Producing high-quality and low-cost die-casting products has always been the goal pursued by die-casting companies. Due to the differences in their specific locations, machine equipment, employee quality, management concepts, raw material supply channels, and product service objects, although their ultimate ideals are similar, the results obtained are very different.

Die casting is a seemingly simple production process, but the processes are intertwined. The problems faced by die-casting companies cannot be solved simply by the methods described in textbooks, such as: the product has slag---the alloy needs to be deslag purified; there are irregular shrinkage holes inside---the local temperature distribution of the mold must be changed; cold shut occurs---ensure the warm flow of the alloy; there is flying material in the mold---check the mold fit/die casting machine clamping force; there are pores on the surface or inside---increase exhaust, etc. The actual problem is much more complicated. Take exhaust as an example. Is it passive exhaust? Or vacuum exhaust? How to choose the vacuum die-casting process?

1. The application of vacuum die-casting process is the natural choice of the product

Just like gravity casting uses riser exhaust, traditional die-casting uses exhaust slots for passive exhaust; but the difference is that the die-casting injection speed is fast. In addition to the inevitable occurrence of alloy melt running faster than the gas and smoke in the cavity to form air entrapment, the air entrapment phenomenon in the injection chamber must also be faced, which leads to low speed and multi-stage low speed control before filling. However, exhaust is subject to many factors, such as time, path, position and size of the gas slot, geometry of the casting, etc. Especially at the moment of injection, the air pressure in the cavity will be greater than the atmospheric pressure, which is undoubtedly a big problem for traditional die-casting production with high quality and low porosity.

The use of vacuum is a revolution in cavity gas control. While reducing the gas content overall, it can also effectively control the porosity of some local areas. With the emergence of full-process vacuum exhaust technology synchronized with the injection cycle, the production of die-casting products with low porosity is no longer a technical challenge. In the increasingly fierce market competition, even if people choose the cheapest products, quality will always be one of the important considerations. As for why vacuum exhaust is needed for die casting and when vacuum exhaust must be used, it depends on what kind of products the die caster produces. For die casters, catering to the market requires meeting product requirements; if the product has low porosity requirements, vacuum exhaust is always the most practical choice.

2. Basic requirements for vacuum die casting production

As far as high-quality die casting production is concerned, there are no special requirements for the application of vacuum: die casting machines with good performance; precision molds; tightly matched pressure chambers and punches; high-quality alloy materials and melting and heat preservation processes; precise mold temperature control; good process lubrication, etc. Different from traditional processes: the cavity should be relatively closed to form a vacuum exhaust environment; and an optimized exhaust design is also required.

The relative closure of the cavity is the most basic requirement for vacuum application. From the appearance, there is no flying material on the cavity parting surface, and no backflow of material at the pressure chamber punch. If the most basic requirements are met, vacuum exhaust will definitely be effective! Vacuum solves the exhaust problem. Regardless of the brand grade of the mold and die casting machine, the exhaust requirements are the same. In addition to the successful examples of imported molds and die-casting machines using vacuum in the market, there are also domestic molds and die-casting machines that use vacuum to produce high-quality products. But on the other hand, the application of vacuum can improve but not solve the deficiencies of other conditions, especially the problems caused by the lack of basic conditions. For example, although vacuum can reduce the injection force and injection speed relatively, this is not the mainstream goal of using vacuum exhaust.

3. Selection of vacuum system

This is a very interesting topic. Including well-known and unknown, there are about 20 domestic die-casting vacuum system suppliers: 4 to 5 in Europe and the United States; 5 to 6 in Japan, South Korea and Taiwan; about 10 domestic ones. The price ranges from tens of thousands of RMB to hundreds of thousands of RMB. As for the standard for measuring good and bad, in addition to the quality, performance and technical content of the system, application services are very important, and the end user has the most say. Due to the lack of a unified definition, die-casting companies are often very confused when choosing a die-casting vacuum system. The die-casting vacuum system includes a vacuum machine and exhaust components. The selection of exhaust components determines the nature and results of the entire die-casting vacuum exhaust. The choice of vacuum machine is entirely based on the system exhaust capacity requirements and is considered to match the exhaust components. Many die-casting companies often go the other way in their initial selection.

