Minimizing Stringing Issues In FDM Printing

Abstract

Among all the additive manufacturing processes, FDM (Fused Deposition Modelling) is the most used due to its high availability and low cost. There is a common issue of FDM printing, which most of the users face is known as stringing or oozing. When the extruder travels from one point to another on the printing bed, string shaped structures are formed on the product's surface. Many of them are hard to remove too, which creates difficulties in gaining a good surface finish. Due to this problem, the quality of the product deteriorates. Most often, people use the retraction settings, where the extruder retracts the filament while moving from one point to another on the board. But, this does not ensure string free prints as different slicer software have different settings. In this study, Ender 3 Pro was used as a FDM printer and PLA filament with a diameter of 1.75mm as raw material. Results from this study will be a useful guide to the additive manufacturing professional in a wide range of personal, commercial, and industrial applications.

Full text

Minimizing Stringing Issues In FDM Printing
Md. Sabit Shahriar Haque
Department of Mechanical and Production Engineering
Ahsanullah University of Science and Technology (AUST)
Dhaka, Bangladesh
sabitshahriarh@gmail.com
Abstract
Among all the additive manufacturing processes, FDM (Fused Deposition Modelling) is the most used due to its
high availability and low cost. There is a common issue of FDM printing, which most of the users face is known as
stringing or oozing. When the extruder travels from one point to another on the printing bed, string shaped structures
are formed on the product's surface. Many of them are hard to remove too, which creates difficulties in gaining a
good surface finish. Due to this problem, the quality of the product deteriorates. Most often, people use the
retraction settings, where the extruder retracts the filament while moving from one point to another on the board.
But, this does not ensure string free prints as different slicer software have different settings. In this study, Ender 3
Pro was used as a FDM printer and PLA filament with a diameter of 1.75mm as raw material. Results from this
study will be a useful guide to the additive manufacturing professional in a wide range of personal, commercial, and
industrial applications.
Keywords
Additive Manufacturing, FDM, PLA, Retraction, Industry 4.0
1. Introduction
Additive manufacturing (AM) is a growing technology enabling the production of complex objects. The additive
manufacturing technology is able to print almost any material (e.g. metals and their alloys, ceramics, polymers,
biological materials, etc.), offering a wide range of products in different range of engineering applications, such as
the automotive, aerospace, civil, medical, energy, sport industries [1]. FDM (Fused Deposition Modelling) is the
most widely used 3d printing technique in present. It provides low cost manufacturing facilities for every type of
consumer. Liquid-based and powder-based processes are used to produce polymers and polymer-like additive
manufacturing materials. Polymers used in additive manufacturing processes are typically thermoplastic filaments,
resins or powders [2]. FDM printers are cheap to manufacture as they are mechanically simple and do not require
complex and expensive components. Their makeup consists of 3 axes that are controlled by 3 stepper motors. These
axis and motors are the main components, beside the print-head and print-bed and are affixed in a constellation that
enables 3 degrees of freedom along these axes. This method offers reduced granularity of the objects compared to
stereolithography. There is a common issue of FDM printing, which most of the users face is known as stringing or
oozing. When the extruder travels from one point to another on the printing bed, string shaped structures are formed
on the product's surface. Many of them are hard to remove too, which creates difficulties in gaining a good surface
finish. Due to this problem, the quality of the product deteriorates. Besides, the user has to spend more time than
usual to ensure a better product finish. Most often, people use the retraction settings, where the extruder retracts the
filament while moving from one point to another on the board. But, this does not ensure string free prints as
different slicer software have different settings. On the other hand, wrong parameters can damage both the filament
and extruder as the extruder uses gear mechanism to retract filament. Besides retraction settings, there are other
factors affecting this phenomenon such as ambient temperature, filament quality etc. The objective of this study is to

