Abstract The effects of hot runner system on injection moulding process and properties of injected part have been investigated from various aspects in comparison with conventional runner system. A two-cavity mould which can be used for both of the runner systems was designed and produced so as to conduct all experiments on the same mould, and to compare the results precisely. In the experiments, acrylonitrile butadiene styrene and polypropylene materials were used as base polymers. Using the results obtained from the experiments, moulding area diagrams were plotted for both runner systems with respect to the process parameters of injection pressure and melt tem- perature changing within a wide range. It was observed that the required injection pressure was considerably lower to produce samples with higher weight in the case of hot runner system. The shrinkage and warpage increased with increasing process temperature, decreased with increasing injection pressure, and occurred at low level when the sample weight was high.67742

© 2006 Elsevier Ltd. All rights reserved.

Keywords:  Polymer (A); Moulding (C); Shrinkage (E); Runner  systems

1. Introduction

The type of the runner system is one of the most influen- tial factors on injection moulding process and the proper- ties of the injected parts. Conventional runner systems (CRS) have some disadvantages such as high cost of energy and workmanship, high scrap ratio, low product quality of surface appearance and requirement of high injection pres- sure. Therefore, the mould designers are attracted to hot runner system (HRS) which is able to provide precisely adjustable process temperature, uniform filling in multi- cavity moulds, even heat distribution within the mould, improvement on mechanical properties of injected  part, and reduction in injection pressure. In addition, HRS allows significant cuts in production costs by saving  mate-

*   Corresponding author. Tel.: +90 264 3460353; fax: +90 264  3460351.

E-mail address: aokapti@sakarya.edu.tr (A.O.  Kapti).

rial as a result of eliminating sprue, shorter mould opening distance because of the absence of sprue, and shorter cycle time. Moreover, it prevents undesirable track of cavity gate [1].

Defects of the injection-moulded part, such as shrink- age, warpage, weld line, sink marks and residual stress, generated by unfavourable process conditions, have great influence on the quality and accuracy of the part. There- fore, effective control of the influential factors is a must. The effects of process conditions on the properties of injected parts have been investigated by several studies. Bushko and Stokes [2] investigated the effects of the mould and melt temperatures and packing pressure on the shrink- age. They reported that the packing pressure has an impor- tant effect on the shrinkage of the injection moulded part. They showed that the shrinkage decreases with increasing packing pressure, and increases with increasing mould tem- perature. Jansen et al. [3] reported the packing pressure and

0261-3069/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.03.015

1468 A. Demirer et al. / Materials and Design 28 (2007) 1467–1476

the melt temperature as the major influential process parameters on dimensional change. Huang and Tai [4] reported that the influential process parameters on war- page are packing pressure, mould and melt temperatures, and packing time, citing in the order of importance. They regarded the filling time  and  gate  dimension parameters as non-significant factors. Liao et al. [5] emphasized the necessity that the geometrical effect of real commercial parts on quality and interaction between the process parameters should be considered in investigating the effect of process parameters on shrinkage and warpage.  They also reported that the mould temperature should be consid- ered among the influential parameters on shrinkage and warpage because of causing the polymer inside the mould cavity to cool down in a higher temperature environment in a longer time. Jafarian and Shakeri [6] investigated the effects of packing pressure in the case of the gate freezing before complete solidification. Their research showed that the molten plastic cannot be added to compensate for shrinkage when gate freezing occurs before complete solid- ification. Thus, shrinkage will increase compared to a fully packed item. They also showed that higher relaxation time leads to less shrinkage and results in lower residual stress. According the literature review, it is noticed that there have been some studies investigating the effects of process parameters on the dimensional and geometrical quality of injected parts. In this research, the effect of HRS on injec- tion moulding process was experimentally investigated in comparison with CRS,  by  conducting  many experiments at different process conditions on a two-cavity  mould which is adaptable to both runner systems. During these experiments relating both HRS and CRS, the clear effect of the runner systems on the properties  of  injected part was determined by keeping the values of mould tempera- ture, injection velocity, gate dimension and cycle time con- stant. The value of packing pressure was determined to be half of the injection pressure. By considering the measure- ments taken from the samples produced in the experiments, the process conditions (i.e., injection pressure, packing pressure and process temperature) minimizing the shrink- age and warpage, and maximizing the weight of the sam-