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Matthews, S., Toghyani, A. E., Eskelinen, H., Kärki, T., and Varis, J. (2015). "Manufacturability of wood plastic composite sheets on the basis of the post-processing cooling curve," BioRes. 10(4), 7970-7984.

Abstract

Extruded wood-plastic composites (WPCs) are increasingly regarded as promising materials for future manufacturing industries. It is necessary to select and tune the post-processing methods to be able to utilize these materials fully. In this development, temperature-related material properties and the cooling rate are important indicators. This paper presents the results of natural cooling in a factory environment fit into a cooling curve function with temperature zones for forming, cutting, and packaging overlaid using a WPC material. This information is then used in the evaluation of manufacturability and productivity in terms of cost effectiveness and technical quality by comparing the curve to actual production time data derived from a prototype post-process forming line. Based on this information, speed limits for extrusion are presented. This paper also briefly analyzes techniques for controlling material cooling to counter the heat loss before post-processing.


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Manufacturability of Wood Plastic Composite Sheets on the Basis of the Post-Processing Cooling Curve

Sami Matthews,a,* Amir E. Toghyani,a Harri Eskelinen,a Timo Kärki,b and Juha Varis a

Extruded wood-plastic composites (WPCs) are increasingly regarded as promising materials for future manufacturing industries. It is necessary to select and tune the post-processing methods to be able to utilize these materials fully. In this development, temperature-related material properties and the cooling rate are important indicators. This paper presents the results of natural cooling in a factory environment fit into a cooling curve function with temperature zones for forming, cutting, and packaging overlaid using a WPC material. This information is then used in the evaluation of manufacturability and productivity in terms of cost effectiveness and technical quality by comparing the curve to actual production time data derived from a prototype post-process forming line. Based on this information, speed limits for extrusion are presented. This paper also briefly analyzes techniques for controlling material cooling to counter the heat loss before post-processing.

Keywords: Cooling rate; WPC post-processing; Forming; Cutting

Contact information: a: Lappeenranta University of Technology, Laboratory of Manufacturing Engineering, Skinnarilankatu 53850, Lappeenranta, Finland; b: Lappeenranta University of Technology, Laboratory of Composite Materials, Skinnarilankatu 53850, Lappeenranta, Finland;

* Corresponding author: sami.matthews@lut.fi

INTRODUCTION

Extruded wood-plastic composite (WPC) sheets are one type of WPC semi-products that have gained industrial interest because of their good environmental impact, together with their reasonable mechanical properties. WPC as a term covers a wide range of composite materials made from different plant fibers using thermosets or thermoplastics as the bonding matrix. WPCs have become a well-studied development in recent years because of the environmental restrictions set on purely polymer-based materials. This is advantageous for WPCs, as it is now possible to use recycled fiber sources and find new uses for waste otherwise going to landfills. In comparison to wood, Klyosov et al. (2007) lists lower maintenance requirements, including no need for staining, sealing, and painting; higher resistance to termites and microbes; the absence of knots and splinters; and environmentally friendly characteristics.

This paper is part of a bigger research project investigating the post-processing of WPCs. Post-manufacturing means material processing after material fabrication by extrusion. This development involves three integrated viewpoints: material, quality, and process, which all influence the end success of the production. Ignoring one area would cause the other areas to fail in the technical or economical aspect. In general, there are two ways to improve the manufacturing stage: optimizing the product and optimizing the manufacturing process. In addition, the development of the manufacturing process is often based on an evaluation of economic and technical aspects. The economic aspects are directly related to the length of the manufacturing stages, which in this case are directly linked to temperature changes and cooling. The longer the product takes to cool, the more space and time is required. On the other hand, if the cooling is too rapid, the temperature will fall below the formable area prematurely and the quality of the formed product will suffer. On the basis of this information, it is important to be able to control the material temperature during this process. In technical aspects, cooling is relevant for the manufacturing stages that demand tight tolerances, such as cutting and forming; therefore, these two manufacturing methods are used as references in evaluating the effects of the material cooling rate. Having the material at the optimal temperature makes it possible to achieve higher productivity and product quality. The aim of this paper is to verify the operation of the post-process production line of an unheated wood plastic composite in terms of material temperature during the post-process stage.

The material investigations are based on actual cooling experiments in a factory-like environment, as the primary aim is not to check the material properties in isolated space, but to check the material behavior in a real production environment. In this study, an example material labelled simply as the composite material is used. This material was selected because it has good formability characteristics and is similar to the commercial materials used in the WPC industry.

