Materials Science and Engineering A301 (2001) 125–130
`
`www.elsevier.com:locate:msea
`
`Short fiber reinforced composites for fused deposition modeling
`
`Weihong Zhong a,*, Fan Li a, Zuoguang Zhang a, Lulu Song a, Zhimin Li b
`a Department of Materials Science and Engineering, Beijing Uni6ersity of Aeronautics and Astronautics,
`Beijing 100083, People’s Republic of China
`b Materials Engineering Program, Auburn Uni6ersity, Auburn, AL 36849, USA
`
`Received 28 April 2000; received in revised form 22 September 2000
`
`Abstract
`
`Addressed in this paper are critical material property issues related to the short fiber reinforced composite used in rapid
`prototyping and manufacturing (RP&M). Acrylonitrile–butadiene–styrene (ABS) copolymer has been a popular choice of
`material used in fused deposition modeling (FDM), a commonly used RP&M process. However, conventional ABS polymers in
`the filamentary form for FDM are known to be of low strength and hardness. In order to overcome this deficiency, ABS was
`modified by incorporating several different property modifiers including the short glass fiber, plasticizer, and compatibilizer. Glass
`fibers were found to significantly improve the strength of an ABS filament at the expense of reduced flexibility and handleability.
`The latter two properties of glass fiber reinforced ABS filaments were improved by adding a small amount of plasticizer and
`compatibilizer. The resulting composite filament, prepared by extrusion, was found to work well with a FDM machine. © 2001
`Elsevier Science B.V. All rights reserved.
`
`Keywords: Composite; Fused deposition modeling; Short glass fiber; ABS
`
`1. Introduction
`
`Low-cost composite manufacturing technology has
`been an important research topic in the area of com-
`posite materials. A wide variety of composite process-
`ing methods have been developed, but most of these
`methods are of a long process cycle, laborious, and:or
`energy-intensive. The resulting high process costs have
`significantly constrained the scope of application for
`composites. Obviously, new and more effective manu-
`facturing technologies for composites are highly desir-
`able [1].
`Rapid prototyping and manufacturing (RP&M) rep-
`resents a group of novel manufacturing methods that
`entail building a 3-D object point by point and:or layer
`by layer. These methods are also commonly referred to
`as layer manufacturing (LM) and solid freeform fabri-
`cation (SFF) methods. A RP&M method normally
`begins with creating a computer aided design (CAD) or
`solid modeling file (e.g. in terms of the .STL format) to
`
`* Corresponding author. Tel.: (cid:27)86-10-82317122; fax: (cid:27)86-10-
`82317127.
`E-mail address: katiehong–2000@yahoo.com (W. Zhong).
`
`represent the object geometry, slicing this file into a
`multiple-layer data format (e.g. a CLI file format), and
`converting this layer-wise data into proper numerical
`control codes (e.g. CNC G- and M-codes). These codes
`are then used to control the X–Y–Z movements of a
`material-depositing nozzle and an object-supporting
`platform. When the nozzle is moved relative to the
`platform on an X–Y plane, a first layer of a solidifying
`material
`is dispensed from the nozzle and deposited
`onto a surface of the platform. Upon completion of the
`first layer, the nozzle is moved away from the platform
`by a predetermined distance in the Z-direction. A sec-
`ond layer of material is then deposited onto the first
`layer and adhered thereto. These procedures are then
`repeated to deposit a plurality of layers for building up
`the 3-D object.
`Little has been done on the fabrication of SFF parts
`with high structural
`integrity [2–4]. Fiber reinforced
`composites are known to have great stiffness, strength,
`damage tolerance, fatigue resistance, and corrosion re-
`sistance. However, currently available SFF technolo-
`gies, in their present forms, do not lend themselves to
`the production of continuous fiber composite parts.
`Some preliminary attempts have been made to use
`
`0921-5093:01:$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
`PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 8 1 0 - 4
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`126
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`W. Zhong et al. :Materials Science and Engineering A301 (2001) 125–130
`
`stereo lithography based techniques to fabricate both
`short and continuous fiber
`reinforced, UV-curable
`resins. In most cases, only composites with excessively
`low volume fractions of fibers were obtained and,
`hence,
`the resulting composites have exhibited low
`strength and stiffness. Furthermore,
`in such stereo
`lithography based techniques, only a laser- or UV-cur-
`able resin can be employed as the matrix material for a
`composite.
