Die casters, especially those who apply vacuum die casting for the first time, are often troubled by this problem: they want to apply it successfully at once, but they are also worried about the unsatisfactory exhaust effect caused by improper selection. It involves the technical nature of the vacuum system, the complexity of the product venting requirements, the proficiency of the design and production operators in the use of vacuum technology, the overall project economics, and the dependability of the supplier’s technical support. The following is an analysis of some of the characteristics of vacuum venting components to provide a reference guide for the practical application of these users.
The role of vacuum venting elements is to end the vacuum venting process of the die casting process, which is the key to vacuum venting! Vacuum exhaust elements are divided into two categories: vacuum exhaust valve and cooling vacuum exhaust block. The vacuum valve is divided into mechanical and conduction types; the cooling block is also known as exhaust block, rubbing board, wash shoving board, a large number of exhaust boards, and so on different, divided into water-cooled and self-cooling types.
Pure mechanical vacuum valve
Its working principle is to use the kinetic energy of the molten alloy to close the mechanical valve at the final filling point. Divided by installation structure: rectangular valve body in line with the mold parting surface; the front and rear part of the return valve body is embedded in the fixed and dynamic mold half each. According to the venting starting point: the starting point is located in the vacuum machine of the conventional valve; the starting point moved forward to the mold of the pre-pumping valve.
Conventional rectangular mechanical vacuum valve
From now on, called mechanical valve, its installation is parallel and consistent with the parting surface of the mold, which is extremely beneficial to the adjustment and cleaning maintenance of the valve surface and improves the repeatability of the valve body use. The exhaust cross-sectional area is annular, and the effective exhaust rate is high. Mostly double spool, the valve body reset for the disc spring reset.
Some mechanical valves are equipped with a spring compensation block, which is theoretically used to compensate for the valve surface gap caused by the deformation of the mold after heat. According to the actual customer use, I think: compensation device for large mold compensation role is more obvious; spring fatigue life of tens of thousands of times, the condition is not stable enough, more frequent replacement; because the compensation device out of activity, its installation can not be locked, on the one hand, may become looser, on the other hand, its lower surface and the mold joint gap, easy to lead to feed and cause compensation surface and parting surface deviation, In addition, the air leakage and running material of the mechanical valve bonding surface are inevitable, so we hope that users should pay attention to it when choosing and using.
The high effective exhaust rate
Synchronization with the injection process, the so-called “full process vacuum exhaust,” can be achieved.
Feedback cavity vacuum measurement can be realized to achieve “instant feedback” and “real-time control.”
Large one-time investment and expensive spare parts
Because it is a mechanical valve, it needs regular cleaning and lubrication, with high maintenance and certain quality requirements for the operator
Requires repeated training
Requires lasting technical support
Conventional rotary mechanical vacuum valve
From now on, referred to as rotary valve, it means that the center line of the vacuum shut-off valve spool is in line with the parting surface, and the whole concave valve body is installed in the fixed mold. In contrast, the other half of the convex valve body is installed in the moving mold. When the mold is closed, the convex valve body is embedded in the hollow valve body, forming a closed exhaust channel connecting the cavity and the exhaust valve hole. The exhaust cross-sectional area is annular, the general exhaust aperture is larger, and the effective exhaust rate is higher. The structure is a single spool, and the valve body reset is usually hydraulic.
Generally speaking, this kind of vacuum valve and the device is integrated with the die-casting machine, or “integrated vacuum unit,” which can be used as the valve closed by the hydraulic power of the die-casting machine to help power. For long-term fixed products, such vacuum units and vacuum valves are fixed together with an integrated die-casting machine, which has specific advantages. Still, at the same time, the mobility and compatibility are poor.
High effective venting rate
Can achieve synchronization with the pressure injection process, that is, the so-called “whole process vacuum exhaust.”
Feedback cavity vacuum measurement can be realized to achieve “instant feedback” and “real-time control.”
Large one-time investment
Large maintenance volume
Relatively troublesome to install, bound to die-casting machine, poor maneuverability
Poor flexibility and versatility
Higher requirements for mold, more complicated to repair the mold
Long-term technical support required
Pre-pumped rectangular mechanical vacuum valve
In the future, called a pre-pumping valve, its installation will be parallel and consistent with the mold parting surface. The exhaust cross-sectional area is ring-shaped. The valve body reset is pneumatic. According to the structure, it is divided into double-core and single-core. The single-core valve has a simple structure and smaller size.
For the cold chamber die-casting process, the gas space to be removed is: exhaust hose + cavity + pressure chamber unalloyed melt remaining space. Then the removal of gas in the hose becomes the “useless” exhaust efficiency part. For conventional vacuum exhaust, its die-casting process vacuum exhaust start point is located in the vacuum machine; the hose gas needs to be removed at the same time, in a given pressure injection time, which will undoubtedly extend the system response time and affect the final exhaust effect.
