Excerpt from the August 2018 article from the International Journal of Engineering & Technology by P. Nagasankar, et.al.
The main objective of this research is to reduce the blowholes by analyzing the factors which are affected during the casting process. The process parameters are optimized and change is made in the design part to reduce the blowhole and to increase the efficiency of the high pressure die casting machines. Product manufactured from every manufacturing process shows some defects. For supplying quality product to the customer these defects must be reduced. In this work, an attempt is made to reduce the rejection due to the blowhole defects found through why-why analysis technique. Process capability of current high pressure die cast manufacturing is checked. The current problem of blowhole defects are solved by making an improvement in design of the die inserts. In-gate directions are changed to obtain modified improved flow patterns. Using magma flow simulation software, existing and modified design is compared. It is found that modified design shows superior results, and using this, the defect of blowholes is minimized satisfactory.
Die casting involves the preparation of components by injecting molten metal at high pressure into a metallic die. Die casting is closely related to permanent mold casting, in that both the processes use reusable metallic dies. In die casting, the metal is forced in under pressure, compared to permanent molding. This proces is also called pressure die casting. As the high pressure is involved in die casting, any narrow sections, complex shapes and fine surface details can comfortably be produced.
In die casting, the die consists of two parts. One is the stationary half or cover die, which is fixed to the die-casting machine. The second part is the moving half or ejector die, which is moved out for the extraction of the casting. The casting cycle starts when the two parts of the die are apart. The lubricant is sprayed on the die cavity manually or by the auto lubrication system so that the casting will not stick to the die. After the two halves of die are closed and clamped, the required quantity of metal is injected into the die. After the casting is solidified under pressure, the die is opened and the casting is ejected. The die casting needs to have the provision of ejectors to push the casting after it gets solidified. It will also have cooling channels to extract the heat of the molten metal to maintain proper die temperature.
There are two types of die casting machines:
- Hot-chamber die casting
- Cold-chamber die casting
The main difference between the two types is that in the hot chamber, the die-casting machine contains a holding furnace, where the cold-chamber machine possesses a separate furnace to melt the metal. The molten metal is poured into the die-casting machine with a ladle for each casting cycle, which is called the shoot.
6.1. Causes of Porosity
Porosity is defined as a void in a cast part where there is absence of cast metal. The main cause of gas porosity is high velocity of the metal injection. Before solving any porosity problem, identification of the type of porosity is essential to obtain an optimum solution for minimizing defects. A blowhole typically shows a series of smooth pores in casting as shown in the figure. There are series of pores near the gate region in final product. Therefore, the mentioned porosity is gas porosity.
After the type of porosity is recognized, the root cause for the issue is found. The North American Die Casting Association lists some causes for gas porosity. Some causes include: trapped air in plunger system, poor gate design, turbulent metal flows inside the cavity, etc... Some secondary causes are coolant leakages in the die, excess lubricant and die coat spray on the die, etc... The figure shows the causes and effect diagram for porosity defect through considering all possible causes. We concluded that maintained blowhole defect problems can be reduced by either modifying process parameters or by modifying gating designs.