A complete vacuum system should have: vacuum manufacturing capability; system exhaust capacity; automatic control capability; vacuum detection capability; system self-cleaning capability; fault diagnosis capability; and wide compatibility.

●Vacuum manufacturing capability---the high vacuum maintenance capability of the vacuum pump and the vacuum storage capacity of the vacuum tank, which have a certain inherent proportional relationship with the size of the castings produced; if economic matching is not considered, the larger the better.

●System exhaust capacity---This refers to the minimum exhaust area of the exhaust component and the resistance of the pipes connected to it. This also involves the vacuum process requirements: whether the vacuum exhaust starts at the vacuum machine connection or the exhaust component, it is very different for processes with demanding exhaust requirements. The matching of exhaust components is very important and must be moderate. Too large or too small is not helpful.

●Automatic control capability---refers to the gapless interface control with die-casting machines, manipulators, etc. Perhaps this is not a requirement for all users, but it should be a must for a good vacuum system.

● Vacuum degree detection capability---refers to the detection of vacuum degree during cavity injection, which is a real-time feedback control. The vacuum degree of the vacuum tank is far different from that of the cavity. In extreme cases, the difference may be more than 300 mbar. Never use the pressure of the vacuum tank to replace the cavity pressure.

● System self-cleaning capability---In addition to the contamination detection when opening the mold, the backwash of the pipeline is also necessary, which can greatly extend the maintenance interval of the vacuum components and pipelines.

● Fault diagnosis capability---The operator will get the fault information immediately, solve the problem directly or respond to the maintenance personnel immediately to reduce downtime; instead of simply reporting that "the vacuum system has stopped".

● Wide compatibility capability---There are many exhaust components. When different die-casting companies produce die-casting products with different requirements, they will use different exhaust components, mechanical valves, cooling blocks, electric or hydraulic valves, etc. Without changing the vacuum machine, the vacuum machine must be compatible.

The selection of die-casting vacuum system is directly related to the exhaust scheme adopted by die-casting companies. Here, the end of cold chamber vacuum exhaust is used as the boundary for introduction and discussion: 3.1 Semi-process vacuum exhaust

When the vacuum valve closing signal is received, the vacuum valve is closed electrically or hydraulically. This method is often equipped with a larger vacuum exhaust pipeline and a vacuum valve with a larger exhaust area. The signal sent by the timer, travel switch or sensor that the alloy melt reaches a certain point before the filling point closes the vacuum valve and ends the exhaust process. It can be seen that the injection is not over at this time, and the smoke during filling cannot be discharged. Since the signal response of any electrical component has a lag of several milliseconds to tens of milliseconds, the vacuum must be ended before filling, so that the exhaust of the whole process cannot be achieved. The total exhaust efficiency of this method is low, and the final cavity pressure may rise to above atmospheric pressure, or even several times the atmospheric pressure.

3.2 Full process vacuum exhaust

Here, the end of vacuum exhaust is to close the vacuum valve by the kinetic energy of the metal itself, or the solidification of the metal itself closes the exhaust gap of the cooling block to achieve synchronization with the injection. It is divided into the following ways: 3.2.1 Simple exhaust

The vacuum starts when the punch passes over the pouring port. Before the vacuum discharges the cavity gas, the gas in the pipeline needs to be discharged first. Taking a 10 kg aluminum casting as an example, dual-channel exhaust, pipeline diameter 25 mm, length 6 meters, punch diameter 120 mm, exhaust stroke 600 mm, punch passes over the pouring port 0.3 seconds, exhaust time 1.2 seconds. The required exhaust gas is 6.8 liters; the additional pipeline gas is 6 liters; the cavity volume is 3.7 liters; the effective exhaust rate is 28.9%. This method is reliable and has a high repeatability. The disadvantage is that the exhaust burden is relatively heavy within a given injection cycle. 3.2.2 Pipeline pre-extraction full process vacuum exhaust

