The difference between bolts, screws and screws

1. Introduction
In the structural design of plastic products, screws and screw posts are small parts, but they play a key role. They are responsible for connecting various parts and ensuring the stability of the overall structure of the product. If the screws and screw posts are not designed properly, it will lead to difficulties in product assembly, unstable structure, and even affect the normal use of the product. Today, I will systematically tell you about the relevant knowledge of screws and screw posts in plastic products, hoping to help you avoid pitfalls in design and improve product quality.
2. Basic knowledge of screws
2.1 Types and application scenarios of screws In plastic products, common types of screws include self-tapping screws, machine screws, etc., which play their respective advantages in different scenarios. Self-tapping screws, which have special threads on the screw rod, can be directly screwed into the plastic material to form the corresponding internal thread in the hole without pre-tapping. According to the shape and function of the tail, self-tapping screws can also be subdivided into pointed self-tapping screws, cut-tail self-tapping screws, etc. Pointed self-tapping screws are more common and are suitable for soft plastic materials such as PP and PVC. They can easily drill into the material for fastening. Cut-off self-tapping screws are often used for hard materials such as POM and PC or glass fiber reinforced materials. The cutting edge at the tail can reduce material stress when tapping. Self-tapping screws are often seen in the connection of plastic shells of some plastic toys and small household appliances. Machine screws need to be used with prefabricated threaded holes or nuts. The crest and bottom of the teeth are similar in size and the pitch is small. Its advantages are high connection strength and repeated disassembly without damaging the thread. It is suitable for occasions with high requirements for connection stability. For example, in the fixing of the motherboard of electronic equipment and the connection of components of large plastic equipment, machine screws can ensure that the components are still firmly connected under long-term use and vibration environment.
2.2 Material properties of screws Screws are made of various materials, including carbon steel, stainless steel, plastic, etc. Different material properties have different effects on plastic products. Carbon steel screws have low cost, certain strength and hardness, and are widely used in general plastic product connections. However, carbon steel has poor corrosion resistance and is prone to rust in a humid environment. This will not only affect the appearance of the product, but the rust produced by rust may also contaminate plastic parts and reduce product quality. In order to improve the rust resistance of carbon steel screws, they are usually subjected to surface treatments such as galvanizing and nickel plating. Galvanized carbon steel screws have a certain rust resistance and the cost is not much increased. They are widely used in some plastic products that are cost-sensitive but have certain rust resistance requirements. Stainless steel screws are known for their excellent corrosion resistance and can maintain good performance in harsh environments such as humidity, acid and alkali. In the connection of plastic products with extremely high requirements for hygiene and corrosion resistance such as medical equipment and food processing equipment, stainless steel screws are the best choice. Common stainless steel screw materials include 304 and 316. 304 stainless steel has good comprehensive performance and can meet the use of most conventional environments; 316 stainless steel adds molybdenum elements on the basis of 304, further improving corrosion resistance, especially suitable for highly corrosive environments such as the seaside. Plastic screws have also gradually attracted attention in recent years. They have the characteristics of light weight, insulation, non-magnetic, and beautiful. In some electronic products with strict weight requirements, such as laptops and tablets, plastic screws can reduce the overall weight of the product in the connection of internal plastic parts. In areas such as electronic equipment and medical equipment that are sensitive to electromagnetic environments, the non-magnetic properties of plastic screws can avoid interference with equipment operation. Plastic screws made of nylon 66 have high toughness and wear resistance and are often used to connect the internal structures of electronic equipment.
III. Key points of screw column design
3.1 Principle of size determination The size design of the screw column is directly related to the stability and reliability of the screw connection. The inner diameter of the screw column must be closely matched with the outer diameter of the screw used. Under normal circumstances, the inner diameter of the screw column of the self-tapping screw can be designed as 0.8-0.85 times the outer diameter of the screw. For example, if a self-tapping screw with a diameter of 3mm is used, the inner diameter of the screw column can be designed to be 2.4-2.55mm. For machine screws with nuts, the inner diameter of the screw column needs to be determined according to the specifications of the nut. It is necessary to ensure that the nut can be smoothly embedded and has a certain amount of interference to prevent the nut from loosening. The design of the outer diameter of the screw column needs to consider the strength of the screw column and the molding characteristics of the plastic material. Generally speaking, the outer diameter of the screw column is 1.8-2.5 times the screw diameter. If the outer diameter is too small, the screw column is not strong enough and is easy to break when tightening the screw; if the outer diameter is too large, it may cause shrinkage and deformation of plastic products during the molding process. For some screw columns that are subject to large external forces, the outer diameter can be appropriately increased, or the strength can be increased by adding reinforcing ribs. The height of the screw column should not be ignored. The height should be determined according to the actual assembly requirements to ensure that the screw can be fully screwed into the screw column to provide sufficient connection strength. However, the height of the screw column should not be too high, otherwise it will increase the amount of plastic material used, and deformation and warping are prone to occur during the molding process. Generally, the height of the screw column should not exceed 4 times the nominal diameter of the screw. For example, for M4 screws, the height of the screw column is generally controlled within 16mm. In some product designs with space restrictions, the height of the screw column needs to be accurately planned to meet the compact structural layout.
