Surface of die-casting part shows uneven soot-like, flow mark-like, and spot-like blackening or yellowing that is different from color of base metal. This is generally caused by excessive release agent, punch oil, smoke and carbides generated by its combustion, which contaminate alloy liquid, cause alloy liquid to be oxidized and discolored. Remaining moisture in mold cavity will also cause alloy liquid to change color and appear black after encountering hydrogen and oxygen decomposed from high-temperature alloy liquid, which oxidizes die-casting alloy liquid. Castings that are well die-cast will also suffer from oxidation, blackening and rust when exposed to moisture or moisture. In view of phenomenon of blackening of die castings, we analyzed phenomenon and causes of blackening of die castings, discussed influencing factors and solutions to blackening of die castings.

Aluminum, zinc, and magnesium die-cast alloys are all active metals. They will oxidize slowly in a dry environment. Under high temperature or humidity conditions, they are easily oxidized and corroded. This is determined by characteristics of alloy itself. After surface of die-casting part is oxidized, color will become gray, even black or moldy. Not only are die-casting parts easily oxidized and contaminated by dirt, but high-temperature die-casting alloy liquids are also more likely to be rapidly oxidized and contaminated.

Oily blackening on the surface of die casting is shown in Figure 1. There seems to be black clouds on the surface of die-casting parts. Black shape is irregular and depth is uneven. This is because when alloy liquid is filling mold cavity, release agent or punch oil does not burn sufficiently when it encounters high-temperature alloy liquid, resulting in a large amount of oil smoke that accumulates in mold cavity, smoke cannot be be completely and quickly excluded from mold cavity. Accumulated oil smoke contaminates surface of alloy liquid and mold cavity, causing surface of die casting to appear blackened like oil smoke.

 

Figure 1: Oily blackening on the surface of die castings

 

Figure 2 Cloud-like blackening on the surface of die casting

Cloudy blackening on the surface of die casting is shown in Figure 2. Surface of die-casting part is black, as if there is a cloud. Black shape is irregular, but black color is relatively uniform. This is because surface of mold cavity is contaminated by carbon black and oil fumes of release agent, and carbon black on the surface of mold is printed on the surface of casting, resulting in a black pattern with a relatively similar shape on the surface of each mold casting. Generally, blackened color is lighter and not as dark as oily smoke.

Black spots on the surface of die-casting parts are as shown in Figure 3. Sizes of spots are different, shape is myopic and round, and color of black spots is darker. This is because black particles formed by release agent or punch oil are on the surface of mold or casting, each black particle alone causes the surface of casting to turn black. These black particles may also be deposits or aggregates of release agent or punch oil. They are relatively viscous and cannot be completely decomposed into black particles by high temperatures.

 

Figure3 Die casting surface blackened like black spots

Oxidized and blackened surface of die casting is shown in Figure 4. Figure 4(a) and Figure 4(b) are die castings whose surface has been naturally oxidized and blackened. Figure 4(a) shows that there is a trace amount of release agent remaining on the surface of casting, color becomes darker and black after natural oxidation in atmosphere. Figure 4(b) A is darker than Figure 4(b)B. Figure 4(b)A is surface of die-casting part that is naturally oxidized and blackened by air after being placed in cleaning workshop for 10 days without packaging protection. This is because atmosphere contains moisture, especially during rainy season or rainy season in the south. A large amount of moisture in the air is deposited on the surface of casting. Hot and humid moisture will quickly oxidize and turn the entire casting black. Figure 4(b)B shows surface of casting that has not been oxidized and blackened after being placed in packaging box for 35 days. It can be seen that good packaging can prevent surface of casting from oxidation and blackening.

 

(a) Natural oxidation and blackening of surface (b) Natural oxidation and blackening of surface comparison
Figure4 Naturally oxidized blackened surface of die casting

Surface condition of die-casting parts after being placed in die-casting workshop for 10 days is shown in Figure 5. After oxidized and blackened surface of die-casting parts is left in die-casting workshop for 10 days, it will be contaminated and oxidized by release agent, refueling fumes and water mist in the air, and black spots will appear. Release agent plus oil fume and water mist aggravate alloy oxidation of casting, oxidized black spots are relatively deep, and black spots cannot be removed by shot blasting. Therefore, after die-casting, it is necessary to prevent sprayed release agent from splashing on the surface of casting. For die-cast and boxed castings, uppermost layer of castings must be covered with a cover plate to prevent release agent water mist suspended in the air in die-casting workshop from falling onto the surface of castings.

