Effect of Molding Parameters on Young’s Modulus of an Injection Molded Low-Density Polyethylene (LDPE) Ce ́ sar Leyva-Porras, †,‡ Miguel A. Esneider-Alcala ́ , † Alberto Toxqui-Tera ́ n, † Alfredo Ma ́ rquez-Lucero, § and Josue ́ A. Aguilar-Martínez* ,† † Centro de Investigació n en Materiales Avanzados S.C. Unidad Monterrey, Alianza Norte 202 Parque de Investigació n e Innovació n Tecnoló gica (PIIT), Apodaca, N. L., Me ́ xico, C.P. 66600 ‡ Centro de Investigació n en Materiales DIP-CUCEI, Universidad de Guadalajara, Av. Revolució n # 1500, Col. Olímpica, Guadalajara, Jal., Me ́ xico, C.P. 44430 § Centro de Investigació n en Materiales Avanzados S.C. (CIMAV), Av. Miguel de Cervantes # 120, Complejo Industrial Chihuahua, Chih., Me ́ xico, C.P. 31350 ABSTRACT: Injection molding is a process employed worldwide to manufacture polymer parts. The final properties of the molded part largely depend on the processing conditions used during the manufacturing process. Therefore, it is necessary to develop empirical approaches that help to understand the relationship between the processing conditions and the final properties of the polymer. In this paper we study the effect of the processing conditions of the injection molding process on the Young’s modulus of a low-density polyethylene (LDPE). The effect of both the barrel temperature and the mold temperature was investigated using analysis of variance (ANOVA) and the effect of the levels of each parameter was examined using the surface response methodology (SRM). The ANOVA results showed that the mold temperature is the parameter that most significantly impacts the Young’s modulus, followed by the barrel temperature, while the combined interaction of both is negligible. SRM showed that the Young’s modulus increases with the mold temperature and decreases with the barrel temperature. Based on the SRM, an empirical equation is proposed which can be used to predict the modulus employing only the barrel and mold temperatures. The changes in the microstructure of the injection molded part are discussed in terms of the crystallinity degree. All this was corroborated with X-ray diffraction (XRD) and differential scanning calorimetry (DSC). 1. INTRODUCTION Low-density polyethylene (LDPE) is a semicrystalline thermo- plastic widely used due to its properties such as thermal, chemical, and mechanical resistance. Usually it is manufactured in products including films, bottles, and other molded parts. 1-4 Due to its good processability and flexibility it is molded by injection, extrusion, and blowing. Of these processes, injection molding (IM) is one of the industrially preferred methods for manufacturing polymeric products due to the high degree of automation, high production rates, and dimensional stability of the molded parts. 5-7 During the IM cycle the polymer undergoes both thermal and mechanical effects through the interaction with the geometry of the IM machine (barrel, nozzle, and mold cavities) and the molding conditions (temperature, pressure, and time). 7 IM process of thermo- plastics basically consists of four stages: filling, packing/holding, cooling, and ejection. 8-10 From these stages, cooling is crucial since in this step the solidification of the polymer occurs. During solidification, the microstructure of the polymer is developed producing regions with different arrangement within the molded part. 11 Likewise, semicrystalline polymers com- monly present two phases well distributed: amorphous and crystalline regions. The crystalline region typically consists of crystal lamellae of folded chains, whereas in the amorphous region, polymer chains are arranged randomly. 12 Because for a semicrystalline polymer the final properties greatly depend on the relative amount and distribution of these two phases, it is possible to modify the degree of crystallinity to tailor those properties; 13,14 where a high degree of crystallinity results in a harder, stiff, and less ductile behavior. In this way, it is also possible to achieve an improvement in the mechanical properties by simply changing the processing variables during the IM process. 9,14-18 When these processing variables are changed, the microstructure of the polymer is affected, 19 which in turn can be quantified by means of mechanical testing and moreover by determining the Young’s modulus. The relationships between the microstructure and the mechanical properties of injection molded polymers have been widely studied and the main results indicate an increment in the mechanical response as the degree of crystallinity in- creases. 14,18,20-23 Therefore, it is of practical significance to understand the development of the microstructure during the IM of semicrystalline polymers, which is also important for optimizing processing variables such as injection rate, melt temperature, mold temperature, packing pressure, and holding time. 11,14,24-27 It is necessary to develop engineering procedures and relationships that even with empirical and phenomenological approaches can contribute to solve that issue. 17,20 The purpose of the present work is to optimize the processing parameters of an injected molded LDPE semi- crystalline polymer and relate the effects of these conditions on Received: November 26, 2012 Revised: February 23, 2013 Accepted: March 19, 2013 Published: March 19, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 5666 dx.doi.org/10.1021/ie3032422 | Ind. Eng. Chem. Res. 2013, 52, 5666-5671