Study on process parameters for filling process of polypropylene microcellular injection molding under supercritical N2

doi:10.19491/j.issn.1001-9278.2022.03.012

Abstract

Abstract:

Employing polypropylene as a raw material and supercritical nitrogen gas as a foaming agent， a flat spiral linear runner mold was designed， and the mold filling length and weight loss ratio of the molded part and the relationship between the cell structures were investigated. An orthogonal experiment was designed to study the effects of gas injection pressure difference， gas injection time， injection pressure， and injection speed on the mold filling length. The results indicated that in a certain range， a higher melt back pressure resulted in a longer filling length for the injection?molded part. The weight loss ratio of the part increased slowly at first and then rose drastically with an increase in filling length. Meanwhile， the number of bubbles was less near the gate， and these bubbles presented a flat ellipsoidal structure. In the middle position， the number of bubbles increased along with a uniform distribution. Their overall appearance presented a relatively regular spherical structure. At the location far from the gate， the cells are very dense. However， the cell merge and rupture phenomenon are serious， and the overall size distribution is ununiform. Based on the orthogonal experiment， the filling length of the part was found to increase with an increase in gas injection pressure difference， injection time， injection pressure， and injection speed.

Key words: polypropylene, supercritical nitrogen, microcellular injection molding, spiral linear runner mold, mold filling length, weight loss ratio, cell structure, process parameter

CLC Number:

TQ320.66⁺2

ZHAI Yujiao, XIN Chunling, HE Yadong, YAN Baorui, QIAO Linjun. Study on process parameters for filling process of polypropylene microcellular injection molding under supercritical N₂[J]. China Plastics, 2022, 36(3): 69-74.

Figures/Tables 11

Fig.1. The flat spiral linear runner mould

注气时间/

注射速率/

~

注气压差/MPa

注气时间/

注射压力/MPa

注射速率/

熔体背压/MPa

0.5

~

Tab.1. Main experiment parameters setting

水平	因素
水平	注气压差（A）/MPa	注气时间（B）/s	注射压力（C）/MPa	注射速率（D）/%
1	0.5	1	75	20
2	1.0	2	105	55
3	1.5	3	135	90

Tab.2. Horizontal design of orthogonal experimental factors

Fig.2. The sampling positions of samples 1~20 at different melt back pressure

Fig.3. Product filling length as a function of melt back pressure

Fig.4. Product gas injection volume as a function of melt back pressure

取样位置	充模长度/mm
1、6、11、16	15.000
2、7、12、17	133.728
3、8、13、18	337.061
4、9、14、19	487.099
5、10、15、20	655.542

Tab.3. Under different melt back pressure conditions， the corresponding filling length of sample No.1~20 at sampling position

Fig.5. Product weight loss ratio as a function of filling length

Fig.6. SEM of PP as a function of filling length

Fig.7. Diagram of relationship between filling length and influencing factors of products

Fig.8. Diagram of relationship between filling length and interaction factors of products

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Study on process parameters for filling process of polypropylene microcellular injection molding under supercritical N₂

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References 18

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