 

Figure 5 Surface condition of die-casting parts after being placed in die-casting workshop for 10 days

One of carbon deposits formed on the surface of casting is shown in Figure 6. Carbon deposit marks are caused by carbon accumulation on the surface of mold. They are relatively rough and look like traces of mold sticking. Generally, there is no sign of mold sticking or tension. If quality of release agent or punch oil is improperly selected, or release agent is impure, wax and organic grease in release agent will burn easily, carbon black residue after burning will accumulate and adhere to the surface of mold, forming carbon deposits. Carbon deposit appears on the surface of mold, which is a layer of dark black dirt adhering to it. Carbon deposits usually appear as a rough surface similar to sticky molds, and color is same as color of adhered aluminum alloy. This is because another layer of aluminum alloy is adhered to the surface of carbon deposit. Carbon deposit and aluminum alloy adhesive mold will be produced one after another and are based on each other.

 

Figure 6 One of carbon deposit marks on the surface of casting
Second carbon deposit mark formed on the surface of casting is shown in Figure 7. There is a rough surface mark on the surface of casting (Figure 7A), a convex and smooth plane appears on the surface of carbon deposited part of casting, which looks like peeling. It is imprint formed after carbon deposits fall off (Figure 7B).

 

Figure7 Carbon deposition mark (ii) formed on the surface of casting
In addition, once carbon deposit layer on the surface of mold falls off, carbon deposit will adhere to the surface of casting, a small dot or a small piece of blackened carbon black will appear on the surface of casting.
Parts where carbon deposits are prone to occur on mold surface: parts of mold with thick hot spots in casting; parts of mold where relatively more heat is accumulated; parts of mold where temperature is relatively high (≥ 220℃); mold comes into contact with part where alloy liquid is at high temperature for a long time; it is far away from inner gate and part where alloy liquid is finally filled. This is because organic matter of release agent is easily sintered and carbon deposited at high temperatures, and carbon deposits are likely to occur in areas where oil smoke accumulates.
Oil black spots that remain after shot peening are shown in Figure 8, and oil black smoke that remains after shot peening is shown in Figure 9. This is because surface alloy liquid of die casting is contaminated by oil smoke and turns black. After shot blasting, surface of casting still remains black with oil stains and cloud-like blackness. Oil smoke does not only adhere to the surface of casting, but oil has contaminated alloy liquid inside, so such blackness cannot be removed by shot blasting.

 

Figure 8 Oil black spots remaining after shot peening

 

Figure 9 Oily black smoke remaining after shot peening

Die casting is blackened as shown in Figure 10. Punch oil on material cake explodes and sprays onto surface of casting at the moment mold is opened. Yellow spots formed on the surface of casting are shown in Figure 10 (a); blackening caused by oxidation of casting when it encounters humid moisture is shown in Figure 10 (b); rust phenomenon after castings are corroded by rain, wind and sun for a long time, severely oxidized and mildewed is shown in Figure 10(c).

 

(a) Diagram of macula caused by punch oil

 

(b) Castings become black due to moisture

 

(c) Oxidative mildew diagram of castings
Figure10 Die casting blackened

Oil stain on conveyor belt that transfers castings contaminates castings, causing castings to turn black. Castings contaminated by oil stains on conveyor belt are shown in Figure 11. If storage boxes, working platforms and tools where castings are stored, trimming dies for trimming, operating gloves, etc. are contaminated with stolen goods or oil, they will also contaminate castings and cause surface of castings to turn black. Oxidation and blackening of surface of casting caused by negative residues of finishing coolant is shown in Figure 12 if water is not blown away after cleaning of casting or finishing. If packaging carton gets damp and absorbs water, parts of casting that contact carton will also become damp and moldy.

