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Die Casting News

Simulation of High Pressure Die Casting Process for Identifying and Minimizing Defects

Posted by Hill and Griffith Company on Oct 7, 2020 4:56:12 PM

Excerpt from the August 2015 article from the International Journal of Engineering Research & Technology by A. Advekar, Y. Arunkumar, and M.S. Srinath

Introduction

High-pressure die-casting (HPDC) process is widely used to manufacture a large variety of products with high dimensional accuracy and productivities. It has a much faster production rate in comparison to other methods and it is an economical and efficient method for producing components with low surface roughness and high-dimensional accuracy. All major aluminum automotive components can be processed with this technology. In this process, the metal is injected into the die at high speeds (30–100 m/s and typically 40–60 m/s for aluminum alloys) and under high pressure through complex gate and runner systems.

The mechanical properties of a die-cast product are principally related to the die temperature, the metal velocity at the gate, and the applied casting pressure. Die casting is a manufacturing process that produces geometrically complex metal parts through reusable molds, called dies. The die casting process involves the use of a furnace, metal, die casting machine, and die. The metal, typically a non-ferrous alloy such as aluminum or zinc, is melted in the furnace and then injected into the dies in the die casting machine. There are two main types of die casting machines - hot chamber machines (used for alloys with low melting temperatures, such as zinc) and cold chamber machines (used for alloys with high melting temperatures, such as aluminum). The differences between these machines will be detailed in the sections on equipment and tooling. However, in both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting.

Exploded View of Die Cast Machine

The castings created in this process can vary greatly in size and weight, ranging from a couple of ounces to 100 pounds. One common application of die cast parts are housings, thin-walled enclosures often requiring many interior ribs and bosses. Metal housings for a variety of appliances and equipment are often die cast. Several automobile components are also manufactured using die casting, including pistons, cylinder heads, and engine blocks. Other common die cast parts include propellers, gears, bushings, pumps, and valves. The work is on optimization of main process parameters in HPDC, namely, die temperature, melt temperature, and plunger velocity. Some other investigators have also worked on optimization of process parameters on die-casting by simulation. However, in many of these works, the geometry of the part is simple and there are few studies on very complex parts in the industry. The aim of this work is optimization of process parameters in die casting of a complex automotive component named ladder frame by simulation.

Process Cycle for Die Casting

Clamping

The first step is the preparation and clamping of the two halves of the die. Each die half is first cleaned from the previous injection and then lubricated to facilitate the ejection of the next part. The lubrication time increases with part size, as well as the number of cavities and side cores. Also, lubrication may not be required after each cycle, but after 2 or 3 cycles, depending upon the material. After lubrication, the two die halves, which are attached inside the die casting machine, are closed and securely clamped together. Sufficient force must be applied to the die to keep it securely closed while the metal is injected. The time required to close and clamp the die is dependent upon the machine - larger machines (those with greater clamping forces) will require more time. This time can be estimated from the dry cycle time of the machine.

Injection

The molten metal, which is maintained at a set temperature in the furnace, is next transferred into a chamber where it can be injected into the die. The method of transferring the molten metal is dependent upon the type of die casting machine, whether a hot chamber or cold chamber machine is being used. Once transferred, the molten metal is injected at high pressures into the die. Typical injection pressure ranges from 1,000 to 20,000 psi. This pressure holds the molten metal in the dies during solidification. The amount of metal that is injected into the die is referred to as the shot. The injection time is the time required for the molten metal to fill all of the channels and cavities in the die. This time is very short, typically less than 0.1 seconds, in order to prevent early solidification of any one part of the metal. The proper injection time can be determined by the thermodynamic properties of the material, as well as the wall thickness of the casting. A greater wall thickness will require a longer injection time. In the case where a cold chamber die casting machine is being used, the injection time must also include the time to manually ladle the molten metal into the shot chamber.

Cooling

The molten metal that is injected into the die will begin to cool and solidify once it enters the die cavity. When the entire cavity is filled and the molten metal solidifies, the casting's final shape forms. The die cannot be opened until the cooling time has elapsed and the casting solidifies. The cooling time can be estimated from several thermodynamic properties of the metal, the maximum wall thickness of the casting, and the complexity of the die. A greater wall thickness will require a longer cooling time. The geometric complexity of the die also requires a longer cooling time because the additional resistance to the flow of heat.

Ejection

After the predetermined cooling time has passed, the die halves can be opened and an ejection mechanism can push the casting out of the die cavity. The time to open the die can be estimated from the dry cycle time of the machine and the ejection time is determined by the size of the casting's envelope and should include time for the casting to fall free of the die. The ejection mechanism must apply some force to eject the part because during cooling, the part shrinks and adheres to the die. Once the casting is ejected, the die can be clamped shut for the next injection.

Trimming

During cooling, the material in the channels of the die will solidify attached to the casting. This excess material, and any flash that has occurred, must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting's envelope. The scrap material that results from this trimming is either discarded or can be reused in the die casting process. Recycled material may need to be reconditioned to the proper chemical composition before it can be combined with non-recycled metal and reused in the die casting process.

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Tags: Die Casting, Die Casting Release Agents, Die Casting Lubricants, Die Casting Release Agent, Die Casting Lubricant, Die Casting Process, die casting lubricant management, Journal of Engineering Research and Technology


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