Aude Plastic Injection Molding Precision Forming Technology and Steam Assisting Technology How to improve the level of injection molding technology, produce high-precision plastic products, and create high value-added products is the goal of all companies. Precision injection molding technology is a combination of many interrelated technologies. It should be based on a series of factors, such as the rationality of product structure, the quality of plastic raw materials, processing methods, processing environment, injection machine performance, mold quality, and injection molding process conditions. To consider comprehensively. In order to get the best results.
Precision molding technology is a high-precision plastic product obtained by injection molding. The accuracy mentioned in this article, in addition to dimensional accuracy, also includes surface quality of the product, such as shrinkage, dents, weld lines, roughness, flatness, and the like.
How to achieve precision forming How to improve the level of injection molding technology, produce high-precision plastic products, and create high value-added products is the goal of all companies. Precision injection molding technology has received wide attention. In order to obtain high-precision plastic products, we need to work hard in four aspects. First, the product structure should be reasonable, second, the manufacture of precision molds, third, the use of high-quality plastics, and fourth, the use of precision injection molding process.
The following is mainly about the design and manufacture of precision molds and precision injection molding process to talk about their own experience for peer reference.
Design and manufacture of precision molds
Precision molds should have the following conditions: excellent mold structure, for the more complex molds, the success or failure, 80% depends on whether the mold structure is reasonable and advanced. Therefore, in order to design and manufacture precision molds, we must first have a team of high-quality mold design engineers.
The final accuracy of the parts and molds also depends on the machining accuracy of the mold parts and the high precision of the mold assembly. To do this, there should be advanced processing equipment and processing techniques. The mold must have sufficient rigidity to withstand strong clamping pressure and expansion force during the injection process. The mold must be rigid enough to not deform. For this purpose, it is necessary to have a good guiding and positioning system, mold size, material, heat treatment method, heat treatment hardness and many other aspects must be the best.
The shaped product is cooled quickly and evenly. The plastic injection mold can be said to be a heat exchanger. After the plastic raw material is heated and plasticized, and then formed by the mold, it is rapidly cooled. There should be certain rules, otherwise the temperature of crystallization , time, speed, will be affected. Uniform cooling prevents deformation of the product due to inconsistent shrinkage rates in various parts of the plastic part. Rapid cooling can reduce the injection cycle and increase the labor productivity of the mold. To do this, the mold must have a good cooling system, and the mold temperature should be controlled within a reasonable range, so that the plastic raw materials are not lost due to excessive or too low mold temperature during filling and cooling. Some characteristics. In precision injection molding, special attention should be paid to the temperature control of the mold.
It is not easy to sufficiently uniformly cool the rubber in the mold, and there is often uneven cooling due to uneven wall thickness. Pay full attention to this when designing the mold. There must be cooling water in the thick-walled part. The latest precision injection technology "Steam Assist Technology" is the most representative mold design control concept. Through the limited analysis of the mold cavity, the design of the cold and heat exchange system along the cavity surface can maximize the heat exchange rate of each stage of the mold molding cycle, thereby maximizing the quality of the product and shortening the molding. cycle,
Reasonable pouring system, usually plastics enter the mold with low temperature from high temperature, so that the plastic raw materials which are cooled to a certain extent can smoothly enter the cavity, and reduce the insufficient filling, joint line and shrinkage depression of the product. Everyone wants to increase the area of ​​the flow channel as much as possible, and the area of ​​the flow channel is increased, and the area of ​​the melting material in the center is also relatively increased. But it ignores the following two points:
First, after the area of ​​the runner increases, the flow rate of the plastic decreases in a square relationship. As the flow rate decreases, the time that the material stays in the flow path increases squarely, which in turn increases the cooling of the plastic, thereby impeding the flow of the melt. If we think about the size of the gate (smaller than the area of ​​the runner), the plastic can still flow into the cavity. Why does the runner need such a large area?
Second, the cross section of the flow channel is increased, the flow velocity is slowed down, and it is easier to cool. On the contrary, if the traditional flow channel area is taken small, the cooling flow can be minimized due to the square relationship of the flow velocity of the melt adhesive in the flow channel. degree.
