The latest injection molding technology

2022-08-16
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New injection molding technology (Part 2)

5. Injection compression molding

injection compression molding medium cavity wall movement direction is perpendicular to the parting line. When molding with this method, pressure driven melt flow is generated according to the process in the mold filling stage, but the depth of this channel is variable. In the deeper channel, the pressure drop is low, so that the melt is not under excessive pressure in the forming of large-area parts, and the instantaneous material response is avoided. These two factors will also hinder the flow of the melt. During the injection molding process, the depth of the cavity may be 14% of the thickness of the final product. After the plastic fills the cavity with about 60% - 75%, stop the injection, and the surrounding of the cavity wall is pushed at the same time until the wall thickness of the final product is formed. The final size of the product is determined at this stage

if the cavity is filled before the mold wall moves according to the process, this process is usually called casting molding. In general, casting molding is to maintain the pressure of the workpiece with constant pressure in a variable volume cavity. The casting stage is the stage of increasing density, and the density changes between melt and solid plastic. The residual stress can be reduced to a minimum by forming a compact disc by casting, and the residual stress on the workpiece can produce variable refraction

the improved movable cavity wall formed by casting and pressing is a new technology, which is used to "keep pressure" parts by injecting all parts through the porous metal cavity wall in the curing stage. This method has been called external gas assisted molding. In fact, this is a misunderstanding, because the gas does not affect the flow of plastic melt in the mold cavity. In conventional injection molding, pressure maintaining is to add more plastic under the action of pressure flow while keeping the cavity volume unchanged. Together with the pressure maintaining flow in the mold cavity, uneven pressure distribution is formed, which may cause workpiece defects at the gate under high pressure

6. Mold cooling

mold cooling is a key process technology, Gerry said. Most of the forming cycle is a process of heat transfer. The energy can be transferred from the hot melt to the cold mold due to the temperature difference. The plastic skin on the mold wall effectively isolates the core layer, which makes this heat transfer method very inefficient. However, mold cooling is usually not noticed until the final stage of design. A better cooling design can shorten the cycle production time by 20%-30% or less and improve labor productivity

during the production cycle, the surface temperature of the mold is constantly changing from "high to low". When the hot melt presses the mold wall, the mold temperature is high. After the ejection of the workpiece, the mold temperature of the empty mold cavity is low before the next injection. In order to shorten the cooling time to the minimum, people have been exploring the minimum mold temperature that can produce qualified parts. The important role of mold temperature is to affect the melt flow in the cavity and the size ratio between the skin and the core. The lower the mold temperature, the thicker the skin size and the greater the pressure drop in the mold cavity. Pulse cooling technology is a technology that usually uses the very frozen frozen liquid in the circulating cooling pipe to regulate the cooling after the injected plastic enters the mold cavity. After the workpiece is ejected, if there is no circulation, the temperature of the cavity wall will rise significantly after the next injection melt enters the cavity. After using pulse cooling method, the temperature of the cavity wall will be higher, but it is slightly lower than the temperature detected by the conventional mold cooling method. Pulse cooling can be widely used in the forming of thin-walled parts; It is required to repeat the molding of parts with accurate surfaces and the sudden change behavior of materials within the range of channel depth. For example, backward flow. For the processing advantages of pulse cooling and the potential advantages of other related characteristics, the debate about whether the cost of pulse cooling is too high has become less important

7. Fusible core forming process

the steel ball of fusible or buffer valve has dirt or excessive clearance in the contact part with the inlet. The core forming method is a relatively new process method, which is convenient for processing parts with cross complex channels, grooves or grooves inside. Manufacturers of small parts such as pipe clamps, pump casings and sports goods began to consider whether to adopt this process as early as 15 years ago. Recently, automobile manufacturing companies have adopted this process to produce large parts, such as lightweight powertrain samples and air intake pipes with a total investment of US $44million. This core has the function of folding core like the conventional mold, and is equipped with an insert core with a limited inner cavity wall. The main difference lies in the structure of the core, which cannot be disassembled in the mold or mechanically extracted out of the mold. In addition, if the core is not melted by inductive heating, such as tin bismuth alloy with low melting point (138 ℃) or other tin alloy, it is cleaned by (soluble acrylic polymer) in the secondary operation. This fusible core molding adopts the conventional process method, and the main difference between the two is that the thermal properties of the core and the cavity materials are different. The thermal performance of the core directly affects the thickness of the flow skin core structure on the workpiece. According to the actual processing conditions, the thickness of the top and bottom skins can be very different. In some cases, warping defects may occur, while in another case, warping of defective parts may be reduced or eliminated

in the fusible core molding method, the core is only used as the mold wall. The filling process does not change due to solid inserts. Because the thermal properties of metal alloy cores or polymer cores are very different, the required pressure is also different. The polymer core acts as a barrier, and produces a very thin fluid skin in the molded parts. The metal core becomes a heat dissipation point and forms a relatively thick fluid skin on the parts. If the surface of the core has melted when the plastic melt flows, there will be a fundamental change in the flow. It has been reported that ice bars are used as cores. This method is feasible for high-precision parts until the complex processing problem of liquid film at the interface between ice and plastic is solved

8. Computer aided molding

using computer aided engineering (CAE) to process design and analysis can help shorten the design cycle and avoid expensive mechanical errors. Commercial simulation codes are often used to mark the size on the runner to balance the flow of molten material in the runner system and cavity, and determine the optimal opening setting and number of gates. The calculation of injection pressure and clamping tonnage depends on different processing conditions and materials. Shrinkage and warpage can also be accurately estimated in combination with the initial flow direction. It is important to make this design tool help skilled analysts to make judgments in a design scheme or processing research. The results must be understood as the premise of the research object and processing citrus family. When this method is considered to input data accurately, great benefits can be achieved. In addition, this analysis economy can make the design cycle shorter and the production time shorter

it should be noted that, Commercial ca[programs are usually not directly available. Mold filling simulation can produce valuable insights, but the results must be reconsidered and estimated for its limitations. The application of modern computers to injection molding simulation tests is limited to pure viscous fluids (excluding viscoelastic molten plastics) 。 It can predict the actual flow rate of the melt flowing into the cavity, and publish the composition, structure and properties, for example, it can carry out high-precision viscoelastic analysis. No other processing method currently used can reach this advanced level, and in recent years, good progress has been made by the industry leaders of simulation equipment and research groups in universities. Several companies are trying to explore simulation technology in order to correctly explain more realistic plastic behavior and processing phenomena, and expand the company's raw material supply channels. For example, the influence of the orientation of the polymer backbone on the local physical properties and property distribution. Processing physics is very complex, and some viscoelastic manifestations are still not fully understood. More perfect and reasonable processing methods are slowly forming. These more powerful ways will gain much more production capacity than currently designed

source: International Plastic Industry

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