Gas injection molding and water injection molding are based on similar process technologies and therefore have a similar scope of application. So, what's the difference between these two technologies? Where are the respective scopes of these two technologies?
As a very mature technology, gas-assisted injection molding has been used in the plastics processing industry for many years, and one of the most important application areas 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 plastic parts with a local thickening area are also important application areas for gas-assisted injection molding.
The corresponding water-assisted injection molding technology is a new technology, only six years after the German Institute for Plastics Processing (IKV) announced the initial results of water-assisted injection molding technology, however, this technology has been developing rapidly. Not long after the invention of water-assisted injection molding technology, people used this technology to process a supermarket trolley accessories. Later, water-assisted injection molding was used to mass-produce handles and rod-shaped plastic parts with large cross-sectional areas. From the actual production point of view, plastic parts with functional space or flow channels began to use water-assisted injection molding technology more and more.
Based on the basic research completed by IKV and its experience in the field of gas-assisted injection molding technology, battenfeld has developed a combined water-assisted injection molding production system. The production system consists of a pressure generator, a pressure control module and a control device. At the same time, special syringe assemblies adapted to special requirements were developed. Battenfeld has two products, labeled "Airmold" and "Aquamold".
Comparative advantage of water and nitrogen
Gas-assisted injection molding technology reduces component weight and cycle time when used in the production of rod parts. Gas-assisted injection molding also helps to significantly reduce or completely eliminate wall thickness areas, 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 causes "internal cooling" of the plastic part, which shortens the cooling time by up to 70% compared to gas, and the plastic part reaches the desired release temperature much faster. At the same time, water is an incompressible and inexpensive medium.
Replacing nitrogen with water will result in better surface quality inside the mold cavity. In addition to machining larger parts, water-assisted injection molding creates a more uniform wall thickness and reduces residual wall thickness.
Water-assisted injection molding and gas-assisted injection molding can be used in different process methods. They do not differ in the use of machines, but in mold design and process control. Water-assisted injection molding is a two-step process similar to gas-assisted injection molding: first, the mold cavity is partially completely filled by the melt; In the second step, the injected water forms a cavity.
Features of water-assisted injection molding equipment
Water-assisted injection molding equipment must be designed to meet conditions similar to gas-assisted injection molding. This is because most process technologies are based on gas-assisted injection molding. However, water-assisted injection molding also has its own characteristics. From the perspective of plastic parts, removing drainage is more complicated than removing nitrogen, and it is necessary to complete the "drainage" of plastic parts by gravity and compressed gas. 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, ensuring uniform wall thickness distribution and high surface quality. For this purpose, battenfeld developed a suitable pressure control model. The water supply unit operates at extremely high flow rates and can reach pressures of up to 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 utilized, which is essential for water to penetrate into the melt at a sufficient speed.
The battenfeld's water-assisted injection pressure generation unit is designed as a stand-alone unit capable of supplying pressure to several injection molding machines at the same time. The hydraulic control components are controlled by means of Unilog B4 mobile controls, which can generally also be used on machines from other manufacturers.
Comparison of the economics of gas auxiliary and water auxiliary
In order to make the right decisions about the economic production of plastic parts, battenfeld, in cooperation with the University of Technology Cologne, compared the following five processes using experimental molds:
Traditional injection molding
Short injection gas auxiliary injection molding
Full injection gas auxiliary injection molding
Short injection water auxiliary injection molding
Full injection water auxiliary injection molding
In order to achieve meaningful results, it is necessary to utilize 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, craters or side grooves. In addition, there are materials that can crack, blister and not replicate due to water. In some glass fillers, glass fibers may be washed off, 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 Crastin T803
DSM's PP.
The plastic parts are processed on battenfeldTM 4500/2800 Unilog B4 injection moulding machines. The press has a clamping force of 4500 kN 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 because the steel used to make the molds is different. The steel used for water-assisted injection molds is of higher quality (a strong nickel plating or titanium nitride coating is essential to protect water-assisted injection molds from corrosion).
The experiment assumes that the operation time is 24h per 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 factored into procurement 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 parts.
In the short injection process, the procurement 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 the critical production volume of the water-assisted injection molding process. In special periods, for example for test parts made of polyamide materials, the critical production volume for gas-assisted injection molding is 38,206 units and for water-assisted injection molding is 43,203 units. The calculation is based on component weights and cycle times obtained in series tests of various materials. The weight of polyamide plastic parts as solid injection molded parts is 224g, gas-assisted injection parts are 114g, and water-assisted injection molded parts are only 104g.
The critical number of plastic parts depends on the process and material
In a direct comparison of gas-assisted and water-assisted injection molding, short-shot water-assisted injection injection molding brought profitability to PA test parts after a production volume of less than 65,000. In this process, material prices play an important role in the absolute quantity of critical production. For PP, the cheapest of the materials tested, 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 water-assisted injection molding is below the cost of gas-assisted injection molded parts. One of the main reasons is that the use of full-injection gas-assisted injection molding requires a license fee. Due to the high production capacity, the variable cost of water-assisted injection molding is lower than that of gas-assisted injection molding. Through experiments, it is found that the critical production capacity of the full injection process is much higher than that of the short injection process. High procurement costs mean that material prices contribute little to the cost of plastic parts.
Water-assisted injection molding technology is an ideal complement 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 times. 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 determined entirely by the production volume and material. Of course, to take full advantage of this advantage, more information on the factors influencing processing must be gathered. For the special requirements of water-assisted injection molding, raw material manufacturers also have to adjust their product grades.
