A brief analysis of the structural design of injection molded plastic parts

Mold Home News: 1·Introduction Plastics, steel, cement, and wood are also called the four major engineering materials. With the advancement of science and technology, the application of plastics has become more widespread. Engineering and technical personnel engaged in mechanical design should have certain knowledge of the design of plastic components. The author summarized several key points in the structural design of injection molded plastic parts, which will help in the design of related products.

Key points of structural design

2.1 Reasonable wall thickness

From the perspective of molding quality, if the wall thickness of plastic parts is too large, defects such as dents and shrinkage holes may easily occur during the molding process; if the wall thickness is too small, it will be difficult to fill the mold cavity, resulting in material shortage. . The wall thickness of plastic parts should be as uniform as possible. It can adopt a gentle transition or a partially hollowed-out structure to make the wall thickness uniform and avoid defects such as warping deformation during the molding process.

2.2 Design principles for reinforced structures

Setting reinforcement ribs on plastic parts can improve the strength and stiffness of the plastic parts and prevent warping and deformation of the plastic parts. Choosing the right rib locations can improve the flow of the plastic melt.

The size of reinforcing ribs generally follows the following principles:

• ① The wall thickness of the ribs is generally 0.4 times the thickness of the main body t, and the maximum is no more than 0.6 times;
• ② The spacing between ribs is greater than 4t, and the height of the ribs is less than 3t;
• ③ Reinforcement of screw columns The ribs should be at least 1.0mm lower than the surface of the column;
• ④ The reinforcing ribs should be at least 1.0mm lower than the surface of the part or the parting surface. When multiple reinforcing ribs intersect, attention should be paid to the local material accumulation caused by the intersection.

The improvement methods are:

• ① dislocate the stiffeners;
• ② design the intersection of the stiffeners into a hollow structure.

If the slender reinforcing ribs are stressed, they should be able to bear the tensile force as much as possible to avoid excessive pressure. Because the elastic modulus of plastic materials is very low, instability problems are prone to occur. This is contrary to the priority pressure principle we follow when designing metal castings, and requires special attention.

2.3 Avoid stress concentration

Special attention should be paid to the structural design of plastic parts to avoid sharp edges and corners. The transition of geometric shapes at edges and corners is discontinuous, where stress concentration occurs and cracks can occur.

Plastic materials generally have low strength and are more susceptible to damage where stress is concentrated. The main measure to avoid stress concentration is to improve the structural form of sharp edges and corners of components. For example, add chamfers, rounded corners, or replace them with gentle transition sections at sharp corners. When the chamfer or rounding cannot be directly added due to the functional requirements of the component, the stress concentration can be reduced by reducing the local structural strength at the sharp corners and taking out the rounded corners inward. Refer to the linear guide shown in Figure 1. Improve design solutions. The tooth shape of plastic threads should be preferably circular and trapezoidal, and avoid triangles and rectangles. This can reduce the notch effect and improve the load-bearing capacity of the thread.

2.4 Design appropriate draft angle

The draft angle is also called the demoulding angle. It is mainly used to prevent plastic parts from adhering to and rubbing on the mold due to cooling shrinkage during demoulding, which will cause damage and deformation. There are 10 parameters set in the demoulding direction of plastic parts. An angle that is conducive to demoulding.

The determination of the draft angle generally follows three principles:

• ① The draft angle is generally an integer, such as 0.5°, 1°, 1.5°, etc. (except matte and textured);
• ② The appearance draft angle of the plastic part is greater than The angle of the inner wall is conducive to demoulding during molding;
• ③ Take a larger draft angle without affecting the appearance. Some materials, such as PP, PE, etc., can be forcibly demoulded, and the amount of forced demoulding generally does not exceed 5% of the maximum cross-sectional area of the core.

