Optimization design of automatic injection blow molding cavity air gap spring

Optimization design of automatic injection blow molding cavity air gap spring Guangxi Institute of Technology (Liuzhou 545006, Guangxi), Gao Zhongyong, Ou Disheng, fully discussed, and compared the results of the optimization calculation for its different objective functions.

1 Introduction 3 The design of the air gap spring is based on the author's description of the mechanical parameters of a fully automatic injection blow mold for the determination of 3.1 mechanical parameters, and pointed out that this airway structure is still necessary to continue to improve. In fact, there are several shortcomings in the airway structure mentioned: 1 If the special heat treatment is not carried out, the compression strength of the support surface of the gas passage groove is low, which will eventually cause the air groove to become shallow; 2 the gas groove is still open when the plastic is injected. It is difficult to avoid the colloid entering and blocking the airway; 3 it is difficult to ensure uniform airflow when blowing the bottle, and it is difficult to obtain a high blowing pass rate. To this end, the author presided over the improvement of the airway structure and achieved a satisfactory blowing effect.

2 The effect of the air gap spring is improved as shown in the air passage structure. The nut 3 axially positions the mandrel 1 on the die holder 2, but it must be ensured that the mandrel and the nut can have an axial movement of 0.2 to 0.4 mm on the die holder. The spiral ring 5 is tightened in the hole of the mold base, so that the air gap spring 4 obtains sufficient pre-tightening pressure to ensure an air gap of 0.2 to 0.4 mm between the shoulder of the mandrel and the support surface of the mold base, and the compressed air is evenly and smoothly. Plastic hollow products are fully formed and accurate.

However, when the tube blank is injected, the high-pressure molten plastic colloid enters the cavity, and the mandrel is subjected to a large axial pressure, forcing the spring to be further compressed. The original 0.2-0.4 mm air gap is completely closed, and the high-pressure colloid cannot enter, neither blocking The airway does not form any tiny flashes inside the mouth of the bottle, which is very beneficial for improving the hygiene of the bottle.

The nominal injection pressure of the injection molding machine is 140 MPa. According to the recommendation, the pressure acting on the projected area of ​​the mandrel is not lower than: 0.35 is the minimum pressure loss coefficient, and 20 is the average diameter of the mandrel.

The outer diameter of the injected plastic tube blank in the vicinity of the air gap is %25mm, the wall thickness is 1mm, and the air gap is opened before the blowing, which causes the tube blank to elongate, and the elongation can be taken as +=0.1. In addition, the tube blank There is an injection gate with an average diameter of 3.4mm' and a length of about 5mm. At the blow molding station, the gate will be flattened to the middle by pressing the core rod under the spring support and the bottom mold. It can basically determine the elastic modulus of the plastic for blowing bottle under the temperature condition of 1109 "=123, then the force of partial stretching of the tube blank and the extrusion of the material head should be the pre-action on the air gap spring. The pressure must be such that the air gap reaches a maximum value of 0.4 mm. In a working cycle, the maximum working load of the air gap spring is !m*=!mn+0.4, where the spring stiffness is the preload force during installation. At the time of design, the initial selection of air gap spring material is selected. The average working cycle is 8 and the machine works 2 times a day, 300 days per year, and the spring life expectancy is 5 years. The total spring stress cycle number is 1.08x107 times. Therefore, the spring material should be selected according to Class I. According to local conditions, the material selected is 65S+2M-WA 3.3 space. In the determination of the size, the external thread of the screw ring is taken as !30mmxlmm, so the outer diameter of the spring yellow should not be greater than 28mm, the inner hole of the nut is 16mmx1mm, the screw ring and the nut are stepped, so that the spring is simultaneously The inner guide column acts, so the inner diameter of the spring should be larger than "20.5mm. The inner hole of the bottom end of the mold base is limited for the length of the assembly nut, the screw ring and the spring, so the length after the spring is pre-tightened should not be greater than 17.2mm. To ensure the predetermined gas Gap size requirements, design and manufacturing must optimize the design of the axial length of the mandrel and the mold base over 4 air gap springs Generally speaking, the design of the spring is a more complicated work, there are many constraints, often need Try it. Conveniently, the predecessors have done a lot of work on the computer optimization design of the spring, and there are many mature programs available. At present, people generally observe the calculation formula of the spring according to the lightest weight 061. Only the four calculation formulas such as weight, natural frequency, stability and stiffness contain all design variables. Therefore, it is advantageous to select one or several of the four as the objective function.

