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Exploration and market prospect of gas assisted injection molding process

for many years, people have been studying the molding technology of hollow plastic products, among which Ernst Friederich of Rohm GmbH in Germany first invented the gas assisted injection molding process. With the rapid development of China's plastic industry, the application scope of plastic products is becoming wider and wider, the demand is growing, and the application market of gas assisted technology is also expanding. Developing gas assisted devices suitable for the application of China's plastic processing industry, meeting the needs of the domestic injection molding processing market, and getting rid of the monopoly of foreign companies on the domestic market and technology has an important impact on improving the quality of China's plastic products and reducing their costs. At present, Beijing Zhongtuo Machinery Co., Ltd. has taken the lead in completing the research, development and production of gas auxiliary devices. Since it was put into the domestic market in 2000, it has assisted many users to successfully carry out the improvement and development of gas assisted molds. At the same time, it also trained a professional gas assisted mold service team to provide customers with technical services including mold flow analysis, gas assisted mold design and so on. According to our discussion on gas assisted molding technology, the following is a brief introduction to the principle, application prospect and market situation of gas assisted molding technology

process principle

gas assisted injection molding process can be divided into four stages:

first stage: plastic injection. The melt enters the cavity and meets the mold wall with lower temperature, forming a thin solidification layer

second stage: gas incidence. Inert gas enters the molten plastic, pushing the uncured plastic in the center into the unfilled cavity

the third stage: the end of gas incidence. The gas continues to push the plastic melt flow until the melt fills the entire cavity

the fourth stage: gas pressure maintaining. In the pressure maintaining state, the gas in the airway compresses the melt and replenishes the material to ensure the appearance of the workpiece

gas assisted molding applications

gas assisted injection can be applied to any thermoplastic and some thermosetting plastics except particularly soft plastics. According to different structural shapes, products formed by gas assisted molding can be roughly divided into three categories: rod products, large wall thickness products such as handles; Plate products, large plane products that are prone to warping deformation and local melt accumulation; Special products are products with special structures that are difficult to be molded at one time by traditional injection molding technology

rod products

gas assisted molding technology shows obvious advantages in the molding of rod products. Generally, the gas assisted process of hollow injection is adopted, that is, the gas penetrates the wall thickness of the whole workpiece to form an airway. The design of the parts is mainly the design of the air duct, and the following aspects should be considered:

(1) the cross section of the products should be close to the circle. Avoid sharp corners and adopt large fillet transition; Avoid the accumulation of melt at the corners; Ensure that the wall thickness of the whole workpiece is uniform

(2) when using rectangular section, the airway is usually oval. In order to ensure the uniformity of gas penetration, the section should meet the aspect ratio ≤

(3) the length of the workpiece should be greater than 5 times the section height of the workpiece to ensure that the gas penetrates as far as possible along the length of the workpiece to obtain a uniform wall thickness

(4) the workpiece at the corner of the airway should have a large enough fillet radius to avoid the wall thickness difference at the inner and outer corners

(5) the change of airway section size should transition smoothly to avoid uneven contraction

(6) the airway inlet should not be set on the appearance surface or where the workpiece bears external mechanical forces

(7) the position of the air inlet should be close to the gate to ensure that the flow direction of gas and melt is consistent, but the distance between the two should be greater than 30mm to prevent gas from entering the gate back

One of the main applications of gas assisted injection molding technology is the molding of plate products. Because the gas always advances in the direction of minimum resistance, it is easy to penetrate in thicker parts. Therefore, in the design of plate products, stiffeners are often used as air passages, which are generally located at the edge of the product or the corner of the wall. The design of products is also the design of stiffeners and ribs, that is, the design of airway. The basic principles are as follows:

for plastic parts with stiffeners, there are two schemes: one is to thicken the root of the stiffener (just pour a big corner), and an airway can be designed along its root, so as to avoid shrinkage marks. Or if conditions permit, change it to the shape as shown in Figure 1, so that in the same case, the size of the total thickness of the plastic part can be reduced, and the deformation and shrinkage are small

Figure 1 stiffeners

(1) when designing and manufacturing stiffeners, we should avoid designing thin and dense stiffeners. The size is too small to provide a good channel for gas, and there will be "bulging" and other phenomena. The design can be improved to be thicker and less stiffeners. However, too large size will cause local melt accumulation, forming surface depression after cooling and shrinkage. Even when the thickness difference between the edge rib used as the airway and the middle thin plate is large, the melt enters the rib first, and the injected gas penetrates into the last filled thin plate and forms air pockets, reducing the local surface strength of the product

(2) "finger" effect is the main problem of large plane parts. In the gas pressure maintaining stage, the lack of material caused by the volume shrinkage of the flat plate is compensated by the melt between the airway and the flat plate, so the so-called "finger" effect is produced, resulting in uneven wall thickness. The main factor causing finger effect is the wall thickness of the plate, because the greater the wall thickness, the greater the risk of finger effect. On the other hand, the thicker the wall thickness of the plate part, the easier it is for the gas in the airway to string into the plate part, resulting in "secondary penetration". Therefore, when designing plate-shaped parts, it should be noted that the wall thickness of the plate part should not exceed 4mm

(3) when the workpiece is fed by only one air needle to form multiple stiffeners or ribs (airway), the airway cannot form a circuit

(4) in order to avoid depression caused by melt aggregation, the shape of the end of the airway should adopt fillet transition

