[Abstract] This article starts with "materials" in quality management, analyzes role of materials in quality control of die-casting workshops, briefly describes impact of materials on quality of die-casting parts, quality control methods and ideas for materials themselves; material management is focus and difficulty of quality management of die-casting parts. Doing this work thoroughly and meticulously will play a guarantee role in improving quality of die-casting parts.
Five major elements in quality management are: people, machines, materials, methods, and environment; "material" among them is basis for producing qualified products. Materials in die-casting production include main materials for die-casting (alloy ingots) and some die-casting auxiliary materials; quality of raw materials determines quality of die-casting parts. Without qualified raw materials, it is impossible to produce qualified products. Following takes aluminum alloy die-casting as an example to make a brief statement on how to manage main and auxiliary materials.

 

Since aluminum alloys have requirements for gas content and hard points, aluminum ingot manufacturers must do a good job of refining, degassing, and slag removal to prevent defects such as high gas content and high impurities in aluminum ingots from being passed on to die-casting aluminum liquid. After aluminum alloy ingot enters factory, appearance is first inspected. Surface of aluminum alloy ingot is required to be smooth, without roughness, without oil stains, mildew spots, or scale; fracture structure of aluminum alloy ingot must be fine and dense; as shown in Figure 1(b), there must be no severe segregation, shrinkage cavities, slag and inclusions. Carry out composition inspection for each batch and each heat number to ensure that alloy composition used is qualified. When producing products with special requirements, other testing items need to be added. For example: when producing products with mechanical performance requirements, tensile test rods are required to be submitted for each furnace when aluminum alloy ingots are delivered. When producing products with air tightness requirements, pinhole testing is required for aluminum alloy ingots.
Types of aluminum alloys used in actual production are often not limited to one grade, and requirements for chemical composition of each grade of aluminum alloy are quite different. Main elements in one brand of alloy may be considered impurities in another brand of alloy; for coexistence of multiple brands, it is necessary to clearly visually distinguish each brand of alloy to prevent occurrence of mixing during use; general measure is to clearly and uniformly specify exterior coating colors of aluminum alloys of different brands and manufacturers, store different materials in separate zones after arriving at factory.

 

Production practice shows that improving quality of alloy melt is a key factor in improving quality of die castings. Therefore, raw material management should be strict so that unqualified raw materials do not enter workshop or be put into production. Operate strictly in accordance with standards required by alloy melting process, strengthen management of various raw materials (new materials, recycled materials, auxiliary materials).
Aluminum alloy melting process must have clear regulations on melting temperature, smelting time, proportion of recycled materials, etc. In particular, recycled materials should be strictly classified and managed and cannot be mixed. Recycled materials should be clean and should not contain oil stains, rust, sediment, moisture, inserts, etc.; recycled materials with oil stains can be remelted by aluminum ingot manufacturer and cannot be directly put into melting furnace for use; those scattered aluminum blocks must be screened to remove dust, sand and other debris before they can be used; Those scattered aluminum blocks must be screened to remove dust, sand and other debris before they can be used; where aluminum liquid from recycled materials is used, proportion of refining agent and slag remover required is larger than when only aluminum alloy ingots are used for melting. When producing in weather with high relative humidity, proportion of recycled materials should not exceed 30%. In dry weather, proportion of recycled materials can be increased appropriately, but it should not exceed 40%.
Slag and degassing of smelted aluminum liquid must be carried out in a timely manner, and corresponding parameters must be recorded as required; chemical composition of each pot of molten aluminum should be tested during centralized melting process to ensure that various chemical compositions of product are qualified during melting process. Long-term melting should also be avoided, which will increase hydrogen content in aluminum alloy liquid, affect strength, air tightness and other properties of die-cast product; time from melting to die-casting should not exceed 4 hours.

Use of release agents can make filling of liquid metal smooth, facilitate molding, prevent mold sticking, make castings obtain bright, smooth, and flat surface quality. It also has a great impact on production rhythm, surface and internal quality of castings. At the same time, it can protect mold, avoid erosion of mold surface by high-temperature liquid metal, reduce thermal conductivity and mold temperature of mold, extend life of mold; when opening mold, it is conducive to smooth ejection of casting, also plays an important role in reducing friction and wear on punch, ejector pin, and movable parts. Control of release agent includes selection and proportion of release agent itself, also includes operator's management of spraying and drying of release agent. Generally, selection of release agent has following requirements:
(a) It has good lubricity at high temperatures; it has a low volatilization point, and moisture in release agent can evaporate quickly at 100-150℃;
(b) It has no corrosive effect on die-casting molds and die-casting parts; stable performance, release agent should not evaporate too quickly and become thickened in the air;
(c) Harmful gases will not be precipitated at high temperatures; no fouling will occur on the surface of die-casting cavity.
At present, spraying of release agents is mainly manual work. Reasonable spraying operations are an important factor in ensuring quality of castings, life of die-casting molds, and production efficiency. Spraying time of release agents also varies with release performance of different types of release agents, dilution ratios, different die casting shapes, different mold temperatures, etc. It takes a long time for release agent to condense from spray gun to the surface of mold to form a protective film. Casting before this process is completed is often source of loose defects in castings. According to regulations of die-casting process, a reasonable spraying time range should be adopted for different types of release agents and different dilution ratios. Die-casting workers must understand meaning of upper and lower limits of process parameters and trend of adjustments, make appropriate adjustments based on the surface conditions of die-casting parts produced.
When using water-based release agents, ratio of release agents must be strictly controlled. If ratio of release agent to water is too concentrated, a thicker film will be formed on the surface of mold, and release agent will slowly accumulate on the surface of mold. Moisture in release agent cannot be completely discharged and gas is generated, making internal density of die casting poor. If ratio of release agent to water is too thin, effect will not be achieved, die-casting parts will be strained and stuck to mold.
Therefore, in quality control of die-casting workshop, management of release agents has a relatively large impact on quality of die-casting parts. It is necessary to strengthen management in this area; designate a dedicated person to be responsible for ratio of release agents. Clearly stipulate ratio of release agent and water in production process of various types of die-casting products. According to release agent preparation process instruction manual, fixed quantity is prepared. When spraying, operators must undergo strict training on spraying movements. They must follow requirements and standardize movements, no more spray, less spray, or missing spray is allowed.