Sonmez and Eyol (2002) have studied the optimal post-manufacturing cooling paths for thermoplastic composites. Their purpose was to determine the optimal cooling scheme to minimize the processing time during the cooling stage in press molding. They noticed that optimal cooling could be analyzed by utilizing heat transfer analysis and the temperature profiles through the thickness of the composite plate and that the heat transfer analysis was mostly connected to the temperature-related material properties of the polymer used in the composite. However, as Sonmez and Eyol focused on reducing the processing time in molding, the viewpoint concerning specific manufacturing stages was not included in their optimization model. This paper considers the special aspects related to optimizing the forming and cutting stages.

Previous research has shown that WPCs as polymer-based materials are greatly dependent on the material temperature and the matrix polymer type and that the performance of high-density polyethylene (HDPE) and HDPE-based composites is strongly dependent on the processing time and temperature of manufacturing (Yang et al. 2013). It is said that the cooling rate of polymers is low because of their poor thermal conductivity (Tan et al. 2012). The thermal expansion of WPCs under elevated temperatures is a well-known phenomenon, and according to Yang et al. (2013), the linear coefficients of thermal expansion-contraction for wood are significantly lower than those of plastics and WPCs. The values for wood are independent of temperatures between -51 and 130 °C. Because HDPE is a continuous phase in WPCs, linear expansion and contraction are related to polymer molecules. When treated at elevated temperatures, polymer molecules begin to move and steadily reach their thermodynamically settled state, resulting in composite expansion. This, in addition to absorption of water, is another reason for WPC products to expand. In addition, shrinkage of WPCs has an important effect during post-processing. Shrinkage happens when a plastic-based board, extruded and pulled from the die, cools too fast. Too fast means that the stretched long polymer molecules coming from the die do not have enough time to return to their thermodynamically favorable coiled form (Klyosov 2007).

Sonmez and Eyol (2002) have presented also another relevant viewpoint, which is similar or at least analogic with the WPC studied in this paper. They have noticed that residual stresses can have a significant detrimental effect on the performance of composite structures by causing defects, e.g., void formation during solidification, reducing strength, and initiating cracks. They underline that the residual thermal stresses should be within tolerable limits to ensure reliability during the use of the product. The tentative observations with the tested WPC showed that under impact forming, the fully cooled product seemed too brittle. According to Wijskamp (2005), who has studied the processing of thermoplastic composites, depending on the initial process temperature, the composite sheet can experience a thermal shock as soon as it is pressed between the pressing tools, as it is rapidly cooled from the outside inwards. The phase formations of polymers in the composite depend on the cooling rate. According to Wijskamp, the selected cooling rate during the thermoplastic composite processing can affect both the shrinkage of the material and its mechanical properties.

Brucato et al. (2002) state that investigating polymer solidification under processing conditions has become a necessary step to predict the final polymer properties. As solidification in industrial processes often involves flow fields, high thermal gradients, and high pressures, the development of a model able to describe polymer behavior turns out to be really complex. The same situation was met with the WPC process studied in this paper. Another analogical aspect can be found in the injection molding of polymers. According to Liu and Gehde (2015), heat transfer is one of the most important segments in injection molding because it significantly affects the temperature distribution of the component and alters the temperature distribution in the mold, thereby affecting the mechanical behavior and dimensional precision of the plastic component, as well as production efficiency. In our case, the question is not of the first molding stage, but the same phenomena are present during the post-processing stage, in which the WPC plate is at an elevated temperature and is pressed into its final geometry.

The temperature-related phenomena highlighted in previous studies (Sonmez and Eyol 2002; Wijskamp 2005; Tan et al. 2012; Yang et al. 2013) needs to be taken into account when designing a successful manufacturing process for extruded WPCs, indicating the importance of monitoring the temperature and controlling the cooling in the development of manufacturability.

EXPERIMENTAL

In this paper, the cooling rates of WPC sheets with 3 mm thickness with a composition of 44% wood fiber, 50% polyethylene (PE), 3% coupling agent, and 3% lubricant are investigated, as the sheet geometry and the material properties offer promising possibilities for multiple post-production techniques; as a polymer-based material, WPCs have the potential problem of premature cooling at room temperature during post-processing. To measure the cooling rate, the specimens were located in a laboratory space at 21 °C. This location represents a typical factory space. In this kind of uninsulated open system, the WPC can transfer heat to the surroundings by conduction, heat gradient-induced convection, or radiation of heat. Conduction was prevented, but heat gradient-induced convection and radiation were unrestrained, similar to a real production environment. In a typical industry setting, the material is often conveyed over metal rollers, which conduct heat. This heat loss was simulated by having a metal grille under the specimens, as shown in Fig. 1.

