For die-casting sites, automated and intelligent equipment have been used in production. However, how to ensure long-term stable production of these equipments, main task is to "maintain stability", find out fluctuation factors in each link, use feasible and reliable methods to restore them to a stable state, "seeking stability first, then seeking improvement", this work is repeated over and over again, there is no one-and-done solution, and there are no shortcuts. Product quality problems encountered at die-casting site may not be caused by one factor. When solving problem, multiple factors need to be integrated to deal with it, so that product can be produced efficiently, with high quality and stably. This article uses Six Management Factors (5M1E) to find out factors that may cause instability on die-casting process site, controls and improves these factors, proposes a strategy for maintaining stability of die-casting process.
Die casting is a technology that produces parts with complex shapes with high automation and high efficiency. Parts produced by this technology have advantages of good density, high precision, small machining allowance, and excellent mechanical properties. They have been widely used in automobiles, mechanical equipment and other fields.
In high-pressure casting industry, problems of pores, shrinkage cavities, and internal cold isolation in die-casting parts have always troubled die-casting people. Although there are prescribed standards for requirements for porosity and shrinkage, quality of die-casting parts is often good and bad due to various uncertain fluctuation factors in production process, which leads to unstable quality of die-casting parts.
Due to rapid filling and forming, a certain amount of gas will remain inside die casting due to various factors. With emergence of advanced process technologies such as vacuum die-casting, oxygenated die-casting, and local extrusion, internal quality of die-casting parts has been improved to a certain extent, but existence of pores, shrinkage cavities, and cold insulation is still unavoidable. Even so, problems such as pores, shrinkage cavities, and cold insulation can be controlled within a certain range.
Die-casting actually refers to collective name of the three elements of die-casting machine, die-casting alloy and die-casting mold. Die-casting process integrates and applies these three elements to production. Hoda Dini et al. studied effect of die-casting pressure on deformation and residual stress of die-cast AZ91D alloy castings. Research results show that die-casting pressure has a greater impact on deformation and residual stress of castings. Increasing die-casting pressure will reduce deformation of casting, but residual stress on the surface will increase. Many scholars have studied issues such as temperature field distribution and temperature gradient of molds and die-casting parts during die-casting process. P.Sharifi et al. conducted experiments to study influence of process parameters on castings during die-casting process. Results show that among many process parameters, punch speed has the greatest impact on porosity of castings. However, for some special alloys, slow injection speed has a greater impact.
Nowadays, more and more companies are using Internet big data to monitor die-casting process in real time and adjust die-casting parameters. Formulation of die-casting process plan is a very important step. Rationality of process directly affects quality of castings and subsequent processing and production links. Quality of die-casting parts is often related to many factors. Reasonable and effective control of quality of die-casting parts requires comprehensive consideration of many factors. Long-term stability of die-casting quality is very important to company's costs and profits. However, complexity of die-casting site makes it more difficult to control and manage quality of die-casting parts. Based on this, a die-casting process stability maintenance strategy based on six factors of on-site management (5M1E) was proposed to systematically solve problem of quality management of die-casting parts at die-casting site.

When quality fluctuations occur in die-casting parts, on-site technicians often first choose to adjust process parameters instead of finding root cause. For example: when internal defects occur, change high-speed switching point, increase fast injection speed, and increase pressure opening; ② When mold is pulled, increase release agent spray volume or lengthen spray time; ③ When punch is stuck, increase amount of lubricant; ④ When temperature of aluminum liquid in holding furnace is not enough, use a flamethrower to bake on liquid surface of pouring port.
Each of above inappropriate adjustments may bring about a 3% defective rate, and more than three inappropriate adjustments may bring about a defective rate of more than 10%. Therefore, we should not think too deeply about problem, but find cause in detail for each problem, use appropriate methods to fundamentally solve problem and maintain it. That’s kind of conviction our field technicians need to have.