`Modified laminated object manufacturing (LOM) has
`been used to prepare polymer matrix and ceramic ma-
`trix composites [5]. The process involves, for instance,
`feeding, laminating and cutting thin sheets of prepregs
`(pre-impregnated fiber preform)
`in a layer-by-layer
`fashion according to computer-sliced layer data repre-
`senting cross sectional
`layers of a 3-D object. The
`process cycle typically consists of laminating a single
`sheet of prepreg to an existing stack, laser cutting the
`perimeter of the part cross section, and laser-dicing or
`‘cubing’ the waste material. After all layers have been
`completed, the part block is removed from the plat-
`form, and the excess material is removed to reveal the
`3-D object. This process results in large quantities of
`expensive prepreg materials being wasted.
`Other SFF techniques that potentially can be used to
`fabricate short fiber- or particulate-reinforced com-
`posite parts include fused deposition modeling (FDM)
`and powder-dispensing techniques. The FDM process
`operates by employing a heated nozzle to melt and
`extrude out a material such as nylon, acrylonitrile–bu-
`tadiene–styrene (ABS plastic), and wax. The build ma-
`terial is supplied in the form of a rod or filament. The
`filament or rod is introduced into a channel of the
`nozzle inside which the rod or filament is driven by a
`motor and associated rollers to move like a piston. The
`front end, near a nozzle tip, of this piston is heated to
`become melted; the rear end or solid portion of this
`piston pushes the melted portion forward to exit
`through the nozzle tip. The nozzle is translated under
`the control of a computer system in accordance with
`previously sliced CAD data to trace out a 3-D object
`point by point and layer by layer. In principle, the
`filament may be composed of a short fiber or particu-
`late reinforcement dispersed in a matrix (e.g. a thermo-
`plastic such as nylon). In this case, the resulting object
`would be a short fiber composite or particulate com-
`posite with improved properties. Presented in this paper
`
`Table 1
`Molding condition of single screw extruder
`
`are the results of a study on short fiber reinforced ABS
`polymers for use as a FDM feedstock material.
`
`2. Experimental
`
`2.1. Raw material
`
`The raw materials used in this study were mainly
`ABS plastic (Qimei-757, Taiwan Qi Mei), short glass
`fiber reinforced ABS (GFABS-30, Beijing Chemistry
`Institution of
`the Ministry of Chemical Industry),
`LLDPE (0209, the Chemistry Institute of the Chinese
`Academy of Sciences; 7042, Qi Lu Petrochemicals) and
`Hydrogenated Buna-N. The ABS matrix in GFABS-30
`was Qimei-757.
`
`2.2. Equipment
`
`A twin-screw extruder TE-34, made by Nanjing Ke
`Ya with a screw diameter (SD) of 34 mm and length:di-
`ameter (L:D) ratio of 34 was used. Also used was a
`single screw extruder with a screw diameter of 30 mm
`and length of 800 mm, respectively. A Model MEM-
`250 Multi-functional RP&M Machine was used in this
`study. This machine, developed by Tsinghua University
`in Beijing, China, provides both FDM and LOM func-
`tions in one machine. Controlled by a computer, the
`whole machine consists of two compartments: the up-
`per one accommodating the motion control and linear
`motion device system and the lower one containing the
`part-building chamber. A German-made ZD-150 tensile
`test machine with a 100 kg load capacity was employed
`to measure the key mechanical properties of ABS plas-
`tics and their composites.
`
`2.3. Sample preparation
`
`The raw materials were mixed in the twin-screw
`extruder and then extruded and granulated into small
`pellets. The pellets were then fed into the single screw
`extruder and drawn into a filament form. The diameter
`of the filament was controlled to fall in the range of
`1.75–1.90 mm by adjusting the rotational speed of the
`single screw and the filament-pulling speed of the trac-
`tor. The molding conditions of the single screw extruder
`in this experiment are listed in Table 1.
`
`Temperature (°C)
`
`Current intensity (A)
`
`Rotational speed (r min(cid:28)1)
`
`Spray nozzle
`
`Measuring section
`
`Compressing section
`
`Providing section
`
`200
`
`270
`
`24
`
`140
`
`3
`
`140
`
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`127
`
`part as well as crystalline part in a semi-crystalline
`polymer. The solid to liquid transformation of the
`amorphous part is a gradual process without a defini-
`tive temperature point. The commonly used term ‘soft-
`ening
`point’
`is
`a
`critical
`temperature
`in
`the
`transformation. A lower thermal expansion is essential
`to achieving the part dimensional accuracy. The
`amount of linear shrinkage in a part between the build
`temperature and the end-use temperature should be less
`than 1%.