 

Figure 11 Castings contaminated by oil on conveyor belt

 

Figure 12 Finishing coolant causes oxidation and blackening of castings

If there is too much punch oil in injection chamber, a large amount of flame, oil smoke and carbon black will be produced when it encounters poured high-temperature alloy liquid. These flames, oil smoke and carbon black will not only cause alloy liquid to oxidize, but also contaminate alloy liquid and cavity wall. Therefore, amount of punch oil should be reduced as much as possible, and amount of punch oil should be used to observe that outer circle of material cake does not turn black, and black color is only on one end of punch.
Because punch oil containing graphite can easily make castings black, punch oil with little or no graphite content should be used. For example, water-soluble punch oil can be used to reduce use of wax, grease and organic matter.
If punch oil produces too much and too thick oil fume, oil fume will flow into mold cavity from inner gate and adhere to cavity near inner gate, causing surface of casting near inner gate to turn black.
Spray punch lubricating oil in the form of mist on the top of punch outside injection chamber, or in the form of mist, spray it from rear of punch into injection chamber after injection, so that accumulated punch oil will be pushed out of injection chamber by returning punch to reduce accumulation of punch oil. If after injection punch is withdrawn, punch oil is sprayed into injection chamber in the form of mist, or dripped into injection chamber in the form of oil, not only should punch oil be sprayed less, but more attention should be paid to using compressed air to blow accumulated punch oil to disperse it on the surface of injection chamber.
If mold core puller leaks oil, hydraulic oil of core puller flows into mold cavity, which will also cause the surface of casting to turn black.

If release agent is too accumulated after spraying die-casting cavity, or concentration of release agent is too concentrated, organic matter and moisture such as oil, grease, wax, and organic silicone oil in release agent will produce a large amount of fumes when encountering high-temperature alloy liquid. Excessive oil fume not only seriously pollutes alloy liquid and surface of mold cavity, but also rapidly oxidizes injection-filled alloy liquid, causing black spots, blackening (Figure 9) and carbon deposits on the surface and inside of casting. Therefore, a release agent should be selected that is effective in preventing mold sticking, resistant to high temperatures, not easy to burn, and not easy to produce oil fumes. A good quality release agent not only prevents oxidation and rust on the surface of die castings, but also does not affect quality of castings.
Using a release agent with a lighter concentration can also reduce generation of oil smoke. Generally, ratio of release agent is about 1:120 ~ 150. For example, heat sink mold has a thin and deep heat sink cavity, which is easy to stick to mold. Amount of spray release agent cannot be reduced, but a release agent with a lighter concentration can be used, and ratio is 1:140.
In order to reduce generation of oil smoke, when spraying release agent on mold, pay attention to spraying position, distance, flow rate and length of time; atomization effect of release agent should be good, amount of release agent and punch oil sprayed should be small and even. Film formed by release agent on the surface of mold should be thin and even, and do not allow release agent to flow or accumulate in cavity. After spraying, use compressed air to blow clean the surface of mold cavity, and be careful to evaporate excess moisture in release agent before closing mold.
It should also be noted that water used to prepare release agent should be pure water or soft water. Hard water cannot be used, because metal ions in hard water not only affect stability, coating properties, demoulding properties of release agent, but also promote oxidation and rust of castings.

Quality of release agent or punch oil is improperly selected, or release agent is impure. Wax and organic grease in release agent are easy to burn, carbon black residue after burning will sinter and adhere to the surface of mold, forming carbon deposits. Carbon deposit layer adheres firmly to the surface of mold and usually requires polishing with oilstone, water-based sandpaper and emery cloth to remove it. Mold surface can be polished regularly to prevent excessive and excessive carbon deposits and blackening defects.
Generally, when mold surface temperature is higher than 240℃ at the moment of mold opening, release agent will be sintered to form carbon deposits. Therefore, it is necessary to reduce pouring temperature, control mold temperature within a certain range, and maintain thermal balance of mold. Reduce mold temperature in overheated parts, increase cooling water flow of mold, keep mold temperature from being too high, prevent release agent from sintering and collecting carbon. In addition, the rougher molding surface, the easier it is for oil smoke to adhere and produce carbon deposits; release agent selected cannot withstand high temperatures. The higher pouring temperature and mold temperature, the easier it is for oil smoke and sintering carbon deposits to occur.
If there is carbon deposit on the surface of mold, speed of heat transfer from alloy liquid to mold will be affected, shrinkage and thermal crack defects will easily form in thick parts of product. At this time, oilstone or emery cloth should be used to grind and polish away carbon deposit on the surface of mold.
In larger hot spots and thick-walled parts of casting, alloy liquid remains at high temperature for a long time, and sintering of release agent is intensified, making it easier to form carbon deposit defects. Therefore, cooling and heating system of mold should be well set up and temperature of mold should be well controlled.

