In the design, using plastic to make plastic springs can simplify the structure and save costs, but the design method of metal springs cannot be simply applied. Generally speaking, plastics can only be used for springs that do not require instantaneous recovery. Plastic springs are best to work intermittently , That is, the spring produces a specified force for a short time, while the rest of the time is in a relaxed state. The elastic recovery time of the plastic spring should be at least equal to the time under load.
The application of plastic springs is not restricted because of this, because most springs are used discontinuously. This issue must be considered when embarking on the design of plastic products. Nevertheless, the application of plastic springs in certain product designs is very successful For example, the spring in the turn lock is only used to adjust the rotation stress. The lock shown in Figure 1 is only locked and released by the spring. In this case, it is very reasonable to use a plastic spring.
1. Advantages of plastic springs
The characteristics of metal are incomparable to plastics. For example, the bending modulus of steel is 30 to 100 times that of commonly used injection-molded plastic springs. However, plastic springs have their specific application space, which has advantages compared with metal springs. as follows:
Integration of parts: At this point, it can compete with metal springs. There is a difference in integration between metal springs and plastic springs. That is to say, designers can use plastic springs creatively. Integration of parts means simplified structure, easy installation, and cost Lower and lighter weight.
Recyclability: Parts with metal springs, if the quality and specifications are not up to standard, will often be scrapped. Plastic springs allow all parts to be recycled.
Corrosion resistant, “no rust”.
Natural smooth, with the convenience of molding and coloring.
Compared with mechanical processing, the manufacturing cost is reduced.
1.1 Spring and its creep
For springs made of plastic, pay attention to the difference in the bending modulus of metal and plastic. Available resins include polyethylene, polypropylene, polyester, ABS, nylon, and vulcanized polyethylene. Material resilience is an important selection factor .
Compared with metal, the spring made of the best resin will quickly deteriorate and fail under continuous load. Acetyl polymer loses 50% of its original elasticity after 1000 hours (about 6 weeks), and after 10,000 hours (about 1 Years), it loses 60% of its original elasticity, and after 100,000 hours (about 11 years) it loses two-thirds of its original elasticity.
Generally, crystalline acetyl and polyphenylene sulfide have better creep resistance than polyethylene and ABS. If the mold temperature is too low, the creep rate of the crystalline material will increase significantly in the final stage of the injection molding process.
1.2 Material selection
The choice of material for the spring depends on the required characteristics. The factors to be considered are creep resistance, load, fatigue limit and chemical resistance. Acetyl polymers are the most commonly used materials. Other available materials and their characteristics are as follows:
L Polyethylene and polypropylene are cheap, but have poor tensile recovery and poor creep resistance. They can only be used for short-term loads, and their fatigue response is moderate, but they have good chemical resistance.
L Acyl copolymer is more expensive, but has good sustained load characteristics and good creep resistance, but it is not suitable for working in an acidic environment less than 4PH.
l Linear vulcanized polyphenylene materials are more brittle than acetyl copolymers, and are more suitable for high load and small displacement applications. It has good creep resistance, good fatigue resistance, and can maintain its toughness at high temperatures. It can work normally for a long time when exposed to chemical erosion conditions.
L Nylon is relatively wear-resistant under dry conditions, but when it is exposed to 50% relative humidity, its rigidity decreases by 25%.
L For simple, occasionally used springs, ABS is suitable, but its creep resistance is poor.
The use of fillers in the material can improve the characteristics of the spring. Filling the glass fiber in the material increases the rigidity and strength, but its deflection is limited. The ambient temperature also affects the performance of the plastic spring. Most plastics have the best characteristics at room temperature, and the ambient temperature The higher the spring material, the closer to the softening point.
In addition, it is necessary to consider the issue of UV protection. If the plastic spring is installed inside the product or installed in the backlight, it can avoid the plastic aging problem caused by ultraviolet radiation. For those products used outdoors, such as garden tools and sports equipment, ultraviolet radiation Can destroy the efficiency of the spring. Under this condition, it is necessary to determine whether the selected material can work under UV exposure according to the typical UV resistance data of the material.
2 Design of plate type plastic spring
In the usual mechanical design, the metal spring is often an independent standard part. The material of the spring is different from the material of other assembly parts. Whether it is a coil spring or a leaf spring, the material is made of good recovery and resistance. Fatigue characteristics. From the perspective of manufacturing methods, stamping or bending is the most common.
Unlike metal springs, plastic springs are often injection molded together with other parts of the parts. Therefore, the elastomer part and other parts are the same material. Obviously, the strength and shear strength of plastic parts are far inferior to that of metal, even in plastic In the era of widespread use, plastics cannot completely replace metal springs.
From the perspective of the particularity of plastic molding, leaf springs are easier to manufacture than coil springs, so plastic leaf springs are preferred.
2.1 Cantilever beam and simply supported beam spring
When comparing the load and stress equations of the two types of springs (see Figures 2 and 3), it can be seen that the stress load of the cantilever beam is simple under the same section (product of W and t) and effective span (L). 16 times the support beam. However, under the same deflection, the stress of the simply supported beam spring is 4 times that of the cantilever beam spring.
From the comparison of the effective amount of spring material, the difference between these two types of springs can be ignored. Under the same load, the simply supported beam spring can be designed with a larger span and a thinner section.
(1) Thickness calculation of leaf spring
transformation formula (1), the thickness of the cantilever spring is calculated as:
(2) Stress check
For the cantilever beam spring, the stress value solved by formula (2) is:
The tensile strength of Celcon M90 at 41°C is 618Kg/cm2. But its ultimate strength of bending fatigue resistance at 41°C is 232 Kg/cm2. Since the calculated strength of the stress check is 320Kg/cm2, the allowable stress should be Reduce to 232Kg/cm2 or lower. In this example, it is assumed that the length and width of the elastic beam can be increased, but the thickness cannot be reduced.
3 Injection molding process issues
After designing the spring product and selecting the appropriate material, we must also pay attention to the impact of the injection molding process on the quality of the spring. When designing the mold cavity, pay special attention to choosing the correct gate position. That is to say, avoid gates or weld seams. Appears in the spring or the moving part of the spring. Because of the weak part caused by the welding seam, the performance of the spring will be greatly affected.
Using injection molding analysis software, you can easily obtain a dynamic view of the flow front of the melt through injection molding simulation. Through the inside of the mold, you can check the mold temperature and pressure at any point, giving the designer access to the truth of the entire processing process Feel . Through injection molding flow simulation, not only can the temperature field and pressure field of the plastic product in the mold cavity be analyzed, but also the formation position of the weld seam and air pocket can be predicted. For the problems of simulation analysis, you can use 3D The computer modeling system modifies the product design or modifies the mold design . Using advanced CAE tools can improve the quality of plastic spring products, avoid mold rework and modify the design, and save time and money.
The cavity layout is designed with the help of injection molding simulation, which is convenient to optimize the runner design and reduce development time and cost. The cavity and runner layout have a great influence on the required clamping force. In some cases, through Injection molding simulation and improved design can greatly reduce the initial clamping force. This has great cost significance.