ensure better printing quality using FDM method by minimizing the amount of strings produced along with the
product to support rapid prototyping by saving time and to analyze the effect of different printing parameters on
weight gain of FDM printed specimens.
Literature Review
Fused deposition modeling (FDM), the name itself also explains the method that the fused material is
deposited in layers to produce a part, is the one of most promising rapid prototyping method considering cost
effectiveness and production speed. It is widely used, and almost half of the 3D printing machines are
in this category [3–5]. It is a quite sufficient and proved way to reduce the time and costs for product development
and production [6]. The FDM method was developed in 1988 by Steven Scott Crump who also founded Stratasys
and commercialized the FDM process after one year later. This company put the first 3D printing device on the
market with the name of 3D Modeler in 1992. After that, several new FDM systems were introduced, and the
common point of these systems was using two nozzles, one is to produce physical part material, and another one is
for the support material [7].
In FDM method, a material is extruded through a temperature-controlled nozzle onto a heated table layer by layer to
manufacture the desired part [8,9]The thermoplastic part material and support material in the form of thin wire is fed
by rollers into heated nozzle which partially melts the wire. As the nozzle moves in a pre-decided path in X, Y and Z
directions, the thermoplastic layers are deposited on table. This layer by layer deposition of plastic material creates
an actual product in extremely small duration (as compared to conventional manufacturing techniques) depending
upon size and shape [10]. Several parameters affect the quality of the manufactured part in FDM method. Layer
thickness and orientation, infill type and rate, support structure and extrusion angle parameters were commonly
investigated to improve the manufacturing time, surface quality, dimensional tolerances and mechanical strength of
the FDM-printed parts[11,12]. Proper feedrate ensures better print quality in FDM printing. The feed rate of the
extruder is maintained by controlling a feeding motor extrusion by a controller. But there are some issues too.
Misprint causes loss of material, time and fund[13]. Printer settings enable to control many variables such as layer
thickness, support angle, extrusion temperature, platform temperature, print speed, extruder flow ratio, nozzle
distance, infill type, infill density, surface layers, supports, seam type and fan speed. They are all very important
parameters for print quality to be considered. The most common problem relevant to printer setting is stringing
problem. It occurs when traces of small strings of polymer are left behind the nozzle when it is not extruding. This
defect can be persistent when fabricating features separated by small gaps. Strings caused by oozing can be
mechanically removed or by chemical surface treatment for some materials. However, that can be challenging to
accomplish for internal features and joints[14]. There are several factors that affect the print quality considering
FDM printing method. Some of them can be understood and learned by experiences[15].
Methods
1.1 Material
Thermoplastic materials in the form of filament are used in the most of FDM machines, especially
Acrylonitrile Butadiene Styrene (ABS) and Polylactide (PLA) thermoplastics are often used in the 3D printing
process[5]. Pure PLA has melting temperature of 130° C to 180° C. But the PLA used in printing is not pure. In this
experiment, PLA was used as filament. In this study, PLA material supplied by Zuhai Sunlu Industrial Co., Ltd.,
China has been used. The optimum extruder temperature prescribed by the manufacturers for printing, using PLA
filament is 190° C to 230°. The PLA filament used here, has a diameter of 1.75mm.

Figure 2. (a) FDM printer (Ender 3 Pro), (b) Extruder, (c) Filament
1.2 Machine
The specimens were printed using Ender 3 pro (Make : Creality 3d, China), which a FDM printer shown in figure 1.
It’s size was 445mm*445mm*465mm and its build size was 220mm*220mm*250mm. The printing process was
performed having four different parameters which were retraction distance, retraction speed, print speed and
extruder temperature. These are one of the most important parameters for print quality. Here, Retraction distance is
the amount of filament the extruder will retract from the nozzle. For example, if retraction distance is set 6mm, it
means that the extruder will pull in 6mm filament inside the bowden tube whenever it travels a minimum distance
set by the user. Retraction speed defines the speed at which the extruder pulls in the filament from the nozzle. On the
other hand, printing speed is the speed at which the extruder deposits melted filament on the heated bed. And
extruder temperature denotes the temperature at which the filament melts before getting out from the nozzle.
Extruding temperature differs from filament to filament.
1.3 Method
At first, the specimen was designed in a CAD software. The specimen was 40mm in length, 15mm in width and
32mm in height. Then it was saved in stl format. Because, slicer software can easily read .stl files than any other
format. Later it was transformed into G-code files using a slicer software named
Ultimaker Cura 4.6. It was done because FDM printers can only read G-code files. At the slicer software, the
parameters were set in accordance to the table 1. The bed level was checked before starting the print. Then the print
bed was preheated to ensure proper adhesion of the extruded filament to the bed. Then the samples were printed
using different values mentioned in the table 1 with the help of the FDM printer shown in figure 2. The samples
were printed using 100% density and cubic shape as infill pattern. The retraction distance was varied from 5.0 mm
to 7.5 mm, as too high or low retraction speed damages the printing filament. The retraction speed ranged from 20
mm/s to 80 mm/s. During retraction process, the filament goes back to the nozzle through a gear movement. So, if
the retraction speed is very high, the extruder may get damaged. The printing speed was varied to 40mm/s to 60
mm/s because higher printing speed degrades the printing quality and lower printing speed takes more time. The
extruder temperature was kept to 210° C and 200° C because this is the optimum temperature for printing PLA
filament. After the print was complete, the specimens were cooled down and weighed using a digital balance as
shown in figure 3.
It was done so that, the amount of strings can be known from the weight of the specimens. The cleanliness of nozzle
was ensured before every print.