(5) when using multi-point air intake, the distance between air passages should not be too close

(6) the air passage shall be arranged evenly and extended to the end of the product as far as possible

special products

special products mainly refer to products with special structures that are difficult to be formed at one time by using traditional molding technology

if there is a large plane in some parts of the plastic parts, or the thickness of a plane is too thick due to special requirements, using ordinary methods will produce shrinkage marks, pores, distortion and other phenomena. If a proper nozzle is added here and gas is used for forming and pressure maintaining here, the above defects can be avoided, but gas sealing marks will be left on the plastic parts. It can be placed in an inconspicuous position. As shown in Figure 2, use the stiffeners around to seal the gas, that is, remove the sealing ring with triangular section

Figure 2 gas assisted injection of large plane parts

mold design

basic principles of mold design

(1) the gating system should adopt point gate, both ordinary runner and hot runner, and the hot runner must be a needle valve type sealable structure

(2) the number of gates can be greatly reduced in gas assisted injection molding because the airway can act as a runner and is easy to fill

(3) generally, the gas often cannot reach the tail of the airway. If the airway must be penetrated, a "overflow well" can be added at the tail of the airway

(4) the gas assisted molding mold is easy to manufacture because the number of bars of plastic parts is reduced. However, mold processing must ensure the wall thickness of plastic parts and air ducts. Because gas is very sensitive to wall thickness, when the wall thickness is out of tolerance, gas may flee. At the same time, the cooling system of gas assisted injection molding mold is very important

technical characteristics of gas assisted molding mold

(1) the design of mold cavity should try to ensure the flow balance to reduce the uneven penetration of gas. Ensuring the flow balance is also a design principle of ordinary injection molding mold, but this is more important for gas assisted molding products

(2) the influence on process parameters should be considered in mold design, because gas assisted molding is much more sensitive to process parameters than ordinary molding. In gas assisted molding, a small difference in mold wall temperature or injection volume will lead to the wrong load drop of gas penetration in symmetrical parts. For example, forming symmetrical products as shown in Figure 3 (a), and starting to inject the same volume of plastic melt into the upper and lower sides (Figure 3 (B)), but assuming that the temperature of mold walls on the upper and lower sides is cold and hot due to the series arrangement of cooling water pipes, the viscosity of the melt on the upper side will be greater than that on the lower side, The gas penetration on the lower side is stronger than that on the upper side, and the effective flow length is shorter than that on the upper side (Fig. 3 (c)). With the injection of gas, this tendency becomes stronger and stronger, and finally gas penetration of different degrees will occur on the upper and lower sides (Fig. 3 (d))

Fig. 3 uneven gas penetration caused by small changes in mold wall temperature

gas assisted injection molding equipment

(1) ordinary injection molding machine (it is better to have a slightly higher counting accuracy)

(2) nitrogen control system, including self sealing gas auxiliary nozzle

(3) high pressure nitrogen generator

(4) industrial nitrogen cylinders and air compressors that provide pressurized power

(5) mold designed and manufactured for gas assisted injection

the relationship block diagram of these five parts is shown in Figure 4

Figure 4 Relationship between various equipment of gas assisted injection molding

(6) gas assisted nozzle

nozzle air inlet mode, that is, use a special self sealing gas assisted nozzle. After the plastic injection is completed, the high-pressure gas directly enters the plastic by relying on the nozzle, forming an extended closed space - air chamber according to the airway and maintaining a certain pressure until it is cold, which will form a great environmental infection. However, before the mold is opened, The nozzle is forcibly separated from the product channel by retreating the pedestal, so that the gas is discharged from the product

(7) air needle

the air inlet mode of air needle is to install the exhaust device air needle at a specific position of the mold. When the plastic is injected into the mold cavity, the air needle is wrapped inside the plastic; At this time, the high-pressure gas is discharged, and the gas needle forms an extended closed space - gas cavity in the plastic according to the airway, and maintains a certain pressure until it is cooled. Before the mold is opened, the gas in the gas cavity is discharged from the plastic by the control device relying on the gas needle

gas assisted process

process characteristics of gas assisted molding

(1) its influence on gas penetration should be considered when determining process parameters. The primary penetration of gas assisted molding mainly depends on the proportion of plastic melt volume in the cavity, and is dominated by the laws of fluid mechanics. The secondary penetration occurs in the thicker part that works on the crest of the resonance curve and extends in all directions. The influence of process parameters on the strength of gas penetration is mainly manifested in the length of gas penetration and the thickness of plastic on the surface of the airway. Generally speaking, higher gas injection pressure, higher melt temperature, lower melt viscosity and shorter switching delay time lead to shorter penetration length and thinner airway wall thickness

(2) it is recommended to use low-pressure gas during mold filling, and increase the pressure to compensate for shrinkage during pressure maintaining, because some gases will be dissolved in the boundary layer between the melt and the gas. If the plastic has not fully solidified after the pressure maintaining, these gases will expand during pressure reducing, resulting in surface bubbles in the airway. The higher the gas pressure during mold filling, the more gases dissolved in the melt surface boundary layer, and the stronger the expansion effect of the gas after the pressure maintaining

(3) pay attention to avoid extensive gas expansion on the inner surface of the airway caused by too steep pressure relief

(4) spray must be avoided as much as possible, because when spray occurs, the melt

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