You Should Know About Die Casting Blackening: Causes and Solutions

The die casting surface shows uneven soot-like flow marks, spot-like blackening or yellowing different from the base metal color, generally due to excessive mold release agent, punch oil, and its burning generated soot, carbide pollution of the alloy liquid, the alloy liquid is oxidized and discolored. The residual moisture in the cavity encounters high-temperature alloy liquid decomposition of hydrogen and oxygen, so the die-casting alloy liquid oxidation will also cause the alloy liquid discoloration, a blackening phenomenon. Die-casting good castings, moisture or encounter moisture, will also appear oxidation blackening and rusting. For the phenomenon of die-casting blackening, the author analyzes the phenomenon of die-casting blackening and the reasons for it and discusses the influencing factors of die-casting blackening and its solution measures.

Table of Contents

Die casting surface blackening phenomenon

Aluminum, zinc, and magnesium die-casting alloys are active metals. In a dry environment, will be slow oxidation. In higher temperatures or humidity conditions, be easy to oxidation and erosion, which are the characteristics of the alloy itself. The color of the die-casting surface will become gray, black, or moldy after being oxidized. Not only are die-casting parts easily oxidized and contaminated by dirt, but high-temperature die-casting alloy liquid is more likely to be oxidized and contaminated quickly.

Blackening of surface soot samples

Die casting surface soot-like blackening, as shown in Figure 1. On the die-casting surface, there seems to be a black cloud of smoke; the black shape is irregular, and the depth is not uniform. Because the alloy liquid fills the cavity, the release agent or punch oil encounters high-temperature alloy liquid, not enough combustion, resulting in a large amount of soot gathered in the cavity, and the smoke can not completely and quickly be excluded from the cavity. The fumes collected pollute the surface of the alloy liquid and the mold cavity, resulting in the surface of the die casting showing a soot-like blackening.

Figure 1 Soot-like blackening of die casting surface

Cloudy blackening of the surface

The cloud-like blackening of the surface of the die casting is shown in Figure 2. Die casting surface blackening, as if a cloud of flakes, the black shape is irregular, but the black color shades are more uniform. It is because the surface of the mold cavity is contaminated with carbon black and soot from the release agent, the carbon black on the surface of the mold printed on the surface of the casting, resulting in the surface of each mold casting, there is a piece of the black pattern of the shape is relatively close. Generally, the color of this blackening is lighter and not as dark as the color of the soot.

Figure 2 Cloudy blackening of the die casting surface

 Surface black spot-like blackening

The black spots on the surface of die castings are shown in Figure 3. The size of the spots is different, the shape is near-visible round, and the color of the black spots is darker. It is due to the black particles formed by the release agent or punch oil on the surface of the die or casting, each black particle individually causing the casting surface to blacken. These black particles, which may also be deposits or aggregates of mold release agents or punch oil, are denser and not easily decomposed completely by high temperature into black particles.

Figure 3 Die casting surface black spot-like blackening

Surface oxidation blackening

The oxidation blackened surface of the die casting is shown in Figure 4. Figure 4 (a) and Figure 4 (b) are the surfaces after natural oxidation blackened die castings. Figure 4 (a) is the casting surface residual trace of the release agent after the natural oxidation of the atmosphere darkened and blackened. Figure 4 (b) A is darker than Figure 4 (b) B. Figure 4 (b) A is in the natural condition of the die casting without packaging protection, placed in the clean shop 10 days after the natural oxidation of the surface by the air black. The atmosphere contains moisture, especially in the southern rainy or rainy season. A large amount of moisture in the air is deposited on the surface of the casting. Hot and humid moisture will quickly make the whole casting oxidation black. Figure 4(b)B shows the surface of the casting placed in the packing box for 35 days without oxidation and blackening, which shows that good packing can prevent the oxidation and blackening of the casting surface.