The use of a pre-pumping valve is to remove the gas in the hose first before the start of the pressure injection. Then the start point of the cavity exhaust is moved to the pre-pumping valve of the mold when it enters the pressure injection cycle, which can immediately carry out the cavity exhaust. The exhaust response time is greatly shortened, and the effective exhaust rate is greatly improved. By the way, for the cold chamber of the die-casting process, the remaining space of the pressure chamber gas removal is also a “useless” exhaust efficiency part. To solve this problem, achieve the so-called “ultra-high vacuum” exhaust, in addition to the vacuum machine and exhaust components, and at least the pressure chamber for the necessary exhaust gas removal. In addition to the optimization of the vacuum machine and the exhaust element itself, at least the necessary transformation of the pressure chamber should be carried out, such as the use of vacuum suction material, similar to the hot chamber die-casting feeding situation, basically eliminating the “useless” exhaust efficiency of the remaining space containing gas in the pressure chamber.
In addition to all the advantages of conventional rectangular mechanical vacuum valves
The effective venting rate within the limited time of pressure injection is greatly improved because the start point of vacuum venting in the cavity is moved forward
The structure is simpler, and maintenance is relatively easy after removing the disc spring reset
100% localization, good economy
Need technical support
Conduction type mechanical vacuum valve
From now on, called conduction valve, its working principle is to use a sensor or limit switch for position control conduction valve closing or a timer for time control conduction valve closing. The reasoning is the same: the vacuum valve will be closed after receiving the position or time signal before the pressure injection is turned into the high-speed filling. This kind of valve is generally simple in structure but mostly uses hydraulic, pneumatic, or electric power to close the valve. The exhaust cross-sectional area is annular. Usually, the conductive valve has a large exhaust aperture, and high efficiency occurs at low speed; at high speed, because it is difficult to control the alloy melt outside the conductive valve spool during the filling period in milliseconds, the exhaust must stop at this time, i.e., no pumping at high-speed filling. Corresponding to the pressure injection process, this is the so-called “semi-process exhaust.”
High exhaust efficiency at low speed before filling
The overall effective venting rate is uncertain and difficult to control
No “instant vacuum feedback measurement” during filling
Cooling vacuum exhaust block
It is mostly used for applications with less demanding exhaust, and the matching vacuum machine is at least double-channel. It is especially suitable for hot chamber die casting, especially for single cavity small parts die casting production. Its working principle is to use the alloy melt to flow in a small gap at the end of the cavity and gradually solidify by cooling, thus closing the exhaust channel and ending the vacuum exhaust process. According to the cooling method, it is divided into water-cooled and self-cooled; according to the different materials, it is divided into steel cooling block and beryllium copper cooling block; according to the gap distribution, it is divided into through-length gap type and small gap type cooling block; according to the exhaust tooth pattern, it is divided into triangle type, trapezoid type, and cosine waveform, etc.
Water-cooled vacuum exhaust block
Hereafter, called water-cooled block, generally made of steel. The exhaust cross-section is flat and long rectangular, which determines the high exhaust efficiency characteristics. The water-cooled block has high condensing efficiency, and its structure is generally short in the length direction.
Simple structure, small size
No special maintenance is required; only surface cleaning can be done
Low effective venting rate, about half the efficiency of a mechanical valve connected to the same venting cross-section
To maintain the same efficiency, the total venting area of the water-cooled block needs to be almost doubled, which will increase the projected area
Due to the open exhaust, the corrugated surface and exhaust channels are more prone to the accumulation of fine alloy powder
Only approximate cavity measurements are possible, and generally, the vacuum values measured are perfect and worth noting
Unstable water throughput will increase the variables of the solidification profile of the alloy solution in the water-cooled block during pressure injection; insufficient water may lead to running material
Once the cooling water leaks at the upper end of the mold, it may cause unexpected process accidents
Self-cooling vacuum exhaust block
The general cooling block refers to this kind, and the principle is to rely on its heat conduction to cool the molten alloy in the exhaust block to stop the vacuum exhaust of the die-casting process. Beryllium copper alloy is the most suitable material for the production of the cooling block, but in reality, the steel material is the most commonly used. The exhaust cross-section is a flat and long rectangle, and the exhaust efficiency is also not high. Beryllium copper conducts heat well, and its structure is generally shorter in the length direction than steel. But with equal clearance and width, the self-cooling type is 50%~100% longer than the water-cooled type in the structure.
Beryllium copper alloy cooling block has good thermal conductivity, elasticity, wear, and corrosion resistance, and processing performance.
