Background

This review covers the available literature pertaining to mechanical testing of rapid prototyped components in support of the Mechanical testing of polymer components made with the RepRap 3-D printer project.

Overview of Rapid Prototyping

A Study of the State-of-the-Art Rapid Prototyping Technologies

C. K. Chua, S. M. Chou, and T. S. Wong, "A Study of the State-of-the-Art Rapid Prototyping Technologies",The International Journal of Advanced Manufacturing Technology, 14(2), 146-152, 1998.

Abstract:

Each rapid prototyping (RP) process has its special and unique advantages and disadvantages. The paper presents a state-of-the-art study of RP technologies and classifies broadly all the different types of rapid prototyping methods. Subsequently, the fundamental principles and technological limitations of different methods of RP are closely examined. A comparison of the present and ultimate performance of the rapid prototyping processes is made so as to highlight the possibility of future improvements for a new generation of RP systems.


Notes:

  • Gives a general overview of current rapid prototyping techniques including droplet deposition (FDM)
  • Droplet deposition (Table 4)
-Advantages:
Low Material Cost
Very little material waste - limited to support material
Low imaging specific energy
Multiple materials or colors possible in same object or layer
-Disadvantages:
Poor surface finish and appearance
Limited to material with low melting point

Rapid Prototyping Mechanical Testing

Anisotropic material properties of fused deposition modeling ABS

Sung-Hoon Ahn, Michael Montero, Dan Odell, Shad Roundy, and Paul K Wright, “Anisotropic material properties of fused deposition modeling ABS”, Rapid Prototyping Journal, 8(4), 248-257, 2002.

Abstract:

Rapid Prototyping technologies provide the ability to fabricate initial prototypes from various model materials. Stratasys Fused Deposition Modeling (FDM) is a typical RP process that can fabricate prototypes out of ABS plastic. To predict the mechanical behavior of FDM parts, it is critical to understand the material properties of the raw FDM process material, and the effect that FDM build parameters have on anisotropic material properties. This paper characterizes the properties of ABS parts fabricated by the FDM 1650. Using a Design of Experiment approach, the process parameters of FDM, such as raster orientation, air gap, bead width, color, and model temperature were examined. Tensile strengths and compressive strengths of directionally fabricated specimens were measured and compared with injection molded FDM ABS P400 material. For the FDM parts made with a 0.003 inch overlap between roads, the typical tensile strength ranged between 65% and 72% of the strength of injection molded ABS P400. The compressive strength ranged from 80% to 90% of the injection molded FDM ABS. Several build rules for designing FDM parts were formulated based on experimental results.

Notes:

  • Studied bead (raster) width, air gap, heating element temperature, and raster angle
  • Found that dog bone samples in accordance with ASTM D638 caused premature failure of specimens due to stress concentrations resulting from specimen manufacture. These stress concentrations occurred at the radius between the sample width at the clamps and the testing width.
  • Followed specimen geometry in ASTM D3039 (229mm x 25.4mm x 3.3mm) to remedy above problem
  • Compared FDM ABS parts to injection molded ABS parts
  • For the design of experiment they only tested air gap and raster angle
  • In tension, all specimens failed in transverse direction except criss-cross pattern.
  • 0 degree raster angle specimen showed failure by breaking individual fibers
  • Compressive strength was not affected by build direction and was higher than tensile strength.
  • Negative air gap (slight overlap of adjacent beads) increases strength and stiffness
  • Author developed 6 build rules for FDM parts

Measurement of anisotropic compressive strength of rapid prototyping parts

C.S. Lee, S.G. Kim, H.J. Kim, & S.H. Ahn, "Measurement of anisotropic compressive strength of rapid prototyping parts", Journal of Materials Processing Technology, 187-188, 627-630, 2007.

Abstract:

Rapid prototyping (RP) technologies provide the ability to fabricate initial prototypes from various model materials. Fused deposition modeling (FDM) and 3D printer are commercial RP processes while nano composite deposition system (NCDS) is an RP testbed system that uses nano composites materials as the part material. To predict the mechanical behavior of parts made by RP, measurement of the material properties of the RP material is important. Each process was characterizes by process parameters such as raster orientation, air gap, bead width, color, and model temperature for FDM. 3D printer and NCDS had different process parameters. Specimens to measure compressive strengths of the three RP processes were fabricated, and most of them showed anisotropic compressive properties.

Notes:

  • Build direction and raster angle are important process parameters
Build direction - orientation of parts as it is being made
Raster angle - direction of deposited material in relation to part loading
  • RP processes result in anisotropic parts
  • FDM - filament material is heated to semi-molten state and deposited through a nozzle. The material fuses with the previously deposited layer.
  • Tested compression of FDM parts by ASTM D695
ABS build material
Axial and transverse orientation with 45 degree raster angle
Axial-built compressive strength was greater than transverse-built strength

Parametric appraisal of mechanical property of fused deposition modelling processed parts

A. K. Sood, R.K. Ohdar, and S.S. Mahapatra, “Parametric appraisal of mechanical property of fused deposition modelling processed parts,” Materials & Design, 31(1), 287-295, 2010.