When equipment, molds, and process technology conditions have met quality requirements of die-casting parts, the most critical thing at die-casting site is to control stability of comprehensive factors. For example: comprehensive condition of equipment and mold, quality of compressed air and range of pressure fluctuations, insulation effect and temperature fluctuation range of alloy liquid, mold temperature and ambient temperature. Only when comprehensive factors are stable can quality of die castings be relatively stable.

Six factors of on-site management refer to Man, Machine, Material, Method, Measurement and Environment, referred to as 5M1E. As shown in Figure 1 fishbone diagram.

 

Figure 1 Fishbone diagram

People are leader in die-casting scene and are the first of all factors. Including production safety, all work surrounding production activities is decided by people. Product quality will always fluctuate. Use management tools to analyze and solve problems, control fluctuations in each link to a minimum, and reduce the rate of defective products. All of this still relies on people.

Machine refers to working machine, mold and supporting auxiliary equipment. Due to importance of mold in die-casting process, it will be analyzed as a separate factor below. Capability of machinery and equipment is decisive factor in production and manufacturing capabilities, so the higher high-end equipment, the higher maintenance requirements.
During production arrangement process, machines and equipment suitable for different products are selected. Carry out daily inspections so that problems can be discovered and repaired promptly. Carry out regular maintenance or scheduled maintenance to ensure that equipment is in good condition and has stable performance. It is strictly prohibited to dismantle one thing to make up the other, and keep equipment intact. Problems that are easily overlooked at die-casting production site: ① Impact of poor matching of injection rod connection part on quality of die-casting parts; ② Impact of unstable feeding of feeder on quality of die-casting parts. As shown in Table 1 (density of liquid aluminum 2.64g/cm³).

Table 1 Relationship between material handle thickness and weight
As mentioned in Table 1, because most of our current die-casting machines use position triggering to set high-speed switching point, if pouring machine is unstable (thickness of material handle is too different), it can directly lead to a change in actual starting point of high-speed filling, affecting stability of die casting quality. Assume that when high-speed switching point has been set at a certain position value, as shown in Figure 2, Figure 3, and Figure 4, flow channel is full and aluminum liquid just reaches gate. This is ideal high-speed switching point, as shown in Figure 2.

 

Figure 2 Normal state when pouring volume is stable
Some molten aluminum has entered mold cavity through inner runner during slow injection process, which can easily form a cold isolation inside casting, as shown in Figure 3.

 

Figure 3 Abnormal state when pouring amount is too much
Liquid aluminum failed to fill flow channel, and gas was trapped in the front end, causing pores inside casting, as shown in Figure 4.

 

Figure 4 Abnormal state when pouring amount is too small
Unstable spray volume and atomization effect of sprayer can also affect quality of die castings. Large fluctuations in compressed air pressure, normal air pressure should be controlled at 5.5 ~ 6.5 kg/c㎡, which is very important; large fluctuations in air pressure cause release agent pressure to also fluctuate significantly. Large fluctuations in air pressure and water pressure cause release agent spray volume and atomization effect to change with changes in air pressure. Mold temperature and release lubrication effect also change accordingly, and product quality also fluctuates. Air humidity in most areas of southern my country is relatively high in summer, and a large amount of emulsified water will be produced in compressed air. This emulsified water will be sprayed onto mold cavity with air flow, which will increase number of pores and cold insulation inside die casting, and will also cause spots and other undesirable phenomena to appear on the surface of die casting. This is an undesirable factor that many people ignore. Therefore, automatic drain valves should be installed at the bottom of gas storage tank and the lowest point of main pipe to automatically drain water regularly to minimize moisture in compressed air.

