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How to improve casting quality, decrease cycle times, and extend die life
Decreased cycle time, low scrap rates, minimized porosity, improved surface finish, long die life, and effective die cooling are goals all die casters strive towards. Recent innovations in die lubrication technology can help you realize these goals while minimizing capital expenditure. Oil-based lubricants adhere to the die at higher temperatures but do not offer surface cooling. Conversely, water-based lubricants applied with a continuous spray pattern enable surface cooling at the expense of casting quality and die life. It’s time for a solution that addresses all of a die caster’s lubrication woes. That solution can come in the form of a water-based lubricant applied with a pulse spray pattern (10 millisecond response time or less) or a hybrid pulse spray system capable of utilizing both lubricant types. By integrating these lubrication method into your die casting process, you can achieve the adhesion you would expect in an oil-based lubricant with the cooling properties of a water-based solution.
Improved Porosity and Surface Finish
To assess the effect of pulse spray patterns and lubricant type on casting quality, we will explore three types of lubrication shown in Figure 1.
When a water-based lubricant is applied with a continuous spray pattern (scenario 1), the lubricant evaporates upon contact with the high-temperature die and water droplets bounce off the surface, causing poor adhesion. This phenomenon is known as the Leidenfrost effect (figure 2). Due to the Leidenfrost effect, the quantity of lubricant applied cannot be easily controlled. Thus, an excess of lubricant might be observed in some areas, which can cause porosity when the lubricant evaporates once the alloy is injected. In areas where lubricant does not properly adhere, the casting can stick to the die, causing defects and damage to the mold. The large application amount with continuous spray causes a significant amount of waste liquid.
As oil-based lubricants do not evaporate when exposed to high temperatures, they allow good adhesion (scenario 2). However, such lubricants provide no cooling properties, thereby increasing instances of improper solidification (unless internal cooling systems are properly installed and maintained). Furthermore, the lack of precision associated with continuous lubricant application can cause poor surface finish and increased scrap rate. In this case, switching to application with a pulse spray pattern can optimize lubricant thickness and improve casting quality.
For die casters that do not wish to invest in internal cooling systems for their dies, pulsed spray patterns will mitigate the Leidenfrost effect observed with water-based lubricants, thereby enabling these lubricants to adhere to the die (scenario 3). The initial spray of water-based lubricant is applied to the die, cooling its surface and evaporating. Immediately after evaporation, a second layer of lubricant is applied. Since the die has been cooled and water vapor has dissipated, the second layer adheres more effectively. This process is repeated until the desired result is achieved. Rapid spray nozzle response time is required to achieve such results (such as RYOEI’s nozzle response time of 10 milliseconds). When switching from continuous to pulse spray with a water based lubricant, air blow time can be reduced up to 50%. Depending on the type of water-based lubricant you use, 80% cuts in air blow time may also be feasible.
For die casters who wish to use both oil- and water-based lubricants on the same die, hybrid pulse spray systems are the answer. These spray systems apply water-based lubricant to the die before applying oil-based lubricant to ensure optimal cooling and coverage.
Increasing Die Life
Die life can be extended by utilizing a pulse spray pattern to apply a water-based lubricant (figure 3).
Such application methods (shown in turquoise) cause a stable, yet significant surface cooling effect. Conversely, water-based lubricants applied in high quantities cause rapid increases and decreases in die temperature (navy). Over time, these rapid fluctuations in temperature damage the die and reduce die life. Oil-based lubricants have essentially no cooling effect (pink).
Reducing Capital Expenditure
Though it may seem counterintuitive, integrating pulse spraying equipment can reduce the need for capital expenditure in multiple ways. Combining a pulse spray method with a water-based lubricant can extend die life, reducing the need for replacements and repair. In some circumstances, pulse spray patterns may also eliminate the need for exhaust hoods and air curtains, due to the precise application method and decreased mist. Finally, using a pulse spray pattern can also reduce or even eliminate excess spray volume, thereby drastically reducing waste management costs (figures 4 and 5).
Reducing Cycle time with Rapid Pulse Spraying
As a rule of thumb, shorter nozzle response time means lower cycle time. Pulse spraying with a nozzle response rate of 10 milliseconds permits faster robot movement without sacrificing lubricant coverage (figure 6). With a robot speed of 20” per second, lubricant thickness can be accurately controlled, as nozzles can achieve up to 6000 pulses per minute.
Conventional spraying with nozzle response times of 500 milliseconds cause undesirable spacing when the robot arm moves at a speed of 20” per second (figure 6). In order to achieve lubricant coverage comparable to that of the pulse method, the robot would need to move much more slowly at just 0.4” per second. Hence, by integrating pulse spraying into your die casting process, cycle time can be significantly reduced.
Spray Simulation: Achieving the Ideal Spray Pattern
Generally, die casters experience a trade-off between spray system versatility and cycle time. Versatile spraying systems that can be used with multiple dies tend to rely on more robot movement and fewer nozzles, thereby increasing cycle time. When die casters choose cycle time over versatility, the number of nozzles used tends to increase and robot movement decreases. By using simulation technology, die casters can now achieve the ideal balance between productivity and versatility, according to their needs (figure 7).
In addition to those benefits, die casters can also simulate spray thickness, allowing them to confirm optimal productivity and application prior to investing in expensive new equipment (Figure 8).
Conclusion
By simply changing your lubrication process, you can experience decreased cycle time, lowered scrap rates, minimized porosity, improved surface finish, longer die life, and more effective die cooling. For die casters who wish to benefit from the cooling properties of a water-based lubricant and the adhesion properties of an oil-based lubricant, water-based lubricants applied in a pulse spray pattern are the optimal solution. Die casters who wish to use oil-based lubricants can also experience increased efficiency and lower cycle time with pulse-spray. For die casters wishing use both oil- and water- based lubricants on the same die, hybrid spray systems are the solution.
Sources:
1. Yushiro Chemical Industry Co., Ltd. (n.d.). 水性最適塗布離型剤『ユシロンフォーム CENTRAY Lube-100』. From Yushiro: https://www.yushiro.co.jp/products_il/feature/lube100.html
About the Author
Monte Swigart is a Sales Account Manager at RYOEI USA. A 38-year veteran of the metal working industry, Monte is an active NADCA Member. Monte currently serves on NADCA’s southwest OH Chapter 14 management team.