 

At present, most oil-based punch lubricants or particle lubrication are used. No matter what form of punch lubrication is used, after aluminum liquid enters barrel, it needs to be fully burned in a short time, and residue must be distributed in upper layer of molten soup. In this way, impact of punch oil on product will not be too great. Otherwise, gas generated by combustion and residue after combustion will enter interior of product, and consequences can be imagined.
A simple way to verify punch oil is to observe color of material handle on module. Generally, thickness of material handle is about 1/3-1/4 of diameter of punch. Length of punch that is burnt and blackened should not exceed 3/5 of thickness of material handle; At the same time, end of material handle should not be obviously blackened (that is, there is too much graphite powder residue). Otherwise, after product is processed, defects such as pores will inevitably increase.
During production process, you should always pay attention to working condition of punch lubrication device. If too much or too little lubricant is sprayed, it will have a negative impact on the life of pressure chamber and punch, as well as quality of die castings. When adjusting amount of lubricant used, it is necessary to consider both lubricity of punch and demoulding property of mold. Especially when particle lubrication is used, no matter how wax pellets are used in die-casting mold, there will be a problem of volatilization of organic compounds. If a large amount of volatile organic compounds cannot be discharged well, it will have a great impact on generation of pores in die-casting parts.

Punch and pressure chamber form an interdependent system. Under normal circumstances, life of pressure chamber can reach 2-3 years, while life of a punch can be as short as one shift, or as long as more than 10,000 times; life difference will cause fluctuations in product quality and significant changes in production costs. Therefore, good management of punches can not only stabilize quality of die castings, but also greatly reduce die casting production costs.

Under normal circumstances, there needs to be a certain matching gap between punch and pressure chamber; practice shows that ideal gap is about 0.1 mm. During injection, if gap is too large, aluminum alloy liquid may enter gap, aluminum liquid leakage and smoke will occur, which will aggravate wear of punch and reduce life of punch; if gap is too small, jamming will occur during punch movement, punch speed will not reach speed predetermined by process, mold filling time will be extended, and die casting quality problems will increase. Therefore, processing and manufacturing of punch must be carried out strictly according to drawings, in order to ultimately control critical gap, extend life of punch, and ensure product quality.

 

Since die-casting process determines that temperature of punch is constantly changing, as temperature changes, gap between punch and pressure chamber also changes; these changes not only affect service life of punch, but also affect stability of injection process during die casting process; to maintain ideal gap, it is necessary to cool punch. When designing, consider fully cooling punch through water core of injection rod, taking away heat of punch to the maximum extent, maintaining stability of ideal gap between punch and pressure chamber; these measures are helpful to ensure stability of each mold injection process and stability of die casting product quality. Practice has shown that an injection system with good punch cooling can not only ensure stability of injection process, but also greatly extend service life of punch. However, sealing work of cooling water path of punch must be done well, leakage of cooling water caused by poor sealing between punch and injection rod must be avoided. If insufficient attention is paid to leakage of cooling water, air tightness of produced products will be seriously affected.

 

Use of crucibles is crucial for aluminum alloy die-casting, especially use of cast iron crucibles. If operation cannot be carried out in strict accordance with crucible usage specifications, it may cause Fe content in aluminum alloy components in subsequent production process to exceed standard; according to our experience, if cast iron crucible can be painted with crucible paint in strict accordance with requirements for new pot use, once every 3 days during use, basically increase in Fe content during the entire production process of die-casting workshop will not exceed 0.2%; Under premise of controlling incoming inspection of aluminum alloy ingots, we can fully ensure that alloy composition of produced die-casting parts meets standard requirements. Although there is no concern about increasing iron when using graphite crucible, it should also be paid attention to. Drying process for first use of graphite crucible must be strictly implemented. These operations will affect service life of graphite crucible in the future. Life of preheated graphite crucible will be greatly affected. Each graphite crucible is expensive, which affects production cost of die-casting workshop. Preheating curve of a general graphite crucible is shown in Figure 2.

Among five major elements of quality management, above mainly discusses issues of "materials" and "people". While managing raw materials and auxiliary materials well, we must also do a good job in personnel training and management. It is necessary to further improve technical level of die-casting workers in order to effectively solve quality problems in die-casting production; applying idea of total quality management to quality control of die-casting workshop and using systematic theoretical knowledge of die-casting production to guide production will surely elevate quality control capabilities of die-casting workshop to a new level.