Modeling of Bond Formation Between Polymer Filaments in the Fused Deposition Modeling Process

Abstract

The bonding quality among polymer filaments in the fused deposition modeling (FDM) process determines the integrity and mechanical properties of resultant prototypes. This paper investigates the bond formation among extruded acrylonitrile butadiene styrene (ABS) filaments in the FDM process. Thermal analysis of the FDM process resulted in an estimation of the cooling profile of the extruded filaments. Sintering experiments were carried out to evaluate the dynamics of bond formation between polymer filaments. Quantitative predictions of the degree of bonding achieved during the filament deposition process were made. The model was used to estimate the effects of different manufacturing parameters in the FDM process. Results suggest that better control of the cooling conditions may have strong repercussions on the mechanical properties of the final part fabricated using the FDM process.

The fused deposition modeling (FDM) machine is a computer-numerical-controlled (CNC) gantry machine carrying double miniature extruder head nozzles. The machine fabricates prototypes by extruding a semi-molten filament through a heated nozzle in a prescribed pattern onto a platform. As the material is deposited, it cools, solidifies, and bonds with the surrounding material. The formation of the bonding in the FDM process is driven by the thermal energy of the semi-molten material. The FDM prototypes are orthotropic composites of polymer filaments, partial bonding between filaments, and voids (Figure /). The quality of the bond formed between individual filaments depends on the growth of the neck formed between adjacent filaments (wetting) and on the molecular diffusion and randomization at the interface. The bond formation process can be modeled following approaches similar to those used to describe polymer welding, where the issue of molecular diffusion dominates. It can also be viewed as a sintering process for which the wetting phenomenon is also of importance.

The overall properties of the prototype can be modeled by the lamination theory (Mallick 1993). At the macro level, the properties are studied as laminates of bonded laminas (Figure /). At the micro level, the properties of each lamina are functions of the properties of the filaments, the quality of the bonds between filaments, and void density. Methods have been proposed to model the lamina properties as a function of constituent properties and geometrical parameters (Tsai 1964). These methods are, however, based on the assumption that the bonding between filaments is such that the properties at the interface are the same as the material used to produce the filaments (that is, perfect bonding). Results obtained from experimental studies on the FDM process have indicated that the bond between filaments is such that the properties at the interface are inferior to those of the material used to produce the filaments (Rodriguez-Matas 1999; Li et al. 2001; Li 2002). A composile mechanics model has been proposed that requires the elastic constant of a laminate with consideration of partial bonding at the interface between filaments (Li et al. 2001). The moduli are expressed in terms of void density and relative bond strength factors. An experimental analysis was carried out by Li and cc leagues (Li et al. 2001 ; Li 2002). The composite m )del showed very good agreement with the experimental results. The values for relative bond strength 'actors were found to vary with the gap size between filaments. The results showed that some degree ol ' wetting is achieved, and confirmed that the quality of the bond is not perfect and varies with the processii g conditions.

FDM prototypes essentially consist of partially bonded polymer filaments and voids (Figure J). The bonding quality among filaments in FDM parts is an importart factor in determining the integrity and mechanical properties of the resultant prototypes. The bonding quality, in this work, is assessed by the degree of Wetting or the size of the neck formed between adjacent filaments. This paper is aimed at modeling the dynamics of bond formation among polymer filaments during the FDM extrusion process and evaluating the effects of different manufacturing parameters on the bond formation.

Thermal Analysis in FDM

The be nding between individual roads of the same layer and of neighboring layers of FDM parts is driven by the thermal energy of the semi-molten material. The required heat transfer is a function of the thermal properties of the liquefier, tip, and modeling materials, as well as the diameter of the filament and volumetric flow rate. The temperature history of interfaces plays an important role in determining the bonding duality. When the filament is deposited and is in contact with surrounding material, the interface's temperature is well above the material's glass transition temperature. This condition favors the rapid development of adhesive bonds. The filaments gradually cool down to the envelope temperature. The cooling process is transient and physically complex in its nature. The heat transfer modeling of the FDM process can provide the temperature profile during the cooling process, which is useful for analyzing the bonding phenomenon.

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A Question about FDM
written by Alan Minasian, April 26, 2010

Hi .thank you for your useful RP course.
I have a dimension sst1200es , and I have the problem of separation of base support material from the build tray after start the build process and building some layers of main part . I use new tray but I don't know what is the reason of this problem . this problem occurs specially for parts bigger than about 12x10 Cm (approx.) . may you have any experience in this issue ?It will be your kindness if you have any information.




Dear Friends , Hi

I have a Dimension SST1200es FDM machine.

I have a problem which really disturb me , so it will be your kindness to tell me any suggestion . thank you in advance

The problem :

After starting the build process and building the base of the part which is made of some layers of support material and then starting to build some 5 - 10 layers of the main part by ABS+ , the part starts to bend upward along one edge for some few millimeters (about 0.5 -1.0 mm)and separates the support material from the build tray and ruins everything .

I use completely new trays , but my support material cartridge manufacturing date is exactly 1 year and it is mentioned that the support and the base material is best to be used before 1 year. I wait for new support material , it will take about 2 weeks but any how I am not sure it will help to solve the problem or not.

please send me your suggestions , I need urgently to solve it.

thank you friends
bye bye
Alan Minasian

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