Gas assisted injection molding and water assisted

2022-08-07
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Gas assisted injection molding and water assisted injection molding as a mature technology, gas assisted injection molding technology has been used in industry for many years. An important application area is the production of thick wall plastic parts, such as handles and similar products. Other application fields include plate shaped parts or other plastic parts with local thickening area

in contrast, it is only six years since the German Institute of plastic processing (IKv), located in Aachen, Germany, announced the preliminary results of water assisted injection molding technology. However, this technology has been developing rapidly. Two years after its release, an early use, supermarket trolleys, was introduced. Then there are mass-produced handles and rod-shaped plastic parts with large cross-sectional area, that is to say, Its predecessor with functional space or flow channel is the technical service organization of the famous Amsler experimental machine company (established in 1854) with a history of more than 100 years (Amsler is the originator of electro-hydraulic servo control technology. Plastic parts are increasingly becoming the use of water assisted technology.

based on the basic research completed by IKv and its own experience in the field of gas assisted injection molding technology, bartonfield has developed an early combined production system for water assisted injection molding. It consists of a pressure generator, a pressure control module and a control device. In addition, special syringe components that meet special requirements are also Developed. Water assisted injection molding process has proved to be suitable for thick wall exposed parts with special requirements for surface quality and functional parts with high requirements for internal surface. Bartonfield has two products labeled "airmold" and "aquamold" by the dealer

Figure 1: bartonfield's water injection component, named aquamold

advantages of water phase over nitrogen

gas assisted injection molding and water assisted injection molding are based on similar process technologies and are attractive for similar types of components. So there is a good reason to compare the two technologies:

what is the difference between the two technologies

when can the process be economically utilized

gas assisted injection molding technology can be used to reduce weight and cycle time when producing rod parts. This is the result of small wall thickness, which can be obtained by injecting gas, usually nitrogen, or by rapid cooling of components. Gas assisted injection molding also helps to greatly reduce or completely eliminate the wall thickness area, deformation and shrinkage traces of planar plastic parts, thus improving the quality of plastic parts. Injection molding machines with reduced clamping force can be used frequently

the thermal conductivity of water is about 40 times that of nitrogen, and the thermal capacity is 4 times. In addition to ordinary mold cooling, water injection will cause "internal cooling" of plastic parts. Compared with gas, the cooling time is shortened by 70%. Plastic parts reach the required demoulding temperature much faster. Water is also an incompressible and inexpensive medium, and its availability is unlimited

using water instead of nitrogen makes the surface quality in the mold cavity much better. The results show that water assisted injection molding can form more uniform wall thickness and reduce the residual wall thickness in addition to using a larger part size

water assisted injection molding and gas assisted injection molding can be used in different process methods. They are not different in machine technology, 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, in which the mold cavity is partially or completely filled with melt. In the second step, the selected injection water forms a cavity. In the two methods, the common variables are short shot process and full shot process

technical features of water assisted injection molding equipment

the design of water assisted injection molding equipment (Figure 1) must meet the conditions similar to those of gas assisted injection molding. This means that most process technologies are based on gas assisted injection molding; However, the transformation of the driving screw rod of some special electromechanical organizations through the reduction gearbox and other 1 Series drives must be considered. Removing water from plastic parts is more complicated than removing nitrogen. Through gravity and compressed gas blowing, the "drainage" of plastic parts can be completed. To prevent corrosion, water must not come into contact with the die surface

water assisted injection molding requires extremely high water injection capacity to ensure uniform wall thickness distribution and high surface quality. For this reason, bartonfield developed a suitable pressure control mode. Water pressure accumulation devices operate at extremely high flow rates and can be used in pressure ranges up to 350bar. In order to inject water into the melt, an injection component 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 and high volume flow rate

bartonfield's water assisted injection pressure generation device is designed as a stand-alone device, which can supply pressure to multiple injection molding machines at the same time. The water pressure control assembly is controlled by the unilog B4 mobile control unit. They can also be used on machines from other manufacturers. Only one interface is required

economic comparison between gas assisted and water assisted

before production can start, some questions need to be answered: which process should be used, gas assisted or water assisted? Which is better, short shot or overflow? What important general conditions must be observed in order to make a correct decision on the economic production of plastic parts? In cooperation with the Cologne Institute of technology, bartonfield completed a special comparison with the experimental mold (Fig. 2), and compared the following five processes:

▲ traditional injection molding

▲ short shot gas assisted injection molding

▲ overflow gas assisted injection molding

▲ short shot water assisted injection molding

▲ overflow water assisted injection molding

traditional injection molding is only used to control the comparison between gas assisted and water assisted

Figure 2: Test molds and corresponding plastic parts applied in the comparative study

in order to obtain meaningful results as of June 15, it is necessary to use materials that are easy to be processed in all processes. However, raw material manufacturers are just beginning to optimize the materials used in water assisted injection molding. When processed by water assisted injection molding, some materials are easy to form foam, shrinkage cavity or side groove. Other difficulties include water induced cracking, blistering in cavities, and non reproducible component performance. In some glass fiber filling materials, the glass fiber may be washed away, resulting in a rough inner surface (Fig. 3)