`Key mechanical properties of a RP&M material in-
`clude strength, stiffness, ductility and flexibility. In the
`process of FDM, the material needs to be fabricated
`into a filament form. The drawn filaments were fed into
`the nozzle of MEM-250 as the feedstock material with
`the leading portion of the filament being melted before
`exiting the orifice of the dispensing nozzle. The solid
`portion of the filament, being driven by a set of rollers,
`acted as a piston to push the fused material out of the
`nozzle orifice. Therefore, the material needs an ade-
`quate strength, ductility and flexibility. At the same
`time, to keep the surface quality of the parts the
`material also requires enough rigidity to prevent the
`surface from wearing and tearing.
`The raw material for the FDM process is normally a
`thermoplastic and the process can be realized by the
`solid-to-liquid transformation of the thermoplastics in-
`side the nozzle. Proper
`rheological properties are
`needed;
`i.e. a low viscosity after
`the filament
`is
`liquefied. A lower viscosity makes it easier for the
`nozzle to dispense the polymer melt. The deposited
`material must be capable of solidifying in a relatively
`short time in order to achieve a good build speed.
`However, a sufficient amount of time is needed to allow
`a solidifying layer to well adhere to a previously de-
`posited layer. In addition, the solidification process
`should result in minimal internal stress in the part.
`
`3.2. Modifications of ABS composites
`
`According to the above requirements, ABS, an engi-
`neering plastic, was selected as the raw material in the
`present study due to its ready availability in the market
`and good balance of processing and performance prop-
`erties. Its softening point
`is approximately 100°C,
`which could meet the heat-resistance requirement of the
`FDM parts. ABS begins to flow at about 200°C so the
`part-building temperature does not have to be too high.
`ABS begins to decompose at approximately 250°C.
`Thus, there is a disparity of 50° between the flowing
`temperature and the decomposing temperature. This
`makes
`the actual heating temperature range wide
`enough to allow for a wide processing window in which
`the material can be heated to flow properly without
`decomposing. Besides, ABS has good mechanical prop-
`erties and fluidity. Pure ABS, however, still exhibited
`
`Fig. 1. Sketch of the samples.
`
`The drawn filament was then wound up onto a drum
`and later fed into the MEM-250 operating in the FDM
`mode. The dispensing nozzle of MEM-250, under the
`control of a computer, was driven to build desired parts
`essentially point by point and layer by layer. The nozzle
`was controlled to move along a square route to form
`two sizes of square boxes or ‘frames’ (Fig. 1): 50
`mm(cid:29)50 mm(cid:29)100 mm and 100 mm(cid:29)100 mm(cid:29)90
`mm. The temperatures of the nozzle and the build
`chamber are 250 and 60°C, respectively. Two types of
`sample were machined from either frame, as indicated
`in Fig. 1. The direction of the sample length was
`parallel to that of the nozzle movement direction (par-
`allel to the X–Y plane or layer plane) for Type 1
`samples and was perpendicular for Type 2 samples
`(sample length is in the Z- or thickness direction). The
`samples are each 90 mm long and 20 mm wide with the
`thickness being equal to the diameter of the feedstock
`filament.
`
`3. Results and discussion
`
`3.1. Basic requirements of the material for RP&M
`
`The materials for use in RP&M must meet the design
`and application requirements of the intended products
`and, in the meantime, be compatible with the RP&M
`process. For FDM, specifically,
`the materials must
`exhibit certain thermo-physical, mechanical, and layer-
`stacking characteristics.
`The requirements on thermo-physical properties in-
`clude a proper range of melting and solidification tem-
`peratures,
`low coefficient of
`thermal
`expansion,
`minimal shrinkage, high heat resistance, no or few
`volatile molecules (when the material is in the liquid
`state), and no phase transformation in the solid state.
`On the one hand, the melting point should not be too
`low in order for the material to have a high softening
`point (or heat distortion temperature). On the other
`hand, the melting point should not be too high to avoid
`a high processing temperature. Generally, a part-build-
`ing zone temperature of 70–100°C is preferred. The
`suitable solidification temperature range is preferably
`5–10°C below the softening point. There is amorphous
`
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`W. Zhong et al. :Materials Science and Engineering A301 (2001) 125–130
`
`excessively large shrinkage, resulting in less-than-satis-
`factory part accuracy.
`In order to improve the properties of ABS, short
`glass fiber was added as a reinforcement. Compared
`with pure ABS, the strength of short glass fiber rein-
`forced ABS composite (GFABS-30) was significantly
`increased, and both the softening temperature and the
`heat distortion temperature were increased as well. In
`the meantime, the shrinkage was decreased and the
`surface rigidity was improved, but the surface tough-
`ness was compromised. This composite could not be
`made into a continuous filament wound on a cylindrical
`drum because it became brittle after being extruded and
`cooled to room temperature. This brittleness makes it
`impossible to feed GFABS-30 into the FDM machine.