Moisture in release agent in mold cavity has not been blown away or evaporated cleanly. There is too much moisture remaining on the surface of mold. When encountering high-temperature alloy liquid, it decomposes into oxygen (O), hydrogen (H) and HO- ions. Oxygen in it oxidizes and contaminates alloy liquid, causing surface layer of casting to oxidize and turn black. At the same time, when there is too much moisture in release agent, release agent cannot be fully oxidized and burned when encountering high-temperature alloy liquid, and smoke will be formed.
When mold surface temperature is higher than 160℃, moisture will evaporate faster and cleaner. Therefore, it is necessary to appropriately increase temperature of mold away from inner gate and mold temperature in parts where temperature is too low; cooling water of mold can be appropriately turned down to prevent mold temperature from being too low and to allow excessive release agent moisture on mold surface to evaporate quickly to avoid excessive oil smoke.
After spraying release agent, release agent moisture on mold surface must be blown away accurately and thoroughly. Excessive release agent must be blown away, blown off, and cleaned with compressed air. Aim at deep cavity, deep hole, deep groove, slider groove and other parts of mold, and use blow tube to stay at close range and blow air for 1 to 3 seconds. Moisture on parting surface of mold should also be blown away with an air pipe. This should not only prevent moisture on parting surface from flowing into mold cavity, but also to prevent excessive moisture on parting surface from blocking parting surface and exhaust groove, affecting exhaust effect.
Regularly check and prevent mold cooling water, release agent water, release agent moisture in slide groove from flowing into mold cavity and injection chamber.
If release agent uses water with high salt content, salt will adhere to the surface of casting, causing oxidation and corrosion of casting. Therefore, release agent for die-casting should be pure water or soft water, or purified tap water can be used.

O2, CO2, CO, H2O produced by oxidative combustion and decomposition of paraffin, vegetable oil, mineral oil, various silicone oils, graphite and other organic matter in release agent react with Al to form Al2O3. At the same time, oil smoke is produced, causing oxidation and blackening of casting. These organic substances are unstable and will condense into viscous particulate oil droplets. If infected by bacteria, they will also condense into flocs and accumulate into piles. When these viscous oil droplets encounter filled high-temperature alloy liquid, they burn and produce fumes. When condensed oil droplets are burned and oxidized insufficiently, oil smoke contaminates oil droplets, causing oil droplets to turn black and turn into black particles of release agent, which are washed away by alloy liquid and flow with alloy liquid, penetrating into interior of casting, causing alloy liquid to oxidize locally or entirely. Therefore, sometimes interior of casting becomes black after partial finishing. When release agent and water emulsify, decompose and dissolve insufficiently, or when high-temperature alloy liquid is burned and decomposed insufficiently, black particles of release agent will also appear.
After die-casting has been stored for a long period of time, if black spots, mildew spots, and white spots (frost-like, black after removal) appear on the surface, it indicates that corrosion has occurred. This may be because release agent contains ingredients that promote corrosion of casting. Surface of newly die-cast casting is slightly blackened. After casting is left in nature for 10 to 30 days, surface of casting will automatically oxidize and become darker.
When there is a large accumulation of punch oil, punch oil encounters high-temperature alloy liquid, is not sufficiently burned and decomposed, black particles of punch oil will also appear. If surface of mold is polluted, such black spots will contaminate alloy liquid on shallow surface of casting. After die-casting, small black dots will form on shallow surface of casting; there are also accumulated contaminations that turn into a small black patch with irregular shapes. Blackening of castings caused by black particles is different from blackening caused by oil fume pollution on the surface of castings. It is not outside surface of castings, but oxidizes inside of castings, which cannot be wiped off with a scouring pad. If internal alloy liquid is contaminated during filling process, the alloy liquid will be polluted and oxidized and discolored, and small black holes or black slag inclusions like small dots will be formed inside casting, which looks like a small black spot to naked eye. Size of small black spots is generally less than 2 mm, appearing singly or scattered. Location, extent and size of small black cavities or small black slag inclusions are related to size, location and filling flow of black particles.
Small black cavities are mainly formed by gas of oil fume. Inner surface of small black cavities is rounder than inner surface of shrinkage hole, but surface of pores without air or hydrogen is smooth and bright. This is because color of oil fume pollution is relatively dark. Generally, color of inner surface of small black holes formed by punch oil is darker than color formed by release agent. Small black slag is oxidized slag formed after alloy liquid is oxidized and contaminated by black particles. Color of oxidized slag is darker than normal oxidized slag due to pollution of oil fumes.
Small black cavities or small black slag inclusions will not only appear on finishing or anatomical surface of casting, but also often appear on fracture surface of inner gate of casting, affecting quality of casting.
Therefore, release agents and punch oils that have good dissolving and emulsifying effects in water, good suspension properties, no precipitation, no aggregation, no scaling, no bacterial infection, and easy dispersion should be used.