 Black spots on the surface

Die casting in the die-casting shop after 10 days of surface condition, as shown in Figure 5. Die casting oxidation black surface in the die casting shop after 10 days will be contaminated by the air release agent oil fumes and water mist fall, oxidation, black spots. Mold release agent plus soot and water mist aggravate the casting alloy oxidation, and oxidation of the black spot is deeper, using the method of shot blasting, also can not spray off the black spot. So, die-casting takes parts, to prevent spraying release agent splash to the casting surface, die-casting boxing good castings, to the uppermost layer of castings, add a layer of cover, to prevent die-casting workshop air suspended release agent water mist landing to the casting surface.

Figure 5 Surface condition of die casting parts after 10 days in die casting workshop

 Carbon marks on the surface

One of the carbon marks formed on the casting surface is shown in Figure 6. The carbon mark is caused by the accumulation of carbon on the surface of the mold, which is relatively rough and seems to be the trace of the sticky mold, and generally, there is no sign of the sticky mold being strained. Suppose the release agent or punch oil quality is not selected properly, or the release agent is impure. In that case, the wax and organic grease in the release agent will burn easily, and the carbon black residue after burning will collect and adhere to the mold surface, forming carbon. Carbon deposits appear on the mold surface as a layer of dark black dirt. Carbon deposits usually appear as a rough surface similar to a sticky mold. The color is the same as the color of the adhered aluminum alloy because another layer of aluminum alloy adheres to the surface of the carbon deposits. The carbon deposits and the aluminum alloy adhering to the mold will be generated one after another and are based on each other.

Two of the carbon marks formed on the casting surface are shown in Figure 7, which shows a rough surface mark on the casting surface (Figure 7A). At the same time, a smooth plane appears on the surface of the carbon part of the casting, shaped like a peeling phenomenon, which is the mark formed after the carbon has been removed (Figure 7B).

In addition, once the carbon layer on the mold surface is off, the accumulated carbon adheres to the surface of the casting and will present a small spot or a small piece of blackened carbon black on the surface of the casting.

Parts of the mold surface that are prone to carbon accumulation:

  • Parts of the mold with thick and large hot joints in the casting.
  • Parts of the mold gather relatively more heat.
  • Parts of the mold with higher temperature (≥ 220℃).
  • Parts of the mold are in contact with the alloy liquid at high temperatures for a longer time.
  • Parts away from the inner gate and the last filling of the alloy liquid.

Because it is easy to make the organic matter of the mold release agent sinter and accumulate carbon at high temperatures, it is easy to accumulate carbon in the parts where the oil and fumes gather.

After blasting still residual oil black spots as shown in Figure 8, after blasting still residual oil black smoke sample as shown in Figure 9, which is the surface of the die casting alloy liquid is blackened by oil pollution. After blasting, the surface of the casting remains black oil spots and cloud-like blackening. Soot is not only attached to the surface of the casting, but the oil has contaminated the internal alloy liquid, so such blackening can not be removed by blasting.

Figure 9 The black smoke sample of oil remaining after blasting

Mildew blackening

The blackening of die castings is shown in Figure 10. The yellow spot formed by the explosion of the punch oil on the material cake at the moment of mold opening and sprayed onto the casting surface is shown in Figure 10 (a); the blackening of the casting caused by oxidation after encountering moist moisture is shown in Figure 10 (b); the rusting of the casting after long-term erosion by rain, wind and sunscreen, and severe oxidation and mildew is shown in Figure 10 (c).

Figure 10 Blackening of die castings

 Oil stain

The oil stains on the conveyor belt for transferring castings contaminate the castings and cause the castings to blacken, and oil stains contaminate the castings on the conveyor belt, shown in Figure 11. If the storage box, work platform, tools, cutting edge die for cutting edge, gloves for operation, etc. have spoiled, the oil will also contaminate the castings, resulting in surface blackening. Castings or finishing cleaning after the moisture is not blown clean, oxidation blackening rust, finishing negative coolant residual caused by oxidation blackening of the casting surface, as shown in Figure 12. Suppose the packaging carton moisture absorption will also cause the casting contact carton parts produced by moisture mold.

Fig. 11 Castings contaminated by oil on the conveyor belt
Figure 12 Finishing coolant causes oxidation and blackening of castings

Causes and solutions for blackening of die castings

Excessive spraying of punch oil

Because the punch oil containing graphite tends to make the casting black, we should choose a punch oil with less or no graphite content. For example, water-soluble punching oil can reduce the amount of wax, grease, and organic matter.

