Posted on :Dec 7 , 2022 By  GREFEE

Vacuum die casting is the necessary technology for high quality aluminum die casting pieces, such as cylinder body, automatic transmission housing, valve body, etc, especially for automotive parts that require T6 Heat treatment and welding. The gas content of castings should be controlled within 0.5cc/100g, which has strictly requirement for high vacuum during mold filling (vacuum degree is below 50 mbar). It is difficult to achieve it under the challenging die casting production environment. For these large scale and complex shaped castings, a high vacuum cavity condition is required.

In addition to a good tightness, the chambers need to be tighten, too. Besides, a better overall punch system (including punch, material pipe, lubrication and splitter cone) also matters in ensuring the stability of injection speed, rapid production cycle, minimum gas production. Indeed, the optimization of the injection process and runner system is also required to avoid the gas trapping. High efficient mold temperature thermal balance system can reduce the amount of spray and local heat. Gas and slag removal of aluminum liquid to reduce gas and slag content. Moreover, avoid the gas generated by the aluminum liquid and residual lubricate oil during the injection process to enter into the castings.

The gas content impacts the weldability of castings largely. The organic matter of release agent will pollute the surface of the castings. Besides, the mold release agent may dissolve hydrogen into molten aluminum after contacting with liquid aluminum, and it becomes microbubble when the castings are cursed. Another substance that is harmful is too much lubricant for the punch. Due to the direct contact between the lubricant oil in chambers and liquid aluminum, too much gas is generated. Too much residues will block the exhaust passage and affect the sliding of valve element. It also may lead to burrs and damage the sealing strips.

During the production, the gap between the standard punch and chambers will slowly lower the tightness of the whole system. Normal sprayer and punch lubrication system are more likely to overuse the amount and cause the gas content increase, which are not beneficial to the high vacuum die casting. Too large gap will make the aluminum to be absorbed into the cavity at an early stage and cause the blockage of vacuum valve. The above are the difficulties in vacuum die casting.

Production issues of punch tubes

To achieve the continual production of high vacuum die casting, the first thing to do is the punch issues during die casting production, including:  

The control of gap between punch and chamber, deformation of chamber, alignment of machine dies and lubrication of punches may all lead to the in smoothness in production, like the short service life of punch material tubers, unstable speed, clamping hammer when the punch ejects the casting, etc. the condition may get worse, especially for large scale castings. There are many reasons behind this, the main reason is clearance tolerance of die/tube/punch, lubrication of punch, temperature control of punch/tube. The chamber is deformed due to the imbalanced heating, the gap difference between the chamber, mold, and punch, aluminum left and accumulated in the chamber, the lubricating issue of chamber/punch, etc.

Uneven temperature and deformation of pressure chamber

There are many factors lead to the deformation of chamber, including feed volume, filling ratio, alloy temperature, cycle time, design of pressure chamber. It is similar to the wall thickness, length, size of soup mouth, etc. the chamber will be heated in the liquid aluminum during the production cycle, and it is also heated by the material cake at the ejection end during the pressurization and casting cooling stages. Hence, the local overheating appears, especially the temperature of the ejecting end and aluminum pouring position of the pressure chamber is very high.

Eg the bottom temperature of the material tube is 300℃，the temperature of the top is 150℃. If the extension length is 400mm, so the difference between the upper and lower lengths is 0.8mm, forming a banana shape. The size of mold holes and material tubes: high temperature will make the mold holes smaller. If the temperatures of four direction are different, the inner circle becomes ellipse. If there is no appropriate control in temperature, there will be many issues, like the unstable speed or large resistance because of the friction caused by the small gaps.

Punch tube clearance control

The liquid aluminum has good fluidity. To prevent the aluminum from running into the feeding tubes and causing wear, the gap must be within 0.1mm  under high pressure.

Uneven head lubrication

Due to the punch oil is generally added on the feeding port or punch, the end of the mold is likely to jam the hammer if the chamber is too long.

To address the tightness of punch and production continuity, sealing ring punch, matching technology of punch lubrication system and temperature control material pipe  can be used to achieve the vacuum die casting production.

In addition to the H13 mold steel, the standard punch material also include wear-resistant beryllium copper punch that is helpful to the production of large die castings with long punch stroke and high fill of feed pipe. Beryllium copper alloy has high thermal conductivity and lubricity, and the punch can be cooled quickly. Due to the low COF, the temperature of punch is not high and it won’t jam the hammer when operating, which is great material for punch. The material of punch can be made of different materials. Commonly used materials include the copper alloy containing nickel chromium silicon has low hardness (HB180-220) and is cheap. The nickel cobalt copper alloy containing beryllium (<1%) has high hardness (HB220-280) and is expensive. While, the nickel cobalt copper alloy with high beryllium (>1%) which is the most expensive option but has the highest hardness (HB>300).