Abstract:

Fused deposition modelling (FDM) is a fast growing rapid prototyping (RP) technology due to its ability to build functional parts having complex geometrical shape in reasonable time period. The quality of built parts depends on many process variables. In this study, five important process parameters such as layer thickness, orientation, raster angle, raster width and air gap are considered. Their influence on three responses such as tensile, flexural and impact strength of test specimen is studied. Experiments are conducted based on central composite design (CCD) in order to reduce experimental runs. Empirical models relating response and process parameters are developed. The validity of the models is tested using analysis of variance (ANOVA). Response surface plots for each response is analysed and optimal parameter setting for each response is determined. The major reason for weak strength may be attributed to distortion within or between the layers. Finally, concept of desirability function is used for maximizing all responses simultaneously.

Notes:

  • Covers mechanical testing results for tensile, flexural, and impact strength of ABS parts
  • Varied five process parameters: Orientation, Layer Thickness, Raster Angle, Raster Width, and Air Gap
  • Detailed lit review covering detailed explanation of temperature gradients and deformations
  • Tensile Test
Tensile strength initially decreased with layer thickness but increased at later thicknesses
Larger air gaps saw greater strength through increased bonding
Brittle failure on plane normal to force
  • A larger number of layers will cause higher temperature gradients which increase diffusion resulting in better strength, but can cause more distortion between layers. Larger number of layers also caused more heating and cooling cycles resulting in residual stresses.
  • Large raster angles preferred because they result in shorter rasters, but small raster angles better align the rasters with loading direction
  • Thick rasters can cause stress accumulation but also cause higher temperatures at the bond with may improve strength
  • It isn't possible to gain an accurate and complete picture of part strength by simply varying process parameters. Parameters interaction is key to part strength.

Mechanical Modeling

Composite Modeling and Analysis for Fabrication of FDM Prototypes with Locally Controlled Properties

L. Li, Q. Sun, C. Bellehumeur, and P. Gu, “Composite Modeling and Analysis for Fabrication of FDM Prototypes with Locally Controlled Properties,” Journal of Manufacturing Processes, 4(2), 129-141, 2002.

Abstract:

Solid freeform fabrication (SFF) technologies have the ability to manufacture functional parts with locally controlled properties, which provides an opportunity for manufacturing a whole new class of products. To a certain extent, fused deposition modeling (FDM) has the potential to fabricate parts with locally controlled properties by changing deposition density and deposition orientation. To fully exploit this potential, this paper reports a study of the materials, the fabrication process, and the mechanical properties of FDM prototypes. Theoretical and experimental analyses of mechanical properties of FDM processes and prototypes were carried out to establish the constitutive models. A set of equations is proposed to determine the elastic constants of FDM prototypes. The models are then evaluated by experiments. An example of FDM prototype with locally controlled properties is provided to demonstrate the ideas.

Notes:

  • Mechanical properties depend on mesostructures
  • Deposition orientation and air gap are the most important parameters
  • Effective stiffness (average stiffness) can be used in analysis
  • To accurately model FDM parts, void density must be considered which depends on air gap
  • Tested parts to ASTM D3039 standard
  • Components with specific mechanical properties can be made by varying orientation and air gap, and by varying lamina orientation on each layer

Design of Fused-Deposition ABS Components for Stiffness and Strength

J. F. Rodriguez, J.P. Thomas, and J.E. Renard, “Design of Fused-Deposition ABS Components for Stiffness and Strength,” Journal of Mechanical Design, 125(3), 545-551, 2003.

Abstract:

The high degree of automation of Solid Freeform Fabrication (SFF) processing and its ability to create geometrically complex parts to precise dimensions provide it with a unique potential for low volume production of rapid tooling and functional components. A factor of significant importance in the above applications is the capability of producing components with adequate mechanical performance (e.g., stiffness and strength). This paper introduces a strategy for optimizing the design of Fused-Deposition Acrylonitrile-Butadiene-Styrene (FD-ABS; P400) components for stiffness and strength under a given set of loading conditions. In this strategy, a mathematical model of the structural system is linked to an approximate minimization algorithm to find the settings of select manufacturing parameters, which optimize the mechanical performance of the component. The methodology is demonstrated by maximizing the load carrying capacity of a two-section cantilevered FD-ABS beam.

Notes:

  • Developed an optimization model to determine the load carrying capacity of an FDM cantilever beam
  • ~1.78 mm diameter filament
  • Can control overall dimensions of part to ~0.125 mm
  • Identifies important parameters of: fiber orientation, air gap, road width, extrusion and envelop temperatures
  • Assumed that interior mesostructure controls the overall mechanical properties
  • In-plane elastic modulus can be found with mechanics of materials (from references). These use the void density and bulk isotropic elastic properties of the deposition material.

Standards

ASTM D638 - 10 Standard Test Method for Tensile Properties of Plastics

ASTM Standard D638-10, 2010, "Standard Test Methods for Tensile Properties of Plastics," ASTM International, West Conshohocken, PA, 2010, DOI: , www.astm.org.

  • Type I is the preferred specimen geometry to be used for with material 7mm thick or less
  • Type II specimen to be used if Type I specimen does not break in the gage section
  • Test 5 specimens for each sample for isotropic materials
  • Test 10 specimens for anisotropic material, 5 normal to and 5 parallel to anisotropic direction
  • Select testing speed to break specimen in 0.5 to 5 minutes
  • 5mm/min, 50mm/min, or 500mm/min

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