On the premise that equipment meets on-site conditions, condition of die-casting mold has main influence on quality of die-casting parts. During die-casting production process, on-site personnel often spend the most time and energy on mold. Therefore, daily maintenance of mold is very important. Following points should be paid attention to.
(1) Ensure that parting surface is intact. If parting surface is not well matched, air edges or flying materials may easily occur. Flying materials will cause pressure loss in cavity and no material feeding, causing large pores and large areas of looseness inside die casting. For quality and safety, strict control is required.
(2) Maintenance of size and shape of gate. At die-casting production site, it is very important to control thickness, width and extension of gate. During production process, as gate continues to be eroded, width and thickness of gate continue to increase, extension of gate becomes shorter and shorter, internal defect rate of die casting will become higher and higher.
Above phenomenon is called runner erosion, which is an inevitable phenomenon during die-casting production process. Adverse consequences caused by sprue erosion are common and often ignored. Due to erosion, width and thickness of gate are increased, which increases cross-sectional area of gate. Consequences of changes in process parameters are as follows: ① Filling resistance decreases, flow rate of ingate changes accordingly, which also changes filling time; ② Filling time is shortened, which increases pressure of gas in cavity and increases exhaust resistance; ③ Exhaust speed changes accordingly, part of gas is not discharged in time and remains inside die casting; ④ Gas that is not discharged in time remains to form pores inside die casting, distribution gradually evolves from regular to irregular. As erosion continues, cold gaps will gradually appear inside casting.
Therefore, it is necessary to control dimensional changes in gate width, thickness and gate extension within a reasonable range. Judging from empirical data, extension of gate should be 1 to 2 times thickness of gate. Its main function is to crush gas in molten aluminum in flow channel during high-speed filling to avoid formation of large pores in casting. It is also helpful in reducing cold shut, as shown in Figure 5.

 

Figure 5 Gate size and gate extension
(3) Cooling water path should be kept clean and smooth and water pressure should be stable. Mold temperature control is an effective means to stabilize quality of die castings. Without cooperation of a mold temperature controller, integrity and effectiveness of mold cooling system is also a link that must be controlled in die-casting production process.
If cooling effect is not good and mold temperature is too high (more than 250℃), release agent will be difficult to effectively adhere due to vapor rebound, resulting in shrinkage holes inside die casting and localized aluminum sticking on the surface, etc., which will also affect production efficiency. Excessive cooling will cause mold temperature to be low (below 180 ℃), which will cause release agent to be unable to effectively adhere, affecting demoulding effect, water will not evaporate normally, which will cause undesirable phenomena such as pores, cold insulation, and local cold material accumulation inside die casting.
There are several problems that are easily overlooked at die-casting production site.
(1) Impact of poor coordination between sprue sleeve and melting cup on quality of die castings.
(2) Impact of clogging of exhaust groove of exhaust block on quality of die castings. At production site, especially in old molds, this situation often occurs: there are many places on parting surface that can be vented, but only vent grooves and vent blocks are blocked and cannot be vented normally. This phenomenon is often overlooked, but it actually changes exhaust sequence. Filling sequence and exhaust sequence should go in opposite directions, must not go opposite to each other. This is also one of factors that mistakenly believe that mold exhaust effect is very good, but porosity of die castings is always high.
(3) Impact of cooling water dripping on quality of die castings. Cooling water dripping is a common problem at die-casting site. It may seem like a small matter, but in fact it hides major hidden dangers: ① Water leaking into mold cavity and melting cup not only causes a local temperature drop, but also causes a drop in local temperature when water encounters molten aluminum. It will quickly vaporize to produce a large amount of hydrogen, which will be absorbed by aluminum liquid and filled into mold cavity. This is also one of factors causing high porosity of die castings; ② Water leaking from cooling holes, centralized distributors, etc. to mold base cannot be underestimated. When amount of water leakage is large, it will lower mold base temperature and make it difficult to maintain mold core temperature. It will also enter mold cavity from fitting gaps. This is also one of factors causing high porosity rate of die castings.
(4) Impact of cylinder oil leakage on quality of die castings. Oil leakage from oil cylinder will not only cause same pore problems as water leakage, but also cause greater oil consumption, directly increase production costs, leave hidden dangers such as safety and environmental protection.

Materials mainly refer to raw materials and auxiliary materials. Raw materials that affect quality of die castings (mainly pores) mainly include following factors.

(1) Internal density of aluminum ingots. According to experience, aluminum ingots with neat fractures and fine and uniform cross-section crystals can be judged as good materials, while those with coarse fracture surface crystals can easily cause internal pores in die casting, as shown in Figure 6. Cross-section of aluminum ingot on the left has thick crystals, while cross-section of right part has fine and uniform crystals.