Therefore, the flow path area is small, which in turn contributes to the flow of the melt in the flow path. Due to the increase in temperature, the quality defects (such as bonding wires) generated during the cavity filling process can be minimized. As for the design of the gate, for medium and large-sized products, the side gate should be used less. Because the molten rubber has a large area and suddenly enters a small area, there will be a short stay, thus causing condensation. Glue is not good.
Properly coordinated mold movements can reduce or eliminate failures. Uncoordinated mold movements can have catastrophic consequences for the mold. In every cycle of production, the mold must have a stable and reliable mechanism to ensure the coordination of its actions, such as the fixed distance division and the first reset mechanism.
High-quality mold steel, internal molded parts will be subject to high temperature, high pressure and high speed melting rubber in the long-term production process. The molded parts must be wear-resistant, pressure-resistant and impact-resistant. The cavity surface must have sufficient hardness to make the mold Have a sufficient life. For injection molds, quality is always in the first place and cost is secondary.
The exhaust gas is good, and in order to obtain a good product surface, a good exhaust structure is required in the high-speed forming process. Everyone knows the importance of the exhaust sump, encounters the problem of poor filling, and soon thinks about the problem of exhaust. However, the design of the exhaust vent should pay attention to the following items:
(1) The front end of the melt adhesive is a very thick latex-like substance, which is easy to block the exhaust groove provided on the parting surface. Especially when the clamping force is large, this phenomenon is more obvious, so the ideal exhaust groove, It should be in a position perpendicular to the parting line, such as the ejector pin and the insert.
(2) It is relatively simple to make the exhaust groove at the end of the stream, on the insert and on the parting surface, but if the air accumulates in the center of the finished product, the exhaust groove is difficult to manufacture. This is due to improper design of the number and location of the gates.
(3) If the trapped air is not obvious, only a weld line is formed on the surface of the shaped product, and a venting groove may be opened on the core type or the insert part.
Precision injection molding process
Many people think that to get precision plastic parts, as long as the design and manufacture of precision molds can be, in fact, this is a prejudice. Precision molds are just one part of precision injection molding technology, and there are many important factors that need to be taken seriously. Sometimes precision molding does not necessarily require high-precision molds. For general-quality molds, if they are matched with correct and stable injection molding conditions, plastic products with high dimensional stability and good performance can be obtained.
Let's talk in detail about the impact of these factors on the dimensional accuracy of the product:
Wind effect
The change in plastic cooling directly affects the shrinkage of the product. We only pay attention to the barrel temperature control of the injection molding machine, and did not notice the influence of the direction speed of the wind on the barrel, nozzle and mold temperature. Strictly speaking, in factories producing precision plastic products, electric fans should be regulated and cannot be used arbitrarily. Otherwise, the temperature of the barrel and the mold will be unbalanced, which will affect the plasticization of plastics and the shrinkage rate of products.
Room temperature
After the plastic raw material is heated and injected into the mold, it is rapidly cooled, part of the heat is taken away by the cooling medium, and one part is radiated and convected into the atmosphere; at the same time, the barrel also emits a large amount of heat to the atmosphere. How to control the temperature of the machine room, how to moderately draw air in the upper part of the plant, or take the hot air above by atmospheric air flow, and inject cold air into the bottom of the plant is very important. If the appropriate air conditioner can be installed in the equipment room and the temperature of the plant is controlled at about 27 °C, it will create very necessary conditions for precision forming.
Environment, climate (rain, weather, temperature, humidity), wind size, direction, greenhouse, air-conditioning, dust, changes in cooling water, changes in water temperature, scale, will have an impact on product accuracy. Therefore, the removal of dust, the capping of the barrel (and electrostatic precipitator), the cleaning of the ground, the pressure of the circulating water flow, and the stability of the voltage are all factors that cannot be neglected.
time
Changes in the climate of spring, summer, autumn and winter, the difference in cooling water temperature will affect the mold temperature, which in turn affects the accuracy of the product. If the quality of the products produced during the day and night is different, or the quality of the products produced on Monday and Saturday is different, it can be judged that the problem is that the mold temperature and the ambient temperature are unstable. After the holiday, the mold temperature has not risen to a fixed range, and production begins. The things that are made are rarely qualified.
material
The stability of material quality, brand differences, the use of recycled materials, drying methods (time, temperature control, etc.), dyeing and color matching methods, etc., will have a great impact on product quality. High-precision products require stricter surface quality (such as flow marks, roughness, transparency, etc.), and special attention is paid to the drying technology of materials. However, generally only pay attention to the drying temperature and time, and even in order to achieve the drying effect, it is absolutely wrong to increase the drying temperature. The temperature is increased, which is easy to cause degradation and deterioration of materials, especially for heat sensitive plastics such as PA, PVC, POM and EVA. The correct method should be to slightly lower the drying temperature and prolong the drying time.