`To improve its toughness, the approach of changing the
`fiber content in the composite was attempted. Glass
`fiber reinforced ABS composites with a glass fiber (GF)
`content of 15, 20 and 25% were obtained by adding
`pure ABS into GFABS-30 which had a GF content of
`30%. The toughness of the resulting composites was
`similar to that of GFABS-30,
`indicating that
`this
`method was not feasible.
`Another approach was then attempted by adding
`LLDPE into GFABS-30 to improve the ductility and
`flexibility. The experimental results have indicated that
`LLDPE, a flexible linear polymer, is an effective tough-
`c
`c
`c
`ening agent for GFABS-30. In the present experiments,
`c
`four compositions, designated as 1
`, 2
`, 3
`and
`4
`(Table 2), were formulated. In these compositions
`the weight contents of GF were 10.2 and 13.2%. Differ-
`ent LLDPE grades (7042 and 0209 from two different
`suppliers), each with two different proportions of
`LLDPE in the short fiber composite, were used. In this
`way, composite filaments were extruded successfully.
`c
`The appearance and toughness of
`the composite
`filaments made from 1
`were found to be better than
`c
`those of the other three compositions. However, the
`content of LLDPE in 1
`was only 10%, while that of
`other three was up to 30%. This difference might be
`
`ascribed to the limited compatibility between LLDPE
`and the ABS composite host. LLDPE could be well
`mixed with ABS when the LLDPE content was only
`10%. Higher LLDPE contents than 30% appeared to
`result
`in extensive phase
`separation between the
`LLDPE-rich phase and the ABS matrix, which had a
`detrimental effect on the appearance and the ductility
`of the filaments. Microscopy observations on the cross
`section of the filament indicated that the surface and
`c
`c
`c
`the core of the drawn filament were separated into two
`layers in Samples 2
`, 3
`and 4
`.
`To overcome this incompatibility problem, hydro-
`c
`c
`genated Buna-N was added to the mixture systems in
`compositions 6
`and 7
`. On the one hand, there are
`butadiene (B) and acrylonitrile (A) groups in Buna-N,
`which are structurally similar to ABS. On the other
`hand, the main-chain structure of hydrogenated Buna-
`N is –(CH2)n–, which is structurally similar to LLDPE.
`Therefore, hydrogenated Buna-N could serve as a com-
`patibilizer between LLDPE and ABS, which signifi-
`cantly improved the properties of
`the composite
`systems. The surface and core areas of the filament
`were no longer separated. Both the toughness and
`appearance of the filament were also improved, even
`when the content of GF was further increased. From
`the appearance the filaments of the seven compositions
`studied, one could not observe any significant differ-
`ence in filament quality between the composite samples
`containing different trades of LLDPE.
`Ethylene-ethyl-acrylate (EEA), an elastomer, did not
`have any obvious effect on the properties of the result-
`ing composites. Possibly due to its lubricity, the wax
`used in the composite systems makes the material extru-
`sion through the nozzle more easily.
`Figs. 2–5 show representative SEM pictures of the
`c
`c
`c
`c
`cross sections of the filaments prepared from composi-
`tions 1
`, 2
`and 6
`, 7
`, respectively. To prepare
`appropriate SEM samples, the filament was soaked in
`liquid nitrogen to freeze the structure so that
`the
`filament could be broken in a brittle fashion.
`
`Table 2
`Modified formula of GFABSa
`
`No. formula
`
`Raw material
`
`c
`c
`c
`c
`c
`c
`c
`
`1
`2
`3
`4
`5
`6
`7
`
`GFABS-30
`(g)
`
`440
`340
`340
`340
`–
`440
`600
`
`ABS
`(g)
`
`440
`340
`340
`240
`880
`430
`270
`
`LLDPE
`(g)
`
`PE (wax)
`(g)
`
`Hydrogenated
`Buna-N (g)
`
`EEA
`(g)
`
`Weight content of GF
`(%)
`
`100 (0209)
`300 (0209)
`300 (7042)
`300 (7042)
`100 (0209)
`100 (0209)
`100 (0209)
`
`20
`20
`20
`20
`20
`20
`20
`
`–
`–
`–
`–
`–
`10
`10
`
`–
`–
`–
`100
`–
`–
`–
`
`13.2
`10.2
`10.2
`10.2
`0
`13.2
`18.0
`
`a The content of GF is received by the grams of GFABS-30 times 30% (g GF) and then divided by the whole grams of the system because there
`is 30% of glass fiber in weight in GFABS-30.