Although die-casting process can solve some casting defects, when process is debugged normally, most casting defects still need to be solved by improving mold. Pouring system and mold structure will directly cause casting to turn black. Generally, blackening is likely to occur on the surface of die castings in areas far away from inner gate, areas with eddy currents, deep cavities of mold, and final filling areas. First, because there is a lot of smoke accumulated in these parts, alloy liquid and surface of mold are in contact with smoke for a long time, and amount of eddy current enveloping gas in alloy liquid is large, which causes alloy liquid on the surface of casting to be oxidized and mold surface to be blackened by oil smoke and carbon black. Second, because alloy liquid flows slowly in last filled part, degree of erosion on mold surface is light, and oil fume accumulated on the surface cannot be washed away by alloy liquid.
Especially for large-surface castings or large-surface parts of castings, most of alloy liquid is spray-filled, which exceeds speed of flue gas flow and discharge. The more fully alloy liquid contacts and mixes with flue gas, the more amount of oil fume gas encapsulated in alloy liquid will increase, the easier it will be for alloy liquid to be oxidized and contaminated, and the more serious blackening will appear on the surface of casting. More inner gates should be installed to allow alloy liquid to be filled with full wall thickness as much as possible so that alloy liquid does not entrap gas. Filling is completed quickly and in a short time, so that oil fume has no time to generate and oil fume has no time to oxidize alloy liquid.
Improving pouring system of mold and changing position, size and direction of inner gate and overflow trough can change flow direction and flow rate of alloy liquid, change generation, flow direction and discharge of oil smoke.
Figure 13 shows heat sink pouring system. Casting in Figure 13(a) is a casting with more heat sinks. Figures 13(b) and 13(d) are heat sinks on both sides of casting (appearance is black). Add a gating system to mold as shown on the left or right side of Figure 13(c), so that alloy liquid can fill blackened parts of casting as early as possible, directly and smoothly. Widen both sides of inner gate to heat sink on the side of casting, allowing alloy liquid to directly fill heat sink on the side of casting. After mold was modified in this way, blackening on the surface of heat sink on both sides of casting was eliminated, and alloy liquid directly filled heat sink on the side of casting, as shown in Figure 14.

 

Figure13 Fin casting system

 

Figure 14 Alloy liquid directly fills heat sink on the side of casting
In addition, cross-sectional area of the overall and local exhaust grooves and overflow openings of mold should be appropriately increased so that exhaust system can drain and exhaust smoothly and quickly, discharge oil fumes and contaminated alloy liquid out of mold cavity in a timely manner, which will also have better effects.

