operation. The use of a tapered felt instead of a round button felt helps minimize surface heat and distortion.
Step 8: When the swirl marks from the previous brush are no longer visible, stop polishing. Over-polishing should be avoided. Use a 3-4 dabs of Green (3-KD-C3) Diamond on the workpiece, moisten the tapered felt with OS Fluid, and slowly move the felt bob. Ensure that the rotary motion goes in the same direction as the paper travelled. This enables the Green felt to polish down into any remaining small lines, making them less noticeable. Apply minimal pressure when using the Green felt. When the fuzzy finish from the Yellow felt is gone, the power felts can be stopped.
Step 9: To remove any light fuzz left by the previous felt, use a 3/4" Felt Bob in one of the hand holders and rub in the same direction as the yellow power felt. Keep the bob moist, and add a little Green Diamond. This step takes little time.
Step 10: Fold approximately four sections of clean toilet tissue into a two-inch square and rub the block. The paper will pick up any remaining diamond slurry on the workpiece. Add a little dab of Green Diamond if necessary to achieve a nice mirror finish
Troubleshooting Mold Polishing
In the realm of surface finishing, troubleshooting is a crucial process in identifying and resolving any imperfections that may arise during the polishing of a block. Should the block not meet the desired standard, a careful assessment is necessary to identify the root cause of the issue. For instance, if lines are observed running in the direction of the last stone, this indicates an incomplete job with the 600 paper. Similarly, if lines are found running in the direction of the paper, the task was not adequately completed with the green diamond brush.
Swirl marks can suggest insufficient use of the Yellow Diamond/felt, and any remnants of these marks can be remedied by employing the Green Kemet Diamond. It is common to observe many lines and poor polishing close to the edges of the block, but this is acceptable for a test block. If the block were part of an actual mold, a piece of steel would be clamped to the edge, allowing for a uniform polish across the workpiece.
This process requires minimal equipment and supplies and can produce predictable results when the steps are followed meticulously and the tools kept impeccably clean. Although there is a range of special tools available to ease the polishing process, the fundamental process remains straightforward. In practice, one may choose to experiment with a softer stone, such as Gesswein's 7002, before employing the paper. Nevertheless, shortcuts should be used with caution, as they often bring more problems than benefits, such as surface finishing problems, which can be resolved through proper troubleshooting.
Felts
The selection of felts plays a significant role in minimizing the occurrence of orange peel during surface finishing. It is highly recommended to refrain from using button felts in power tools, as they generate substantial amounts of heat and stress. Additionally, applying excessive pressure while working straight down on the workpiece can exacerbate heat and stress problems, especially when dealing with high chrome content tool steels. Therefore, care must be taken to avoid overs-tressing the surface.
Mounted tapered felts are ideal for flat work and should be used almost exclusively, while bobs are suitable for manually rubbing out the power marks. Tapered felts in a right-angle handpiece can access even deeper cavities. In the transition from Step 6, i.e., moving from 600 wet paper to Yellow Diamond Compound, it is crucial to employ a brush to eliminate the paper lines. Brushes are significantly less stressful to the surface than felts, further minimizing the potential for undesirable surface effects.
Techniques for Lapping Slots, Parting Lines, Ejector Pin Holes, and Inside Radii
The lapping of slots that are greater than one inch deep and less than 1/16 inch wide requires careful attention to detail. The use of long reach diamond files is the fastest method, and can be paired with a Diprofil Mini Filing Unit for optimal results. Alternatively, diamond sheets and foils can be fashioned into homemade tools and affixed to thin metal strips. These versatile tools can be utilized in reciprocating action machines, and are compatible with pencil type reciprocators that offer enhanced operator comfort. The Diprofil transverse motion handpiece is another tool that can aid in polishing up to the bottom of slots. In the absence of specialized equipment, a copper lap with brown 14 micron diamond compound followed by yellow 6 micron diamond compound can be used in conjunction with an old cable Diprofil to achieve the desired finish.