`
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`
`129
`
`and the dispersion of LLDPE is more uniform after the
`compatibility agent is added to the material system.
`c
`c
`Hence, the surface quality and mechanical properties of
`c
`c
`Compositions 6
`and 7
`are better than those of
`1
`and 2
`.
`
`3.3. Mechanical properties of the modified GFABS
`samples made by FDM
`
`Two types of tensile samples were made from the
`modified GFABS by FDM. Type 1 was used to mea-
`sure the longitudinal strength of glass fiber reinforced
`ABS to assess the effect of modifications. The tensile
`strength of Type 2 samples was used to mainly deter-
`mine the adhesive strength between the composite lay-
`ers. In order to reduce the effect of the clamping force
`from the clamp on the precision of the test results, a
`layer of adhesive tape was adhered onto the clamped
`part of the sample and a layer of rubber was adhered
`onto the rim of the clamp. The frictional coefficient
`between the sample and the clamp rim was also in-
`creased in this way. Besides, the distribution of the
`clamping force is more even because the rubber reduces
`the stress concentration on the sample.
`From Tables 2 and 3,
`it can be found that the
`strength of the modified GFABS is markedly higher
`than that of the unmodified counterpart. Moreover, the
`strength of GFABS is much higher than that of ABS
`when Type 1 samples were tested. The data of Type 2
`samples show that the properties of the modified mate-
`rials are much higher than those of the unmodified
`
`Fig. 5. Cross section of the filament from system 7
`
`c
`
`by SEM.
`
`Table 3
`Tensile strength of GFABS composites by FDM
`
`No. formula
`
`Type of sample
`c
`c
`
`1
`
`2
`
`Type 1 (kg)
`Type 2 (kg)
`
`38.93
`1.19
`
`29.16
`0.72
`
`c
`
`5
`
`24.50
`12.25
`
`c
`
`6
`
`52.37
`8.81
`
`c
`
`7
`
`58.60
`11.15
`
`Fig. 2. Cross section of the filament from system 1
`
`c
`
`by SEM.
`
`Fig. 3. Cross section of the filament from system 2
`
`c
`
`by SEM.
`
`Fig. 4. Cross section of the filament from system 6
`
`c
`
`by SEM.
`
`In these micrographs, white LLDPE phases were
`found to be dispersed in the black ABS matrix. Com-
`pared with the phase morphology in Figs. 4 and 5 for
`the composite compositions containing the interfacial
`compatibilizer, Hydrogenated Buna-N,
`the LLDPE
`phase particles in Figs. 2 and 3 are much larger in sizes.
`In particular, the length of LLDPE phase particles is up
`to 10 mm in Fig. 3 which shows the phase morphology
`c
`of Composition 2
`with a LLDPE content of 30%. By
`contrast, Figs. 4 and 5 demonstrate that the compatibil-
`ity between ABS and LLDPE increases significantly
`
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`
`ones. It can also be found that adding short glass fiber
`to ABS causes the adhesive strength between the layers
`in FDM-made ABS samples to fall, but that between
`the layers in FDM-made GFABS samples increases
`with the increasing GF content. This might be due to
`the speculation that a higher GF content provides a
`better chance for glass fibers to bridge together adjacent
`layers prior to the solidification of the ABS matrix.
`Another two key issues to consider in polymer com-
`posite extrusion, whether for FDM or injection mold-
`ing, are the force required for extrusion:injection and
`potential tool wear caused by glass fibers [6]. An in-
`creased fiber content in a polymer composite will in-
`crease the forces required for extrusion:injection and
`will increase the tool wear rate, so the determination of
`an appropriate fiber content in the composite for FDM
`must strike a compromise between processing difficulty
`and performance
`characteristics of
`the
`resulting
`composites.
`
`4. Conclusions
`
`A series of experiments were conducted to investigate
`the processability of properties of ABS and short glass
`fiber reinforced ABS composites for use as a feedstock
`filament in FDM. Glass fibers were found to signifi-
`cantly improve the strength of an ABS filament at the
`expense of reduced flexibility and handleability. The
`latter two properties of glass fiber reinforced ABS
`
`filaments were improved by adding a small amount of
`plasticizer and compatibilizer. The resulting composite
`filament, prepared by extrusion, was found to work
`well with a FDM machine.
`
`Acknowledgements
`
`This work was supported by the National Science
`Foundation of China (No. 59803001). The authors
`gratefully acknowledge the support of NSFC and Hs-
`inghua University.
`
`References
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