If the punch oil produces too much and is too thick, the soot will flow into the cavity from the inner gate and adhere to the cavity near the inner gate, resulting in the blackening of the casting surface in part near the inner gate.

Punch lubricating oil is sprayed in mist on the top of the punch outside the pressurized chamber or sprayed from the back of the punch inside the pressurized chamber after the pressurized shot so that the accumulated punch oil will be pushed out of the pressurized chamber by the returned punch to reduce the accumulation of punch oil. If the punch oil is sprayed in a mist inside the pressurized chamber or dripped inside the pressurized chamber in oil form after the return of the pressurized punch, not only should less punch oil be sprayed but more importantly, should use compressed air blow the accumulated punch oil to disperse it on the surface of the pressurized chamber.

If the mold core extractor leaks, the hydraulic oil of the core extractor flows into the cavity, which will also make the surface of the casting black.

Too large spraying amount of release agent or too thick concentration of proportioning

If the die-casting cavity spraying release agent is too stacked, or the concentration of the release agent is too thick, the release agent in the oil, grease, wax and organic substances such as silicone oil and water will produce a lot of oil smoke after encountering the high-temperature alloy liquid, too much oil smoke not only seriously pollute the alloy liquid and mold cavity surface, but also make the injection filling alloy liquid rapid oxidation, causing the casting surface and internal black spots, blackening (Figure 9) and carbon accumulation. Therefore, choose the mold release agent to prevent good adhesion, high-temperature resistance, not easy to burn, and not easy to produce fumes. A good quality mold release agent not only prevents oxidation and corrosion on the surface of die casting but also will not affect the quality of the casting.

Figure 9 The black smoke sample of oil remaining after blasting

Using a lighter concentration of release agent can also reduce the generation of oil fumes. Generally, the ratio of release agents is about 1:120~150. For example, the mold of a heat sink has a thin and deep cavity of the heat sink, so it is easy to stick to the mold, so cannot reduce the spraying amount of mold release agent, but can use a lighter concentration of mold release agent, and the ratio of 1:140 can be used.

To reduce the generation of oil fumes, when spraying mold release agent to the mold, we should pay attention to the location, distance, flow rate, and time duration of spraying; the mold release agent spraying atomization effect should be good, and the spraying amount of mold release agent and punch oil should be small and even. The film formed by the release agent on the surface of the mold should be thin and, even, do not let the release agent flow and accumulate in the cavity. After spraying, use compressed air to blow the surface of the mold cavity clean, and pay attention to evaporating the excess water in the release agent before closing the mold.

Note also that the water used to prepare the release agent should be pure water or soft water, not hard water because the metal ions of hard water not only affect the stability of the release agent, coating, mold release, and other properties but also will promote the oxidation of castings, rust, and corrosion.

 Carbon blackening on the casting surface

The wax and organic grease in the release agent will burn easily, and the carbon black residue after burning will sinter and adhere to the surface of the mold, forming carbon. The carbon layer adheres to the mold surface firmly. It usually needs to use oil stone, water sandpaper, and abrasive cloth polishing to remove. The mold surface can be polished periodically to prevent excessive and large carbon deposits and blackening defects.

Generally, when the mold surface temperature is higher than 240 ℃ at the moment of mold opening, the mold release agent will be sintered to form carbon. Therefore, we should lower the pouring temperature and control the mold temperature within a certain range to keep the mold thermal balance. Reduce the mold temperature in the overheated part and increase the cooling water flow of the mold so that the mold temperature is not too high to prevent the release agent from sintering and collecting carbon. In addition, the rougher the mold-forming surface is, the easier it is to make soot stick and produce carbon; the selected release agent cannot withstand high temperatures. The higher the pouring temperature and mold temperature are, the easier it is to produce soot and sintered carbon.

If there is carbon on the mold’s surface, the heat transfer speed from the alloy liquid to the mold is affected. It is easy to form shrinkage concave and thermal crack defects in the thick parts of the product, so it is necessary to apply oil stone or sand cloth to grind and polish off the carbon on the surface of the mold. In the larger hot section parts and thick wall parts of the casting, the alloy liquid is at a high temperature for a longer time. The sintering of the release agent is intensified, so it is easier to form carbon defects. So it should set up the mold well but should regulate the heating system and the mold temperature.