 

Figure 6 Internal quality of parts
(2) Melting temperature: Temperature of molten aluminum in insulation chamber of central melting furnace should be controlled at 720~740℃. If temperature is too high, it will easily cause material crystals to become coarse, causing pores inside die casting. If temperature is too low, it will be difficult to maintain accumulated temperature drop in each link during turnover process.
(3) Insulation temperature: Insulation temperature in holding furnace should be maintained at 650 ~ 680 ℃. For small die castings, when material temperature is lower than 650 ℃ (especially lower than 640 ℃), pores and cold insulation are likely to occur inside die castings. .
Problems that are easily overlooked at die-casting production site: whether temperature of melting furnace insulation pool reaches above 720℃; whether transfer process causes too much heat loss; whether insulation furnace has sufficient insulation and heating capabilities.
Whether holding furnace has sufficient insulation and heating capabilities, pay attention to following points: whether seal between furnace body and furnace cover is intact; whether feeding trough has a cover seal; whether sealing between feeding trough and furnace body is intact.
Negligence in each of above steps will cause heat energy loss and result in low aluminum pouring temperature.
(4) Auxiliary materials: Auxiliary materials include refining agents, slag breakers, release agents, punch oil (particles), etc. Quality of these auxiliary materials, or whether they are used timely and effectively, will also directly affect quality of die castings.

Dharma refers to decision-making, methods and techniques. This is one of management tools that runs through the entire process of production activities. If there is a problem in each of four factors above, you can use this tool to analyze it, find out reasons, formulate improvement plans and implementation measures, then follow up and summarize effects. This cycle continues, but you need to pay attention to following points.
(1) For production safety, equipment management and maintenance work, daily inspection process and production process must meet response speed and countermeasures for discovered problems, ensure that equipment operating conditions meet needs of production technology and technology. This is an unshakable premise.
(2) For use, maintenance and upkeep of molds before and after production, during production process, analyze potential failure modes (FMEA), formulate a storage plan for wearing parts, formulate and implement a spare mold plan based on purchase volume.
(3) Formulate, implement and review die-casting process, and conduct daily inspections to ensure seriousness of process card.
(4) Control of temperature of molten aluminum is an unavoidable issue, and lower limit of 650℃ is a red line that cannot be exceeded.
Most technicians at production site only focus on data of fast injection and boosting parts. In fact, improper setting of slow injection speed has a great impact on quality of castings. Problems that are easily overlooked at die-casting production site: impact of slow injection speed on quality of die-casting parts; difference between slow-speed filling state theory and practice.
Slow injection is front part of fast injection. Theoretically, function of slow injection is to fill flow channel with molten aluminum and push it to vicinity of gate. As can be seen from Figure 7, this is not the case. In fact, when a small amount of front peak of aluminum liquid has crossed inner runner, runner is not completely filled and is also mixed with gas. This phenomenon becomes more serious the faster injection speed, the more serious it is. Therefore, it is important to choose a suitable slow injection speed.

 

Figure 7 Slow filling theoretical calculation and actual state
Air blowing is an effective method to remove moisture residue, but it is not a good thing to blow too much air from sprayer and for too long. Over-reliance on air blowing not only loses mold temperature, but also wastes resources. The lower mold temperature, the more moisture remains, and the more it needs to be solved by blowing air, and so on. Therefore, concentration of release agent should be as high as possible to enhance lubrication of mold while reducing amount of water sprayed out.