The punch may be worn if being used for a long term. Besides, if the gap between the punch and chamber is large, the aluminum may run into the gaps or left on the inner walls during the injection under the high pressure. After a long term operation plus the uneven lubrication of chute, the punch and tubes may be worn or jam the hammer. To address the gaps, we have the seal ring punch, which is to add the seal copper ring at the front of punch. After the continual optimization, its durability and cost performance will raise when it develops into double sealed steel ring+ATS-RS steel punch with copper sleeve.

Working principle of seal ring punch

Many great seal ring punch is a combined design, including the open steel ring, steel punch, copper sleeve and body at the front end. The diameter of the open steel ring is slightly bigger than the rated inner diameter of pressure chamber, which can make the steel ring attached on the chambers. During the injection process, the open steel ring and steel punch at the front end are impacted by the aluminum liquid, and there is a gap between steel rings and steel punches so that the aluminum will run into “A” during injection and spread the steel ring further. Thus, the steel ring and chamber will be more closer, and the tightness is higher and no burrs on the inner wall of pressure chamber.

Copper rings are copper alloy material with high strength and elasticity is also the design of opening, which not only increases the tightness of punches, but also improves the lubrication between punches and pressure chambers. The attached figure shows that burrs will not appear on the outer circle but the inner circle if use the cake of seal ring punch.

The design of punch should have sufficient strength to against the high pressure impact. The cooling of punch should also be considered. If the temperature is too high, the service of life of punches will be shortened and the cooling time of cake will be extended. After the modification of designing, the cooling effect is great. Under the normal production, the surface temperature of punch is under 70C. If the service life of punch can be ensured, it could be 50-60 thousand.

Another benefit of seal ring punch is steel rings are more easily to change with special pliers without removing the punches. Under suitable conditions, the life of D120mm steel rings can be above 15000 modulus。

Pressure chamber design and temperature control

Temperature, tolerance control of mold and template are essential in addressing the tightness of material pipes. When the temperature difference of different locations in the pressure chamber is over 50C, the pressure chamber will be deformed. Large deformation not only makes the movement of punch not smooth, but also shortens the life of punch and chamber, especially for the long stroke of punch of large die-casting machine. The filling ration of liquid aluminum is high and the chamber is likely to deform. It is better to have the cooling pipes in the chambers to control the temperature. If the temperature of material pipes is controlled by cold water, cold mold is required for preheating and thermocouple and temperature controller connection shall be installed in the pressure chamber. Relatively speaking, it is better to control the temperature of the material pipe with a hot oil mold thermostat even though the cost is high, but is simple to operate. The hot oil can use preheating pipes. Adjusting and balancing the overall temperature to make the temperature difference lower than 50C. The life of chamber can also be extended to 3-5 times. The stability of injection increases, the service life of punches and steel rings also increases, the scrapping rate of cold start drops.

The designing of material pipes is the key of the whole injection kit. Seal ring punch and integrated material pipe are preferred. The advantage of it is that the service life is longer and the process stability is higher. In addition, the alignment of the material pipe/machine/mold is easy, with tolerance and built-in pipe temperature control. A successful designing of material pipes is extremely important for the seal punch ring.

The factors to be considered in the design of the pressure chamber include thermal expansion, strength and alignment.

A: The temperature of the material pipe near the mold is high, so the integrated design with the sprue sleeve should be avoided to affect the deformation.

B: One in and one out oil pipes are processed and arranged, and the pipes are distributed in the lower half of the material pipe. The distribution area should be based on the filling ratio of aluminum liquid, which can efficient cool the heating location.

C: placing the central ring so that the material pipe easy to align during production.

D: there should be enough space between the material tube and die/machine plate, which allows the material pipe can expand outwards. If the space is too small, the pressure chamber cannot inflate towards outside but oppositely, which narrows the inner diameter. Meanwhile, the friction between the punch and pressure chamber will increase, which shortens the service life significantly.

E: The thickness of the pressure chamber should be 1/3 of the punch diameter to maintain the strength of the pressure chamber.

Follow type cooling of splitter cone

To reduce the using amount of mold release agent and shortens the production cycle, the splitter cone is essential as this is obviously the thermal centre. To cool quickly, what can do is only by spraying heavily at this position. However, this will raise the risks of gas trapping of castings. Another method is to make random cooling pipe at the splitter cone, which is not limited by the traditional processing due to its pipe shape. It is designed according to the complex shape of splitter cone. The random cooling pipes should be distributed close to the mold surface. Moreover, the large heat exchange area greatly improves the cooling speed of the insert without heavily spraying. This technique also can achieve the balanced thermal field distribution even with complex shaped molds not only avoid over-spraying, but also address the shrinkage cavity, aluminum bonding, short die life, etc.

