Blow moulding

Abstract

Blow moulding, which is also called blow forming, is a manufacturing process for production of hollow-form plastic products. The present report is intended to deal with the effect on bottle weight, bottle dimensions and machine output of the various process factors. By reference to Rheological properties of plastics, the correlation between the share rate and screw speed, melt temperature, die swell, bottle thickness were discussed. Also, the influences of elongational viscosity were suggested.

1. Introduction

Blow moulding, also known as blow forming, is a process used to produce hollow productions by “blowing” thermoplastic molten tube into the shape of a mould cavity.

Generally, blow moulding can be divided into three main types including stretch blow moulding, injection blow moulding, and extrusion blow moulding. In this experiment, extrusion blow moulding was studied to examine the effects of process variables on bottle weight, bottle dimensions and machine output.

In extrusion blow moulding, plastic particles are melted into fluid with heat applied. Then the melt plastic is extruded through a die, forming a hollow tube, which is usually called a parison. After that, the parison is captured by closing it around a mould. Next, air is pumped in to the parison when the ends of the parison keeps sealed at the mould parting line during forming. The parison deforms, forming a shape very closed to the mould. The mould is opened after the component is well cooled. Finally the component is ejected and the procedures are repeated. More and more products are made.

In this experiment the plastic particles are High-density Polyethylene, the density of which ranges from 0.941 to 0.967 g/cm3. The High-density Polyethylene is preferred for production by blow moulding as it is more rigid and usually has a matt finish compared with Low-density Polyethylene.

2. Experimental

Equipment

Hayssen extrusion blow moulding machine

Materials

High density polyethylene (HDPE), Blow moulding grade, BS2581, Borealis.

Procedures

Firstly, HDPE particles were pumped into the hopper though a pipe. After that, the parameters of process variables and the temperatures of different zones were set according to Table 1& 2, respectively. Then the machine was set in automatic mode and continuous cycle. Each group needs 10 samples, marking 1 to 10. Before the Process Variables were changed, the weight of parison extruded per unit minute was measured. Finally weight of each bottle, was measured along with thickness distribution along the length circumference.

3. Results And Discussions

Experimental Results

The original records including weight of bottle, bottle thickness distribution, output rate and crew speed are shown in Appendix I. In addition, the calculation of share rate and modified prison length are shown Appendix II and the general results are summarized.

Experimental Discussions

Influences Of Machine Variables On Bottle Weight And Dimensions

Generally the bottle weight and dimensions is influenced by screw speed, melting temperature, and vent time.

Screw speed. By comparing group A and group C, it is clear shown that output rate increasing with the increasing screw speed due to low viscosity and high die head pressure. By referring to the rheological properties of plastic, thickness and weight of group A should be higher than that of group C, because the higher sagging brings decreasing of weight and thickness. However, the records of the experiment do not accord with the theoretical analysis. The reason is that the machine is too old.

Melting temperature. By comparing Group C and Group D, it is found that the bottles of group D are lighter and thinner than those of group C. The higher the melt temperature is, the lower the viscosity of polymer is. Lower viscosity reduces bottle weight and dimensions.

Vent time. By comparing Group A and Group B, the result is that the bottles of group B are lighter and thinner than those of group A. If the vent time is too short, it will cause insufficient cooling and less sagging.

Share Behaviours

According to the equation γ= (6Q)/ (WH²), the results of apparent share rate are given in Table 4 (all steps in calculation is shown in Appendix II).

Table 4: Apparent shear rate

Generally shear rate is related to screw speed, melt temperature, die swell and bottle thickness.

Screw speed. The output rate is proportional to the screw speed. According to the equation γ= (6Q)/ (WH ²), as the mean circumference (W) and die gap (H) are constant in this equation, the shear rate (γ) increases as output rate (Q) increases, in other words, screw speed increases.

Melt temperature. The viscosity of polymer becomes lower at higher melt temperature. Lower viscosity results in high output rate (Q), which brings out a higher shear rate (γ).

Die swell. “An increase in die swell results in a lower linear output rate. Since extrusion is usually continuous, further adjustments to the process dynamics are sometimes inevitable. Parison length sensors are available, to ease the effects of the problem”. In consequence, larger die swell results in larger output rate, which brings out larger shear rate. In one word, shear rate is proportional to die swell.

Bottle thickness. Bottle thickness is related to the viscosity of polymer. Higher viscosity results in lower shear rate. So, shear rate increase as bottle thickness decrease..

Elongational Behaviours

According to the equation ΔL= (ρgtL²)/ (2λ), the results of Modified Parison Length are (all steps in calculation is shown in Appendix II).

According to Table 5, it can be found that the longer the cycle time is, the longer the parison becomes. The result can be explained in this way: the parison will be elongated if more time is given under the gravity force.

The elongational viscosity is influenced by molecular weight and
temperature. High molecular weight and high temperature cause a decreasing of
the elongational viscosity.

4. Conclusions

Blow moulding is a manufacturing process for production of hollow-form plastic products. Process variables have the effects on bottle weight, bottle dimensions and machine output. Specifically, high screw speed, low melt temperature, and short vent time results in the increasing of bottle weight and thickness. High screw speed, high melt temperature, large die swell and thin bottle thickness lead to high shear rate. Longer cycle time results in larger parison length. High molecule weight and high temperature cause a decreasing of the elongational viscosity

References

[1] A. W. Birley, B. Haworth and J. Batchelor, Physics of plastic, Hanser, 1991

[2] Edwin G. Fisher, Blow moulding of plastics, The Plastics Institute, 1971