Too much residual moisture of mold cavity surface release agent

Mold cavity release agent water is not blown clean or not evaporated clean, mold surface residue too much water, encounter high-temperature alloy liquid that decomposes into oxygen (O), hydrogen (H) and HO- ions, where the oxygen oxidation and pollution of alloy liquid, so that the casting surface layer oxidation black. At the same time, when there is too much moisture in the release agent, the release agent will not get sufficient oxidation and combustion when it meets the high-temperature alloy liquid and will form. smoke

The mold surface temperature is higher than 160 ℃. The water will evaporate faster and be cleaner. Therefore, the temperature of the mold away from the inner gate part and the temperature of the mold in the low-temperature part should be increased appropriately; the cooling water of the mold can be turned off appropriately so that the temperature of the mold will not be too low and can evaporate the excessive moisture of the release agent on the surface of the mold quickly to avoid excessive fumes.

After spraying the mold release agent, we should accurately and thoroughly blow clean the mold release agent’s moisture on the mold surface, and the excessive mold release agent should be blown out, blown off, and blown clean with compressed air. Aim at the deep cavity, deep hole, deep groove, and slider groove of the mold, and use the blowing tube to stay close to blow for 1 ~ 3 sec. For the water on the mold parting surface, also use the air tube to blow clean, not only to prevent the water on the parting surface from flowing into the cavity but also to prevent the excessive water on the parting surface from blocking the parting surface and exhaust slot, which affects the effect of exhaust.

Regularly check and prevent the mold cooling water, mold release agent water, and mold release agent water in the slider slot from flowing into the cavity and pressure injection chamber.

If the mold release agent uses water with high salt content, the salt will adhere to the casting surface, prompting the casting to oxidize and corrode. Therefore, the mold release agent for die casting should be pure water or soft water, and purified tap water

Poor quality of mold release agent and punch oil

Paraffin wax, vegetable oil, mineral oil, many kinds of silicone oil, and graphite in the mold release agent, such as oxidation and combustion of organic matter decomposition of O2, CO2, CO, H2O, and Al reaction to produce Al2O3, while producing oil smoke caused by the oxidation of castings and blackening. These organic materials are unstable and will condense into sticky particle-like oil droplets. If bacteria are infected, they will condense into a flocculent mass and collect into a pile. These viscous oil droplets meet the filling of high-temperature alloy liquid burning oil smoke when the condensation of oil droplets is burning. Oxidation is not sufficient. Oil smoke contamination of oil droplets particles, so that the oil droplets particles black into the release agent black particles, and by the alloy liquid flushing and with the alloy liquid flow, penetrate the casting, so that the alloy liquid local or overall oxidation, so sometimes there is a local finishing of the casting, the internal black phenomenon. Release agent and water emulsion decomposition and dissolution are not sufficient, or when encountering high-temperature alloy liquid combustion decomposition is not sufficient, there will also be release agent black particles.

Die castings stored for a longer period, the surface, such as black, mold, and white spots (frost, black after removal), indicating that corrosion has occurred, which may be because the release agent has the ingredients to promote the corrosion of the casting phenomenon. Just die-casting out of the casting surface is slightly black. Castings in the natural world, after 10 to 30 days, the casting surface will automatically cause oxidation, and the color becomes darker.

Punch oil accumulation more, punch oil encounter high-temperature alloy liquid combustion decomposition is insufficient, the same will appear punch oil black particles. Suppose the surface of the mold is contaminated. In that case, such black spots will contaminate the shallow surface layer of the casting alloy fluid, and after die-casting, the shallow surface layer of the casting will form black dots; there is also a gathering of contamination into a small piece of blackened, irregular shape. Black particles caused by the casting blackening and the casting surface soot pollution blackening are not the same, not outside the surface of the casting, but the oxidation of the casting internal, with a hundred clean cloth, can not be wiped off. Suppose the filling process contaminates the internal alloy liquid. In that case, it will make the alloy liquid polluted, and oxidation discoloration, the formation of small black cavities or small black slag-like dots inside the casting, the naked eye looks like a small black dot. The size of small black dots is generally below 2 mm, single or scattered. The location, extent, and size of the black cavities or black slag are related to the black particles’ size, location, and filling flow.

