Under background of increasingly fierce market competition, various plastic products with peculiar shapes stand out, leading to more and more diversified injection mold structures. When making 3D data for large injection molds, it is necessary to assemble tens of thousands of parts together. There are often some interferences in the movement of certain parts. Such interference problems are difficult to discover in time, will be exposed during mold assembly and mold testing. Some corrective measures need to be taken, and sometimes even re-machining is required to eliminate problem. This increases manufacturing cost of mold, and at the same time extends manufacturing cycle, which affects reputation and long-term development of company. Therefore, it is particularly important to establish a motion simulation for large scale injection mold.
In the process of establishing a motion simulation for a large injection mold, compared with previous simulation models, it usually takes a long time to establish a motion simulation solution for a mold with a side core pulling mechanism. However, it only takes a few seconds to establish a motion simulation solution for a mold without a side core pulling mechanism. Therefore, it is side core pulling mechanism that mainly affects speed of motion simulation. In following, by establishing a motion simulation model of side core pulling mechanism, comparing and analyzing, find out reasons that really affect calculation speed, and propose a method to reduce amount of calculation, thereby improving calculation speed.

Because large scale injection mold has many parts, shape and structure of each part is more complicated, overall digital mold memory is relatively large. If motion simulation is directly established in overall digital model, it will cause a huge amount of calculation, a slow solution speed, and even a computer crash. First step to increase calculation speed is to export parts that need to be simulated and build digital model separately. When exporting components, it is necessary to remove parts that will not cause interference, such as screws and pins that sink into surface of part, so as to minimize storage of motion simulation digital model. Since motion simulation of side core pulling mechanism is only established here, only parts related to side core pulling mechanism need to be derived. Side core pulling mechanism is shown in Figure 1.

 

Figure 1 Side core pulling mechanism

1. Fixed template 2. Wedge block 3. Limiting block 4. Inclined guide column 5. Wear plate 6. Slider 7. Movable template

 

Figure 2 Enter motion simulation interface

After importing digital model that needs to be simulated, click on motion simulation in start drop-down menu to enter simulation interface, as shown in Figure 2.

Steps to establish a motion simulation model are as follows.

 

(A) Establish connecting rod interface

 

(B) Build a model

Figure 3 Establishing a fixed link

Set fixed mold plate, wedge block, and inclined guide column as a connecting rod, name it "dingmu"; set movable mold plate, limit block, and a connecting rod as a connecting rod, name it "dongmu". Rod does not move, it can be set as a fixed link, as shown in Figure 3. Set slider and wear plate as a connecting rod, name it "huakuai".

 

(A) Establish connecting rod interface

 

(B) Build a model

Figure 4 Establishing a sliding pair

1) First establish sliding pair of "dingmu" link, select link, specify any point on link, specify direction of mold opening, and name motion pair "kaimu". Since link is an active link, a drive needs to be added to motion pair, as shown in Figure 4.
2) Then create a sliding pair of "huakuai" link, select link, specify any point on link, specify sliding direction of slider, and name motion pair "huadong". Since this connecting rod is a driven connecting rod, there is no need to add a drive.
3) Finally, a connector needs to be established to simulate real movement of slider. Find contact surface (force surface) during movement, including contact between slider and inclined guide column, contact between wear plate and wedge surface (surface between wedge block and fixed mold plate), contact between slider and limit block . Need to establish 3 groups of 3D contacts, try to choose "small plane" when selecting type, which can reduce amount of calculation, as shown in Figure 5.

 

Figure 5 Establish 3D contact

 

Figure 6 Establishing spring

In addition, a spring connector needs to be established between slider and movable mold plate. Stiffness and free length can be filled in according to spring parameters used in real mold. Origin selects beginning and end of spring, connection direction of two points must be consistent with direction of spring expansion, as shown in Figure 6.

 

Figure 7 Establish a solution

Select solution type as conventional drive, analysis type as kinematics/dynamics, set appropriate time and number of steps (to facilitate later viewing of motion animation, number of steps can be set to an integer multiple of time), specify direction of gravity and click OK to perform solution, as shown in Figure 7.

After clicking OK, motion simulation starts to be solved. Calculation speed of motion simulation established by above method is very slow, and it takes nearly 140s to get calculation result.

 

Factors that affect speed of motion simulation solution may include connecting rods, sliding pairs, 3D contact, and solution schemes. In view of these influencing factors, same computer is used to establish different connecting rods, sliding pairs, 3D contact, and solution schemes. Calculation time required for statistics is shown in Table 1.

① Increase in the number of connecting rods or sliding pairs does not affect solution time; ②Number of 3D contacts increases, and solution time increases; ③ Time of core-pulling movement in solution plan increases, and solution time increases.

Factors affecting solution speed of motion simulation are number of 3D contacts and time of core-pulling motion in solution scheme. The more 3D contacts or the longer time of core-pulling motion in solution scheme, the slower solution speed of motion simulation.

According to above conclusions, since core-pulling motion time is generally fixed (same as core-pulling motion time in real production process), solution speed of motion simulation can only be improved by reducing number of 3D contacts and simplifying structure of connecting rod. Since 3D contact type established is a small plane, above conclusion can be verified by reducing number of small planes of connecting rod. For above-mentioned side core pulling mechanism, build a containment block to wrap fixed mold plate, wedging surface on wedge block and inclined guide post, then use "intersection" command to select containment block, fixed mold plate and wedge block, obtain a simple intersecting body (an entity where containment block intersects fixed mold plate and wedge block), add it to original "dingmu" connecting rod. When connector is established, entity can establish 3D contact with wear plate, replacing original 3D contact between fixed mold plate and wear plate, wedge block and wear plate, reducing number of 3D contacts and simplifying structure of connecting rod in 3D contact, as shown in Figure 8.

 

(A) Before simplifying

 

(B) Simplified

Figure 8 Model before and after simplified mold plate

1. Wedge block 2. Inclined guide column 3. Fixed mold plate 4. Simplified model
Through this operation step, complex structure of "dingmu" connecting rod is simplified into a simple entity. In same way, wear-resistant plate and slider can be summed into one entity, 3D contact between slider and inclined guide post can be reduced; same method can be used to establish a containment block on limit surface to obtain intersecting body with limit surface, thereby simplifying 3D contact between limit block and slider.
Performing calculation again, it is found that calculation time is shortened to about 30s, and motion state of all connecting rods in motion simulation is consistent with original, which proves correctness of above conclusion.