Improvement in Power Efficiency of Injection Molding Machine by Reduction in Plasticization Losses

1. Introduction

Injection molding is one of the most commonly used manufacturing processes for producing identical parts(Muhammad Khan, 2014)(Muhammad Khan 2014). Injection molding machines have been in use since 1872 for batch production of parts having complex geometries. A typical injection molding machine has two units; an injection unit and a clamping unit. Injection unit consists of a hopper, a rotating screw and a heated barrel, whereas the clamping unit contains mold base (Figure 1).

Figure 1.

Reciprocating screw injection molding machine

Once the injection temperature is reached, molten plastic is ready to be molded; this melting of polymer is called as plasticization(Zheng, Tanner, & Fan, 2011). This molten plastic is pushed into the molds by forward motion of a screw where it cools down to ejection temperature. The part is taken out once it solidifies and this process continues.

2. Literature Review

Traditionally, barrel heating is done by wrapping heater bands around the barrel. The heat of fusion is conducted to the plastic material through barrel, which melts the polymer to injection temperature. For high quality parts the control of these heating bands is very necessary. Different types of heaters are used for the plasticization including cast aluminum heaters, calrod electrical elements, and ceramic and mica heaters(Rosato, Rosato, & Rosato, 2000).

Researchers have worked to optimize the injection molding process through Design of Experiments (DoE). Variation in process parameters should be reduced for high quality parts and increased reliability. Surface-quality, flow length and the aspect ratio of the end product is influenced by six main process parameters i.e. barrel temperature, mold temperature, injection speed, holding pressure, the existence of air evacuation and the width of micro-legs. The results have revealed that the barrel temperature and injection speed are the most significant process parameters. The proper control and reduced variations in these two parameters will lead to high quality parts(Packianather, Chan, Griffiths, Dimov, & Pham, 2013).

Alireza et al. has researched on dimensional changes in solid part due to shrinkage and warpage. They have used the existing data to investigate the effect of injection molding parameters on the dimensional changes in polypropylene (PP) and polystyrene (PS). Statistical methods i.e. regression methods and Analysis of Variance (ANOVA) technique are used to study relationship between input and output of the process. They studied four process parameters i.e. melting temperature, injection pressure, packing pressure and packing time. During the study, it was found that these parameters have significant effect on warp-age and shrinkage of parts. So use of optimum values and control of parameters will result in better quality parts(Akbarzadeh Tootoonchi, 2011).

Generally, six thermocouples are mounted on the barrel surface to measure melt temperature and maintain the temperature of heaters (Figure 2). The temperature settings of the heaters is called as temperature profile and there are four different temperature profiles that can be used for injection molding i.e. horizontal, ascending, descending and ascending with descending at the nozzle (Table 1)(Goodship, 2004).

Figure 2.

Placement of heaters on barrel