Conventional injection molding (CIM) and rapid heat cycle molding (RHCM) methods are used to prepare ABS and (acrylonitrile-butadiene-styrene)/glass fiber copolymer (ABS/GF) plastic parts. Pentaerythritol stearate (PETS) and silicon particles are chosen as lubricants. Effect of resin viscosity and lubricants on properties of plastic parts was investigated. The results show that resin viscosity has some effect on surface quality of plastic parts in CIM process but no effect in RHCM process. ABS/GF plastic parts molded in RHCM process exhibit higher tensile, higher flexural strength and lower impact strength. Silicon is more effective in RHCM process than in CIM process.

Keywords: injection molding, rapid heat cycle molding, properties, viscosity, lubricants

Glass fiber reinforced polymer has been of great interest to researchers for decades.^1,2 Conventional injection molding (CIM) method is an important technology to fabricate complex shaped plastic parts. However, mold temperature is controlled by circulating coolant through the cooling channels in CIM. In order to shorten the cooling stage which usually occupies more than half of molding cycle, the set mold temperature is often lower than the ejection temperature of plastic parts.³ The rather cold mold makes resin melt to freeze in advance during filling stage, which brings a series of appearance defects of plastic parts,⁴ such as weld line, flow marks, crazing defects and floating fibers, especially for glass fiber reinforced plastic parts. These defects are usually removed by using second processing technology, such as polishing, electroplating and spraying, which aggravates environmental pollution, waste of energy and increasing of cost.
Rapid heat cycle molding (RHCM) is an advanced injection molding technology and it can overcome inherent defects of conventional injection molding parts. A dynamic mold temperature control method is introduced into RHCM process. The mold cavity is rapidly heated up to the glass transition temperature of polymer before filling stage. During filling and packing stage, the mold should be always kept at high temperature in order to improve the flow ability of the polymer melt. At the end of packing stage, the mold cavity is cooled down quickly so as to freeze the polymer melt for demolding. And then the mold will be heated again for the next injection cycle. Since the mold cavity temperature is always kept at a high level during filling and packing stage, the flow ability of melt is significantly improved and the premature freezing of polymer melt during filling stage is avoided. Products made in RHCM process are usually put into practical use without painting. So RHCM process has a broad development prospect due to its environmental protection and low cost.
Recently, the researches of RHCM process are almost concentrated on heating methods of mold^5-16 and the effect of processing parameters on surface and mechanical properties of plastic parts.^17-23 Xie¹⁷ investigated the influence of processing parameters on weld line mechanical properties of polypropylene (PP) in RHCM process. It was found that the influencing significance order of processing parameters from strong to weak were mold temperature, melt temperature, injection speed, ejection temperature, packing pressure and injection pressure. The V notch size in the specimens’ middle part was larger and deeper than that in the edge. Wang¹⁸ researched the reduction of sink mark and warpage of the molded part in RHCM process. The external gas assisted packing was also proposed to reduce the sink mark in RHCM process and the warpage of plastic parts was reduced effectively. Liu¹⁹ investigated the influence of mold temperature on shrinkage of plastic part in RHCM process. Results revealed that the shrinkage of RHCM part was reduced obviously compared with conventional injection plastic part. In addition to mold temperature, both packing pressure and packing time also had important influences on the shrinkage of plastic part. Wang²⁰ found RHCM process can greatly increase the surface gloss of fiber-reinforced plastics. Li²¹ studied the influence of mold temperature on tensile strength of plastic parts in RHCM process. The relationship between dynamic mold temperature and tensile strength of plastic parts was demonstrated. Zhang²² investigated the effect of mold temperature on surface quality and mechanical properties of ABS/PMMA/nano-CaCO₃ in RHCM process. The results revealed surface quality of ABS/PMMA/nano-CaCO₃ parts was improved significantly by increasing mold cavity temperature in RHCM process. And weld line was eliminated in RHCM process. Wang²³ researched the effect of mold temperature on crystal structures, morphology of polypropylene and surface quality of plastic parts. The crystallization status in surface layer, subsurface layer and core was studied under different mold temperature in RHCM process.
In a word, the replication ability of resin matrix is the key to determine final surface quality of plastic part.^24,25 Mold temperature is one of the most important parameters which influence the replication ability of resin melt in injection process. The high mold temperature can improve the surface quality of plastic parts by improving the replication ability of resin matrix in RHCM process. However, mold temperature is not the only parameter related to the replication ability of resin matrix. The viscosity of resin matrix is another parameter related to the replication ability of resin matrix. It is of great significance to clarify the relationship between viscosity of resin matrix and surface quality of plastic parts in RHCM process. However there has been little research about the impact of viscosity on surface quality of plastic parts in RHCM process.
In addition, additives are essential in the process of injection molding. The influence of additives on the properties of plastic parts cannot be ignored. Compared with CIM process, RHCM technology involves more processing parameters and more complex process control. The high temperature of cavity wall in filling stage changes the filling process of resin melt and the RHCM forming mechanism is quite different from CIM’s. Whether the effect of additives in RHCM process is the same as it in CIM process is not clear.
In this paper, SAN (styrene acrylonitrile) was incorporated into ABS based on twin screw compounding in order to decrease the viscosity of ABS and then the LV-ABS (low viscosity ABS) was prepared. Then the effect of rein melt’s viscosity on surface and mechanical properties of polymer plastics in RHCM and CIM processes was investigated. Lubricant is one of the most important additives in glass fiber reinforced polymers. Pentaerythritol stearate (PETS) and silicon particles were chosen as lubricants for ABS and ABS/GF in our study. Then the effect of PETS and silicon on properties of ABS and ABS/GF plastic parts in RHCM and CIM process was studied.