What is thin wall? A simple view is called thin wall when the wall thickness is less than 1 mm. More comprehensively, the definition of thin wall is related to the process/wall thickness ratio, the viscosity of the plastic, and the heat transfer coefficient. The process L from the main flow path of the mold to the farthest point of the finished product, divided by the wall thickness t of the finished product, is called the process/wall thickness ratio. When L/t>150, it is called thin wall. If the thickness of the process is inconsistent, it can be calculated in stages. The flow/wall thickness ratio PP has a viscosity factor of 1. The disposable lunch box has a flow of 135 mm, a wall thickness of 0.45 mm, and a flow/wall thickness ratio of 300. The viscosity factor of the PC is 2. The flow of the mobile phone battery case is 38 mm, t = 0.25 mm, and the flow/wall thickness ratio = 152. Multiply the viscosity factor by 304, which is similar to the lunch box. Generally, the plastic has poor thermal conductivity. In order to increase the heat dissipation effect or achieve electromagnetic wave compatibility, some outer casings use plastics with high thermal conductivity. Metal powders are also highly thermally conductive. The above formula is the cooling time formula of the injection molded product, where t = wall thickness, Tm = melting temperature, TW = mold wall temperature, T = demoulding temperature, and α = plastic heat transfer coefficient. The definition of L/t should include viscosity factor and heat transfer factor. Let me improve it? Briefly, why do you want thin-wall injection molding first? The cost of plastic raw materials usually accounts for a large proportion of the cost of the product, such as 50-80%. Thin walls help to reduce this ratio. Due to the miniaturization and portability of consumer electronic devices such as mobile phones, MP3 players, digital cameras, and handheld computers, the related plastic parts are becoming thinner and thinner. Thin walling has become a new research hotspot in the plastic molding industry due to its advantages of reducing product weight and external dimensions, facilitating integrated design and assembly, shortening production cycle, saving materials and reducing cost. The design ideas and methods of thin-walled products are more complicated and further affected by molding limitations and material selection. Thin-walled products are required to have high impact strength, good appearance quality and dimensional stability, and can withstand large static loads, and the flowability of the molding material is good. The design process should focus on the rigidity, impact resistance and manufacturability of the product. Specially designed thin-walled products are specially designed for forming thin-walled products. Compared to standardized molds for conventional articles, molds for thin-walled articles have undergone major changes from mold structures, gating systems, cooling systems, exhaust systems, and demolding systems.
Mainly in the following aspects: (1) Mold structure: In order to withstand the high pressure during molding, the thin-wall molding die has a large rigidity and high strength. Therefore, the moving and fixed stencils of the mold and the supporting plate are relatively heavy, and the thickness is usually thicker than that of the conventional mold. There are many support columns, and there may be more internal locking in the mold to ensure accurate positioning and good side support to prevent bending and offset. In addition, the high-speed injection speed increases the wear of the mold, so the mold should use higher hardness tool steel, and the hardness of high wear and high erosion areas (such as gates) should be greater than HRC55. (2) Gating system: Forming thin-walled products, especially when the thickness of the product is very small, a large gate is used, and the gate should be larger than the wall thickness. If it is a sprue, a cold well should be installed to reduce the gate stress, assist the filling, and reduce the damage when the product is removed from the gate. To ensure that there is sufficient pressure to fill the thin cavity, the pressure drop should be minimized in the runner system. For this reason, the flow path design is larger than the conventional one, and at the same time, the residence time of the melt is limited to prevent degradation of the resin degradation. When it is a multi-cavity, the balance of the casting system is much higher than that of conventional molds. It is worth noting that two advanced technologies, hot runner technology and sequential valve gate (SVG) technology, have also been introduced into the casting system for thin-walled product molds. (3) Cooling system: Thin-walled products can not withstand the large residual stress caused by uneven heat transfer like traditional thick-walled parts. In order to ensure the dimensional stability of the product, and to control the shrinkage and warpage within an acceptable range, it is necessary to strengthen the cooling of the mold to ensure the cooling balance. Good cooling measures include the use of non-closed cooling lines in the core and cavity modules, increasing the cooling length, and enhancing the cooling effect. High-conductivity metal inserts are added where necessary to accelerate heat transfer. (4) Exhaust system: Thin-wall injection molding molds generally need to have good venting properties, and it is best to perform vacuuming operations. Due to the short filling time and high injection speed, it is very important to fully exhaust the mold, especially the exhaust in the flow front gathering area, to prevent