The gas of soot mainly forms small black cavities. The inner surface of small black cavities is rounded to the inner surface of the shrink hole but not as smooth and bright as the surface of the cavities formed by air or hydrogen, which is because the color of the pollution of soot is darker. The color of the small black cavities formed by the general punch oil is a little darker than the color formed by the release agent. Black small cavities are oxidation cavities formed after the alloy liquid is polluted by black particle oxidation. Oxidation cavities are darker than normal oxidation cavities due to the pollution of oil and fumes. Small black cavities or small black slag will not only appear on the surface of the casting finishing or dissection but also on the fracture surface of the inner gate of the casting, affecting the quality of the casting.

Therefore, the mold release agent and punching oil should have a good emulsification effect dissolved with water, good suspension, no precipitation, no accumulation, no scaling, no bacterial infection, and easy dispersion.

Pouring system and mold structure

Although the die-casting process can solve some casting defects, in the case of normal process debugging, most of the casting defects still need to be solved in the mold. The pouring system and mold structure will directly cause the casting blackening. Generally, parts with vortex are away from the inner gate parts, parts with vortex, in the deep cavity of the mold, and the last filling parts of the die casting surface are prone to blackening phenomenon. One is because these parts gather more fumes, alloy liquid and mold surface contact with fumes for a long time, the amount of alloy liquid vortex wrapped around the gas, resulting in the oxidation of alloy liquid on the surface of the casting increased, the degree of blackening of the mold surface adhesion soot carbon black increased. Secondly, because of the slow flow of alloy liquid in the last filling part, the degree of scouring on the surface of the mold is light, and the alloy liquid cannot flush out the soot adhering to the surface of the mold.

Especially for large plane castings or large plane parts of castings, most of the alloy liquid is jet-filled, which exceeds the speed of fume flow and discharge. The more the alloy liquid and the fume contact, the more fully mixed, the more the amount of alloy liquid wrapped with soot gas increases, the easier the alloy liquid is oxidized and polluted, and the more serious the phenomenon of blackening on the surface of the casting. More internal gates should be set up to allow the alloy liquid to be filled with full wall thickness as much as possible so that the alloy liquid does not encapsulate the gas. Quickly finish filling in a short time so that the fumes are too late to be produced and so that the fumes do not have time to oxidize the alloy liquid.

Improve the pouring system of the mold, and change the position, size, and direction of the inner gate and overflow groove, which can change the alloy liquid’s flow direction and flow rate and change the production, flow direction, and discharge condition of the soot.

Figure 13 shows the heat sink pouring system. Figure 13(a) casting is a casting with more heat sinks. Figure 13(b) and Figure 13(d) are heat sinks on two sides of the casting (with a blackened appearance). Add a pouring system to the mold, as shown on the left or right side of Figure 13(c), to allow the alloy liquid to fill the blackened areas of the casting early, as quickly, directly, and smoothly as possible. Widen both sides of the inner gate to the side of the casting where the heat sink is, and let the alloy fill the heat sink on the side of the casting directors. After this modification, eliminate the blackening on the surface of the heat sink on both sides of the casting, and the alloy directly fills the heat sink on the casting side, as shown in Figure 14.

In addition, appropriately increasing the total and local exhaust slot and overflow port cross-sectional area of the mold so that the exhaust system can smoothly and quickly discharge liquid and exhaust and timely discharge the oil fumes and contaminated alloy liquid outside the cavity will also have a better effect.