Xebec Diamond Ceramic stones are ideal for lapping small slots and are effective in removing sparking. They can be applied manually or with reciprocating handpieces, as they are resistant to snapping. For optimal performance, use the NSK Sheenus ultrasonic polisher with Xebec heat-resistant and diamond stones. A brass lap with diamond paste can be employed if a lap with less flexibility is required.
Parting lines can often be finished with the core and cavity together to create an almost invisible parting line, if possible. In some cases, this ideal approach may not be possible. Ejector pin holes can be plugged with soft cold-rolled steel pins or actual ejector pins cut to length to facilitate stoning and polishing directly over the holes. This may be necessary when holes are located in slots that make it impossible to protect the edges of the hole. In some cases, holes may need to be plugged even when in open sight, if only a flawless result is acceptable. However, typically, with extra care in this area, an acceptable result can be achieved without plugging the hole.
When working with an inside radius, several common mistakes should be avoided. Using too coarse a stone may result in lines that sink in the radius by the time they are smoothed out. Attempting to stone the radius without first smoothing it out can also lead to undesirable results. When stoning the radius, it is best to use a single direction and avoid cross-stoning. It is also recommended to stone approximately 1/4 to 1/2 inch parallel to the radius up to 600 grit. This will help to alleviate any difficulty when stoning the sides with the final stone. In some instances, an G-Flex wheel with power may be used in the corners, but it is important to exercise caution and avoid overuse of power tools. Radii do not necessarily have to be absolutely perfect, as the diamond polishing felts can help to refine and enhance their appearance.
Burnishing tools
One effective yet lesser-known polishing aid is the burnishing tool. Comprising a hub, a mandrel, and a variable number of slender, 1-1/4" long metal wires or fingers, this tool is akin to an end brush, with its diameter determined by the number of fingers it possesses. These tools are especially useful for tackling dead-end holes, deep radii, and hard-to-reach corners. By combining them with a small amount of diamond or lapping compound, one can even eliminate EDM pits from seemingly impossible-to-reach locations.
In conclusion, this 10-step system for achieving a superior finish is highly reliable, and those who are not frequent polishers can easily follow the steps to achieve an acceptable finish. However, for professional mold polishers, it is highly recommended that they use these techniques for motivation and skill-building, either by training new mold polishers or outsourcing polishing work to specialized polishing shops.
Review of basic steps
Step #1: Begin by using a 320 grit, 630x* stone to eliminate the hardest and worst areas of the EDM burn.
Step #2: Proceed to use a 320 grit, 530x* stone to eliminate the remaining pits.
Step #3: Cross stone the 530x* with a 260x* type stone. If the steel already has a ground finish, this step may be the first.
Step #4: Use a 600 grit, softer bond like the 160x* type stone to cross stone the 260x* stone. This makes the 600 paper process easier and consumes almost the same time.
Step #5: Use 600 wet/dry paper under felt bobs or shaped sticks for corners to eliminate any remaining paper lines.
Step #6: Clean the part thoroughly and apply the Yellow Diamond Paste. Utilize large or medium cup brushes for flat areas and end, wheel, or bottle brushes for special areas. Continue until the paper lines vanish.
Step #7: Eliminate the yellow brush marks using a rotary felt, with tapered felts being the safest option. For special places, button or end felts may be used with caution.
Step #8: Remove the yellow felt marks with Green diamond and tapered felts. Follow the same direction as the 600 paper while traveling the rotary action of the felt.
Step #9: Use a shoulder bob or felt stick in the same direction as the Green power felt for hand rubbing.
Step #10: Hand rub with tissue and Green diamond in the same direction as the previous step.
Furthermore, cleanliness is of the utmost importance and should not be overlooked. Ensure to wash your hands before starting with the diamond compound, and whenever you switch between different diamond pastes. Clean your rotary tool and the cable that drives it before starting with a diamond. The lamp above your work should also be cleaned to avoid dropping any grit on it. Cover your work area with clean shop towels, paper towels, or any other clean materials. Finally, ensure that no one blows compressed air in your area while using diamond compounds, as it may lead to contamination of the work surface.
