Helping Die Casters Overcome Productivity and Quality Challenges
The function of a die lubricant is to facilitate production of high quality castings productively while minimizing the environmental impact in the work place and in the effluent stream. From a lubrication perspective, anything that prevents the easy release of the casting from the die is a problem. Through our long associations within the Die Cast industry, including our work as far back as the late ‘60s when we were recognized for our leadership by the Die Casting Research Foundation, and our years of shop floor experience working shoulder-to-shoulder with our customers, we have an extensive knowledge of the problems you face in your die cast operations. Problems such as:
- In-Cavity Buildup/Carbon
- Cold Shut
- Fish Eye
- Environmental Challenges
Soldering is caused by the reaction of the molten alloy with the tool steel. Soldering is accelerated by high temperature and erosion and is commonly seen in those areas of the die where such conditions are present. Soldering can be prevented (or at least minimized) by forming a barrier coating between the molten metal and the tool steel. Certain alloys are more susceptible to soldering. Chem-Trend’s advanced technology makes a wide variety of die lubricants designed to function optimally at different temperature ranges and alloys. Chem-Trend’s proprietary polymers provide exceptional performance at high temperatures, while silicone-free options are available for cooler die surfaces.
Porosity is one of the biggest problems in die casting. The die casting process is very often used to make lightweight components out of light metals to replace steel or iron parts. Since the intrinsic strength of these lighter metals is less than that of steel, anything that can adversely affect the strength of the component is undesirable. The presence of small pores within the cross section of the casting can have a big impact on the tensile strength.
There are two main types of porosity, namely gas porosity and shrinkage porosity. Gas porosity is the consequence of the very high velocities at which the metal is injected into the mold. However, it can also be aggravated by the presence of water on the die and the decomposition of any organic compounds within the die or the shot sleeve. Since all die and plunger lubricants tend to contain water, organics or both, they can contribute to this problem. Chem-Trend’s well-designed products produce extremely small amounts of decomposition products and all liquid water is evaporated or blown off from the die before molten metal is admitted, thus minimizing any direct contribution to gas porosity.
|Gas porosity defects||Shrinkage porosity defects|
Gas porosity can be also minimized by providing overflows and vents, which allow both the air in the cavity and any decomposition products to escape from the body of the casting before the metal solidifies. Using the proper diameter shot sleeve and fill speed can also minimize porosity, but the proper location and sizing of vents is essential to make good castings. In addition, the choice of die lubricant can impact the effectiveness of the vents.
Die lubricants are usually sprayed into the die cavity, but with any spray set-up, a certain portion of the spray will end up outside the target area such as in the vents. This is usually referred to as overspray. As the water evaporates a film of lubricant can be formed in the vents. Unlike the film in the cavity, this film is not completely removed when the castings are extracted and, over time, can restrict the flow path of the escaping gases. Chem-Trend’s recent advances have focused on this area of interest and our latest range of die lubricants specifically minimizes the amount of over-spray that is generated by the die lubricants, thus further improving the quality of castings.
Shrinkage porosity occurs because most cast metals have a slightly lower density in the liquid state than in the solid state. This means that, as the metal solidifies, it will occupy less volume. As long as liquid metal can enter the casting as it solidifies, shrinkage will be minimized.
Unfortunately, when a casting has many different cross-sections, a ‘hot spot’ can develop. This portion of the casting will remain liquid after all the other parts have solidified, and so will create a shrinkage pore when it solidifies. The best way to control shrinkage porosity is by improved design of the die cavity, intensification pressure and metal temperature. Chem-Trend die lubricants can help solve this problem through improved cooling characteristics that can control the incidence of ‘hot spots’.
The presence of dark extraneous material within the die cavity is referred to as ‘carbon’ or in-cavity buildup. This is very different from solder, in that it is not bonded to the metal, but rather is a layer on the surface that can be removed without having to chemically dissolve the alloy. When a die lubricant is sprayed onto the die, the water evaporates leaving behind a thin film of die lubricant that provides release and prevents solder. Usually this film is removed when the casting is ejected. In some cases – especially when the die is running under its designed temperature – this lube film does not get completely removed and so it gradually builds up within the cavity. The best way to correct this is to reduce the amount of die lubricant being applied. However, since the local temperatures on different parts of a complex die can be quite different, reducing the lubricant spray quantity to eliminate build-up may result in solder. Chem-Trend die lubricants offer superior balance between high temperature and low temperature film formation, providing more uniform protection and mitigating this problem.
A common cause of in-cavity build-up is the dilution water. Generally, softened water is recommended for diluting die lubricants. If hard water is used, the dissolved calcium and magnesium salts will precipitate when the water evaporates. These materials are not decomposed by molten metal and so they can build-up within the cavity. Chemical analysis of the residue from within the cavity will almost always show the presence of die lubricant components, but the presence of hardness salts will identify the dilution water as being the root cause. Upsets in the water softening system can lead to hardness deposits in the die cavity.
Occasionally, castings may show dark lines on the surface that appear to conform to the flow path of the molten metal. This is particularly true in magnesium alloys and in very thin sections. In extreme cases, the casting may not fill completely. In other instances, the surface of the casting may show what looks like a crack but in actuality is two layers of metal solidifying on top of each other. This is sometimes referred to as ‘cold shut’. This is an indication that the casting has started solidification before the die cavity has been completely filled. This could be due to low mold temperatures, low melt temperatures or excessive cooling. Chem-Trend die lubricants are engineered to regulate the rate of heat transfer to the die and thus have been very successful in averting fill problems in castings.
A common concern about die lubricants is their effect on operations like painting, plating or gluing that are carried out on the castings. As release agents are designed to prevent adhesion between the casting and the die surface, they can interfere with paint adhesion or plating, resulting in a ‘fish eye’ defect, if they are not properly removed by the cleaning process. Chem-Trend’s release agents are readily cleaned by the alkaline washes typically used before a painting or plating process.
The earliest release agents were oils that were manually applied to the hot die before the molten metal was introduced. This constituted a major health and safety hazard and Chem-Trend was a pioneer in the development of safer alternatives. Today water-borne die lubricants are used in most die-casting operations. They are usually supplied as concentrated emulsions and are diluted with water at the casting site just before use. Chem-Trend has been an industry leader in the development of new and unique formulations to serve the growing technological challenges faced by the industry. Increased dilution ratios and lessened environmental impact have been major areas of product improvement over the past decade. Recent innovations include products that can operate with extremely high die temperatures, drastically reduce the overspray build-up on the die and significantly improve productivity in a die casting operation. For real-world examples of the benefits of our die casting process aid, please read the following case studies: Ancillary Products Add Value, Improved Productivity through Product Innovation and Product Performance Leads to Operational Savings.
The use of dilution water does generate a significant volume of liquid waste. To tackle this problem, Chem-Trend has a range of dry powder die lubricants that can be applied electrostatically or pneumatically to the die surface. Upon contact with the hot surface, these materials melt and flow across the surface providing a thin and uniform protective layer. Powder die lubricants do not create a thermal shock to the die surface, thus minimizing fatigue and extending tool life.
For applications with very fast cycle times and low die temperatures, Chem-Trend offers a number of solvent-based die lubricants as well. Designed to function very efficiently, the small application rates and low operating temperatures of these materials minimize the risk of flammability while providing clean, bright castings.
With our many years of experience solving these types of die casting problems we are confident that we can help you. Why not contact us and talk to one of our experts?