[Abstract] Several common problems of thread forming in injection molds are analyzed, and solutions to each problem are given.

Molding thread with a thread-off structure in injection mold is a common injection mold structure, some problems often occur in production, which make mold unable to produce normally. Following takes unthreaded structure of injection mold of a car expansion kettle as an example to illustrate how to solve these problems. Common problems: ①Plastic part thread size control problem; ②Plastic part thread surface break-off problem; ③Injection mold thread mechanism jamming problem; ④Thread starting point control problem. Thread of car expansion kettle is generally divided into external thread (see Figure 1) and internal thread (see Figure 2).

 

Figure 1 External thread

 

Figure 2 Internal thread

Under normal circumstances, there are tolerance requirements for thread of expansion kettle, as shown in Figure 3 and Figure 4. According to our experience, after injection molding, thread mouth is generally large and root is small. Therefore, when designing and manufacturing braces, you need to consider above factors: Generally, braces (external threads) are designed as shown in Figure 5, top diameter and bottom diameter of tooth should be drawn at 0.5°.

 

Figure 3 External thread

 

Figure 4 Internal thread
Specific design size requirements for braces:
Plastic part tooth bottom diameter ϕ 46.8 mm, tooth top diameter: ϕ 49.6mm, shrinkage rate: 1.8%, according to shrinkage braces size: tooth bottom diameter ϕ 47.64mm, tooth top diameter ϕ 50.49mm. Because of lower tolerance, take: ϕ 47.64-0.2 = ϕ 47.44mm, ϕ 50.49-0.2 = ϕ 50.29mm. Take tolerance down to prevent plastic part from being out of tolerance after molding. According to draft angle of 0.5°, theoretical dimensions after molding are: ϕ 50.07/1.018= ϕ 49.18mm, ϕ 47.17/1.018= ϕ 46.33mm.
Actual molding results: ϕ 49.4mm, ϕ 46.56mm, mouth diameter is qualified, actual plastic part size will be about 0.2mm larger than theoretical value, as shown in Figure 5.

 

Figure 5 External thread braces

Reasons for surface of thread to be damaged during demolding process are as follows:
(1) There is no demoulding angle on the top and bottom of tooth.
(2) Tooth surface is not polished enough or surface is rusted.
(3) Tooth extraction speed is too fast.
For above reasons, measures are taken in mold design:
(1) Draft angle is directly made during processing. Generally, threads are non-standard threads after being shrunk. The best processing method is to CNC lathe once, turning tool moves axially while moving longitudinally. After doing so, surface of thread will also have a draft angle.
(2) Put sandpaper directly on turning tool for polishing.
(3) Following phenomena occur during use of mold, oil cylinder cannot enter a large amount, but cylinder pressure needs to be large, strain can be improved. Main reason is that ratio between gear and big gear that mold meshes with rack cannot exceed 1/4.

Main reason for jamming of extraction mechanism is that it is easy to burn between molded braces and cavity. As shown in Figure 6, main reason for burns is that inertia of extraction mechanism during movement is not considered. Due to inertia, molded braces will move a little longer in axial direction. Most mold factories consider installation and positioning when designing. Taper and steps are designed in axial direction. After disassembling mold, they will find that these positions are roughened. Improved thread-off structure is shown in Figure 7.

 

Figure 6 Original thread structure

1. Inner hole insert 2. Molded braces 3. Guide braces 4. Cavity 5. Guide braces

 

Figure 7 Improved thread-off structure
1. Inner hole insert 2. Molded braces 3. Guide braces 4. Cavity 5. Guide braces
In Figure 6 I1, positioning step is designed on molded braces 2, which is wrong. Positioning steps on molded braces are easy to burn. Correct design is shown in Figure 7 I 1, positioning surface for installation is designed on guide brace.
In Figure 6 I 2, taper is designed between molded braces 2 and cavity 4, which is wrong, it is easy to burn between molded braces and cavity. Correct design is shown in Figure 7 I 2 and it is designed to be straight. Matching gap is controlled to ensure that molded braces are not burned.

Starting position of screw thread of expansion kettle is very important. Customer has requirements for feel of cap screwing, so starting point of screw thread is very important. As shown in Fig. 8, since molded mouthpiece 3 is a cylinder, starting position of screw thread is random during processing. Therefore, starting position should be checked against 3D mold during installation and aligned with marked position on mold cavity. After molded braces 3 is installed, top surface is flush with cavity, guide braces 4 should be properly rotated to make molded braces lower than molding surface of cavity 2 by 0.1mm. After above-mentioned parts are installed in place, positioning block 5 is installed and locked to solve problem of starting position of screw.

 

Figure 8 Thread start position control
1. Fixed template 2. Cavity 3. Molded braces 4. Guide braces 5. Positioning block