Die Spray Techniques for Forgers - Part II
In the first part of this article we discussed how important automation is to the forging industry. Starting with the most basic of manual spraying or swabbing of dies between press or hammer strokes, we additionally reviewed stationary spray systems and reciprocator spray systems, at each step moving up the ladder of coating complexity, uniformity and quality. In this final article we complete the journey through the various processes by which forging dies are coated with lubricants and coolants during forging cycles.
Robotic Spray SystemsRobotic spray offers the forger the most options to solve die spray issues. The robots come standard with four or six axes of servo-controlled movement, travel rates in excess of 100 inches per second, accuracy of +/-0.002 inch and uptime rate of 98%. Robots can be among the most durable and reliable pieces of equipment on the plant floor.
The spray robot allows all the spray movements required for the most difficult forged parts: sweep spray for dies that require it, stop spray for pinpoint-type spray tips and pattern spray for parts that require a spray pattern that duplicates the part being forged (connecting rods, front wheel spindle and control arms). The forger is not limited to two planes, and the spray head can be rotated in any direction to accommodate the spraying of dies. All of these movements and spray patterns can be stored for future use, and most new spray jobs can be modified from existing spray programs. This makes robotic spray automation economical for job shops as well as those that have long-running jobs.
Those considering the use of robotic stations should be aware of safety requirements that must be met when using robotic automation. ANSI dictates that a man and a robot cannot share the same space. Hence, safety fencing is required. In the case in which an operator is moving the part through the dies, the operator must be out of the die area when the robot is in the die area. There must be safety circuits to guarantee that if the operator enters this area, the robot is disabled. This can be done with a light curtain, limit switch or pressure switch.
Walking BeamsWalking-beam die spray mechanisms usually employ stationary spray tips mounted to the bolster or die block. These do not always allow for the best spray angle. This technique ends up flooding the dies to get the required cooling and coating.
A solution to this problem is to achieve the same angle of spray as a reciprocator or robot by mounting the spray tips on the walking-beam arms. The clamping motion of the arms can often be used to carry the spray tips over the die area and then spray the dies as the part is being transferred to the next station. We added reciprocating mechanisms to the walking-beam arms so that when the walking beam got the signal to move in (to clamp onto the parts), the air-actuated reciprocator moved an additional distance to cover the dies. We then sprayed the dies as the part was being transferred to the next station. When the walking beam reached the point to set the part into the next die, the reciprocator retracted out of the die area. This should result in better die cooling, better coating, less lube usage and better die life.
A robotic walking-beam system uses a series of clamping mechanisms that clamp onto the part along the centerline of the forging press (not perpendicular to the centerline as with a standard walking beam). This means the part clamps are on the centerline of the press when the robot comes in to pick up the part. This allows the spray tips to be mounted in between the clamping mechanisms. As the part is being moved to the next station, the dies can be cooled and coated, which eliminates the need for another piece of die spray equipment to spray the dies, reducing the part-to-part cycle time.
Spray Tips and NozzlesOur definition of a spray tip is the exit point from which the atomized spray comes out. Spray tips are used to create a pattern of spray for cooling and coating forging dies.
For sweeping spray, using off-the-shelf tips with either fan-shaped or conical nozzles is possible because you are passing over the die. This type of spray can be used, with slight modifications, for many different die configurations. For stop-and-spray or pinpoint spray tips, the configuration of the exit spray conforms to the shape of the part. This makes the spray tip much more complicated, and it is configured for one die set. The advantages of this type of spray are fast travel into the die area, rapid execution of the spray sequence and fast retraction from the die area, all of which decrease spray cycle time.
For pattern spray, the tips are set up in one position and the robot follows the shape of the part. With many part configurations, this is the most efficient way to cool and coat the dies.
There are three basic types of spray nozzles: flood, internal atomized and external atomized.
In a flood-type nozzle, no atomization is created. It basically is a water hose with the lube poured onto the dies. Systems like this are used mostly on walking beams. The disadvantage of this type of spray is that you only use a small percentage of the lube itself. There is a lot of overflow. To make this approach practical, you need to recycle and reconstitute the lube. This requires removing tramp oil and metal particles, as well as analyzing and adding the chemicals to make the lube whole. This creates a huge set of issues in itself.
With internal atomized spray nozzles, the lube/water meet the atomizing air in a chamber in the nozzle body. The atomized spray then travels to the spray tip. The advantage of internal atomization is that you can create a very fine spray with velocity. The spray can travel 10-15 feet past the spray tip, assuring penetration of the thermal barrier and shorter spray cycle time. It also allows for an endless variety of spray tips. The goal of this type of spray is to apply only what is required. The water carrier of the lube is flashed off, the solids are left on the dies at the correct thickness and the spray is consistent cycle after cycle. The downside of internal atomized spray is the limited distance between the nozzle and the spray tip. If the two are separated by 10 feet or more, you begin to lose the atomization in the line and, naturally, the effect of the cooling and coating.
In external atomized spray nozzles, the lube does not meet the atomizing air until the spray tip (exit point). Although this technique is used by some suppliers, it is relatively new and its advantages and disadvantages are unclear. Our understanding is that one can have the spray tip longer distances from the nozzle and that it does not plug as easily. Although we are still evaluating the performance of externally atomized spray nozzles, we are not yet convinced they offer better atomization.
We have looked at the different types of spray systems and nozzles, but unless you get maximum efficiency out of the spray system, you will not get the desired uptime that you will require.
Auto Water Purge SystemThe purpose of the water purge system is to keep the die spray system clean and free of lube clogs by purging it with water. There are various designs of water purge systems on the market, but these systems are a must for applications in which you are spraying solids (such as graphite) in suspension. If they are not agitated or moved through the lube lines by spraying, diluted lubes will settle and cause the system nozzles to plug.
Water purging the spray system (pumping the mixed lube back into the tank and filling the lube lines with water) when the press is down for repairs or die change assures that the lube will not settle in the lines and prevents plugging of the nozzles. When the repairs or die change are complete, you then charge the spray system with mixed lube and are ready to continue with the forging process.
We have reduced spray plugging issues by as much as 99% with water purging, an excellent rate of success.
Proportioner SystemProportioners are designed to maintain the lube concentrate at the desired dilution ratio for the product you are forging. The concept underlying the use of proportioners is that the dilution ratio determines the coating thickness on the dies. We do not believe that one must spray longer to get a thicker die coating if required. A full understanding of the spray process will lead you to achieve the optimal coating thickness without having to spray longer. When you flash off the water carrier from the lube formulation, the solids will be left on the die surfaces. By controlling the lube dilution ratio, you can control the thickness on the dies.
This leads us to a very important point. The proportioner must be reliable and offer repeatable results, maintaining the dilution ratio of the mixed lube as consistently as possible.
ConclusionThere are many different techniques and specialized systems and equipment dedicated to keeping dies lubricated and cool between forging cycles. The effective application of lubricants is vital to successful and cost-effective hot-forging processes and extended die life.
Author Michael D. Forster has 30 years of experience as an automation integrator for forging installations. He may be reached at 614-371-0150.