Development of water assisted injection molding te

Development of water assisted injection molding technology

Abstract: This paper introduces the principle of water assisted injection molding technology, summarizes the research and application status of water assisted injection molding technology, points out the key problems to be solved, and looks forward to the future development direction of this field

introduction

injection molding, as one of the important molding methods in plastic processing, has been developed and applied quite mature. With the increasingly widespread application of plastic products, people put forward higher requirements for plastic products in terms of precision, shape, function and cost. Therefore, based on the traditional injection molding technology, some new injection molding processes have been developed, such as gas assisted injection, multi-point feed injection, layered injection, molten core injection, low-pressure injection, etc., to meet the needs of different fields. As a method of manufacturing hollow products or part of hollow products, water assisted injection molding has become a new research direction of injection molding technology, and is developing towards commercialization

the working principle of water assisted injection molding

since the early 1970s, gas assisted injection molding developed for the purpose of reducing warpage, avoiding dents and reducing raw material consumption has been proposed and gradually applied. The use of gas assisted injection molding technology in the production of products with large areas of reinforcement and products with both thin-walled and thick-walled areas is showing an increasing trend. Its basic principle is to inject accurately measured melt into the mold cavity first, and then inject gas (generally nitrogen) into the melt through a special nozzle. The gas pushes the melt forward to produce hollow shaped products. After the mold filling is completed, the pressure of the gas in the melt remains unchanged or increases, and the pressure maintaining and feeding are carried out. After cooling, the gas in the melt is removed, and the product can be demoulded. The advantage of gas assisted injection is that on the one hand, the gas pressure can be evenly distributed in the mold cavity, which can improve the quality of products; on the other hand, the hollow grooves produced by gas flow can increase the strength of products. However, due to the high-pressure equipment and high-pressure nitrogen required by gas assisted injection molding, the production cost is also increased. In recent years, it can be queried, displayed, printed and deleted at any time according to the needs of verification. With the development of science and technology, the molding technology of using injection liquid, such as water, to form hollow products came into being

the water assisted injection molding process is to use the supercharger or air compressor to produce high-pressure water, inject the high-pressure water into the mold cavity that has been partially pre filled with melt through the piston nozzle, and use the pressure of water to push the melt forward to fill the mold cavity: inject in such a way that water will not evaporate, and the leading edge of water acts on the molten core of the product like a displacement plunger, from the leading edge of water to the transition section of melt, A thin plastic film is solidified, which is like a high viscosity core, further pushing the polymer melt forward: after the melt is cooled, compressed air is used to press water out of the product, and then the product is ejected to form a hollow molded product. Taking advantage of the fact that water will not evaporate in the process of high-speed melt flow, combined with perfect process control means, the advantages of water will be fully displayed, and the recycling of water will be ensured. Compared with gas assisted injection molding, water assisted injection molding has the following characteristics:

(1) one of the advantages of water assisted injection molding is to directly cool the interior of products. Gas assisted injection molding is accompanied by a large residual wall thickness and the danger of melt foaming. In order to avoid foaming, the pressure holding b~ f room of gas or the pressure release time of gas are forced to be extended, which also increases the cooling BF room. The thermal conductivity of water is 40 times that of gas, and the specific heat capacity is 4 times that of gas. Due to the cooling capacity of water assisted injection molding, the cooling time can be reduced to 25% of that of gas assisted injection molding

(2) the biggest difference between water and gas is that gas can be compressed, but water cannot, so water assisted injection molding can withstand high injection pressure. Moreover, gas can mix with polymer and easily escape, so the working pressure of gas assisted molding is generally 300 ~ 2500psi, while that of water assisted molding is generally 4350psi. Generally speaking, thicker and longer products are more suitable for water assisted injection molding technology

(3) gas can be mixed with polymer, but when the gas exudes again, the inner surface of the product will become very rough. At the same time, unlike water, gas can be divided into small tributaries in products, causing an unwelcome finger effect; At the same time, the unpredictability of gas movement makes it difficult to control the wall thickness, while water can make the wall thickness uniform and repeatable. Water assisted injection molding technology can not only produce uniform thin-walled products, thereby saving materials and expanding the application scope of injection molding, but also produce a smooth inner surface inside the parts, which is difficult to achieve by gas injection molding technology, and also makes injection molding more competitive than blow molding