In the case of simple full process exhaust, pre-extract pipeline gas first. The vacuum exhaust of the pipeline starts when the mold is closed or even immediately after the mold is closed; the vacuum exhaust of the cavity also starts when the punch passes over the dump port. In this way, the vacuum directly acts on the location of the mold exhaust component. Taking 2.1 as an example, the pipeline gas is pre-extracted before the injection cycle, and the effective exhaust efficiency is 54.4%. This method has the same characteristics of reliable operation and high repeatability as 2.1, and the exhaust efficiency is greatly improved.

The pipeline pre-extraction of semi-process vacuum exhaust has little effect due to the exhaust process abandoned halfway. 3.2.3 Special full-process exhaust

First of all, it should be noted that this method has very high requirements for the coordination of the pressure chamber and the punch. The pressure chamber needs special modification, which has exceeded the concept of independent die-casting vacuum system exhaust. Its application is not widely accepted and is further divided into: 3.2.3.1 Pressure chamber bottom air pressure feeding type

After the mold is closed, the alloy melt is pressed into the bottom of the pressure chamber by air pressure, and the vacuum starts, which is similar to the hot chamber feeding method. The advantage is that the defect of vacuum exhaust when the pouring port is open is eliminated, and the exhaust time is longer, at least as long as the time when the punch passes the pouring port during ordinary pouring, which is 0.3 seconds in case 2.1. Do a simple calculation: in the case of 2.1, the exhaust time is 1.5 seconds, and the efficiency is about 36.1%. This method has good continuity and high efficiency, but it requires a good air pressure insulation furnace and a special pressure chamber. Please note: In order to prevent blockage, the feed port under the pressure chamber may have insulation requirements! The key to repeatability lies here. 3.2.3.2 Auxiliary vacuum exhaust at the top of the pressure chamber

It is not a new concept, but a new attempt. Close the mold, pour the material, and the punch passes the pouring port. At this time, the vacuum starts to exhaust at the top of the pressure chamber, and the exhaust components on the mold can start with it. Taking 2.1 as an example, the auxiliary exhaust is calculated as a single channel, the time is 0.6 seconds, but an additional 3 liters of pipeline gas is also added, and the total efficiency is about 29.3%. This method will make the initial exhaust more balanced, but the exhaust point at the top of the pressure chamber will soon be covered by the punch. The exhaust point should be set away from the filling point, generally between the starting point and the filling point. The disadvantage is that the pressure chamber needs to be modified, and the exhaust point at the top of the pressure chamber is easy to be blocked. Other changes include that the punch stops for a while after passing the pouring port, and the exhaust at the top of the pressure chamber is controlled separately from the exhaust of the mold exhaust element: the exhaust at the top of the pressure chamber can be regarded as pre-exhaustion, but this will increase the injection cycle time; or directly exhaust the top of the pressure chamber first, and then exhaust the mold exhaust element after passing this point. When doing this, the continuity of the system may not be good. The prominent disadvantage is that the repeatability is not high. (Figure 7)

Fourth, high vacuum and ultra-high vacuum system

If defined according to the general level, high vacuum refers to the pressure between 1.3x10-8 and 1.3x10-12 millibars; ultra-high vacuum is the pressure below 1.3x10-12 millibars; the range of die casting vacuum is between 50 and 500 millibars, which is considered rough vacuum at most according to the above definition. What is mentioned here is the high vacuum and ultra-high vacuum in the die-casting process: 50 to 100 millibars is die-casting high vacuum,

Below 50 millibars is die-casting ultra-high vacuum.

