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The strength of plastic parts is always a subject of concern for mechanical engineers. Compared with metal parts, the strength of plastic parts is generally low, but through reasonable part design, the strength of plastic parts can be increased significantly, which can expand the range of applications of plastic parts.
Part design can be increased by increasing the wall thickness to improve the strength of the part, but this is often unreasonable. Increased wall thickness of the part will not only increase the weight of the plastic parts, and easy to make the parts produce shrinkage, bubbles, and other defects, while increasing the injection production time, reducing production efficiency. To improve the strength of the part, the correct method is to add reinforcement ribs, rather than increasing the wall thickness of the part. The addition of reinforcement ribs can improve the strength of the part, but also avoid the occurrence of shrinkage, bubbles, and other defects, as well as lower productivity problems. Of course, the design of reinforcing ribs must follow the design principles of reinforcing ribs, and excessive thickness of reinforcing ribs can also cause defects such as shrinkage and air bubbles in the part.
Two methods of increasing the strength of the part by a factor of two are shown in Figure 1. One is to increase the wall thickness; the other is to keep the wall thickness unchanged and add reinforcement ribs. To increase the strength of the part by a factor of 2, the method of increasing the wall thickness of the part requires a 25% increase in the volume of the part, while the method of adding reinforcement ribs requires only a 7% increase in the volume of the part. This shows that adding reinforcement ribs is the best way to increase the strength of the part.
It is important to note that the strengthening ribs can only strengthen the plastic part in one direction. The direction of the strengthening rib needs to consider the direction of the load, otherwise it cannot increase the ability of the part to resist the load, as shown in Figure 2.
If the part is subjected to a load in multiple directions or twisting load, consider adding X-shaped strengthening ribs or diverging strengthening ribs to improve the strength of the part, as shown in Figure 3.
In daily life, the back of plastic stools is often strengthened by X-shaped reinforcement ribs or diverging reinforcement ribs to improve the strength of the part.
The design of multiple reinforcement ribs to improve the strength of the part than a single thicker or higher reinforcement ribs effect, while avoiding the part surface shrinkage or reinforcement rib top injection dissatisfaction and other quality problems, therefore, when a single reinforcement rib is too high or too thick, you can use two smaller reinforcement ribs to replace, as shown in Figure 4.
The strength of plastic parts can be improved by designing the shape of the part reinforcement profile. Common part enhancement profiles include V-shaped, serrated, and circular arcs, as shown in Figure 5. The disadvantage of this method is that the part does not provide a flat plane and cannot be used in some cases.
Avoid flat plastic part designs. Flat plastic parts have very low strength, and the strength of the part can be improved by adding sidewalls all around, as shown in Figure 6. The shape of the sidewall can be a simple straight wall. Where possible, curved sidewalls or sidewalls with reinforced profiles can improve the strength of the part, as shown in Figure 7.
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Part stress concentrations often occur at the sharp corners of the part, where the part wall thickness changes dramatically, part holes, slots and metal inserts. Part stress concentrations can significantly reduce the strength of the part and cause the part to fail under impact loading. Parts should be designed to avoid the occurrence of stress concentrations.
During the part injection process, the melt will flow in two or more directions as it passes through holes, slots, pillars and large part sizes, or when multiple gates are used. When the two directions of plastic melt meet, a melt mark is created in this area.
The melt mark area is one of the lowest strength areas of the part and is the most prone to failure. Therefore, the location and number of gates must be set reasonably to avoid loading the part in the melt mark area. As shown in Figure 8, in the original design, the gate is positioned so that the fuse mark is just at the load on the part, and the part is prone to failure under load; in the improved design, the gate is adjusted so that the fuse mark is positioned away from the load on the part, and the reliability of the part is greatly enhanced.
The location of the melt marks can be predicted by mold flow analysis software such as MoldFlow. Mechanical engineers can request the mold supplier to provide a mold flow analysis report when the part is opened, so that the location and number of gates can be selected appropriately.
(1) Glass fiber reinforced plastics are often used to replace common plastic materials to improve the strength of plastic parts. It should be noted that glass fiber reinforced plastic only improves the strength of the part in the direction of the glass fiber.
(2) Plastic parts are more capable of withstanding compression loads than tensile loads.
(3) When subjected to tensile loads, design consistent part profiles to evenly distribute the load.
(4) Avoid parts subjected to circumferential load. Parts subjected to circumferential load (such as metal inserts at), it is easy to rupture and failure.
(5) In the impact load, to maintain the integrity of the part profile, to avoid notches and stress concentrations in the impact load direction of the part profile.