Because an online extruder could not be used during the study, prefabricated WPC sheets were used. To simulate the post-process, the sheet was placed in an electrical oven at 150 °C (+-3 °C) for 15 min. The temperature was measured using a National Instruments USB-TC01 transducer and included J-type exposed thermocouple. The measurements started 10 s after the sample was taken out of the oven. The temperature was measured at the center and 10 mm from the edge of the specimen. The size of the specimen was 300 mm2 with 3 mm thickness, as shown in Fig. 1.

C:\Users\matthews\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Outlook\B5PMO2Y3\WP_20150218_13_18_50_Pro.jpg

Fig. 1. Heat sensor attached and temperature measured from the edge of a WPC sheet

The detailed analysis of the measured cooling rate curves made it possible to use mathematical tools to estimate the best curve fitting to describe the behavior of the material. Further on, it will be possible to find the critical time and temperature values to show the areas in which the forming and cutting process should be made. The curve fittings can be made either for the whole set of measured values or stepwise to strengthen the importance of the time period when the cooling starts. The curves were fit using the curve fitting toolbox version 3.3.1 in MathWorks MATLAB 2013a.

Although fibers, coupling agents, and lubricant play a large part in the material properties of WPCs, the overall material properties are not dramatically changed by these ingredients at different temperatures (Klyosov 2007), and they were therefore not investigated in this study. For example Stokke et al. (2014) stated that dry wood fibers in WPCs endure thermal degradation only at temperatures over 200 °C. The scope was the thermal behavior of the polymer part, and if there was no direct information available on the behavior of WPCs, information available on pure HDPE polymers was used.

During the preliminary testing of the composite material, visible cracks started to form at a temperature proximate to the melt temperature, as seen in Fig. 2.

Fig. 2. The effects of temperature on product quality. The test specimens were heated from 150 and 125 °C before pressing. At 125 °C, visible cracks were observed and the surface finish was rough in comparison to the specimen at 150 °C.

This finding supports the idea that the melt temperature can be marked as a reference for the production. The extrusion temperature of the composite material was 170 to 190 °C, while the typical melt temperature of HDPE is at 130 to 137 °C (Askeland et al. 2009).

For further information, the flexural modulus of the composite material was tested with a three-point system where a weight was positioned at the center of the specimen and the temperature of the environment was increased slowly while the displacement was measured. The measurement could only be reasonably measured up to 120 °C because material creep behavior was observed to noticeably distort the results. The dashed line in Fig. 3 represents the extrapolation of the measured curve.

Fig. 3. Measured flexural modulus of the composite material in terms of temperature. The dashed line represents the temperature where the material started to have significant creep behavior, extrapolated from the measured results.

Forming and cutting were selected as different stages in the evaluation of the cooling, as they are heavily temperature-influenced operations. The technique in forming WPCs resembles compression molding closely, and it was used as a reference at this stage. Stokke et al. (2014) stated that compression molding is a process in which a heated polymer is compressed into a preheated mold, taking the shape of the mold cavity and cured by heat and pressure applied to the material. A pre-weighed amount of a polymer mixed with additives and fillers is placed into the lower half of the mold. The upper half of the mold moves downward, pressing on the polymer charge and forcing it to fill the mold cavity. Long (2007) list two possible methods for forming material analogical to WPCs, a thermoplastic composite (TPC). The two methods are isothermal and non-isothermal. In the isothermal method, both tools are heated over the melt point of the desired material, while in the non-isothermal method, both tools are kept cold. Long (2007) list the advantages of the non-isothermal process to be a shorter cycle time and improved productivity over the isothermal process; therefore, it was used in this study.

In the cutting operation, a flat product is punched or trimmed out from the material using the shear force of tools. This process can be either matched metal punching with matched die molds or done using a steel rule die. Engelmann (2012) lists better accuracy of products and larger production volumes possible with matched metal punching, and it was used in this study. The total process time was divided into specific timeslots in each process stage to determine the optimal temperature for each stage. Similar partial analysis of the cooling curve is presented also in Sonmez and Eyol (2002), where the time intervals were established between specific adjacent key points describing the progress of the manufacturing process.

A prototype production system called LUT KompoLine shown in Fig. 4 was used in the evaluation of manufacturability. The system consists of two moving press units, each consisting of an Exlar model GSX60-1005 electric actuator for pressing with 55 kN of press force and a Tecnotion linear motor TL12 with 1 kN of linear force for moving the press units. Both units move on a 2-m magnetic track. The press units work in the flying shear principle, in which both units steadily follow a constantly moving raw material web.