Product diagram of double-threaded nut is shown in Figure 1. Maximum size of product is 29.00 mm * 29.00 mm * 32.00 mm, average thickness of plastic part is 5.15 mm, material of plastic part is PP, shrinkage rate is 1.018, and weight of plastic part is 11.20 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling of injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, etc. In addition to dimensional tolerances, plastic parts also need to ensure dimensional accuracy of their triangular threads to meet assembly requirements.

 

Figure 1 Product Picture of Double Thread Nut
Product diagram of double-threaded nut is shown in Figure 1. It can be seen from figure that plastic part has a simple structure, similar to two polygonal nuts overlapping each other. Therefore, to form an undercut in the shape of plastic part, it is necessary to design Hough slider for molding. There is no continuity between two sections of thread inside plastic part, and middle is separated by 2mm. Plastic part needs to be threaded off from both ends. Specifications of two internal threads are both inch thread PT1/2. Basic feature of plastic part is that threads at both ends are exactly same. Therefore, when determining mold opening plan, one end of plastic part is unthreaded in the front mold, and the other end is unthreaded in back mold.

 

Since shape of plastic part needs to be formed by design of Hough slider, choice of mold opening direction of this set of molds can also be designed by laying down plastic part. At this time, threaded core is on the slider. Because slider threading mechanism is complicated, this mold design solution for threading is not selected.
There are many types of thread-removing mold structures. Power source of automatic unthreading mechanism is to use mold opening force to unthread, including mold opening force to drive rack to drive gear to unthread and screw rod to unthread; relying on hydraulic cylinder to drive rack to reciprocate, thread core is rotated by gear to realize internal thread demolding; cycloidal hydraulic motor unthreading uses a variable-speed motor to drive gear, and gear drives thread core to achieve internal thread demolding; cycloidal hydraulic motor can also be used to drive sprocket through chain, and finally drive thread core to unthread; generally, motor drive is mostly used in the case of a large number of threads.
When housing products are unthreaded, their ranking generally depends on size and shape of product. For single-cavity molds, threaded part needs to be arranged in a position where unthreading mechanism is easy to be arranged. For multi-cavity molds, threaded parts of plastic parts need to be arranged according to a certain regularity. Motors and hydraulic motors often use plastic parts to arrange their threads in a circle. Bottle caps and nut plastic parts are small in size, and are mostly multi-cavity threading molds. There are two types of linear and circular arrangements when threading.
Significant difference between unthreading of casing products and unthreading of bottle caps is:
1) For large casing plastic parts, mold opening direction is determined by casing itself, and thread is only a small structural element of plastic part. Therefore, thread-removing mechanism must obey structural characteristics of housing mold. Movement of unthreading mechanism is isolated from movement of the rest of mold. For example, threading mechanism and plastic part ejection are carried out in isolation. At this time, threading is mostly used to drive rack and pinion by oil cylinder.
2) Nuts and bottle caps are unthreaded. Unthreading mechanism is related to ejection mechanism and mold opening movement of mold, mold must have a certain degree of mold opening stability. It is necessary to prevent screw teeth from being damaged during mold opening movement.
Use cycloid hydraulic motor or electric motor to unthread, not limited by number of thread turns. It should be noted that travel switch must be designed to effectively control two extreme positions of thread core after mold opening or closing. Final position of thread core must be connected to electrical contacts to start mold closing or injection process. Through this electrical control technology, only time when thread core is unthreaded can be determined, it has nothing to do with opening and closing of mold.
When power source adopts cylinder to unthread, molds of European and American customers will also require cylinders with a limit switch to accurately set thread core exit distance.
Comparing various thread-removing mechanisms, oil cylinder plus rack and pinion has stable thread-removing performance and accurate return, but operating efficiency of oil cylinder is low, injection cycle will be lengthened. Usually pumping force of oil cylinder is used to unthread thread, and pulling force of oil cylinder needs to be calculated. With help of mold opening force, it is convenient and concise to use rack and pinion or screw rod to unthread, mold size can be reduced. With help of mold opening force to unthread mold, mold opening force is huge at the moment of mold opening, which is mainly to overcome static friction force. It is necessary to pay attention to rigidity of mold base, positioning between mold plates of mold base is good, and moving mold plate is a good guide mechanism. Rack should be designed with a 3-sided guide mechanism and have sufficient strength.

 

 

Cavity layout of mold design is 4 cavity. Front and rear molds are designed with independent thread-removing mechanisms. Thread-removing mechanism of front and rear molds is used for thread-removing by hydraulic motors. In the front mold unthreading mechanism, cycloidal hydraulic motor drives sprocket, then drives sprocket coaxial with gear 17 through chain 25, so that gear 17 (number of teeth is 60, module is 2) rotates and gear 16 (number of teeth is 50, modulus is 2) is driven to rotate, speed is increased. Gear 16 respectively drives two front mold thread cores (the number of teeth is 25, the module number is 2) to rotate, and the thread is released. During rotation of thread core, tail retreats into thread sleeve. Internal thread of this thread sleeve has same specification parameters as internal thread of plastic part. The whole process of unthreading adopts 2-stage transmission.
Thread of back mold adopts same unthreading method as thread of front mold. Gating system is designed with a latent gate in the middle of Hough slider. Two sliders are driven by oblique guide posts. Cooling inserts are designed inside threaded cores of front mold and back mold, cooling water is designed to ensure normal progress of injection molding. If thread core of unthreaded mold is not designed to cool and transport water, mold will heat up after a period of injection molding, thread core will easily burn to death due to high temperature, resulting in abnormal production.

 

 

 

Figure 2 Die diagram of double-threaded nut

 

Figure 3 3D mold water transportation diagram

 

Figure 5 3D drawing of dynamic mold