Another point must be paid attention to: drying in a closed container, moisture is not removed by filtration, and the incoming air is not dehumidified. After heating, the relative humidity of the air is lowered, and the absolute humidity is not changed. Since the moisture in the air is not reduced, how can the drying effect be achieved? Therefore, how to achieve dehumidification and drying is an indispensable part of precision forming technology.
Injection machine
Injection molding machine performance, brand difference, injection molding machine wear, aging, use method, measuring instrument, meter method, temperature controller type, performance, cooling medium (oil, water), cooling medium flow rate, flow rate And the stability of the voltage, etc., will also affect the quality of the product. An automated injection machine can make up for the lack of forming technology. However, if you have skilled and superb injection molding technology, you don't necessarily need an automated injection machine. At present, there are two items in the factory that use the injection machine to be negligent:
One is to use an oversized injection machine to form. When the injection machine is too large, the volume of the material is also increased, so that the plastic stays in the barrel for too long, and the heating time is too long and deteriorates, directly affecting the precision of the product.
Another negligence was the failure to pay attention to the "nominal injection volume: x grams" in the specifications of the injection machine. Suppose that the injection volume of an injection machine is 300 grams, and the weight of the product is 200 grams. There is no problem on the surface, but the unit of the rated injection amount is negligible. Therefore, it is necessary to recalculate the production weight per minute of the product, whether it exceeds the rated injection amount? If it exceeds, the material will be plasticized unevenly in the barrel. If it is not fully melted, it is shot and formed, and the result must affect the quality of the product.
Mold temperature control
The mold temperature control is very important for the improvement of product quality. In addition to the mold design, we must pay attention to the following points:
(1) The temperature of the cooling water should not be too low. If the temperature of the cooling water is too low, the temperature of the mold will be relatively low, which is unfavorable for the filling and flow of the melt, and finally has a great influence on the quality of the product. The commonly used water temperature is room temperature and cold water of about 5 °C, which is much lower than the water temperature that should be used. This has a great influence on crystalline plastics (such as nylon, POM, PBT, PPS, etc.).
(2) In order for the cooling water to fully remove the heat from the mold, the correct approach is:
A. The flow rate of the water and the surface area of ​​the cooling water pipe are calculated according to the standard of R e = 800 00 to 10000 (the standard Reynolds number of turbulent flow).
B. Instead of lowering the water temperature and taking away the heat with a large temperature difference, the heat of the mold can be taken away at a speed that produces turbulence. Because the temperature difference (the difference between the mold temperature and the water temperature) is too large, it is easy to cause uneven mold temperature, resulting in deformation of the molded article.
C, when the mold temperature is very high, close to 100"C, pressurized water can be used for heat exchange work, but not with oil. Because the viscosity of the oil is very high, the specific gravity is light, Reynolds number (R e = Dvp / n) difficult to achieve the standard of turbulence, while laminar
In this case, it is difficult to take away the heat of the mold.
The temperature of the injection molding conditions, the molding pressure, the speed, the cycle, the stability of the molding conditions, and the like directly affect the quality of the product.
(1) Forming pressure
Because the filling is not easy, generally the injection pressure can be increased to overcome, but the pressure is large, it is easy to produce flash, even the best mold can not avoid the production of flash. The pressure is too large, in addition to easy to fly, but also due to the increase of internal stress, it is easy to cause deformation of the product.
In addition, appropriately reducing the material temperature (preventing the material from overheating and deteriorating), while increasing the rotational speed of the screw, and utilizing the heat generated by shearing and friction, also contribute to the filling of the finished product and improve the molding quality of the product.
(2) Forming cycle
If the forming cycle is too long, the time for the melt to stay in the barrel is too long, and the plastic is prone to degradation, thereby destroying the original characteristics of the plastic. The long forming cycle also reduces the productivity of the mold.
