Manufacture of polyurethane foam parts for automotive industry using FDM 3D printed molds

https://doi.org/10.1016/j.cirpj.2021.01.019Get rights and content

Abstract

Metal molds have traditionally been used in the manufacture of polyurethane foam parts. This type of molds presents several disadvantages: have a high cost; need a long time for their manufacture; involve the use of release agents during the demolding operation. As an alternative, the use of molds manufactured by fused deposition modeling is proposed. These 3D printed molds have a poor surface finish, which complicates the demolding operation. To overcome this handicap, materials that can be chemically polished have been studied: acrylonitrile butadiene styrene and high impact polystyrene. Acrylonitrile butadiene styrene can be polished by exposure to acetone vapors and high impact polystyrene by immersion in limonene. This post-processing operation allows to reduce the surface roughness of the 3D printed mold. To simulate the industrial use of the mold, seven molding-demolding cycles have been performed. To measure the ease of demolding of each material, a pull-off test has been used. The results indicate that high impact polystyrene has less affinity with polyurethane foam than acrylonitrile butadiene styrene. In addition, although release agent is used in cycle 1, with high impact polystyrene specimens, it is not necessary to use it in cycles 2, 3 and 4; this means a saving of release agent. Furthermore, the chemical polishing of high impact polystyrene by immersion in limonene has several advantages: it is performed in a short time; it allows to obtain an excellent finish regardless of the initial surface roughness; it is done using a biodegradable solvent of organic origin. To experimentally demonstrate the viability of the proposed solution, a mold for the back of a car seat has been printed in high impact polystyrene via fused deposition modeling and chemically polished by immersion in limonene (1 min); the geometrical deviations generated were less than 1 mm.

Introduction

The polyurethane (PUR) foam parts is commonly used in the manufacture of everyday items (chairs, sofas), for elements related to rest (mattresses, pillows) or leisure (chairs for cinemas or theatres), in the construction sector (as insulation in roof and facade panels) and in the transport sector (car seats) [1], [2], [3], [4], [5].

The foam is obtained after the reaction of product A (polyol) and product B (isocyanate) [6]. Once mixed, a mold is needed to obtain the final shape. Traditionally, metal molds have been used for this purpose; however, the manufacture of foam parts in metal molds presents several handicaps. On the one hand, metal molds are very expensive, both environmentally and economically: metal molds are produced by milling prismatic blanks [7]; in addition to raw material costs, this operation usually requires many machining hours [8], [9], [10] and a large amount of energy [11]. After the machining stage, there is also a polishing stage, which is usually done by hand [12]. The total cost of the metal molds is amortized only for long batches, so it is necessary to look for new ways to manufacture prototypes or short series [13]. On the other hand, polyurethane foam presents a high fixation to metal molds; to avoid problems during demolding operation, release agents should be used [14]. Most of these agents are very contaminating [15], so the reduction of release agent application is a topic widely studied in the recent literature [5], [14], [15], [16], [17], [18].

Today, an alternative to metal molds is 3D printed molds [19], [20]. Additive manufacturing is increasingly used in the industry for tool (rapid tooling, RT) and fixture manufacturing [21], [22]. RT has several advantages [23]: it allows direct manufacturing of the element; the manufacturing cost is low compared to other technologies; the investment pays for itself quickly, even with small batches; there is full material utilization (as opposed to chip removal processes); the energy consumed for small batches is less than with conventional manufacturing processes [24]; it allows up to ten times more iterations in a six-month period during the development stage [25].

Fused deposition modeling (FDM) is one of the most widely used 3D printing techniques [26]. FDM is used in the industry to manufacture all types of molds and tools [27]. However, there is hardly any work that studies the use of FDM technique in the manufacture of molds for PUR foam parts. This fact may be caused by the rough surface finish obtained on FDM printed parts [28].

The work of Martens at al. [29] is the only work found in the literature about the use of FDM 3D printed molds for PUR foam casting. They have used molds printed on polyphenylsulfone for centrifugal casting of polyurethane. One of the conclusions reached by these authors is that a suitable surface roughness of the mold must be achieved to avoid problems during the demolding operation. An excessive fixation of the foam to the walls of the mold usually means that the PUR manufactured part is discarded and obliges cleaning work to be carried out on the mold to be able to continue with its use. To improve the surface finish, Martens et al. sanded the inside of the mold. This mechanical polishing process requires equipment, qualified personnel, time, and a consumption of electrical energy.

Certain materials used in FDM support chemical polishing processes [27]: acrylonitrile butadiene styrene (ABS) can be polished by acetone vapors [30]; high impact polystyrene (HIPS) can be polished by immersion in limonene [31]. These post-processing treatments improve significantly the surface finish of FDM 3D printed parts [32], do not require mechanical equipment or qualified personnel, and do not consume electrical power.

The authors propose the use of FDM 3D printed molds for PUR foam casting. For this purpose, specimens have been printed with different surface roughness, using two different materials (ABS and HIPS). Once chemically polished, the specimens have been exposed to several moulding and demoulding cycles. The fixation of the PUR foam has been measured using a pull-off test [16], [33]. The objectives of the work are: (i) evaluate what time is necessary to carry out the chemical polishing of the ABS and HIPS specimens and what surface roughness is obtained in every case; (ii) measure which material has the highest affinity with PUR foam, via pull-off test; (iii) assess the number of molding-demolding cycles that can be performed in each case, without the need to use a release agent; (iv) experimentally check the results obtained by manufacturing a scale model of a car seat back.

Section snippets

Materials and methods

The methodology followed in the work is shown in Fig. 1. Initially, 27 prismatic specimens (25 × 25 × 1 mm3) were printed with each of the materials (ABS and HIPS), following an orthogonal experimental design, with three factors and three levels (Table 1). The design of experiments (DOE) was only used to generate 27 test specimens with different surface roughness.

The specimens were designed using SolidWorks. Once the STL file was generated, the G-code program for the 3D printer was obtained using

Results and discussion

The average values of Ra for each specimen are presented in Table 3. As can be seen, the specimens with the best surface finish are number 2 and number 11, for both ABS and HIPS specimens. In contrast, the specimen with the worst surface quality is number 27. Specimen number 18 has been included in the study because it presents intermediate values of surface roughness. Table 3 also includes the time invested in printing each specimen. The specimens with the best surface finish (2 and 11) take 14

Conclusions

This paper proposes the use of 3D printed molds using fused deposition modeling technique for the manufacture of polyurethane automotive parts. According to the literature, one of the most important factors to achieve a good demolding of the foam part is the surface roughness of the mold. Therefore, two materials that can be chemically polished have been tested: acrylonitrile butadiene styrene and high impact polystyrene. On printed and polished specimens, several molding-demolding cycles have

Funding

This research was funded by University of Cordoba, through its Plan Propio de Investigación (2018–2020).

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgments

The authors would like to thank Grupo Copo for having ceded the materials to make polyurethane foam for experiments. The authors would also like to thank Smart Materials 3D for their help in providing filament material to print the test specimens used in the tests.

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