trapping of the gas. Gas is usually discharged through cores, rams, ribs, studs, and parting surfaces. The end of the flow path should also be fully vented. Japan's Sumitomo uses small tool steels as small inserts to solve the problem of exhausting small items. (5) Demoulding system: Because the walls and ribs of thin-walled products are very thin, they are very easy to damage, and the shrinkage in the thickness direction is small, so that the reinforcing ribs and other small structures are easily bonded, and the high pressure holding pressure makes the shrinkage more small. To avoid topping and sticking, thin-wall injection molding should use a larger number of ejector pins than conventional injection molding. Conventional injection molding machines are difficult to use in thin-walled plastic injection molding. For example, the filling time of thin-wall injection molding is very short, and many filling times are less than 0.5 s. It is impossible to follow the speed curve or the cut-off pressure in such a short time, so it is necessary to use a high-resolution microprocessor to control the injection molding machine; During the entire injection molding process of the product, the pressure and speed should be controlled independently at the same time. The filling process of the conventional injection molding machine is controlled by speed, and the method of controlling the pressure maintaining phase to pressure control is not applicable. Therefore, mechanical equipment manufacturers and research institutions have worked together to develop special injection equipment. Such as Taiwan's zhongjingji VS-100 thin-wall injection molding machine, Germany Dr. The Boy series injection molding machine developed by Boy and the special injection molding machine developed by famous injection molding machine manufacturers such as Battenfeld, Arburg and JSW. Thin-walled injection molding materials have good fluidity and must have a large flow length. It also has high impact strength, high heat distortion temperature and good dimensional stability. In addition, the heat resistance, flame retardancy, mechanical assembly and appearance quality of the materials are also examined. At present, thin-wall injection molding is widely used as polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS) and PC/ABS blends. The filling process and the cooling process of conventional injection molding are intertwined. When the polymer melt flows, the melt front encounters a relatively low temperature core surface or cavity wall, which forms a condensation on the surface. In the layer, the melt continues to flow forward in the condensing layer, and the thickness of the condensing layer has a significant effect on the flow of the polymer. A more in-depth and comprehensive study of the properties of the condensed layer in thin-wall injection molding is needed.
Therefore, the numerical simulation of thin-wall injection molding needs to do a lot of work in the following aspects. (1) A more in-depth study of thin-wall injection molding theory, especially the nature of the condensation layer, in order to propose more reasonable assumptions and boundary conditions. From the above analysis, many conditions are quite different from conventional injection molding in the thin-wall injection molding process. Many of the assumptions and boundary conditions of the melt flow mathematical model need to be properly adjusted during thin-wall injection molding during simulation. (2) Determine the factors added in thin-wall injection molding and consider these factors correctly. Some factors that can be neglected in conventional injection molding tend to have a large effect on the thin-walled melt flow. For example, in thin-wall injection molding, viscosity has a significant dependence on pressure, but not in conventional injection molding; weld line strength has a great influence on the performance of plastic parts, especially thin-walled plastic parts, weld line strength and temperature and pressure. Related, but the conventional numerical simulation does not take into account the effects of pressure; the specific heat of the material, the heat transfer coefficient and the pressure loss. Existing commercial numerical simulation software ignores these influencing factors, so there is an inconsistency in predicting thin-wall injection molding filling. (3) Apply true 3D numerical simulation. The existing commercial numerical simulation software is a simplified model that uses two-dimensional and two-dimensional semi-elements to represent three-dimensional geometric figures, without considering the change of physical quantity in the thickness direction. The three-dimensional flow area, that is, the flow at the corner, the thickness change area, and the front-end fountain effect of the melt are not represented in the existing numerical simulation software, and they play an important role in thin-wall injection molding. (4) Simulation of the whole process of injection molding. The current simulation software mainly includes modules such as filling, flow, pressure holding, cooling, and warpage analysis. The development of each module is based on independent mathematical models, ignoring the influence of each other. However, from the perspective of the injection molding process, the filling flow, holding pressure and cooling of the plastic melt are intertwined and mutually influential, which is particularly evident in thin-wall injection molding. Therefore, mold filling flow, pressure holding and cooling analysis and warping modules must be organically combined for coupling analysis to fully reflect actual injection molding.