Casting structure

Generally, in the casting wall thickness of thin parts, mold temperature is relatively low, the mold release agent water evaporates more slowly, residual release agent and water more, so more soot generated; can not smoothly castings filled, and difficult to exhaust deep cavity parts, blind hole parts, residual soot in the cavity is also more. Due to the shape or structure of the casting resulting in a greater clamping force on the mold, there will be the phenomenon of sticky mold, the need to spray a thicker, more mold release agent. The cavity will remain with too much release agent, increasing the blackening of the casting. So some blackening can not reduce the release agent so that it can eliminate. blackening

Heat sink castings blackened more on one side of the wall thickness is relatively thin. Alone away from other heat sinks, the amount of alloy liquid gathered in the cavity is relatively small, so the mold and the temperature of the slider here are not high enough, the cavity surface release agent evaporated not completely, and more residual release agent to meet the high-temperature alloy liquid produced relatively more soot. The solution is to increase the temperature of the mold.

In the cavity parts that cannot be filled directly by the alloy liquid and the parts where the filling process appears vortex, the surface of the casting is prone to vortex-like blackening. The solution is to change the location and flow direction of the filling, direct filling, and eliminate the eddy current phenomenon.

Blackening often exists simultaneously with the hard partition and flow marks. Because of the low temperature of the mold or alloy liquid in the hard partition and flow marks, the mold release agent burns and evaporates slowly when it meets the alloy liquid. The fumes are in contact with the slower-flowing alloy liquid for a long time, and fumes and oxidation pollute the alloy liquid for a long time. The solution is to increase the temperature of the mold.

For large plane castings, the alloy liquid will jet overfill to the distal end or overfill to the surrounding parting surface and will not be able to make the soot gas in the cavity be discharged to the outside of the cavity in sequence, like complex castings; therefore, the surface of large plane castings is prone to blackening. The solution is to set more internal gates to allow the alloy liquid to finish filling quickly and for a short time.

Die Casting Alloy Materials

Most die-casting alloys are easily oxidized aluminum alloys, especially in the high-temperature state are very easy to oxidation. In the process of pouring and the moment of die-casting filling, it will be oxidized, and the defect of oxidation slag will appear.

In the process of pouring a spoon to take the alloy liquid from the furnace and pouring, the alloy liquid will be oxidized by the oxygen in the air during the flowing and tumbling. An aluminum oxide film will be formed on the surface of the alloy liquid. The longer the time to take the alloy liquid, the more the alloy liquid tumbles, and the more serious the oxidation is.

When the alloy liquid is poured into the press room, the high-temperature alloy liquid makes the punch oil sprayed into the press room burn and smoke. The oxygen atoms decomposed in these fumes and burning gases will accelerate and intensify the oxidation of the alloy liquid. Therefore, the more punching oil is used, the more serious the alloy fluid is oxidized and polluted.

Aluminum alloy dies castings are stored in the natural state. The surface will soon be oxidized and darkened; if stored in a humid environment, the surface will also appear with rust defects in a few days. After the shot blasting treatment of die casting, the casting in die-casting high-temperature molding forms a dense oxidation layer, and the speed and degree of surface oxidation will be increased.

Different chemical compositions of the die-casting alloy and its ability to resist oxidation are not the same, so using different grades of alloy die-casting out of the castings, the surface blackening is not the same. So, you can use the die-casting alloy that is not easily blackened by oxidation. For example, ENAC-434000 alloy is more easily blackened by oxidation than ADC12 alloy.

In the aluminum alloy liquid slag refining, add 0.1% to 0.3% of the alloy liquid quality of sodium fluoroaluminate or sodium fluorosilicate, which is also conducive to preventing the casting oxidation discoloration.

Die castings after blasting (blasting) surface blackening discoloration

Stainless steel shot to regular screening and dust removal, this is because the stainless steel shot after some time, not only will make the steel shot is worn smaller, the steel shot surface will also be attached to aluminum oxide dust, in the shot blasting hit die castings at the same time, the steel shot will also hit the aluminum oxide dust to the surface of the casting, resulting in the overall color of the casting surface darkened, or become matte color.

Zinc shot material: bright appearance, high specific gravity, blasting force is large enough; low hardness, can remove large burrs while not wearing the surface of the product; zinc shot produces dust that is not easy to burn and explode, is a safer shot material, is widely used in zinc, aluminum products blasting deburring and cleaning. However, zinc shot itself is easy to oxidation and easy to produce oxide dust, resulting in the casting surface color darkened, darkened.