(4) because water assisted injection molding uses water as the medium, it is easier to obtain and cheaper than nitrogen, and it can be recycled, so the production cost can be reduced

therefore, water assisted injection molding can reduce the wall thickness and residual wall thickness of products in a shorter molding cycle. For large and thin products, more uniform and lower pressure can be used for molding. It is suitable for molding such as tubular parts, automobile oil pipes and other fluid systems, handles, luggage racks, instrument panels on automobiles, buffers, door handles, clutches and driving rod support frames. Other products include baby walkers, handles of kitchen appliances, office furniture, outdoor activity equipment, brackets and other appliances

water assisted injection molding process methods

at present, there are four main methods of water assisted injection molding technology, and its process steps are similar to gas injection molding technology. The difference is that open the oil return or reset system (oil return valve or software control) to inject melt, introduce water, and discharge water by gravity or air, as shown in Figure 1

(1) short short short method: the process is to first inject part of the plastic melt into the mold cavity, then inject water, and then inject the remaining melt to push the plastic melt to the end of the cavity and maintain the pressure. The flow of melt from the injection unit and water from the water injection hole is controlled by various valves. Open the drain valve so that the water can be discharged out of the product. A valve installed at or near the end of the cavity can facilitate the discharge of compressed air

this method is considered to be a good method for manufacturing thicker parts. There is no waste in this method, and the inlet and outlet of water can be the same. The disadvantage of this method is that it requires precise control: if too little melt is injected in advance, water may break through the melt and enter the mold cavity; The injection pressure of water must be greater than that of the melt, so that the melt can be pushed to the end of the cavity; Melt injection and water injection valves will leave traces on the products, making it difficult to get a high-quality surface. In addition, the melt is easy to form a thicker section at the end, which may prolong the molding cycle.; (2) Pushback: first, fill the mold cavity with the melt completely, then open the water injection hole at the end of the melt flow, and the water pushes back the excessive melt into the head space of the injection unit. The advantage of this method is that there is no waste and a high-quality surface can be obtained. As for the disadvantages, it requires a special nozzle and a check ring to mediate the return material into the injection unit. Users should be particularly careful not to let water penetrate into the top space of the injection unit (this is a big problem for hygroscopic materials, such as nylon). Each stage of the process must be under pressure control to obtain a stable and known melt return. The pressure and temperature of the returned melt may be different from the melt that has been in the barrel, which will cause changes in the process, which will affect the next injection process. This method requires independent air and water delivery systems

(3) ovemove method: first, the mold cavity is completely filled with melt and closed by a valve. Then an independent water injection hole is opened, and the valve at the end of the cavity is opened, thus opening a path from the main cavity to the auxiliary cavity or overflow cavity. The melt pushed into the sub cavity is replaced by new water, and the overflow cavity valve is closed to maintain pressure. Water is discharged by gravity or evaporation

this method is reported to produce high-quality surfaces. This method is very close to the traditional molding method and requires much lower water pressure than the underinjection method. The disadvantage is that the condensate in the overflow chamber has to be recycled, and it also needs to be separated from the product by shearing

(4) flow process: This is a method combining under injection method and overflow method. As the water passes through the products quickly, the cooling effect is improved. The cavity is partially filled with melt, and then water is injected, pushing the melt into the end of the cavity. A special valve is set at the end. Water will break through the melt and enter the water circulation system through the valve for recycling. The advantages of this method are material saving and high cooling efficiency. The disadvantage is that there are defects on the top of the product, and low pressure can cause water to seep out on the inner surface of the mold and the outer surface of the product

development history of water assisted injection molding

as early as the early 1970s, someone proposed the concept of injecting fluid (water, oil, etc.) into molten polymers to form a hollow, which was later called water assisted injection molding technology. However, this technology has been shelved due to the emergence and wide application of gas assisted injection molding ∵ in recent years, due to some deficiencies exposed in the use of gas assisted injection molding technology, people have turned to the research work of water assisted injection molding technology. At present, the research and development of this technology has made great progress and has been commercially available

according to relevant data [41, the first commercial application of water assisted injection molding technology is the all plastic shopping cart manufactured by sulo GmbH company in Herford, Germany. This project began in 1998, and it uses PP materials and PME (Project Management Engineering) of Schulman company The company ◇ water assisted injection molding technology to mold a product with three water channels with a diameter of 20-60mm and a length of 800-1500mm. The original gas assisted injection molding cycle of this PP shopping cart required 280s, while water assisted injection molding only required 68so

in addition, companies and research institutions in many countries in Europe have also carried out research and development work on water assisted injection molding technology, such as EngD company of Austria, Battenfeld company, ferromatik company of Germany, And the famous plastic fjn Institute of Technology (IKv) of Aachen University of Technology (r~ Aachen), Germany. Some achievements have been made in shortening the molding cycle and improving the process methods, and have been applied to practical production. At the exhibition held in Dusseldorf, Germany in 2001, water assisted injection molding technology caused quite a sensation

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