2.5 Consider the structural design of plastic parts from the perspective of mold structure

The process equipment for injection production is the mold, which reflects the shape of the plastic part. Due to the complex structure of plastic parts, the mold has to be structurally complicated, or even have an impossible structure. This should be fully considered when designing plastic parts. On the premise of ensuring the appearance and function, strive to make the mold structure as simple as possible. This saves time and costs and improves product quality. For example, there are many side grooves, side holes and other structures on plastic parts that prevent the product from being ejected in the ejection direction. Generally, structures such as core pulling and sloping tops must be adopted. If the groove and side holes are designed to be consistent with the ejection direction or designed as a collision structure on the premise of realizing the function and ensuring the appearance, the mold structure will be greatly simplified. Therefore, excessively complex structures should be avoided when designing plastic parts structures.

The design of plastic parts should avoid inscribed structures. Plastic parts with inscribed structures cannot be demoulded directly, making it impossible to design the mold or having to use mold cores, hidden structures, or separate the mold. However, this increases the complexity of mold production, reduces the reliability of the mold, and produces The possibility of scrap increases, increasing manufacturing costs. Therefore, inscribed structures should be avoided as much as possible when designing structures.

The design of plastic parts sometimes requires lateral demoulding due to appearance or assembly requirements. This requires that the structure of the mold and the impact of the mold structure on the product itself should be fully considered during design.

• ① Problems with the inclined roof and the slider: The inclined roof and the slider move in both the mold parting direction and the direction perpendicular to the mold parting direction. There must be no glue blocking the movement of the inclined top and rows in the direction perpendicular to the parting mold, and there must be enough space for movement;
• ② Treatment of vertical surfaces:

The appearance of some plastic parts requires no slope, and the side walls must be vertical. It is necessary to design sliders or sloped roofs on vertical surfaces. There are wirings when running the slider. In order to avoid obvious wirings, the wirings are generally placed at the intersection of surfaces. The particularity of this location must be taken into consideration when designing plastic parts.

2.6 Design considering the non-isotropic characteristics of plastics

Plastics are sometimes not isotropic like metals, in which case the strengths and weaknesses should be exploited in the direction. For example, for some plastics with reinforced materials, the glue flow direction should be consistent with the direction in which the component carries the greatest load, because the axial direction of the reinforcing fibers flowing in with the molten plastic is the same as the flow direction. Figure 2 shows a beam-like structure with reinforced ribs. It is made of plastic with reinforced fibers. Its main load-bearing direction is in the length direction of the reinforced ribs, so the correct direction of glue feeding should also be in the length direction.

When the component adopts multi-point glue injection, the force direction should be avoided to be parallel to the fusion line. Because at the place where two or more glue flows meet, the drop in melt temperature will cause the bonding strength to decrease and breakage will easily occur. The correct design method is that the force direction is perpendicular or at a certain angle to the direction of the fusion line.

2.7 Consider the structural design of plastic parts from the perspective of assembly

Since the elastic modulus of plastic materials is small, that is, the material is soft, and the molding process is different from that of metal parts, the tolerance accuracy of plastic parts is generally much lower than that of metal parts. Therefore, this feature should be paid attention to when designing the structure, and large dimensions and small tolerances should be avoided. The larger the size, the greater the accumulated deformation of the component, and the greater the impact on the tolerance accuracy. Bonding is one of the commonly used assembly methods for plastic parts. When bonding plastic parts, you should avoid the bonding interface being subjected to tearing tension, because its tear resistance is poor. The correct approach is to make the bonding interface bear shearing force. The bonding strength under positive tension is not as good as the bonding strength under shear, because the bonding interface under positive tension is subjected to tearing tension at its root; while the area of the bonding interface under shear is Generally larger than the area of the bonding interface under positive tension, it has stronger tear resistance.

Bolted connections are also one of the commonly used assembly methods for plastic parts. Since the strength of plastic is very low and is usually not strong enough to bite the screw, self-tapping screws cannot be directly embedded in plastic when the force is large. In addition, flat-head bolt connections or riveted connections should have larger lining plates to increase the stress-bearing area. #p#Paging title#e#

3·Conclusion

The author summarizes the problems that are often encountered and easily ignored in the structural design of plastic parts. Only by conducting research and summary in actual work and continuously accumulating experience in practice can we design plastic parts with reasonable structure and excellent performance.

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