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1. Co-injection molding (core injection molding)
Co-injection molding helps to observe the unique structure of the part. The plastic "A" is injected into a part of the cavity first, then the plastic: "B" is followed by the "A" injection into the cavity and maintains the initial push of the flow pressure field. According to the size of the skin area and the core layer, the amount of material used for “A†and “B†can be measured in the correct proportional relationship, and one inner core layer can be made into a “A†outer surface completely wrapped by “Bâ€. Pieces.
In addition, in cosmetic applications, a small portion of the skin "A" material is injected after the "B" material to allow the skin of the gate portion to be completely closed. Co-injection molded parts from two different colors of resin form an easily distinguishable skin and core section (recognizing that it is important to have a similar skin and core layer in all injection molded parts.) Advanced detection techniques often make it difficult to distinguish between the skin-core layer and its interface.
Co-injection molding is not a new process technology. The British ici company began to apply this technology as early as the 1970s, and has obtained several patents including basic theory, production products and machinery. The commonly used ici production process is similar to “sandwich moldingâ€. Since the material of the outer skin of the molded one is different from the material of the middle or core layer, the two materials must have certain compatibility, and the core material is required to have High radiation, foam molding and 100% recycling. The choice of materials should be based on a variety of options. After 15 years of co-injection molding, it was really popularized. A cross-section is produced by co-injection of a thick tooth.
The skin material is unfilled nylon and the core material is glass-bead-filled nylon. The glass beads in the core layer have extremely low shrinkage and good dimensional stability. The nylon skin imparts good lubricity to the gear teeth and avoids the abrasive problems that are easily caused by the beads.
Based on the basic theory of co-injection molding, several new processing improvement methods have been developed. For example, in-mold "painting" and gas-assisted molding expand the range of processes employed. In-mold lacquer processing uses a low molecular weight polymer as the outer layer material, while gas assisted molding uses nitrogen or another gas as the core (or partial core) material. With the continuous improvement and improvement of product design and production processing equipment, it will meet the needs of various new applications and new technologies. Co-injection technology will become a promising industrialized mass production process.
2. Gas-assisted injection molding
Gas-assisted injection molding technology has been developed primarily to reduce weight and/or save cycle time.
In common co-injection molding, the outer layer material is first injected and the cavity is only partially filled. The gas is then injected through the nozzle or directly into the cavity, the core portion of the cavity. The liquefied gas can also be injected into the core portion of the article to be formed. In general, the gas pressure in the core layer pushes the melt forward until it completely fills the cavity, and prevents the surface layer of the part from being recessed from the cavity wall during the curing stage, and the connected skin layer is in close contact with the cavity wall, gas It is then stored in the core layer of the molded part. Since the pressure of the injected gas is higher than the atmospheric pressure, the pressure of the gas must be lowered before the workpiece is ejected to avoid deformation of the workpiece when the wall of the cavity functioning as a limit moves.
Highly compressed gases are difficult to control their shape and position the gas core, but as processes and processes continue to improve, qualified parts can be produced repeatedly.
Under the condition of machine control, various process programs are usually used for control. With the deepening of the basic theory research of injection molding processing, the method of eliminating the control pressure under the control method adopts the real process program control, and the response of the material is monitored, adjusted and even controlled. Improvements in processing parameters such as injection rate will have a significant impact on the molded part, especially on its mechanical and surface properties. It is critical to understand that the shooter speed is not equal to the melt injection speed. The response of the viscoelastic material is related to the watering process and must be performed simultaneously. In short, it is neither possible nor realistic to directly observe the response of the material. In the past, devices have been created that process or control objects in a repeatable order. The key step in this process flow is the frequent use of dry cycle measuring machines. It is of course important to have a repetitive molding process, but the reproducible melt characteristics should be given the highest priority. Thus, it is often required to add an auxiliary detector to the controller to preserve the recorded plastic melt pressure and temperature, which is an important parameter for the melt condition, but not sufficient to adjust the non-linear response of the material. Auxiliary control and control equipment is constantly being developed for the injection molding of highly complex injection molded parts. For example, mechanical valve-controlled gates are used in hot runner systems to better regulate dispensing pressure in the flow path and eliminate weld marks and warpage.
3. Low pressure injection molding
Low pressure injection molding has emerged in recent reports on processing techniques. In fact, this is not a new process, but the processing method used can make the equipment process better match the expected melt response. Under conventional molding conditions, the melt initially produces a large destabilizing effect due to excessive compression. This causes a sharp increase in viscosity, while the melt stores elastic energy due to compression. In contrast to the low pressure injection molding process, the melt flows through the nozzle and the flow path. Since the melt viscosity increases with an increase in pressure, the viscosity of the low pressure injection molded melt is lower, so that the viscous flow characteristics of the melt can be better controlled. In addition, the faster the pressure of the melt in the cylinder is increased, the more the solid body response resembles a solid state. Viscoelastic plastic melts have broadband response characteristics from pure liquid to pure solid state. Specific characteristics such as the response or relaxation time of the melt are determined by the chemical composition of the polymer backbone. Avoiding sudden changes in flow conditions or large amplitude changes in a moment is more conducive to the formation of similar liquid properties required. In fact, low pressure injection molding is just a processing method that controls or adjusts the viscoelastic properties of plastics. Resin manufacturers generally lower the molecular weight of high flow resins in order to reduce their viscoelasticity, which is suitable for the production of thin-walled articles and the like. With the deep understanding of the processing environment, the use of low pressure injection molding will make the plastic melt more adaptable to the requirements of the production environment.
