Inter-filament Bonding in Fused Deposition Modeling 3D Printing

Brandon Spradlin

Chemistry 400 Research

Abstract: Fused deposition modeling can be used to quickly create objects with complex

geometries. ABS is commonly used as filament for 3D printing. During the deposition

process, the plastic is repeatedly heated above its glass transition temperature resulting in

inter-diffusion between deposited layers. The extent of inter-diffusion corresponds to the

size of void space, which relates to the strength of the printed object.

Introduction:

Recent development in technology has lead to an increase in additive

manufacturing processes. Additive manufacturing, or 3D printing, has become an ideal

process to rapidly design objects with complex geometries. 3D printing encapsulates

many varieties of additive manufacturing. Specifically for printing plastic, a method

called fused deposition modeling is utilized. During this process plastic filament is heated

and extruded layer by layer to create an object. Diffusion between one layer and another

is essential to creating a solid object. Since the plastic is not continuously heated above

its glass transition point, complete diffusion does not occur and open spaces are observed

in the object. These open spaces, or voids, decrease the strength of the completed

structure. The orientation of the printed object, as well as the bed temperature, affects the

structural integrity of the object. Controlling the amount of inter-filament diffusion

enables optimization of the physical properties of the 3D printed object.

Sample Preparation:

Commercially bought ABS plastic

filament was used in a Solidoodle 3 3D printer

to create sample pieces using three different

print geometries. Small bars (50 x10 x 3mm)

were printed horizontal (Fig. 1a), perpendicular

(Fig. 1b), and vertical (Fig. 1c) with respect to

the bed. The temperature of the bed was set to

100º C and the extruder 210º C. Each sample

was printed three times and mechanical

analysis of the samples led to the confirmation

of the strongest geometry.

Once the strongest geometry had been

determined, it was printed over a varying range

of bed temperatures to explore the inter-

diffusion between layers. A thin foil

thermocouple was calibrated at 100º C, 0º C,

Figure 1: 3 print geometries using Solidoodle 3 and ABS filament

a) Horizontal print geometry

b) perpendicular print geometry

c) Vertical print geometry

Figure 1: 3D Print Geometries

and 70º C to ensure proper temperature measurements. The thermocouple was secured to

the base and a sample was printed on top. A continuous temperature profile was taken of

samples printed with a bed temperature of 80º C, 100º C, and 120º C. Cross sections of

the samples were observed by cleaving. Submersion in liquid N2 avoided crystallization

during the cleaving process. Microphotograph images were taken of the cross sections to

analyze the temperature/bonding relationship.

Discussion:

The mechanical analyses performed on the initial samples printed at 100º C

indicate that the orientation of the printed object controls its physical properties. From the

DMA results (Figure 2), the vertically printed sample (Fig. 1c) has the lowest storage

modulus, and the perpendicularly printed sample (Fig. 1b) has the highest storage

modulus. Because the perpendicular

sample was the strongest, that

geometry was used for the

temperature profile samples.

According to the thermal

history of the bottom layer, the

temperature of the deposited

structure varies as the extruder

passes over layers and deposits more plastic. The extruder causes the deposited plastic to

repeatedly heat above its glass transition temperature when new layers are deposited.

This repeated heating/cooling increases the amount of inter-filament bonding.

Figure 2: Storage Modulus of Varying Sample Geometries

The amount of inter-filament bonding is related to the size and amount of voids

that are created as the filaments fuse

together. Void space can be seen when the

object is properly cut in half. For the

perpendicular samples printed at different

bed temperatures, it is apparent that the

temperature affects the amount of inter-

filament bonding. Figures 3, 4, and 5 are

microphotographs taken of the inside of the

perpendicular samples that were printed at bed temperature of 80º, 100º, and 120º C

respectively. The samples are orientated in the images so that the side in contact with the

bed is to the left, and the top is to the right.

As the temperature of the bed is increased, it

is apparent that the size of the voids

decreases. Figure 3 shows wide and

continuous void space beginning close to the

base of the sample (left side) and increasing

as towards the top of the sample (right side).

The voids in Figure 4 are somewhat smaller

and start higher from the base. Figure 5

shows the smallest void space, and also

shows the inter-diffusion of deposited

filaments is better up to a greater distance

Figure 3: Halved perpendicular sample printed at bed 80ºC

 

Figure 4: Halved perpendicular sample printed at bed 100ºC

Figure 5: Cross section of perpendicular sample printed at bed 120ºC

from the bed. This phenomenon occurs because the heat is diffused up through the

sample, which increases the amount of layers that are raised above the glass transition

temperature.

Conclusion:

Fused deposition modeling is becoming increasingly efficient at creating plastic

objects. Prototypes can be quickly created, replacement parts can be fabricated, and a

multitude of other objects can be printed and used. Because many objects are being 3D

printed, it is important to understand the variables that attribute to the object’s structural

integrity. The results from the experiment indicate that orientation of the object, as well

as the bed temperature, affect its strength. This is due to the extruder passing overhead of

the deposited layers and increasing the temperature above the glass transition point. As

seen in Figure 6, the temperature of the bed also corresponds to the amount of inter-

diffusion, which is directly related

to the strength of the object.

Heat from the bed diffuses

up through the object, and raises

the temperature of the layers. This

increase in temperature allows the

plastic to reach its glass transition

temperature. Thus, increasing the

temperature of the bed increases

the amount of heat that is diffused through the object. As the temperature is increased,

layers higher from the bed are exposed to heating, and as a result the layers can reach the

Figure 6: Bed temperature effects on storage modulus

glass transition temperature, increasing the amount of inter-diffusion. These results show

that optimization of inter-diffusion between layers is important when manufacturing

robust polymeric objects by 3D printing. Developing new materials and control of the

deposition of layers is key to enhancing the structure of 3D printed objects.