Injection molded fastening screws
Injection molding fastening screws are essential standard components for mold assembly and plastic part connection. They secure mold components (such as the mold plate, core, cavity, guide pins and sleeves) together or serve as a secure anchor for later assembly of plastic parts. Their selection and use directly impact the mold’s structural stability and the reliability of the part’s assembly. Mold fastening screws must withstand loads such as the clamping force and injection pressure during the injection molding process, requiring high strength, high wear resistance, and good fatigue resistance. Fastening screws for plastic parts, on the other hand, must consider compatibility with the plastic to avoid stress concentration and cracking in the part. For example, a car bumper mold used insufficiently strong screws to secure the cavity. The screws broke under the clamping force, causing cavity displacement. The resulting bumper exhibited flash and dimensional deviations, resulting in direct losses of 100,000 yuan.
The selection of injection molding fastening screws requires specifications based on the force applied, connection thickness, and operating environment. These specifications primarily include thread diameter, length, precision grade, and material. The thread diameter selection is determined through force calculations. The diameter of mold plate connection screws typically ranges from M8 to M20. Based on the plate thickness and clamping force distribution, a “dense at the edges, sparse in the center” layout is adopted. For example, for a 300mm×300mm plate, M12 screws are used with a spacing of 100mm at the edges, and M10 screws are used with a spacing of 150mm in the center to ensure uniform stress distribution. The screw length must ensure an effective thread engagement length of at least 1.5 times the diameter. For example, the engagement length of an M10 screw should be at least 15mm to prevent thread slippage due to insufficient engagement. Regarding precision grade, 6g precision is used for key mold connections, while 8g is used for common areas to ensure a tight connection.
The material selection for fastening screws must meet strength and corrosion resistance requirements. Mold screws are often made of high-strength alloy structural steels, such as 45 steel (tensile strength 600 MPa), 40Cr steel (tensile strength 980 MPa), or 35CrMo steel (tensile strength 1080 MPa). After quenching and tempering, they achieve a hardness of HRC 33-38, ensuring both strength and toughness to prevent brittle fracture. Molds used in humid or corrosive environments require stainless steel screws (such as 304 or 316 stainless steel) to prevent rust and failure. Fastening screws for plastic parts are selected based on the type of plastic. For hard and brittle plastics (such as polystyrene), lower-strength brass screws are used to reduce stress damage to the plastic. For tough plastics (such as polypropylene), high-strength steel screws can be used, but washers must be used to distribute the stress. For example, in the assembly of a polystyrene appliance casing, M4 brass screws and nylon washers are used to reduce the stress in the plastic part from 80 MPa to 40 MPa when the screws are tightened, thus preventing cracking.
The installation process of fastening screws is crucial to the quality of the connection, and pre-tightening force control is the core. Too loose will cause the connection to become loose, while over-tightening may cause the screw to break or the threaded hole to be damaged. The pre-tightening force of mold screws is usually 60%-70% of its yield strength . The tightening torque can be controlled by a torque wrench. For example, for M12×50 40Cr screws , the tightening torque is set to 350-400N · m to ensure that the pre-tightening force reaches 65% of the yield strength . Before installation, the threaded hole must be cleaned of iron filings and oil stains, and anti-loosening glue (such as Loctite 243 ) must be applied to the threaded surface to enhance the anti-loosening performance of the connection. For screw installation in plastic threaded holes, the tightening speed must be controlled (no more than 10r/min ) and ” multiple step-by-step tightening” must be used. This method avoids damage to plastic threads caused by one-time tightening. For example, the M6 threaded hole of a plastic part was tightened in steps: first tightening to 10 N · m , pausing for 10 seconds, and then tightening to 15 N · m . This gradually relieved the stress in the plastic thread and reduced the thread damage rate from 12% to 1% .
Specialized fastening screws are required for special applications. The connection between the mold’s movable platen and backing plate, which is subject to frequent impact loads, should be secured with screws equipped with retaining washers to prevent vibration and loosening. High-temperature-resistant screws (such as Inconel 718 alloy) should be used to secure heating elements in hot runner systems. These screws can withstand long-term use in environments exceeding 300°C. Concealed connections to plastic parts can utilize countersunk screws, ensuring the head is flush with the part surface, improving appearance quality. For example, the hot runner mounting plate of a high-temperature mold (operating at 250°C) utilizes M8 Inconel 718 screws with copper washers. These screws have remained stable and free of loosening and oxidation over time, improving the stability of the hot runner system by 50%. Furthermore, regular inspection of fastening screws—rechecking the tightening torque every 10,000 molds produced or every 1,000 parts assembled—and timely replacement of deformed or worn screws can effectively prevent quality incidents caused by screw failure. Through scientific selection, standardized installation, and regular maintenance, injection molding fastening screws can fully function as connections, ensuring the reliability of molds and plastic parts.