Generally, in parts with thinner wall thickness of castings, mold temperature is relatively low, moisture of release agent evaporates more slowly, there is more release agent and moisture remaining, so more fumes are generated; castings have deep cavities and blind holes that cannot be filled smoothly and are difficult to exhaust. There is also a lot of oil fume remaining in cavity. Due to shape or structure of casting, tightening force on mold is large, and mold will stick. A thicker and more release agent needs to be sprayed. Excessive release agent will remain in cavity, which will increase blackening of casting. Therefore, some blackening cannot be completely eliminated because release agent cannot be reduced.
Wall thickness of the more blackened side of radiator casting is relatively thin, and it is far away from other heat sinks. Amount of alloy liquid accumulated in cavity is relatively small, so that temperature of mold and slider here is not high enough, release agent on the surface of cavity is not completely volatilized. When larger residual release agent encounters high-temperature alloy liquid, a relatively large amount of oil smoke is generated. Solution is to increase mold temperature.
In parts of mold cavity where alloy liquid cannot be directly filled and where eddy currents occur during filling process, surface of casting is prone to appear blackened like eddy currents. Solution is to change filling position and flow direction, fill directly, and eliminate eddy current phenomenon.
Blackening often coexists with cold shut and flow marks. Because temperature of mold or alloy liquid at cold seal and flow marks is low, release agent burns and evaporates slowly when it encounters alloy liquid. Flue gas is in contact with slow-flowing alloy liquid for a long time, alloy liquid is oxidized and polluted by oil fume for a long time. Solution is to increase mold temperature.
For large-plane castings, alloy liquid will be sprayed and filled to far end in advance, or to surrounding parting surface in advance, instead of being able to sequentially discharge fume gas in the mold cavity out of cavity like complex castings; Therefore, surface of large flat castings is prone to blackening. Solution is to install more internal gates to allow alloy liquid to be filled quickly and in a short time.

Most die-cast alloys are aluminum alloys that are easily oxidized, especially at high temperatures. During pouring process and moment of die casting filling, it will be oxidized and cause oxidized slag inclusion defects.
When pouring ladle takes alloy liquid from furnace and pours it, alloy liquid will be oxidized by oxygen in the air during flow and tumbling, forming an aluminum oxide film on the surface of alloy liquid. The longer it takes to extract alloy liquid, the more alloy liquid rolls and the more serious oxidation is.
When alloy liquid is poured into injection chamber, high-temperature alloy liquid causes punch oil sprayed into injection chamber to burn, fire, and smoke. Oxygen atoms decomposed in oil smoke and burning gas will accelerate and intensify oxidation of alloy liquid. Therefore, the more punch oil used, the more seriously alloy liquid will be oxidized and polluted.
When aluminum alloy die-casting parts are stored in a natural state, surface will be oxidized quickly and color will become darker; if stored in a humid environment, rust defects will appear on the surface within a few days. Die castings that have been shot peened will spray away dense oxide layer formed when castings are demoulded at high temperatures, speed and degree of surface oxidation will increase.
Die-cast alloys with different chemical compositions have different oxidation resistance capabilities. Therefore, castings die-cast with different grades of alloys have different degrees of blackening on the surface. Therefore, you can choose die-casting alloys that are not easily oxidized and blackened. For example, ENAC-434000 alloy is more easily oxidized and blackened than ADC12 alloy.
When cleaning and refining aluminum alloy liquid, adding 0.1% to 0.3% of weight of alloy liquid as sodium fluoroaluminate or sodium fluorosilicate can also help prevent oxidation and discoloration of castings.