For high vacuum, it is not difficult to achieve the high-quality die-casting production conditions mentioned above; but the challenge of ultra-high vacuum is great. First of all, in addition to the above conditions, a vacuum machine with at least twice the capacity of ordinary matching is required; high-precision molds need to be set up with pressure-resistant seals that can withstand temperatures of 300oC; the ejector pin must have a precision guide of at least 40 to 50 mm; a clamp-type punch is used and a high repetition rate that is closely matched with the pressure chamber is guaranteed; the lubrication of the punch and the cavity must be accurate, and the cavity must be dry and clean before each cycle of injection.

The ultra-high vacuum system is not just an independent vacuum concept, but an efficient vacuum system, plus efficient sealing and precise lubrication of other related connecting parts. If this is the requirement of the product, this investment may have to be borne no matter how large it is, such as requiring the casting to have excellent airtightness, T6 treatment, high weldability after casting, etc., otherwise it is really unnecessary. Especially for super large molds, the investment outside the vacuum system may increase geometrically. For requirements beyond the norm, the investment in the vacuum system has become secondary, and the cost requirements in other aspects are much higher.

Considering the compatibility of the system and the particularity of product exhaust, various characteristics can be combined to derive and combine more different types of vacuum systems. Here, the economy of the vacuum system selection must also be considered: it is nothing more than the investment in the vacuum system, plus the energy consumption and maintenance costs, as well as the additional investment brought by ultra-high vacuum, and put them into each die casting according to the system's discounted life to see whether the improved yield and the saved waste recycling costs can offset the above costs in the short term. This is well known. Vacuum is not a panacea. Proper use can only reduce the scrap rate caused by exhaust problems. Generally speaking, it is normal to increase the yield by 5-10%. When other conditions are met, the first yield should be 97%-98%. In order to increase the yield by another 1-2%, it is not cost-effective to introduce ultra-high vacuum.

V. Comprehensive comparison (omitted)

VI. Other forming processes and treatments 6.1 Hot chamber die casting process

The application of vacuum in hot chamber die casting is similar to that in cold chamber, but due to the short injection time and effective exhaust time of the hot chamber, there are certain requirements for the stability of the hot chamber die casting machine. In addition, due to the effect of pressure difference, the hot chamber vacuum die casting process is more likely to "absorb" the alloy melt into the cavity prematurely, which requires more accurate parameter settings. It is worth noting that hot chamber die casting products, such as zinc alloy castings, generally require surface post-treatment processes and are more sensitive to porosity, but in terms of the current domestic market, the vacuum die casting process has not yet attracted enough attention from domestic hot chamber die casting companies. 6.2 Semi-solidification and extrusion molding

These processes can basically eliminate the phenomenon of air entrainment in the "pressing chamber", but there will be gas and smoke exhaust problems during filling, just like hot chamber die casting. When using vacuum, pay attention to mold temperature control, and cooling blocks with matching exhaust area are economical and suitable exhaust component selection. 6.3 Low-pressure and low-speed die casting

The application of vacuum is feasible, but the selection of exhaust components should be cautious. For example, mechanical valves that rely on metal kinetic energy to close are not applicable here. Compared with high-pressure die casting, this process requires a large adjustment of related parameters when vacuum exhaust is applied. 6.4 Impregnation treatment

As a post-casting chemical treatment, there is no shortage of measures to improve the yield rate. The following are the additional considerations that die-casters need to consider when using impregnation treatment: impregnation treatment requires a separate process production line, equipment, consumables, environmental protection, additional working hours and labor, etc. And not all products can accept this treatment. Regarding investment costs, the calculation is not complicated.

Seventh, Summary

The process selection of vacuum die casting should be based on the requirements of the product, and the key to the selection of the vacuum system is its practicality and economy. Simply put, vacuum pump + vacuum tank + cooling block can be called a vacuum system, but the problem is that as a user, you need to know what you really need, or who provides a system that is suitable for your product requirements.