Quality Analysis of Internal Thread of Injection Molded Pipe Fittings and Design of Mold Core Pullin

Time:2021-10-04 10:03:38 / Popularity: / Source:

Internal threaded pipe fittings are one of common connection methods in plastic piping systems, their types include internal threaded elbows, internal threaded tees and other different forms. For forming of internally threaded pipe fittings, it is generally necessary to use form of rotating unthreaded for internal thread demolding. At present, this kind of products usually use form of slider + spring to complete core pulling while rotating. It has characteristics of simple structure and convenient production. However, there are strains, deformations and damages when internal thread is demolded during use. Following analyzes causes of such problems, and proposes a design plan to improve internal thread core pulling mechanism of mold.

Analysis of Causes of De-mold Deformation of Plastic Parts

Design of Mold Core Pulling Mechanism 

Take φ32mm three-head internally threaded tee pipe as an example to analyze and explain strain phenomenon during demolding. Overall dimensions of plastic part are shown in Figure 1. In order to complete core-pulling action, threaded structure needs to be rotated and core-pulled, slider + spring is used to pull core while rotating, as shown in Figure 2.
Design of Mold Core Pulling Mechanism 
In slider spring core-pulling structure shown in Figure 2, during rotation of threaded core, spring pushes slider to complete core-pulling action. This structure has following problems in production: ①When spring force is too large, core is rotated to final stage of threading, thread locking force between core and plastic part is less than spring force. At this time, slider directly bounces, causing plastic part to strain and deform; ②When spring force is insufficient, slider cannot retreat with rotation of core. At this time, thread of plastic part is damaged (threaded teeth).

New core-pulling structure design

Key to design of mold structure is core pulling of internal threaded pipe while rotating, and key to design of core pulling mechanism is how to smoothly complete core pulling while rotating. Ideal design is: when core is pulled, slider moves back by a standard pitch every time core rotates once. Therefore, it can be considered to use screw rod to cooperate with slider to complete internal thread core pulling.

01 Core-pulling mechanism design

Design of Mold Core Pulling Mechanism 
Thread Rc1-14" of three-way pipe shown in Figure 1 is a 55° sealed pipe thread, standard pitch is 2.309mm, material is PP, shrinkage rate is 1.6%, and standard ball screw with a pitch of 5mm is used. According to size of plastic part, gear meshing arrangement of designed screw core pulling structure is shown in Figure 3. Molding of plastic part adopts a layout of 2 cavities for injection.
Calculation of rotating core pulling and sliding core pulling is as follows: driving gear and core driven gear transmission ratio i1:
Internal Thread of Injection Molded Pipe Fittings 
Transmission ratio i2 of driving gear and lead screw driven gear:
Internal Thread of Injection Molded Pipe Fittings 
Taking into account factor of Rc1-14″ thread shrinkage factor, mold core pitch P is: 2.309 * 1.016 = 2.346mm; when core is pulled, pitch P1 for each turn of core is: 2.346 * i1 = 2.9324mm; when core is pulled, pitch P2 of slider per rotation of ball screw is: 5*i2=2.941mm. Pitch of P1 and P2 differ by 0.0086mm, difference is extremely small, which meets design requirements. Mold structure is shown in Figure 4.
Internal Thread of Injection Molded Pipe Fittings 

02 Working principle and process of core pulling mechanism

Core-pulling principle of new internal thread core-pulling mechanism: driving gear drives driven gear (core driven gear and screw driven gear) to complete core pulling and resetting of slider while rotating. Pitch of thread core should be controlled to be same as retracted pitch of slider during core-pulling.
When mold is opened, first fixed mold insert 4 and movable mold insert 7 are separated, gate condensate is released. When mold is opened in place, hydraulic motor drives driving shaft 19 to rotate through hydraulic motor mounting sleeve 25. At the same time, driving shaft 19 distributes rotational power to thread core 16 and ball screw sleeve 22 through driving gear 20, driven gear (core) 18, and driven gear (ball screw) 27; so that while threaded core 16 completes rotation, driven gear sleeve (ball screw) 26 drives slider 8 to cooperate with slider pressing plate 5 and bearing plate 6 to complete core-pulling action of threaded core 16, which meets use requirement of core pulling while rotating. Then, plastic part is pushed out through push rod fixing plate 14, push plate 15 and push rod 30.
In order to avoid problems such as strain and deformation of internal thread when internal threaded pipe is demolded, internal thread core pulling structure of ball screw slider is designed. Because ball screw drives slider core pulling and thread core to rotate core pulling same pitch, so that core pulling action of plastic part in mold runs smoothly, which can effectively solve problems of tooth damage and strain caused by internal thread plastic part during core pulling. Core-pulling structure has been used in actual production, action is stable during production process, effectively avoiding generation of unqualified products, reducing number of mold repairs, and having a good use effect.

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