Steel cooling block has good hardness and strength
A steel cooling block is more economical
No need for special maintenance
No risk of water leakage
Low effective venting rate, about half the efficiency of a mechanical valve with the same venting cross-section
To maintain the same efficiency, the total exhaust area of the water-cooled block needs to be doubled, which will increase the projection area
Due to the open exhaust, corrugated surfaces and exhaust channels are more likely to accumulate fine alloy powder
Only approximate cavity measurement is possible, and the vacuum value measured is generally perfect, which is worth noting
The cost of beryllium copper alloy is not expensive
The surface strength of beryllium copper alloy is not strong enough compared with steel, and the surface is more likely to be damaged after the alloy material is interspersed
Corrugation gap of cooling vacuum exhaust block
The corrugated gap is the basic element to determine its exhaust capacity, the larger the cooling path required the longer. A too small gap is too poor to resist pollution. Generally, less than a 0.3 mm gap is rarely used in practice.
The tapering gap is an optimized combination of exhaust and solidification, but it should be noted that: the exhaust area should be counted by the smallest bottleneck gap. The processing difficulty of this form will be slightly increased, and it is recommended that the ratio of maximum and minimum gaps of the asymptotic structure does not exceed 2:1; through-length gaps are the most straightforward to process, repair, and calculate. It is recommended that the maximum gap should not exceed 1.2 mm, and it is better to be within 1 mm. The larger the gap, the greater the length of the exhaust block.
Effect of corrugation on cooling vacuum exhaust block
The principle of corrugation design is to facilitate gas removal and molten alloy solidification; it should be easy to process and facilitate the demoulding of the solidified alloy in the exhaust block. The following is a basic comparison of these three typical corrugations
The above relative comparison makes it easy to see that no corrugation shape is absolutely perfect. In practice, triangular corrugation is the least expensive and is widely used, while the integrated type is slowly becoming the optimized corrugation trend.
Cooling block recommended exhaust area
For aluminum castings, as an example, see the table below; the pressure injection cycle is long, and the geometry of simple castings can be appropriately reduced by 20%; the opposite pressure injection cycle is short, and the shape of complex castings increases by 20%.
The overall efficiency comparison of vacuum exhaust component
Mechanical and rotary valve – blue; pre-pumping valve – red; conduction valve – pink; cooling block – green; no vacuum exhaust – black.
The venting curves of the above various vacuum venting elements are shown in the figure above. It can be seen: that if the mechanical valve and rotary valve as the basis (blue), after the curve of the pre-pumping valve (red) in a given limited time, the cavity is soon in a high vacuum state (low air pressure); and the curve of the conduction valve (pink) in the low-speed zone of the exhaust efficiency is very good, but into the high-speed zone, due to the vacuum exhaust stopped, the vacuum in the cavity will be rapidly reduced (air pressure increased), the final exhaust is not ideal the end stage of the vacuum exhaust process is more important than the beginning stage; the exhaust curve of the cooling block (green) shows that its exhaust efficiency is nearly about half of that of the mechanical valve, and it is difficult to break the 60% efficiency of the mechanical valve.
Application selection of vacuum exhaust components
Selection It is recommended that the steel self-cooling type is preferred.
The first application of vacuum exhaust users is advised to use cooling block exhaust, which is easier to master and smoother production.
Product exhaust requirements are not demanding.
Multi-cavity small parts or single-cavity small parts, single weight or total net weight, is small (such as aluminum castings with a net weight of 500 grams or less)
Hot chamber die casting more choice of cooling block exhaust
Products subject to comprehensive economic constraints
Products with relatively small batch sizes
In terms of efficiency, although the efficiency of the cooling block is low, the problem can be solved by increasing the number of exhaust blocks or increasing the total area of exhaust. Here it is important to pay attention to the negative effects of the resulting increase in the projected area. It is better to choose a vacuum machine with dual channels.
In practice, due to the low price of the cooling block, the most suitable exhaust element is generally designed according to the specific conditions of the product and is not used as an interchangeable part for different molds but follows the fixed mold. The maintenance of the cooling block follows the same cycle as the mold. In addition, the same mold cooling block can be different. Due to each designer and mold maker’s different design and manufacturing concepts, it is difficult and unnecessary to require each cooling block to be interchangeable, but the basic principle is the same.f cooling blocks
Selection of vacuum valves
In terms of repeatability and mobility, it is recommended that purely mechanical vacuum valves of rectangular construction be preferred.
where the product is extremely sensitive to air holes or air tightness
products with high surface quality requirements
Dies with multiple slides, compact structure
Precision die castings
complex castings with thin and thick walls and asymmetrical structure
Die casters who are familiar with the vacuum die casting process
Die casters with personnel who have strong maintenance and operation skills
highly automated die casting cells that require precise feedback of cavity vacuum and process control that displays cavity vacuum profiles
Pre-pumped vacuum valves can greatly increase the effective venting rate, especially for “ultra-high vacuum” applications
Mechanical valves are generally available in series sizes to facilitate the user’s choice due to their complexity and high cost. The same specifications of the same brand are interchangeable.
As for the conduction vacuum valve, it is recommended to use it carefully because the secondary flue gas influences its final effect; especially if the vacuum suction process has been adopted, the conduction vacuum valve is almost useless. For the specific selection and use of vacuum exhaust components, consult with the vacuum system supplier for specific products.