Fig. 3: the effect of materials on the quality of plastic parts in water assisted injection molding process: washing out from glass fiber

(a), formation of side channel (b), foaming compared with evoque sliding inner wall (c) (d)

three materials that can be successfully treated by gas assisted and water assisted injection molding are compared with each other:

■ PA66 Durethan BKV 30gh of Bayer

■ DuPont PBT crastin t803

■ DSM PP

higher purchase cost and lower raw material consumption

plastic parts are made on bartonfield TM 4500/2800 unilog B4 injection molding machine with a clamping force of 4500kN. It is equipped with interfaces for gas assisted and water assisted injection molding modes. The overflow process requires two additional core pulling valves for the clamping plate, and the two water assisted injection molding processes require an additional core pulling valve for the nozzle plate. The two purchases for the process are pressure generator, respective pressure control components, nitrogen and water injection components

water assisted injection mold is generally more expensive than gas assisted mold. This is due to higher quality steel or hardened surfaces (a strong nickel or titanium nitride coating is essential to protect the water assisted injection mold from corrosion)

the feasibility study assumes that the operation time is 24 hours a day, the number of working days is 300, and the system utilization rate is 90% The depreciation period is assumed to be 8 years. Variable costs - labor, energy and other costs (cooling water, cleaning costs, etc.) - are included in this calculation. In water assisted injection molding, the cost of water must be added. The cost of nitrogen formation is taken into account in the work procurement value and energy cost. The purchase cost of gas assisted and water assisted injection molding is 100000 to 145000 euros higher than that of solid plastic parts

in the short injection process, the purchase cost of gas assisted injection molding is much lower than that of water assisted injection molding. This means that the critical production of gas assisted injection molding process is 5000 ± 500 units lower than that of water assisted injection molding process. During special periods, for example, for test parts made of polyamide, the critical production of gas assisted injection molding is 38206 units, and that of water assisted injection molding is 43203 units (Fig. 4). The calculation is based on the component weight and cycle time obtained in a series of tests for various materials. Therefore, the weight of polyamide plastic parts as solid injection molded parts is 224g, that of gas assisted injection molded parts is 114g, and that of water assisted injection molded parts is only 104g

Figure 4: the production cost of Durethan BKV 30 GH injection molded parts is based on the number of plastic parts

the critical number of plastic parts depends on the process and materials

in the direct comparison between gas assisted and water assisted injection molding, after the production volume is less than 65000, short shot water assisted injection molding brings profits to the PA test plastic parts. In this process, the material price plays an important role in the absolute quantity of critical production. For the cheapest PP among the tested materials, the critical production of water assisted injection molding and solid plastic injection molding is 75000

in the overflow process, the cost is different. For all three materials, the production cost of water assisted injection molded parts is lower than that of gas assisted injection molded parts (Figure 5, taking PP as an example). One of the main reasons is that the use of overflow gas assisted injection requires license fees. Due to high production capacity, the variable cost of water assisted injection molding is lower than that of gas assisted injection molding, so it is worth using water assisted injection molding in many cases. The critical production of overflow process is much higher than that of short shot process. The overall high procurement cost means that the material price has no great effect on the cost of plastic parts

Figure 5: production related manufacturing costs of natural color DSM PP injection molded parts

Outlook: 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. Moreover, the patent situation of water assisted injection molding is more clear. From an economic point of view, water assisted injection molding is more economical than gas assisted injection molding because it does not involve license fees. If the short shot process is used, the more economical process is completely determined by the production volume and materials

however, in order to take full advantage of this advantage, it is necessary to collect more information on the influencing factors of processing and optimize the newly applied syringe Technology (Fig. 6). According to the special requirements of water assisted injection molding, raw material manufacturers will adjust their product grades

Figure 6: not every plastic part can be directly converted from gas assisted injection molding to water assisted injection molding for the new water injector successfully tested by bartonfield. It must be checked on a case by case basis for suitability and feasibility. It is necessary to always consider the location of the injector and whether the polymer used is suitable for water assisted injection molding. However, water assisted injection molding has considerable potential, because this process can produce plastic parts that can not be made by solid plastic injection molding or gas assisted injection molding

Figure 7: process diagram

variable of water assisted injection molding

short shot process: in the first step, the mold cavity is only partially filled by the melt. Through injection, the water injected into the melt completes the remaining filling and pressure maintaining steps in the following two steps: digital display hydraulic universal testing machine, screen display hydraulic universal testing machine and microcomputer controlled electro-hydraulic servo hydraulic universal testing machine. Before mold opening, the water pressure is reduced and the water is discharged from the plastic part

overflow and melt push back process: in the two blowing out processes, the mold cavity is first completely filled by the melt

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