The random cooling used by die casting can be achieved with 3D Additive Manufacture or Metal Fusion Bonding, depending on insert size, shape and production cost. The cooling pipes of splitters are not complicated. Metal welding technology is one of the options, of which the cost is low. Due to the even distribution of cooling, the internal stress of inserts is significantly reduced. The service life is much longer than the traditional ones, over 100 thousand modulus. Also, this technology can be applied into the complex cavities.

Punch lubrication system

A smooth movement of punch promos a stable injection speed. Thus, the lubricate of puns is essential. With great punch lubricate system, the service life of material pipes can be ensured. The following work must be done well:   

-a proper position to place at, such as steel ring, and directly lubricate the whole punch, which is suitable for shorter journeys.  The one that generally used is <500T die-casting machines.

Use lubricating oil with good quality and proper viscosity.

The punch oil containing much impurities and poor quality will produce excessive gas and organic volatiles when contacting with aluminum liquid. If the volatilization lasts for a long time, especially when the mold is closed, gas will be generated, which will easily contaminate the exhaust channel and increase the gas content of the casting.

Minimum lubricant usage can largely reduces the pollution of machines and environment. Spraying of atomized lubricating oil is highly recommend.

For long stroke of the punch, the punch oil or punch bead will be generally dropped in the material trough. After being heated, the liquid will volatile and adhere on the inner wall of trough to lubricate the punch. However, for the whole stroke is not lubricate sufficiently ( the front end is lubricating evenly, but end is poor), a high amount of lubricating oil is required and thus not beneficial to the high vacuum die casting.

The lubrication of steel ring is very important to smoothly use with steel ring punch, especially for the long material pipes. Insufficient lubricating of end will increase the friction or jam the punch. Too much lubricating will also contaminate the castings. What worse is that direct contact between lubricating oil and aluminum liquid leads to combustion and gasification, which increase the gas content in castings. What ideal is that only use a small amount of oil can lubricate the long material pipe by spraying atomized lubricating oil evenly throughout the feed pipe. We recommended that install the spray head at the connecting rod position at the rear of the punch, and spraying staged. By doing so, the pressure chamber can be lubricate evenly. Moreover, the punch will not get stuck in the front of the pressure chamber and this only requires a small amount of spray. The control system can be decided according to the diameter and length of pressure chamber. By uploading the time and intervals of spraying, the lubricate oil amount can be accurately controlled (accuracy is within 0.2ml). Providing the minimal required amount of lubricate oil. Take the pressure chamber (D: 120mm, length 800mm) as an example, the overall lubricate oil amount in each cycle is only around 1ml.

Matching of vacuum valve

During the vacuum die casting, Only aluminum can fill the blind hole position and reduce the air content when the mold cavity and pipes are in a high vacuum status. Thus, mold in multi-slide (easy to leak)can utilize efficient vacuum valve (large extraction area) is extremely important. Most efficient high vacuum valves have many movable components, requiring regular maintenance and cleaning so that it can work smoothly, which is inconvenient to use. Although it is convenient to use the flat washboard for vacuum pumping, the pumping area is small and the vacuum degree is not enough. The Australian Kester 3D washboard is a good option, for it ensures the production efficiency.

Conclusion  

When producing thin-walled structural parts, the vacuum degree in mold cavity should be within <50 mbar under air pressure, requiring the following conditions to fit:

-The sealing property of the ram pressure chamber should be high to reduce air leakage.

Do not spray too much release agent to avoid residual moisture in the mold cavity.

Sprayer atomization should be qualified, and it is better to use micro spray head.

Using little mold release agent to avoid accumulation and residue in the exhaust duct, affecting the movement of the vacuum valve or reducing the area of the exhaust duct.

Using high-purity refining release agent with less wax. Avoid excessive residue on the mold surface, causing burrs, damaging the sealing strip and affecting production.

High injection speed for producing thin-walled parts, requiring good lubricity of punch.

Avoid to put the punch oil into the pressure chamber directly, affecting the gas content of castings. The connecting rod pulse spray mode is recommended.

The punch should be cooled for a long period of time and the water pressure and flow amount should be ensured.

-Tolerance control and temperature control of the material pipe are essential, careful calculation and temperature control are required.

To achieve the above effect, the sealing of punch, design of the material pipe, lubrication of the punch, and the vacuum valve must be handled at the same time. Great seal ring punch system is an innovative technology and provides a comprehensive solution, making the vacuum die casting product more stable or lowering the scraping rate. Increasing the utility rate of machine, the cost would be reduced. While, less oil contamination of die-casting machine will make the environment of workshops more clean. Thus, vacuum die casting production is an inevitable trend for automobile structural parts.

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