Environmental factors refer to environmental factors, which can affect all above links, mainly safety, which is also a factor that cannot be ignored. Changes in ambient temperature will affect personnel's emotions, pose safety risks, also affect equipment accuracy and performance. Equipment failure rates are always higher in summer. For precision cutting processes and inspection processes, ambient temperature has a great impact on dimensional accuracy, and some small details are often not paid attention to.
Problems that are easily overlooked at die-casting production sites: For production sites without mold temperature machines, release agent concentration, spray time, and cooling water flow must be adjusted immediately according to ambient temperature changes in summer and winter to keep mold temperature within a suitable and relatively stable temperature difference range. For example, if ambient temperature is high in summer, release agent can be used with a lower concentration and an appropriately long spray time. When ambient temperature is low in winter, a higher concentration of release agent can be used and spraying time should be shortened appropriately. According to ambient temperature changes in summer and winter, cooling water flow rate is increased in summer and reduced in winter. Clean cooling water channels regularly to maintain cooling effect.
For areas with high relative air humidity, when relative air humidity reaches 90% or higher, in addition to fact that die castings are prone to blackening and moldiness, gas generated by air compressor will also contain a large amount of emulsified water, which can cause pores in die castings.
High humidity seasons can easily cause electrical failures. Equipment that has been suspended from production should be kept powered on regularly. In particular, weak current control system can be powered on to keep it dry by self-heating.

Measurement is quality inspection, which mainly refers to measurement tools, measurement methods, trained and authorized measurers. In modern production process, testing is an important factor. Without testing and control, it is difficult to ensure product quality. When measuring and controlling, you need to pay attention to: ① Whether responsible person is designated; ② Whether prescribed measuring tools are used; ③ Whether it is at designated measuring point; ④ whether correct measurement method is used; ⑤ Whether measurements are carried out at a certain frequency; ⑥ whether there are records. Improving ease of operation of measuring instruments and ensuring measurement accuracy is very helpful for product quality control.

Application examples of separately proportioned release agents. For valve body shown in Figures 8 and 9, due to small draft angle of individual holes, only 5027 release agent can be used, and proportion concentration is too high (1:160 ~180), so that product quality is better and stability of on-site production can be achieved.

 

Figure 8 Draft angle of individual holes in valve body

 

Figure 9 Valve body and valve port surface quality requirements are high

3.8JB flame distributor is shown in Figure 10. Product has a diameter of 140 mm and a height of 28 mm. It is required that there should be no pores, micro-spots, flow marks, chromatic aberration, etc. after large-area mirror processing on the surface. Gate size: The two points in the middle are 18mm * 1.2mm (limit 1.4mm), the two points on both sides are 16mm * 1.1mm (limit 1.3 mm), punch diameter is 55 mm, injection head area ratio of gate size is 1:30.3, and fast injection speed is set to 2.3m/s (actual speed 1.9~2m/s). By adopting such a gate size, product will have better quality and achieve stable production.

 

Figure 10 3.8JB flame distributor
Under premise that set conditions remain unchanged, when actual speed of fast injection reaches more than 2m/s, pores and micro-spots will appear on processed surface.

368B type flame distributor is shown in Figure 11. This piece has 7 core-pulling die-casting parts. It is required that there are no pores, micro-spots, flow marks, chromatic aberration, etc. after large-area mirror processing on the surface. During die-casting production process of this part, if cracks appear on parting surface, pores will appear on machined surface of die-casting part, and this has been verified many times. Therefore, keeping parting surface intact, die-casting parts without gaps, and keeping size of ingate within established range are focus of maintenance work for this set of molds.

 

Figure 11 Type 368B flame distributor

Among six major factors, according to analysis results of last five major factors, factors affecting production are wide-ranging, diverse, changing at any time, and unpredictable.
But when undesirable factors occur, key is whether problem can be discovered or taken seriously in time, whether it can be dealt with in time, how to deal with it, who will deal with it, what results will be after treatment, etc. All spearheads here point to people, so talent is core element among six elements.
For die-casting site, main job is to "maintain stability", find out fluctuation factors in each link, use feasible and reliable methods to restore it to a stable state. "Seek stability first, then seek improvement." This work is repeated, there is no one-time solution, and there are no shortcuts either.
A product problem at die-casting site may be caused by joint action of several of six elements. When solving problem, product problem is not solved by just solving one of elements, but by integrating six elements and solving it comprehensively. Therefore, product problems can be solved efficiently and with high quality according to "stability maintenance" strategy of die-casting process.