There are currently several industrial products that use low pressure injection molding. Most design projects have focused on combining low pressure injection with reinjection plastic molding. For example, the molding of the automobile door trim panel is to place the textile or non-woven fabric into the mold, and then directly inject the melt into the mold. The in-mold labeling method is another molding method that goes beyond simple printing. At the beginning of each cycle of production, a separate label or successive film can be switched in position within the mold. In addition to being printable, the film has a lot of kinetic energy from time to time (such as high resistance to impact, tough resin) or the film may contain additives and stabilizers to protect the surface of the molded part.
4. Alternating injection molding
In comparison, alternating injection is a relatively new injection molding selection parameter. The biggest difficulty with this technology is that little is known about how the plastic melt will change when the processing conditions suddenly change. The basics of melt rheology are not just fixed shear viscosities. Specifically, the melt response (viscous and elastic behavior) requires characteristics of expression, not only the normal steady state flow rate or shear rate and temperature, but also pressure and instantaneous flow rate. These features include a lot of content and are very difficult to figure out. However, if substantial progress is made in the injection of profiled materials, specific operational procedures for a variety of different plastics will need to be developed. There is also a need to add a common method of trial and error in order to obtain a mature and accurate control method.
In conventional injection molding, the cavity walls are fixed, and in some cases, the mold walls are moved in the filling and holding traps. Two different methods can be used: moving the cavity wall direction perpendicular to the parting line; rotating or sliding the cavity wall. The core is rotated during the filling phase to increase the axis-orientation of the molecules of the article, particularly the skin portion. Through this processing, the bending properties and other mechanical properties of the workpiece are greatly improved. Polystyrene cups and polypropylene syringes are two products that have undergone significant changes in this processing method.
5. Injection - compression molding
The injection-compression molding medium cavity wall moves in a direction perpendicular to the parting line. When molded by this method, in the filling stage, a pressure is generated to drive the melt flow, but the depth of the one flow path is variable. In deeper flow passages, the pressure drops lower so that the melt is not over-compressed in large-area parts and avoids instantaneous material response, which also hinders the flow of the melt. During the injection molding process, the cavity depth may be 14% of the thickness of the final part. After the plastic is filled with approximately 60%-75% of the cavity, the injection is stopped and the wall around the cavity is pushed at the same time until the final part is Wall original molding. The final dimensions of the part are determined at this stage.
If the cavity is filled before the mold wall is moved by the process, the process is often referred to as cast molding. In general, cast molding uses a constant pressure to hold the part in a variable volume cavity. The casting phase is the stage of increasing the density, which is followed by a change between the melt and the solid plastic. The compact disc is formed by casting, which can minimize the residual stress, and the residual stress on the workpiece can cause variable refraction.
The cast-formed, modified, movable cavity wall is a new technology that "holds" the part through the porous metal cavity wall from the solidification stage of the injection system. This method has been called external gas-assisted molding, which is a misunderstanding because the gas does not affect the flow of the plastic melt in the cavity. In conventional injection molding, the pressure is maintained while the volume of the cavity is kept constant, and more plastic is added under the action of the pressure flow. In conjunction with the pressure-preserving flow in the cavity, an uneven pressure distribution is formed, which may cause workpiece defects at the gate position subjected to high pressure.
6. Cooling of the mold
Cooling of the mold is a key process technology. Most of the molding cycle is caused by the conduction heat transfer process. The transfer of energy from the hot melt to the cold mold is due to the temperature difference. The plastic skin on the side of the mold effectively isolates the core layer, making this heat transfer method very inefficient. However, mold cooling is usually not noticed until the final stage of the design. Better cooling design styles can reduce cycle production time by 20% to 30% or less and increase labor productivity.
During the production cycle, the surface temperature of the mold is constantly "high-low" cyclically. When the hot melt is pressed against the mold wall, the mold temperature is high. After the product is ejected, before the next injection, The mold temperature of the empty cavity is low. In order to minimize the cooling time, people have been searching for the lowest mold temperature that can produce qualified parts. The important role of the 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 and the greater the pressure drop in the cavity. Pulse cooling technology is a technique that uses a very frozen liquid in a circulating cooling tube to regulate cooling after injection of plastic into the mold cavity. After the part is ejected, if there is no circulation, the temperature of the cavity wall rises significantly after the next shot melts into the cavity. After the pulse cooling method, the cavity wall temperature will be higher, but slightly lower than the temperature detected by the conventional mold cooling method. Pulse cooling can be widely used in the molding of thin-walled parts; it requires repeated precision surface forming, as well as material variation in the range of flow depth variation. For example, reverse stagnation. For the potential advantages of pulsed cooling processing advantages and other related limiting characteristics, related to pulse cooling
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