Therefore, in order to save costs; to increase production, few people will increase the forming cycle for no reason, but the following is another:
(1) In order to improve the deformation and shrinkage of the molded article, it is often overcome by increasing the cooling and holding time (i.e., extending the forming cycle).
(2) The thickness of the product is uneven. In order to achieve a sufficient cooling effect for a portion having a large thickness, it is often overcome by extending the forming cycle.
(3) Forming temperature, in order to fully melt the plastic in the material, the temperature is increased to contribute to the degree of plasticization, but the temperature is increased, which is likely to cause degradation of the material. It is best to moderately reduce the temperature of the material, which is 5 to 10% lower than the usual temperature. The insufficient part is supplemented by increasing the rotation speed of the screw (and thus increasing the injection speed). Because of the increase in the rotational speed of the screw, it is possible to increase the heat generated by the shearing and friction between the plastic molecules, which is enough to make up for the shortage of the material temperature. Since the friction heat is only instantaneous, the plastic will not degrade, and the friction heat generated by the rotation of the cylinder is relatively uniform, and there will be no local overheating, which is worthy of a try.
A new type of precision injection molding technology for plastics is being paid more and more attention: steam-assisted technology, steam-assisted technology is a set of complex factors in the production of injection. It starts with the design and processing technology of the mold structure, and uses the scientific coordinated control of steam, cooling water, compressed air, vacuum pumping, etc., so that the mold temperature and rapid heat dissipation can be quickly improved.
Compared with the ordinary mold production technology, the temperature of the mold can be rapidly raised above the thermoforming temperature of the resin because the cooling and heating pipes inside the mold are maximally increased, so that the resin is filled into the cavity. In this case, the solidified layer adhering to the mold wall is not formed by the rapid cooling of the flow path or the surface of the mold, so that the sequential filling can be more conveniently performed. At the confluence of the multiple melt streams, the surface of the parts can be ensured due to the good fluidity of the resin.
There are many advantages that can be obtained by using steam assist technology, such as avoiding the generation of flow marks, solving the deformation and crack of the product, and the good recovery of the mold surface. Since its working principle is based on the physical change of the resin, it can be applied to almost all resin materials. In a molding production cycle, the filling, feeding and cooling of the resin are carried out under ideal conditions, and all aspects of the injection production are scientifically and rationally controlled to obtain a product more in line with the ideal molding conditions. It is an ideal solution for epoch-making plastic precision injection molding.
The main application areas are
o Thick outer casing and exterior parts
o Office automation equipment, weak electricity, housing or casing of home appliances
o Parts for optical instruments such as large-area light-transmissive panels and large-diameter plastic lenses
o The shell of the book wall is molded
oPrecision quality requirements of injection molding products At this stage, the application of steam assist technology in China is basically limited to LCD display frame products. One of the reasons is: PHLIPS company first uses steam assist technology for LCD display frame products. On the other hand, the use of ordinary resin such as ABS or ABS+PC to produce ultra-thin surface frame products, its perfect high-gloss effect and flawless product appearance, has gradually become the industry standard for LCD display frame products. . The domestic related supporting factories have to be used to compete. In foreign countries, steam-assisted technology is also widely used in appearance products that need to add glass fiber or other filling materials, and the high temperature brought by steam assisting technology enables the mixed resin material to diffuse freely under the influence of density difference. . The appearance of the product is fine and bright, and its toughness and strength. Very good products.
There are many similar products on the market today, such as a mold temperature machine using a heating tube for mold temperature control, an oil temperature machine using high temperature oil control, a water temperature machine using high temperature and high pressure water, and a hot air mold temperature machine using hot air. . However, the evaluation of the effect is undoubtedly the most economical and practical steam assist technology. Steam assisted technology has great advantages, both in terms of shortening of the molding cycle, from the stability of the product, or from the deformation of the finished product.
in conclusion
Precision molding technology is a combination of continuous, interrelated, and many technologies that represent the overall technical capabilities and level of the enterprise. The application of technologies such as steam assist also requires coordination and cooperation in many aspects, including the design, processing, material application, equipment configuration, and operator's technical level of the mold under the new concept.
Scientific control and coordination of these factors can lead to high quality and low cost products. Precision injection molding technology is a combination of many interrelated technologies, so we should take the rationality of product structure, the quality of plastic raw materials, processing methods, processing environment, injection machine performance, mold quality, injection molding process conditions, etc. A series of factors to consider.