Gas-assisted injection molding and water-assisted injection molding are based on similar process technology, their scope of application is also similar.
What is the difference between these two injection molding technologies? Where are the applicable scopes of these two injection molding technologies?
As a very mature injection molding technology, gas-assisted injection molding has been used in the plastics processing industry for many years. One of the most important applications of this technology is the production of thick-walled plastic parts, such as the production of handles and similar products. Plate-shaped parts or other injection molded plastic parts with local thickened areas are also important application areas for gas-assisted injection molding.
From the actual production point of view, injection molded plastic parts with functional spaces or runners have begun to use water-assisted injection molding technology more.
The comparative advantage of water and nitrogen
Gas-assisted injection molding technology is used to produce rod-shaped parts to reduce part weight and cycle time. Gas-assisted injection molding also helps to greatly reduce or completely eliminate the wall thickness area, deformation, and shrinkage marks of flat plastic parts, thereby improving the quality of plastic parts.
The thermal conductivity of water is about 40 times that of nitrogen, and the heat capacity is 4 times that of nitrogen. In addition to ordinary mold cooling, water injection will cause internal cooling of the plastic parts. Compared with gas, the cooling time is shortened by 70%, and the plastic parts reach the required demolding temperature much faster. At the same time, water is also an incompressible and inexpensive medium.
Using water instead of nitrogen will make the inner surface of the mold cavity better. In addition to processing larger parts, water-assisted injection molding creates a more uniform wall thickness and reduces the residual wall thickness.
Water-assisted injection molding and gas-assisted injection molding can be used in different process methods. They are not different in the use of machines, but they are different in mold design and process control. Water-assisted injection molding is a two-step process similar to gas-assisted injection molding: first, part of the cavity is completely filled with melt; in the second step, water is injected to form a cavity.
Features of water-assisted injection molding equipment
The design of water-assisted injection molding equipment must meet conditions similar to those of gas-assisted injection molding. This is because most process technologies are based on gas-assisted injection molding.
Water-assisted injection molding also has its own characteristics. From the perspective of plastic parts, the removal of drainage is more complicated than the removal of nitrogen. It requires gravity and compressed gas to complete the drainage of the plastic parts. In order to prevent corrosion, water must not come into contact with the mold surface.
Water-assisted injection molding requires extremely high water injection capacity to ensure uniform wall thickness distribution and high surface quality. To this end, Battenfeld has developed a suitable pressure control mode. The water supply device operates at a very high flow rate and can reach a pressure of 350 bar. In order to inject water into the melt, injection components with a larger cross-sectional area than gas-assisted injection molding must be used, which is essential for water to penetrate into the melt at a sufficient speed.
Battenfeld’s water-assisted injection pressure generator is designed as a stand-alone device that can provide pressure to multiple injection molding machines at the same time. The hydraulic control components are controlled by the Unilog B4 mobile control device. Generally speaking, they can also be used on machines from other manufacturers.
The economic comparison of gas-assisted and water-assisted
In order to make correct decisions on the economic production of plastic parts, Battenfeld cooperated with the Cologne University of Technology to compare the following 5 processes using experimental molds.
Short shot gas-assisted injection
Full injection gas-assisted injection
Short shot water-assisted injection
Water-assisted injection molding
In order to obtain meaningful results, it is necessary to use materials that are easy to handle in all processes. However, raw material manufacturers are just beginning to optimize materials for water-assisted injection molding. When plastics are processed by water-assisted injection molding, some materials tend to form foam, shrinkage cavities, or side grooves.
In addition, there are some materials that can cause cracking, foaming, and non-reproducible properties due to water. In some glass fiber filling materials, the glass fiber may be washed away, resulting in a rough inner surface. Therefore, the following three materials were selected for this experiment.
Bayer’s PA66 Durethan BKV 30GH
DuPont’s PBT Cristin T803
The plastic parts are processed on the Battenfeld TM 4500/2800 Unilog B4 injection molding machine. The molding machine has a clamping force of 4500kN and is equipped with interfaces for gas-assisted and water-assisted injection molding modes.
Water-assisted injection molds are generally more expensive than gas-assisted molds. The reason is that the steel used to make the molds is different.
The steel used in water-assisted injection molds is of higher quality (a strong nickel plating layer or titanium nitride coating is essential to protect the water-assisted injection mold from corrosion).
The experiment assumes that the operating time is 24 hours a day, the working day is 300 days, and the system utilization rate is 90%. The depreciation period is assumed to be 8 years. Variable costs such as labor, energy and other costs (cooling water, cleaning costs, etc.) are included in this calculation.
In water-assisted injection molding, the cost of water is also included. The cost of nitrogen formation is taken into account in purchasing and energy costs. The procurement cost of gas-assisted and water-assisted injection molding is 100,000 to 145,000 euros higher than that of solid plastic injection molding.
In the short injection process, the purchase cost of gas-assisted injection molding is much lower than that of water-assisted injection molding, which means that the critical production volume of the gas-assisted injection molding process is 5000±500 units lower than that of the water-assisted injection molding process.
In a special period, for example, for the test parts of polyamide materials, the critical production volume of gas-assisted injection molding is 38206 units, and the critical production volume of water-assisted injection molding is 43203 units. The calculation is based on the part weight and cycle time obtained in a series of tests of various materials. The weight of the polyamide plastic part as a solid injection part is 224g, the gas-assisted injection part is 114g, and the water-assisted injection part is only 104g.
The number of critical plastic parts depends on the process and material
In a direct comparison of gas-assisted and water-assisted injection molding, after a production volume of less than 65,000, short-shot water-assisted injection molding has brought profit for the PA test plastic parts. In this process, material prices play an important role in the absolute quantity of critical production. For PP, which is the cheapest among the tested materials, the critical production volume of water-assisted injection molding and solid plastic injection molding is 75,000.
In the full injection process, the cost situation is different. For all three materials, the production cost of plastic parts made by water-assisted injection molding is lower than that of gas-assisted injection molded parts. One of the main reasons is that the use of full injection gas-assisted injection molding requires license fees.
Due to the higher production capacity, the variable cost of water-assisted injection molding is lower than that of gas-assisted injection molding. It is found through experiments that the critical throughput of the full injection process is much higher than that of the short injection process. The high purchase cost means that the material price does not contribute much to the cost of plastic parts.
Water-assisted injection molding technology is an ideal supplement to gas-assisted injection molding technology. The advantages of water-assisted injection molding include better and more uniform distribution of residual wall thickness and shorter cooling time.
From an economic point of view, the water-assisted injection molding process is cheaper than gas-assisted injection molding because it does not involve license fees. If a short injection process is used, the more economical process is completely determined by production volume and materials.
To take full advantage of this advantage, it is necessary to collect more information about the factors affecting processing. In response to the special requirements of water-assisted injection molding, raw material manufacturers must also adjust their productivity levels.
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