3.2 Structural optimization design In order to improve the performance of the screw column, some structural optimization designs are often used. The crater structure is a common and effective design method. It adds a raised ring structure around the root of the screw column, which can effectively improve the uneven cooling of the plastic material at the root of the screw column and reduce the shrinkage caused by the difference in wall thickness. During the injection molding process, the root of the screw column is prone to shrinkage marks due to the concentration of plastic and the slow cooling speed. The crater structure can disperse the plastic at the root, speed up the cooling speed, and improve the surface quality of the product. On the shells of some electronic products with high appearance requirements, the screw column is designed with a crater structure to avoid shrinkage marks affecting the appearance of the product. The setting of reinforcing ribs is also an important means to enhance the strength and stability of the screw column. When the screw column is high or needs to withstand a large external force, the reasonable arrangement of reinforcing ribs around the screw column can effectively prevent the screw column from deforming and breaking when subjected to force. The layout and size of the reinforcing ribs need to be determined according to the actual force of the screw column. Generally, the height of the reinforcing ribs is similar to the height of the screw column, and the thickness is 0.5-0.8 times the thickness of the plastic wall. For example, in the connection of structural parts of large plastic equipment, by setting multiple reinforcing ribs around the screw column, the strength of the connection part can be significantly improved, ensuring the stability of the equipment in long-term use and vibration environment.
4. Common problems and solutions
4.1 Cracking of screw column Cracking of screw column is a common and difficult problem in plastic products, which seriously affects the structural integrity and service life of the product. In terms of materials, brittle materials, stress-sensitive materials or materials that are prone to internal stress, such as PS, ABS, PC, PC/ABS alloy, etc., especially PC materials, are more likely to crack in the application of screw columns. When too much nozzle material is added to the material, it may cause the material to be too brittle; if the elongation at break of the material is too low, it is also easy to cause the screw column to crack. For materials with a glass fiber content of more than 40%, it is not recommended to tap the screw column, because the presence of glass fiber will make the material brittle and increase the risk of cracking. In addition, if the molding temperature is too high or the drying is insufficient, the material will contain moisture, causing the material to degrade, which will also reduce its strength and cause the screw column to crack. Unreasonable structural design is also an important reason for the cracking of the screw column. If the inner diameter of the screw column is smaller than the inner diameter of the screw, the possibility of cracking will increase significantly. If the outer diameter of the screw column is too small, that is, the wall thickness is too small, the screw column will be insufficient in strength and will easily crack due to the pressure when the screw is tightened. When designing, be sure to select the appropriate inner and outer diameters. You can refer to the relevant design standards and empirical data for design. If necessary, use reinforcing ribs to enhance the strength of the screw column. Excessive internal stress generated during the injection molding process can also cause the screw column to crack. In terms of structural design, if the root of the screw column and the top of the pin are not rounded, it is easy to form a stress concentration point, thereby causing cracking. In terms of injection molding technology, parameters such as melt temperature, mold temperature, holding pressure, holding time and injection rate will have a great influence on the internal stress of the part. Generally speaking, using higher melt temperature and mold temperature, smaller holding pressure and holding time, and slower injection rate can help to obtain smaller internal stress, among which the mold temperature has the most significant effect on internal stress. When it is necessary to embed copper threads in the screw column, it is best to use a higher mold temperature so that the heat of the mold can be transferred to the copper nut in a shorter time, or preheat the copper thread first to eliminate the internal stress caused by low temperature and avoid cracking of the workpiece. To solve the problem of cracking of the screw column, the following measures can be taken: In terms of material selection, try to avoid using materials that are easy to crack, or modify the material to improve its toughness. In the structural design stage, accurately calculate and design the inner and outer diameter dimensions of the screw column to ensure that it matches the screw, and reasonably set the reinforcing ribs and fillet transition. Optimize the injection molding process parameters, find the most suitable process conditions through experiments and simulation analysis, and reduce the internal stress. Adding a concave platform or chamfer at the entrance of the inner diameter of the screw column to reduce the initial stress when tapping the screw can also effectively reduce the risk of cracking of the screw column.
4.2 Slipping phenomenon Slipping phenomenon will cause the connection between the screw and the screw column to loosen, reducing the fastening effect of the product. Material properties have an important influence on slipping. If the material is too tough and not rigid enough, it will be difficult for the screw to form an effective thread bite when it is screwed in, which is easy to cause slipping. For example, some rubber materials or soft plastics with high elasticity are more likely to have stripping when used in screw columns. If the inner diameter of the screw column is designed to be too large, the thickness of the meat bitten by the screw will be thinner, and it will not be able to provide sufficient friction and tightening force, thus causing stripping. This situation may occur if the inner diameter of the screw column is not accurately calculated in strict accordance with the screw specifications during design. During the assembly process, excessive torque is also a common cause of stripping. When using a tool to tighten the screw, if the applied torque exceeds the bearing capacity of the screw column material, the inside of the screw column cannot withstand the torque, while the outer part is strong enough and not damaged, which will cause the inner thread to be destroyed and cause stripping. To avoid stripping, materials with moderate rigidity and toughness should be selected.