Basic Die Spray Techniques for Forgers

Stationary spray nozzles are used to lubricate and cool forging dies between strokes

In this first of two pages on die spraying techniques and equipment, some basic methods of coating dies with solutions that both the cool and lubricate the dies are examined. Advantages and disadvantages of each technique are considered. In the next page, sophisticated and more highly automated techniques will be discussed.

In the current business climate there are demands on forgers to produce more parts per hour, hold closer tolerances, generate less scrap, decrease overhead and reduce costs. As if these weren’t all tall orders in and of themselves, add in the current problems within the automotive and other industries served by forgers and it is fair to say that, in some ways, the forging industry is under siege. This is why automation is so important for the forging industry. Die spray automation can have a positive effect on all of the issues above. Forgers today have many options for spraying their dies, from swabbing to robotic die spray. Every forger has to determine which option is best for their operation. Spraying dies between forging cycles is critical to successful and cost-effective hot-forging processes. Our definition of die spray is multifaceted and includes: the controlled cooling and coating of forging dies; the controlled application of atomized water for cooling (if needed) to maintain a die surface temperature between 300°F and 450°F or to the specifications of the tool steel being used; and the controlled application of atomized lube at the correct dilution ratio to assure the consistent and even coating thickness (constant % solids) on the dies. The objective is to eliminate die temperature peaks and valleys to prevent surface heat checking. We do not want to use the lube to cool the dies because of the expense. However, we do want to maintain the coating thickness on the die through the dilution ratio of the lube, not by spraying longer. Again, using excess lube can cause non-fills and other part defects. We want to make the die spray as efficient as possible. In this article, we will discuss die spray options starting with the least sophisticated, noting that all of these spray options are being used today.

Manual Application

Manual spraying or swabbing are the most basic techniques of applying die lubricants and coolants. These enhance die life and part quality by improving the die surface to make the forged metal move better over the dies. At its simplest, the process is nothing more than a swab or rag tied to a broom handle, dipped into a bucket of lube and splashed onto the dies. The lube used most is an oilbased product. However, water-extendable products are also used. In an effort to get a better coating, the process moved to dipping and blowing, in which the lube technician swabs the die where the lube is required and uses an air gun to blow off scale and blow air over the swab to get an atomized spray for even coating (and some cooling) on the dies. The process itself is inconsistent from hour to hour and certainly from worker to worker. It also requires two people to do the forging process. A more modern approach is to use spray guns that spray lube mixtures directly onto the die surface (Figure 2). Most spray guns on the market are two-stage air blow-off and atomized spray, which use pressurized lube. However, these are still manual applications of the lube and scale blow-off and are used on both hammers and presses. In hammer applications, the spray gun replaces the swabbing, dipping and blowing method. This allows for a more consistent atomized spray coating, and in many applications you would not need a luber. Instead, the operator could forge the part and then spray the dies. This is especially true in press applications. This does not, however, eliminate the inconsistencies of the operator or the inconsistencies from operator to operator. The use of atomized spray does allow for a better coating on the dies and helps with cooling. Atomized spray with velocity will penetrate the thermal barrier faster, flash off the carrier (water) and leave the solids on the dies. Most forging lubes are designed to be dry-film lubricants.

Figure 2. A typical hand-held die spraying device

Stationary Spray Systems

Stationary spray systems (see lead graphic) are often the most desired way to spray dies because they do not have any moving parts and remove operator inconsistency from the process. Nevertheless, there are some drawbacks to stationary spray. The exit points are normally mounted on the bolster or on the spraying device when changing dies. They can be even more difficult to set up on the next set of dies. The exit points are normally mounted so the angle of spray can be adjusted from die to die. This leads to exit points being knocked out of alignment, resulting in poor coverage on the die and causing poor part quality and shorter die life. There is also a problem of die spray turbulence in the die area. If all exit points are on at the same time, it is impossible for the sprays to go through other sprays (turbulence), requiring more lube and flooding of the dies and resulting in poor die life and scrapped parts.

We have used two bank-type stationary spray systems with some success. This system sprays the back half of the top and bottom dies at one time then the front half of the top and bottom. This prevents turbulence in the die area, but stationary spray is really only good for flat dies and hammers. The hammer has a large die opening, giving a good angle of spray, and can cover the complete die as the ram moves up and down. All the operator has to do is trigger the spray system when he requires spray, and the spray duration is controlled automatically. The most efficient angle to cool and coat a hot die is 90 degrees to its surface. In a stationary spray system there is no way to achieve that angle, which means there is always a shadow area that cannot be reached. If the die has a deep cavity or a plug, it is almost impossible to achieve complete coverage of the dies without flooding.

Figure 3. A reciprocating device applying lube and/or water onto the surface of an upper forging die

Reciprocator Spray Systems

Reciprocators come in 1-axis (in/out) or 2-axis (in/out and up/down) units. They can be air or servo actuated, and both can deliver sweep or stop and spray at a better spray angle. Air-actuated units, though less expensive, offer limited programmability.

They are controlled by a limit switch or linear transducers with limited repeatability because of the compressibility of air. Air-actuated reciprocators are either sweep or stop and spray, but not both.

Servo-actuated units offer much more programmability. They can sweep, stop and spray, or any combination of the two, but they are much more expensive because of the electronics and programming required. Reciprocators bring a much better angle of spray to cool and coat the dies. They also bring repeatability to the forging process and take the operator out of the die spray equation. The downside of reciprocators is that they can only spray in two planes (in/out or up/down).

Figure 4. A representation of an entire reciprocating

Conclusion

There are many different techniques used to spray dies
between forging cycles. The effective application of lubricants
between forging cycles is vital to successful and cost-effective
hot-forging processes and extended die life. This article covers the most basic and simple methods used to coat and cool forging dies between cycles.