Stainless steel shot should be screened and dusted regularly. This is because after a period of use, stainless steel shot will not only cause steel shot to be worn and become smaller, but also alumina dust will adhere to surface of steel shot. When shot peening hits die casting, steel shot will also hit alumina dust to the surface of casting, causing the overall color of casting surface to darken or become matte.
Zinc shot material: bright appearance, high specific gravity, strong impact of shot blasting; low hardness, can remove large burrs without wearing product surface; dust generated by zinc shot is not easy to burn and explode. It is a relatively safe shot blasting material, is widely used in shot blasting to remove burrs and clean zinc and aluminum products. However, zinc shot itself is easily oxidized and produces oxide dust, causing surface color of casting to darken and deepen.
Aluminum shot material: Surface of casting after blasting with aluminum shot is the most beautiful. However, because aluminum shot is lighter in weight and impact force is relatively small, small burrs cannot be sprayed out, and shot peening efficiency is not high enough      blasted away, and efficiency of shot blasting is not high enough. Moreover, dust produced is flammable and explosive, so it is rarely used.
Mixing 20% to 30% aluminum shot into stainless steel shot can achieve a whiter and brighter surface effect than using stainless steel shot alone.
There is a patented low-carbon bainitic alloy steel shot that not only takes into account advantages of low carbon content and good toughness, but also ensures sufficient hardness through micro-alloying and special heat treatment processes to ensure high efficiency. At the same time, because carbon content is close to that of stainless steel shots, castings will be smoother and whiter after shot blasting, which significantly improves surface quality of castings after shot blasting.
During shot peening process, not only will moisture and dust in the air be sprayed onto surface of casting, but casting will also be heated by shot. Increase in temperature will accelerate oxidation reaction on the surface of casting, so casting will quickly turn black.
For products after shot blasting, gloves should be worn during operation. If castings are directly touched with hands, hand marks will be left on the surface of castings and cause blackening.
Die-casting parts are in a high-temperature state after demolding, and an oxide film will quickly form on the surface. After shot peening, oxide film disappears, and oxidation defects are more likely to occur on the surface of casting. Therefore, die-casting parts after shot blasting must be dust-proof, moisture-proof, waterproof and oil-proof, sealed and packaged, and surface coating treatment must be carried out quickly.
If casting is smoothed by vibration grinding, it can only remove dust, oil and tiny burrs on the surface of casting, but cannot remove blackening of casting matrix. Before and after finishing, castings must be sealed and stored dry to prevent casting matrix from getting damp, oxidized and blackened. During finishing process, attention should be paid to cleaning powder deposited by abrasive, castings should be cleaned, baked and dried immediately after finishing.

Surface of die-casting parts must not have defects such as cracks, looseness, shrinkage cavities, pores, bubbles, pinholes, cold shuts, flow marks, etc. Otherwise, surface of castings will easily blister after electroplating and electroplating layer will separate from substrate.
During die-casting and solidification process, a dense chilled hard layer is formed on the surface due to rapid cooling, while internal structure may have defects such as pores and shrinkage cavities. If this good surface layer is removed during grinding, defects such as pitting and bubbles will appear during electroplating. When polishing die-casting parts, do not press polishing wheel too tightly and do not overheat casting to prevent abrasive from adhering to casting, causing product to fail to be electroplated and cause blackening or poor peeling. In other words, grinding and polishing of die castings before electroplating should not be excessive.
Die castings are easily oxidized. If stored for a long time, an oxide film will form on the surface, which will also affect electroplating performance. Before electroplating, oxide film needs to be removed with reducing substances.

Air in the southern plum rainy season or rainy season is humid. During blowing process after spraying release agent, moisture in compressed air is sprayed onto mold surface, resulting in more moisture on mold surface. Therefore, it is necessary to dry compressed air.
When air humidity is high, if die castings are not properly sealed when stored indoors or during transportation, moisture in the air will condense on the surface of castings. In mild cases, surface of castings will become darker and black. In severe cases, surface of castings will oxidize, turn white and produce mildew spots.
In winter, when die castings are transported from outdoors to indoors for finishing or storage, if there is air conditioning in the room and indoor temperature is significantly higher than outdoor temperature, moisture in the air will condense on the surface of castings and frost will form. Frosting will cause rapid oxidation of castings. Therefore, do not open packaging immediately when castings enter room; or perform finishing processing immediately after opening packaging to prevent castings from frosting, oxidation and blackening caused by long-term storage.
During high-temperature season, temperature of products in machining and air-conditioning factory is relatively low and cannot be loaded directly for shipment. They must enter warehouse buffer zone for transition, otherwise water will condense on products.
Aluminum alloy is an amphoteric metal, will oxidize and change color when encountering acidic or alkaline substances. Therefore, neutral cutting fluid should be used during finishing and ultrasonic cleaning.
Clean finished castings in hot water, then immediately use compressed air to blow off water droplets on the surface of castings, and leave them for a period of time before packing them into boxes. If castings are stored in packaging boxes immediately while they are hot, oxidation, blackening, and mold spots will quickly appear on castings.

