The molding process of plastic injection molding machine mainly includes four stages, such as filling - holding pressure - cooling - demolding, etc. These four stages directly determine the molding quality of the products, and these four stages are a complete and continuous process.
Understanding the whole injection machine molding process and flow of injection molding machine is very helpful to the business and maintenance of injection molding machine.
1. Filling
Filling is the first step in the whole injection molding cycle, and the time is counted from the time the mold is closed to start injection until the mold cavity is about 95% filled. Theoretically, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is subject to many conditions.
High speed filling, high shear rate at high speed filling, plastic due to shear thinning effect and the presence of viscosity decline in the case, so that the overall flow resistance to reduce; local viscous heating effect will also make the thickness of the curing layer thinner. Therefore, in the flow control phase, the filling behavior often depends on the volume size to be filled. That is, in the flow control phase, due to high-speed filling, the shear of the melt becomes thin effect is often large, while the cooling effect of thin walls is not obvious, so the utility of the rate prevails.
Low-rate filling. Heat transfer control low-speed filling, shear efficiency is low, local viscosity is high, and flow resistance is high. Due to the slower rate of thermoplastic replenishment, the flow is slower, so that the heat transfer effect is more obvious, heat quickly for the cold indifferent wall away. Together with a smaller amount of viscous heating phenomenon, the thickness of the curing layer is thicker, and further increase the flow resistance at the thin wall.
Due to the fountain flow, in front of the flow wave of plastic polymer chains lined up almost parallel to the front of the flow wave. Therefore, when the two strands of plastic melt meet, the polymer chains at the contact surface are parallel to each other; together with the different nature of the two strands of melt, resulting in poor structural strength of the melt intersection area on a microscopic level. When the parts are placed at a suitable angle under the light and observed with the naked eye, it can be found that there are obvious joint lines, which is the formation mechanism of melt marks. This is the formation mechanism of fusion marks. The fusion marks not only affect the appearance of the plastic parts, but also cause stress concentration due to the loose microstructure, which makes the strength of the part decrease and fracture occurs.
2. Holding
The role of the holding pressure stage is to continuously apply pressure to compact the melt and increase the density of the plastic to compensate for the shrinkage behavior of the plastic. In the process of holding pressure compaction, the injection molding machine screw can only slowly move forward to do a small movement, the flow rate of plastic is also relatively slow, the flow at this time is called holding pressure flow. As the plastic is cooled and cured by the mold wall during the holding pressure stage, the melt viscosity and therefore the resistance in the mold cavity is high. In the later stages of the holding pressure, the material density continues to increase, the plastic is also gradually formed, the holding pressure phase to continue until the gate curing seal, at this time the holding pressure phase of the mold cavity pressure reaches its highest value.
In the pressure-holding stage, because the pressure is quite high, the plastic shows some compressible characteristics, in the higher pressure area, the plastic is denser and the density is higher; in the lower pressure area, the plastic is looser and the density is lower, thus causing the density distribution to change with the location and time. In the pressure-holding process, the plastic flow rate is very low, and the flow does not play a dominant role; the pressure is the main factor affecting the pressure-holding process. During the holding process, the plastic has been filled with the mold cavity, and the gradually cured melt is used as the medium to transfer pressure. The pressure in the mold cavity is transferred to the surface of the film wall with the help of plastic, which has the tendency to hold up the mold, so the proper clamping force is needed to lock the mold. The clamping force will slightly hold the mold open under normal circumstances, which is helpful for the exhaust of the mold. Therefore, when choosing an injection molding machine, you should choose an injection molding machine with a large enough clamping force to prevent the rise of the mold and to effectively maintain pressure.
3. Cooling
In the injection molding mold, the design of the cooling system is very important. This is because the molded plastic products can only be cooled and cured to a certain rigidity, after demolding, to avoid deformation of plastic products due to external forces. Since cooling time accounts for about 70%-80% of the whole molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity and reduce costs. Improperly designed cooling system will make the molding time longer and increase the cost; uneven cooling will further cause warping and deformation of plastic products.
The factors that affect the cooling rate of the product are: plastic product design, mainly the wall thickness of plastic products. The greater the thickness of the product, the longer the cooling time. Generally speaking, the cooling time is about proportional to the square of the thickness of the plastic product, or proportional to the maximum runner diameter. That is, doubling the thickness of the plastic product increases the cooling time by 4 times.
The design requirements of the cooling system of injection molding machine:
The cooling channels of the injection molding machine should be designed to ensure uniform and rapid cooling. The purpose of designing the cooling system is to maintain proper and efficient cooling of the mold, and the cooling holes should be of standard size to facilitate processing and assembly.
When setting up the cooling system, the selection of the mold needs to be based on the wall thickness and volume of the plastic part to determine the following design parameters - the location and size of the cooling holes, the length of the holes, the type of holes, the configuration and connection of the holes, and the flow rate and heat transfer properties of the coolant.
4. Demolding
Demolding is the *** last part of the molding cycle of an injection molding machine. Although the product has been cold-set molding, demolding still has a very important impact on the quality of the product. Improper demolding may lead to uneven force during demolding and cause product deformation and other defects when ejecting. There are two main ways of mold release: top bar release and stripper plate release. When designing the mold, we should choose the appropriate release method according to the structural characteristics of the product to ensure the quality of the product.
For the mold with top bar, the top bar should be set as evenly as possible, and the position should be chosen in the place with the biggest release resistance and the biggest strength and stiffness of the plastic part to avoid deformation and damage to the plastic part. The demoulding plate is generally used for the demoulding of deep cavity thin-walled containers and transparent products that do not allow the trace of push rod. This mechanism is characterized by large and uniform demoulding force, smooth movement and no obvious traces left behind.
After understanding the whole process of injection molding machine, and then choose to buy injection molding machine or maintenance of injection molding machine, you can have a general grasp of the whole production business to bring a lot of help.
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