However, for the air gap springs described herein, there are no instability and resonance problems, so weight and stiffness can be prioritized as objective functions.

4.1 Consider the lightest weight to optimize the target objective function: 1.5 is the number of tight turns.

(10() and (11() are fatigue strength conditions and stiffness conditions respectively; (1.5 in 10() is the fatigue safety factor; (320 in U() is the initial setting 4.2 considering the maximum stiffness as the optimization target The air gap spring is always in the pre-compacting state. Only when the injection is performed, the spring will continue to compress by 0.2~0.4mm, so that the air gap is completely closed. Therefore, the author hopes that the spring will have maximum rigidity when resetting, so that the air gap can be fully opened. According to this, the objective function constraint is unchanged except for the original (11() becomes the objective function.

4.3 Considering the highest fatigue strength as the optimization target Although taking the fatigue strength as the optimization target involves only two variables, 1 and 2, since the spring is under the action of variable stress for a long time, it is still meaningful to optimize the fatigue strength. The objective function is at this time, only (10() becomes the optimization target, and the remaining constraints are the same as before.

4.4 Multi-objective optimization calculation Combine the above-mentioned objective functions considered separately, and take the weighting factors as 1, 1, and 0.1 respectively. The weighted combination method is used to establish the optimized unified objective function: there are 9 constraints at this time, namely the aforementioned The (1()4.5 optimization result analysis in 4.1 is convenient for comparison, and the calculation results of the above four optimization targets are summarized in Table 1.

Table 1 Optimization calculation results of air gap springs Calculation condition Weight lightest stiffness Maximum fatigue strength Maximum three targets Comprehensive rounding It can be seen from the data in Table 1 that the calculation results under the three-object comprehensive condition can be directly adopted except for some adjustments. . In the improved design of the original automatic injection-blowing air passage structure, the author used this set of rounded data to commission local spring manufacturers to manufacture air gap springs in batches. The pitch of the air gap spring is taken as 7mm, and the free height on the drawing is 25mm after the two ends are flattened. Although this size reduces the actual number of tight turns from the design 1.5 to the actual 1.14, it does not affect The effect of the spring. Under the control of the air gap spring, the air gap is completely closed when the tube blank is injected; and in the blowing step, the air gap is automatically fully opened. With this new airway structure, there is no longer a bottle-unfilled or bottle-shaped skew asymmetry due to poor airway, which has been highly appreciated by users.

5 Typical parts with improved mold structure Since the air gap spring is introduced into the air passage structure of the mold, the mold itself must be modified accordingly. Of course, the outer shape and size of the mold are basically unchanged, and the biggest change is inside the mold base. In addition, the annular surface on which the shoulder of the mandrel is supported is no longer opened with a radial air groove, thereby multiplying the bearing capacity of the annular surface. The variation of the main dimensions of the mold base is given. This change facilitates the installation of the air gap spring and the preloaded screw. Except for the "16mmx1mm thread position" on the mandrel, the remaining parts are not affected by this change.

(a) Before the improvement (b) The improved main dimensions of the die holder are improved. The positioning and pre-tightening parts of the opposite ends of the comparative air gap spring are as shown.

6 Conclusion In the automatic injection-blowing mold, the optimal design of the airway structure should be: the airway is closed when the blank is injected, the high-pressure melt can not enter the airway, and the blockage can be absolutely avoided; in the blowing process, the airway should be It can be fully turned on automatically to make the airflow unobstructed. After improving the airway structure of the mold in a fully automatic plastic bottle blowing machine, the author completely eliminated the defects such as bottle filling and skewness, and ensured the bottle forming rate of 1002.

The air-gap spring of the key part of the new airway structure adopts the optimized design method. After comparing the single objective function optimization and the multi-objective function optimization result, the author believes that the multi-objective function optimization calculation result obtained by the weighted combination method is more Reasonable and feasible. These design results are fully validated by field applications.

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