Conduct a 100% inspection of blackened surface of castings. If individual blackened castings are found, they can be polished with a polishing machine or polished off manually with a scouring pad.
The most effective way to deal with oil stains and mildew stains on castings is to use die-cast aluminum alloy mildew stain removal cleaner to thoroughly clean oil stains and mildew stains at one time.
Although aluminum is easily oxidized, a thin oxide film will form on the surface after oxidation. Oxide film is very dense and can prevent aluminum alloy inside from further oxidation. Therefore, after cleaning castings, passivation treatment should be carried out immediately to form a passivation film on the surface of aluminum alloy castings to achieve anti-corrosion and antibacterial purposes. In humid weather or environment, it can also prevent mold corrosion of castings.
Castings that have been oxidized and dried should be placed in a dry and ventilated place and inspected regularly. If it is found that castings are exposed to water or moisture, and mildew spots appear again, they should be cleaned and anti-oxidized in time. Most castings can be brand new.

Die castings are easily oxidized and corroded under high temperature or humidity conditions. After surface of die-casting part is oxidized, color will become gray, even black or moldy. Good die-casting parts, or die-casting parts after cleaning, if they are not treated with anti-oxidation in time and are piled in a humid warehouse for a long time, which can be as short as one or two months or as long as half a year, they may oxidize, turn black and grow mold. Therefore, appropriate anti-oxidation measures and warehousing management are required for die castings.
Due to different humidity levels, ventilation conditions and temperatures inside storage warehouse, oxidation, blackening and moldiness of aluminum alloy die-casting parts will be different depending on location and height of warehouse. Therefore, warehouse where castings are stored should be dry, ventilated in sunny weather, closed in humid weather, and provided with necessary dehumidification. If warehouse is on the ground, bottom floor of storage should be elevated and supported to prevent moisture from returning to ground.
Castings should be sealed and packaged and dehumidified with desiccant. Castings should not get damp during storage and transportation. Special protective measures should be taken during yellow plum season to prevent castings from getting wet and moldy. Plastic bag packaging can be added, castings can be sealed and packaged in a small plastic bag, or they can be stored in a plastic bag as big as a large packaging box, put in desiccant, sealed and packaged.
Die-casting parts should be cleaned and dried after die-casting and machining processes. If no cleaning treatment is performed, corrosive substances such as release agents, cutting fluids, saponification fluids, and water stains will remain on the surface of die-casting parts. These stains accelerate growth of mold spots and blackening of aluminum alloy die-casting parts. Therefore, after finishing die casting, ultrasonic cleaning should be used and it must be dried. However, if cleaning agent selected is inappropriate, it will also cause corrosion and oxidation of die castings.
Wet cartons, cardboard, turnover boxes and other items cannot be used. Plastic boxes and plastic boards can be used. Box should be sealed immediately after packing and top layer should be covered with a cover.
For castings that require special rust protection, stagnation time of castings during circulation process should be reduced. For castings with steel or iron inserts, apply anti-rust oil on inserts without omissions.
During transportation, they should be protected from rain and covered and tied with protective raincloth. When castings are exposed to rain, batch of castings should be immediately isolated, cleaned and dried. Moreover, castings must be left for 7 days to confirm that they are qualified, and they can only be circulated if there is no further rust.
In order to prevent water droplets splashed when spraying release agent on die-casting from falling on castings, castings should be moved to other areas for storage in a timely manner.
Conveyor belt next to die-casting machine, trimming mold for trimming and cleaning, and workbench should be wiped clean regularly. Operator's gloves should be clean and free of moisture and oil to avoid contaminating castings.
Unpack and inspect castings before they leave warehouse, especially products packed in the first two or three months. For castings that have been boxed for 6 months, boxing must be rechecked.

Commonly used surface coating methods for die castings are as follows:
(1) Apply paint.
(2) Anodizing can prevent aluminum alloy from continuing to oxidize.
(3) Porcelain is anodized, with beautiful surface, high hardness and good decorative properties.
(4) Micro-arc oxidation has high hardness and good corrosion resistance. Disadvantage is rough surface and poor aesthetics.
(5) Hard anodizing, high hardness, wear resistance, unsightly color, gray.
(6) Electroplating.
(7) Chemical oxidation.
(8) Organic coating, plastic spraying.
(9) Physical vapor deposition (PVD treatment), the effect is good.
(10) Ceramic coating, high cost.

There are many reasons for blackening of die castings. Corresponding measures need to be taken according to each cause of blackening in order to effectively eliminate blackening phenomenon on the surface of die castings and improve quality of die castings.