Injection molded limit screws
Injection molding limit screws are key components used to control the position of moving parts in the mold. Their primary function is to limit the travel of components such as the template and slider, preventing mold damage or dimensional deviation of the plastic part due to excessive movement. During the injection molding process, precise position control is required for the opening and closing of the mold, the core pulling and resetting of the slider, and other operations. Limit screws, by counteracting the contact surface, limit the movement range of the component within the designed range, thereby ensuring the stability and reliability of the mold’s operation. Whether it is a large automotive panel mold or a precision electronic component mold, limit screws play an indispensable role, and their performance directly affects the mold’s service life and the production quality of the plastic part.
The structural design of the limit screw must take into account both functionality and durability. Common limit screws consist of a head, a shank, and a threaded portion. The head usually adopts a hexagonal or countersunk design for easy installation and adjustment. The diameter of the shank needs to be determined according to the impact force it withstands and the mold space. A diameter that is too small can easily cause deformation under stress, while a diameter that is too large may increase the weight of the mold and manufacturing costs. The precision of the threaded portion is particularly important. It must fit tightly with the threaded hole on the template to avoid loosening of the limit due to excessive clearance or difficulty in adjustment due to interference fit. In addition, the transition between the head and the shank of the limit screw should adopt an arc chamfer design to reduce stress concentration and prevent breakage under long-term stress. For occasions where frequent adjustment is required, scale markings can also be added to the screw head to facilitate operators to quickly set the limit position.
Material selection is a key factor in ensuring the performance of limit screws. Because limit screws are subject to frequent impact and friction during operation, high-strength, high-wear-resistant materials such as 45 steel and Cr12MoV alloy tool steels are required. After quenching and tempering, 45 steel can reach a hardness of HRC28-32, exhibiting excellent overall mechanical properties and suitable for general working conditions. After quenching and tempering, Cr12MoV can reach a hardness of HRC58-62, offering superior wear and heat resistance, making it suitable for use in high-speed, high-pressure molds. For precision molds with higher requirements, limit screws can also be made of stainless steel to prevent rust from affecting adjustment accuracy. The material’s heat treatment process must also be strictly controlled to ensure that the screws possess high strength without breaking due to excessive brittleness.
The installation and adjustment of the limit screws directly affect their working effect. During installation, it is necessary to ensure that the axis of the screw is perpendicular to the direction of movement of the template to avoid uneven force due to tilt, which in turn causes eccentric wear or jamming of the mold parts. The processing accuracy of the threaded hole must match the screw. Taps are usually used to ensure the verticality and smoothness of the thread. Before installation, the iron filings and impurities in the hole need to be cleaned to prevent jamming during assembly. When adjusting the limit position, the screw should be rotated slowly while observing the movement of the mold part until the designed limit point is reached, and then locked with a nut to prevent the screw from loosening due to vibration during the operation of the mold. For the use of multiple sets of limit screws, it is necessary to ensure that the limit height of each screw is consistent to avoid damage to individual screws due to excessive force.
The maintenance and replacement of limit screws are important aspects of mold maintenance. During long-term use, the head and contact surface of the limit screw will wear due to frequent impact, resulting in a decrease in the limit accuracy. At this time, the wear of the screws needs to be checked regularly. If problems such as head deformation, thread stripping, or contact surface depression are found, they should be replaced in time. In daily maintenance, it is necessary to regularly clean the oil and impurities on the surface of the screws, apply grease to reduce friction, and pay attention to rust prevention treatment, especially in humid environments. When replacing limit screws, products with the same specifications as the original should be selected to avoid affecting the limit effect due to size differences. In addition, for key molds, a replacement record of the limit screws can be established, and a preventive replacement plan can be formulated based on the frequency of use to ensure that the mold is always in good operating condition.