Aluminum shot material: the surface of the castings after blasting and spraying with aluminum shot is the most beautiful, but because of the lighter specific gravity of aluminum shot, the strike force is relatively small, can not spray off small burrs, and the efficiency of the shot is not high enough, and because the resulting dust is flammable and explosive, so the application is relatively small.

The stainless steel shot mixed with 20% to 30% aluminum can achieve a better white surface effect than the stainless steel shot alone. There is a patented product of low carbon bainite alloy steel shot with both low carbon content and good toughness, but also through the micro-alloying and special heat treatment process to ensure sufficient hardness, thus ensuring high efficiency. At the same time, due to the carbon content and stainless steel shot close, castings, after blasting, are more bright and white, significantly improving the surface quality of castings blasting.

The blasting process will not only spray the moisture and dust in the air to the casting surface, but the shot will also hit the casting to heat up, and the temperature will accelerate the oxidation reaction on the casting surface, so the casting will quickly turn black.

After the shot blasting products, the operation should wear gloves. If direct hand contact with the casting, the casting surface will leave a hand imprint caused by blackening. Die casting is in a high-temperature state. After die casting off, the surface will quickly form a layer of an oxide film. After shot blasting, the oxide film disappears, and the casting surface is more likely to produce oxidation defects. Therefore, the die casting after shot blasting, to dustproof, moisture-proof, waterproof and oil-proof, sealed packaging, and fast surface coating treatment.

If the casting uses vibration grinding for light finishing, only light finishing off the casting surface dust, oil, and tiny burrs can not light finishing off the casting matrix blackening. Before and after finishing, the castings should be sealed and kept dry to prevent the casting matrix from moisture and oxidation blackening. In light finishing, attention should be paid to cleaning the powder deposited by abrasive, and the casting should be cleaned, baked, and dried immediately after light finishing.

Casting shot blasting

Poor die casting causes plating treatment blackening, bubbles, peeling

Poor die casting causes plating treatment blackening, bubbles, peeling
The die-casting surface can not have defects such as cracks, looseness, shrinkage, porosity, bubbles, pinholes, cold compartments, flow marks, etc. Otherwise, the surface of the casting is easy to blister after plating, and the plating layer is separated from the substrate.

Die casting is the process of die-casting solidification, the surface due to rapid cooling, and the formation of a layer of the dense cold hard layer. At the same time, the internal organization may have pores, shrinkage, and other defects. If this good surface layer grins, the plating will appear with pitting, bubbles, and other defects. When polishing dies, castings should not be pressed on the polishing wheel too tightly, and the casting should not be overheated to prevent the abrasive from sticking to the casting and causing blackening or poor peeling of the product plating. In other words, the die casting before plating, grinding, and polishing can not be excessive.

Die castings are easy to oxidation, and if stored for a long time, the formation of a layer of oxide film on the surface will also affect the plating performance, plating before the use of reducing substances to remove the oxide film.

The moisture in the air drives the oxidation and blackening of the casting

The moisture in the air drives the oxidation and blackening of the casting
Southern rainy season or rainy season air humidity, in the blowing process after spraying release agent, the moisture in the compressed air sprayed to the mold surface, resulting in more moisture on the mold surface, so it is necessary to dry the compressed air.

In the air humidity, die castings stored indoors or in transit, if the seal is not good, the moisture in the air will condense on the surface of the castings, light so that the casting surface color darkened, black, serious so that the casting surface oxidation white mold spots.

In winter, when the die casting from outdoor transfer to indoor finishing or storage, if the indoor air conditioning, the indoor temperature is significantly higher than outdoor, the casting surface will also condense air moisture frost, frost will cause rapid oxidation of castings. Therefore, when the castings enter the room, do not immediately open the package; or open the package immediately after finishing to prevent the castings from frost and long storage and oxidation blackening.

In the high-temperature season, the temperature of the products in the machined air conditioning plant is relatively low. They are not directly loaded and shipped. They must enter the warehouse buffer zone for transition. Otherwise, the product will condense water.

