When output of production and service provision process cannot be verified by subsequent supervisors or measurements, so that problems do not become apparent until after product is used or service is delivered, organization shall confirm any such process in real time. From above definition, it can be known that die casting process is a special process. Quality of die casting cannot be actually controlled during die casting process. All process parameters are only indirect measurements of external control. True quality of die casting can only be verified by post-mortem inspection; At this time, quality of die-casting parts has been formed, inspection results cannot change quality of die-casting parts in any way.
In order to ensure quality of die casting products, it is necessary to take necessary control measures for die casting process, such as strictly controlling stability of process parameters and consistency of personnel operations in die casting process. However, it is particularly important in production process whether it is possible to find unqualified die castings in time and minimize waste losses. In order to guarantee and improve quality of product, meet requirements of mass production, some quality control measures must be taken from production preparation stage to exert some benign influence on quality of die casting to ensure reliability and stability of quality of mass produced die castings. This article intends to briefly discuss quality control measures of die casting from three stages of die casting preparation, die casting process, and sample inspection (before, during, and after).

 

Practice of aluminum alloy composition and melting control shows that improving quality of aluminum alloy melt is the key factor to improve quality of die castings. Aluminum alloy melting process must have clear regulations on melting temperature, melting time, and proportion of regrind. In aluminum alloy smelting process, proportion of refining agent and slag remover required for remelting of molten aluminum is larger than that when only aluminum alloy ingot is used for melting. Proportion of increase must correspond to proportion of remelting relationship. Molten metal should be deslagged and degassed in a timely manner. During centralized melting process, chemical composition of each pot of molten metal should be tested to ensure that various chemical components of product are qualified during melting process. In addition, it should also avoid melting for a long time, otherwise it will increase hydrogen content in alloy liquid, affect strength and air tightness of die casting products. For production of castings with airtightness requirements, degassing effect needs to be monitored to ensure that gas content in alloy liquid can meet requirements for use. If necessary, a hydrogen detector is used to qualitatively measure gas content of aluminum liquid, density measurement to monitor quality of molten aluminum.
Verification of slow injection speed
Function of slow injection is to use slow motion of punch to deposit metal melt on the front of  punch without generating a jumping wave. As punch advances, melt liquid level gradually rises to top of pressure chamber, gas in pressure chamber is eliminated from mold exhaust groove. For this reason, it is necessary to verify slow injection speed set by process in slow injection preheating mold stage. Generally, a simple method can be used to confirm slow injection process parameters. When slow injection of injection piston reaches fast injection start position, injection rod automatically stops moving forward (this function can be customized on some die casting machines). After alloy liquid is cooled, it is ejected, cutting and turning are observed and judged. At correct slow injection speed, when metal liquid reaches inner runner, metal liquid in pressure chamber has completely become a complete cylinder. When internal tissue is found to be dense and there are no pores visible to naked eye, slow injection speed at this time generally suitable. Improper slow injection speed will cause severe entrainment. It can be seen from Figure 2 that when slow shot velocity is 0.17m/s, there are serious holes in material handle. This will adversely affect quality of casting.
Due to generally low degree of automation of die casting productions, consistency of production process operations is basically in a state of semi-manual operation. Quality of die castings has a great relationship with operator's operating skills, working environment, and mental state. In order to make these factors as stable as possible, we recommend main measures to be taken in this regard are "three specific" of specific people, machines, and products, that is, arranging specific people to produce specific products on specific equipment. Once any of these three factors have changed, they must be identified and monitored.

 

Confirmation of consistency of multiple adjustments of process parameters
In general, production of each product is performed in batches, consistency of process adjustment after each new mold replacement must be clearly confirmed. Speed adjustment method that is relatively common on current die casting equipment is relatively simple. Slow injection speed is basically not detected and controlled. Fast injection speed is often adjusted by hand wheel.
Check process parameters according to handwheel scale provided by manufacturer and two-speed comparison table. This control method itself has great quality risks, which mainly manifest as follows:
First, handwheel opening of equipment from different manufacturers is not consistent with two rapidity comparison curves.
Second, actual speed of same type of equipment from same manufacturer and opening of handwheel are inconsistent.
Third, there is a discrepancy between actual speed and opening of handwheel at different stages of same device. These inconsistencies will make technicians at a loss. At production site, die castings produced by original process are qualified, but according to same production process, qualified products cannot be produced. Actual reason for this performance is that process parameters of these two productions are inconsistent. Simple method to solve this problem is: during production process, use a linear velocity meter to measure slow injection speed and fast injection speed, check each time according to determined value. Of course, for equipment with injection curve, injection speed can be easily detected on device, each adjustment can be performed.
Trial processing of internal quality of die-casting parts is the most convenient and fast testing method through X-ray inspection, but it has disadvantages of large investment and unintuitive results. Generally, the most direct inspection method is trial processing. After trial processing, quality of machined surface can be observed more intuitively, a reasonable judgment can be made on quality of product. For an enterprise that simultaneously produces blank and performs subsequent machining, it is feasible to send produced blank directly to production line for trial processing to determine internal quality of blank. However, for a factory that only produces blanks, although method of product trial processing inspection, although product quality of blanks is checked, trial processed products will be scrapped directly. If product can not be well controlled during trial addition process, it will also be a considerable loss. Trial processing is mainly for those products with high internal quality requirements and air tightness requirements. Products of automotive air-conditioning compressor casings produced by our company are subjected to 100% air tightness test after machining. At the same time, more stringent helium inspection is required after assembly of complete machine. If a leak is found in helium inspection, the whole machine will be discarded and losses caused by this will be borne by responsible party. For such products, strict, efficient, and practicable real-time inspection methods must be adopted to ensure quality of product parts. For such compressor housing products, our company generally only provides rough parts to be machined by host plant. Machining cost of a single product is much higher than value of blank. Host plant has strict control on rejection rate for each batch of products. In order to control internal quality of die casting parts, it is necessary to carry out a leak test on product parts. Since leak test process can only be performed after blank is machined, blank must be processed at corresponding location before leak test.

 

In order to speed up inspection speed, we have adopted simplified processing during machining. First, all threads are treated with round holes of equal diameter.
Second, inner hole is processed according to required size.
Third, machining allowance should be not less than design allowance. By adopting method of leak testing after machining, we can find problems of product in time and adjust die casting process accordingly to ensure quality of product.

（2) Destructive inspection

So-called destructive inspection is self-evidently a method of inspecting some dimensions and properties of a product after it is destroyed. Direct consequence of this inspection method is that products are scrapped and cannot be used again. However, it can enable inspectors to more intuitively observe and measure some internal characteristics of die castings. As we all know, die casting process is a high-temperature, high-pressure, high-speed metal liquid filling process, which will inevitably cause a large liquid flow impact on cantilever core here. Factors such as long-term impact of molten metal and inadequate removal of mold parting surface adhesion will cause core bending and inaccurate positioning. Once core is bent or misaligned, product will be discarded. Due to inconvenience of disassembling core on mold, in order to find potential quality hazards in a timely manner, we adopted a method of destructive inspection of steering gear housing. Produced die casting parts is sawn from the smallest end of core. Shape after sawing is shown in FIG. 5c. Wall thickness can be used to determine whether core is bent or deformed.