Figure 3. Weight measurement of specimen
Table 1. Printing parameters for the specimens
Specimen no.
Retraction
distance (mm)
Retraction speed
(mm/s)
Print speed
(mm/s)
Extruder
temperature (°C)
Weight (g)
1
7.5
50
60
210
1.7804
2
7.5
80
60
210
1.7870
3
5.0
45
60
210
1.8974
4
5.0
35
50
210
1.8964
5
6.0
30
50
210
1.6598
6
6.5
30
50
210
1.6655
7
6.5
20
50
200
1.6157
8
6.5
30
40
200
1.7371
9
7.5
35
40
200
1.7781
10
7.5
25
40
200
1.7516
11
5.5
25
40
200
1.6674
12
5.0
20
40
200
1.6591
13
6
25
40
200
1.6501
14
6
40
40
200
1.8062

The table 1 represents the four different parameters for printing the specimens, which are: retraction distance,
retraction speed, printing speed and extruder temperature. It also shows the measured weight of the specimens.
Results & Discussion
Four parameters were kept constant all the time as the quality of the print does not depend very much on them. The
parameters are layer height, infill, infill pattern and nozzle diameter. The figure 4 represents specimen 1,5,7 and
Figure 4. Presence of stringing issue in the printed specimens (a) Ongoing print (b) Specimen 1, (b) Specimen 5, (c)
Specimen 7, (d) Specimen 8
8 from left to right. After analyzing the specimens, it was seen that specimen 1, 2, 3, 4, 5 had almost the same result.
But, from specimen 6, the output quality started changing. In specimen 6, 7, 8, 9, 10, 11, 12, 13, 14, the stringing
issue is much lower than that of the first five specimen. Among all the specimens, specimen 7,12 and 13 has the
lowest amount strings where the retraction distance was 6.5, 5.5, 6 and the retraction speed was 20, 20, 25.
When stringing issue occurs, more filament gets used along with the actual product. So, the specimens where the
weight is greater, more strings has been produced and more filament has been used. It can be seen from table 1 that
the specimens 7,12,13 has the lowest weight among all the specimens.
From the graph shown in figure 5, it can be seen that the specimen 7,12,13 possesses lowest weight among all
specimen. So, they contain the lowest amount of strings too. As a result, it can be said that, the retraction settings
contained by them are more preferable.

Figure 5. Change in weight of specimens
It was observed that, extruder temperature plays a great role in stringing. As high temperature liquifies the filament,
so there remain higher chances of forming string. Besides, due to liquification, more filament gets used in the
printing process. As it can be seen from the table 1, when the extruder temperature was set 200° C from 210° C, the
amount of strings became lower as the weight of the specimens gradually decreased. As the cleanliness of nozzle
was ensured before every print, so there no extra filament at the tip of the nozzle. Besides, for proper retraction,
lower printing speed is necessary. So, according to this study, retraction distance of 5 – 6.5mm, retraction speed of
25-20mm/s, print speed of 40-50 mm/s and extruder speed of 200° C can provide a better retraction result. These
parameters may vary in case of different printer, filament, slicer software or environment.
4. Conclusion
This paper investigates different techniques of getting string free 3d prints. During this study, PLA filament was
selected for FDM printing as it is the most commonly used filament. This paper summarizes the guidelines for
retraction settings, the users are recommended to follow while printing. The case study demonstrates the
disadvantages of stringing issue. In conclusion, this study can be further expanded to find out the impact of these
parameters on other FDM printing defects.
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