Aluminum alloy is an amphoteric metal. It will oxidize and change color when encountering acidic or alkaline substances, so neutral cutting fluid should be used in finishing and ultrasonic cleaning.

Put the castings after finishing in hot water for cleaning, then immediately blow off the water droplets on the surface of the castings with compressed air and put them in boxes after some time. If immediately put into the packing box for storage while hot, the castings will soon appear oxidation, blackening, and moldy spots.

The method of removing the black surface of die casting

The operation, management, and storage of die casting surface oxidation to prevent blackening die casting in higher temperature or humidity conditions are very easy to oxidation and erosion.
Die casting surface is oxidized after the color becomes gray, black, or moldy. Good die castings, or die castings after cleaning, if not promptly done anti-oxidation treatment, long-term stacked in a humid warehouse, as short as one or two months or as much as six months, may oxidize black mold. Therefore, it is important to have suitable anti-oxidation measures and storage management for die castings.

Due to the different degrees of humidity, ventilation conditions, and temperature inside the storage warehouse, aluminum alloy dies castings are stored in the warehouse. The height is different, its oxidation and blackening, and moldy condition will be different. Therefore, the warehouse where the castings are stored should be dry, ventilated in sunny weather, and sealed in wet weather with necessary dehumidification.  If the warehouse is on the ground, the bottom of the storage should be erected to support the shelf height to prevent the ground from returning to moisture.

Castings should be sealed and packed, and dehumidified with desiccant, and castings should not be exposed to moisture during storage and transportation. It should carry out special protective measures in the yellow plum season to prevent castings from dampness and mold. It can add Plastic bag packaging to put the castings in a small plastic bag, sealed packaging, stored in a plastic bag as large as a large packing box, and desiccant, sealed packaging.

Die casting in die-casting, machining process should be cleaned and dried after, if not do any cleaning treatment, die casting surface residual mold release agent, cutting fluid, saponification fluid and other corrosive substances and water stains and other stains, these stains accelerate the speed of aluminum die casting long mold spot blackening. Therefore, die casting after finishing, the application of ultrasonic cleaning, and the drying process must go through. However, if the choice of cleaning agent is inappropriate, it will also cause corrosion and oxidation of die castings.

Can not use wet cardboard boxes, cardboard and crates, and other items. You can use plastic boxes and plastic plates. It should seal the box immediately after loading and cover the top layer with a flap.

For castings that especially need rust prevention, it should reduce the stagnation time of the castings in the flow process. For castings with steel and iron inserts, anti-rust oil should be applied to the inserts, and there should be no omission.

Rain should be prevented, covered, and tied with a protective rain cloth during transportation. When the castings are wetted by rain, they should be isolated immediately and cleaned and dried. Moreover, the castings should be placed for 7 days to confirm that they are qualified and that no more rusting occurs before they can be circulated.
To prevent the water drops from splashing on the castings when spraying the release agent for die casting, move the castings to other areas for storage. The conveyor belt next to the die-casting machine and the cutting edge cleaning of the cutting edge die should regularly clean the workbench. The operator’s gloves should be clean, not damp and slippery, so as not to pollute the castings.

Before the castings are discharged from the warehouse, the castings should be opened and inspected, especially the products packed in the first two or three months. The boxing must check again for the castings that have been boxed for 6 months.

Surface coating treatment methods to prevent oxidation of die castings

The commonly used surface coating methods for die casting are as follows.
(1) Paint coating.
(2) Anodic oxidation, which can prevent the aluminum alloy from continuing to oxidize.
(3) Porcelain anodizing, with a beautiful surface, high hardness, and good decorative properties.
(4) Micro-arc oxidation, high hardness, good corrosion resistance. The disadvantage is that the surface is rough, poor aesthetics.
(5) Hard anodizing, high hardness, wear resistance, unattractive color, gray.
(6) Electroplating.
(7) Chemical oxidation.
(8) Organic material coating and spraying.
(9) Physical vapor deposition (PVD treatment), good effect.
(10) Ceramic coating, high cost.


There are many reasons for the blackening of die castings, and it is necessary to take corresponding measures according to the causes of each blackening to effectively eliminate the blackening phenomenon on the surface of die castings and improve the quality of die castings.

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