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Tool Treatment Extends Core and Pin Life in Die Cast Operationsby
Dr. Tohru Arai, Technical Advisor Producing intricate shapes quickly with excellent precision and good surface quality at a low cost is one of the advantages of die casting. But, maintaining this advantage is difficult. A die casting die can cost more than the machine that operates the die. Die casting is hard on dies and die components and pulling the dies and pins for repair and/or to polish them is costly. To help maintain the advantages of die casting and production up time, die life needs to be extended by reducing repairs. A tool treatment process called thermal diffusion (TD) can help die casters reduce maintenance and repairs and extend core and core pin life. The TD process prolongs die life by decreasing the damage caused by thermal and mechanical cyclic loadings and the reaction with active cast materials that severely damage dies and die components. It reduces the soldering, corrosion, erosion, wash-out, wear and heat checking from the loadings and reactions with other materials. It also provides excellent peel and adhesion strength. Die life is significantly extended and repairs and polishing are reduced. The process also helps protect aluminum die casting dies which are especially vulnerable to die casting problems. One of the reasons it works effectively in die cast operations is found in the process itself. During the treatment process, the substrate surface of the core and core pins is modified, creating a metallurgically bonded non-porous vanadium carbide (VC) layer.
The VC layer is infused into and onto the dies and die components (Fig. 1 & 2). This VC layer is .0001- to .0006-inch thick and has a hardness of 3200 Vickers at room temperature. Even under the high working temperatures used in die casting, the VC layer retains its hardness level. And, once the die is cooled to room temperature, the original hardness of VC is restored to previous levels. For example, at 800° Centigrade, the hardness of the VC layer drops to approximately 900 Vickers, which is still above the hardness of other treated surfaces. As the VC layer cools to room temperature, it returns to near its original hardness of 3200 Vickers. Its ability to maintain its hardness under high temperature applications allows it to perform well in casting operations. The process also is effective in die cast operations because the hardness of the VC layer is higher than abrasives such as aluminum oxide, which is found in hone stones and sandpaper used for polishing cores and pins. This allows any aluminum that mechanically sticks to a pin to be easily removed without significant damage or dimensional change to the pin. Polishing time and the resultant wear on the pin are greatly reduced. Another reason it is effective in die cast operations is that it does not react chemically with aluminum. It protects dies from the aggressive nature of aluminum and prevents the chemical interaction between the casting alloy and the die. Therefore, corrosion and soldering are reduced. With approximately 75% of die cast parts being produced from aluminum, the process can make a significant difference in other die cast operations utilizing zinc and magnesium. The TD process is not widely used in die cast operations in the United States. Typically, casters do not use tool treatments at all, although a limited number have begun trial tests. Japanese and European die casters do use tool treatments and have been using the process for many years with excellent results. In Britain, zinc die casters are also using it. (British results were published in Die Casting Engineer.)1
In Japan, the treatment process has been shown to be especially effective with conventional hot working die steels such as H13, a tool steel typically used to make die cores and core pins (see Fig. 3). Damages caused by die casting were significantly reduced. The results from European die casters tooling treatment have been detailed in several European pulblications as well as Die Casting Engineer.2
Fig. 3. Core die which has been treated with the process. By using treated tooling, the Japanese also realized other benefits. They reduced interruptions in the produciton process, helping to eliminate the reconditioning of the casting condition. This reduced cast scrap and energy usage. The surface quality and dimensional accuracy of cast products were also improved. Using the treatment process meant less sticking of cast materials on pins. The improved surface quality and dimensional stability of the cored surfaces also allowed thread tools to last longer. With the use of the tooling treatment process, the Japanese also reduced the casting distortion caused by excessive stresses in ejection of cast producs. And, finally, the life of tools was increased. Today, in Japan, over 8000 pins, insert cores and other die casting components are treated each month, Japanese die cast plants based in the USA also use the treatment process. In the United States, when die casters do use a tool treatment to help reduce die wear, it is typically nitriding, a tool surface treatment. In comparison tests and long term usage tests with nitriding and TD in casting operations, TD was shown to be more effective than nitriding (Fig. 4).
Fig. 4. Comparison tests between nitriding and TD show that TD significantly increases pin life.
The process also significantly reduced the number of pins abandoned per machine and the time required to polish pins with and without die reset. In comparison with conventional nitriding, the process significantly extended core pin life, decreased pin maintenance costs and improved the surface quality of the end product. In a field test between 1972 and 1979, a Japanese company completely replaced nitriding with the process. During this time period, the company increased the total number of castings produced by 90%, reduced the time spent on die repair work by 80%, and increased pin life an average of 4 times by using the process (Table 1 and 2). Profits were increased as well, since downtime was reduced. Table 1. Results from a field test conducted by a Japanese company between 1972 and 1979. In the test, the company completely replaced nitriding with the tool treatment process.
Table 2. Test results between nitriding and the TD process show that TD increases pin life and significantly reduces polishing time and the number of pins abandoned in comparison to nitriding.
* Numbers per machine Test results from an American die caster show the same results. Prior to using the process, the company pulled a core die every 4 hours to clean the soldered material. Each cleaning took an average of 30 minutes. This meant one hour of lost production time for every eight hour shift. The use of treated core pins completely eliminated the need for periodic cleaning of the core.
The tests further showed that by treating only one die, the company increased die life by more than 10 times and realized more than $20,000 in savings by reducing downtime and labor to clean dies. The treatment cost less than 10% of the savings realized by the company, making it an economically viable solution to die wear problems. The process can work for United States die casters, too. It can extend die life, reduce maintenance, polishing and repairs. References
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