Thin-walled cover product is shown in Figure 1. Maximum external dimension of product is 66.10 mm * 40.69 mm * 42.00 mm, average thickness of plastic part is 0.80 mm, plastic part material is PA66+30GF, shrinkage rate is 1.005, and plastic part weight is 5.43 gram. Technical requirements for plastic parts are that there shall be no defects such as peaks, underfilling, flow lines, pores, warpage deformation, silver lines, cold materials, and spray lines.

 

 

Figure 1 Product map of thin-walled cover sheet
As can be seen from Figure 1, structure of plastic part is a simple plastic part with an L-shaped shape, and wall thickness is 0.8, which is a thin-walled plastic part. Plastic part itself has no complicated structure except for thin walls. Thin-walled plastic parts are mainly to meet needs of some special functions, such as light transmission, magnetic permeability or to facilitate passage of certain rays to meet functional requirements of product. Another purpose is to save materials, especially for disposable tableware, aviation tableware and other industries. Thin-walling has become goal pursued by plastics industry due to its advantages of reducing product weight and external dimensions, facilitating integrated design and assembly, shortening production cycles, saving materials and reducing costs, which has driven development of injection molding technology and high-speed precision injection molding machines in plastic molding industry.
Design and manufacture of thin-walled products is more complex and is affected by molding process limitations, material flow, and performance of high-speed injection molding machines. Thin-walled products are required to have high impact strength, good appearance quality and dimensional stability, can withstand large static loads, flowability of molding material is better. In design process, rigidity, impact resistance and injection molding performance of product should be considered. Disposable tableware, bowls, cups, spoons, etc. are mostly made of PP and PE. Engineering plastic parts are mostly made of nylon. This paper takes thin-walled cover sheet as an example to introduce design characteristics of injection molds for thin-walled plastic parts of engineering plastics.
Cylindrical thin-walled plastic parts, such as disposable aviation cups, deep-cavity fast food boxes, etc., belong to closed containers. In addition to difficulty of molding, thin-walled plastic parts also have difficulty in ejection, that is to say, it is easy to form a vacuum inside cavity, plastic parts are easy to be deformed when demoulding. Bernoulli's principle is mostly used, plastic parts are demolded by blowing them inside and outside. Molding cycle of this kind of plastic part is very short, only a few seconds. For the key points of mold design, please refer to "Application of Bernoulli Effect in Air Ejection of Thin-walled Plastic Parts.
Thin-wall cover sheet is simple in product and large in production volume, and belongs to mold exported to Europe. Mold design cavity ranking is 4 cavties. Mold design diagram is shown in Figure 2. Compared with mold of conventional products, mold of thin-walled products has all aspects such as mold structure, pouring system, cooling system, exhaust system and demolding system. with large differences. Gate is a side gate, and gate size is 3x0.3. In order to achieve automatic injection molding, plastic parts are ejected by ejector pins and straight ejectors.
In order to withstand high pressure during molding, rigidity and strength of thin-walled molding die should be high. Therefore, thickness of movable and fixed mold plate of mold and its supporting plate should be thick. There are more support heads, four corners of front and rear mold cores are designed to be interlocked to ensure accurate positioning and mold clamping, prevent bending and offset. In addition, high-speed injection speed increases wear of mold, so mold adopts a higher hardness tool steel of 1.2767, and heat treatment hardness is HRC54.
For a multi-cavity mold, balance requirements of gating system are much higher than those of conventional molds. It is worth noting that two advanced technologies have also been introduced into gating system of thin-walled product molds, namely hot runner technology and sequential valve gate (SVG) technology. In final stages of runner design, mold flow analysis software can assist in confirming sensitivity of flow rate to runner system design and determining appropriate forming conditions. For example, when designing a runner system with multiple cavities arranged in a straight line, different filling rates will result in different filling patterns. Generally speaking, cavity that is far from vertical sprue will be filled first with a low feed rate; see Figure 4(a), cavity close to vertical sprue will be filled first with a high feed rate, as shown in Figure 4 ( b). Reason is that when melt with a low feed rate flows to the first gate, it will flow to other parts of runner due to flow resistance. After runner system is filled with melt, the first upstream gate will have a large flow resistance due to solidification of part of melt. Roughly balanced runner design after runner modification is shown in Figure 4(c). A correct understanding of this diagram is helpful for making relevant choices when designing mold cavity ranking.
Thin-walled products can not bear large residual stress caused by uneven heat transfer like traditional thick-walled parts. In order to ensure dimensional stability of product, control shrinkage and warpage within an acceptable range, it is necessary to strengthen cooling of mold to ensure a balanced cooling. Thin-wall injection molding molds generally need to have good venting. Due to short filling time and high injection speed, adequate venting of mold, especially in gathering area of flow front, is very important to prevent air trapping. Gas is usually discharged through core, thimble and parting surface. End of runner is also adequately vented.
Because walls and ribs of thin-walled products are very thin, they are very easy to damage, and shrinkage in thickness direction is very small, making ribs and other small structures easy to bond, and high holding pressure makes shrinkage smaller. To avoid punch-through and mold sticking, thin-wall injection molding should use a larger number and size of ejectors than conventional injection molding. This set of molds is designed with a straight top ejector in the center.
For example, filling time of thin-wall injection molding is very short, and many filling times are less than 0.5s. It is impossible to follow speed curve or cut-off pressure in such a short time, so a high-resolution microprocessor must be used to control injection molding machine; During the entire injection molding process of product, pressure and speed should be controlled independently at the same time. Therefore, thin-wall injection molding requires use of special injection equipment. Such as Taiwan's VS-100 thin-wall injection molding machine company in precision machine, Germany Dr. Boy series injection molding machines developed by Boy Company and special injection molding machines developed by famous injection molding machine manufacturers such as Battenfeld, Arburg and JSW.
Identify added factors in thin-wall injection molding and properly consider them. Some factors that can be ignored in conventional injection molding tend to have a greater impact on melt flow of thin-wall molding. For example, viscosity has a significant dependence on pressure in thin-wall injection molding, but not in conventional injection molding; weld line strength has a great influence on performance of plastic parts, especially for thin-walled plastic parts, weld line strength is related to temperature and pressure. Existing mold flow analysis software ignores these influencing factors, resulting in inconsistencies in predicting fill for thin-wall injection molding. Existing mold flow analysis software all use 2D and 2.5D elements to represent simplified models of three-dimensional geometric figures, and do not consider change of physical quantities in direction of wall thickness of plastic parts. Three-dimensional flow area, that is, flow at the corner, thickness change area, and fountain effect at the front of melt cannot be represented in existing mold flow analysis software, but they play an important role in thin-wall injection molding. Current simulation software mainly includes modules such as filling, flow, packing, cooling, and warpage analysis. Development of each module is based on its own independent mathematical model, ignoring mutual influence. Therefore, filling flow, packing and cooling analysis and warpage modules must be organically combined, and coupled analysis can be carried out to comprehensively reflect actual injection molding.

 

 

Figure 2 Thin-walled cover sheet mold diagram

 

Figure 3 3D drawing of mold

 

Figure 4 Illustrated diagram of runner balance