J. Pac .Sci. Tech.

Vol.24No.6（2015）

Technical Report～～～～～～～～～～～～～～～～～～～～～～～～～～～

Study on the Effects of Various Factors on Side Seal Strength
Tsuyoshi NISHIZAKA*, Takeshi DOI ** and Hiroyuki SHIRAIWA**
The purpose of this study is to improve the strength of side seal bag. For that purpose, the form ofheat cutter, material of heat cutter and operating conditions were examined. Side seal method is one of
the bag-making methods. Flat films put on two sheets, such as CPP (Cast Polypropylene film) and OPP(Oriented Polypropylene film), are cut thermally and welded by the specified width. The relation between
the intensity of a side seal bag and each operating condition was investigated. A breaking load of CPP
film increases with increasing thickness of film. When a thickness of CPP film becomes over 30
micrometers and over the breaking load become almost steady value. A breaking load increases withdecreasing the number of times per minute of decision. A breaking load increases with increasing preset
temperature of a heater. Additionally, as a result of executing a two-dimensional heat conduction FE(Finite Element) program which changed the material (thermal conductivity) of the heat cutter, it was
ascertained that thermal conductivity of cutter affects greatly temperature near the tip of cutter.
Furthermore, the shape of the heat cutter which may be able to improve temperature near the tip of cutter
was examined.
Keywords：cutter, side seal, thermal conductivity, finite element program

1.

Introduction

Bags have become an indispensable part of our daily life. The most common type of bag iscurrently plastic. There are even many different types of plastic bags depending on the manufacturingmethod and the materials. These bags are cheap, thin, flexible, come in plenty of sizes and therefore, havebecome an indispensable item in our daily lives for uses such as when shopping or disposing of garbage.
However, insufficient side seal strength has become a serious problem in bag manufacturing, and as aresult, research and development is being conducted daily to improve this strength.1,2)
The side sealing method is used in this study as a representative of the many bag-production methods.
Bags manufactured with this method are used mostly for wrapping stationary and accessories. However,if the conditions during production, such as the temperature of the heat cutter or the number of shots(cutting speed: shot/min), are not optimal, then the weld can be either insufficient or excessive, whichcauses problems such as lines and pinholes in the exterior, and greatly affects the deterioration of thestrength of the bag, in addition to the deterioration of strength of the weld. It is extremely important to*Department of Mechanical Engineering, National Institute of Technology, Kure College
2-2-11 Aga-minami, Kure City, Hiroshima Prefecture, 737-8506** Department of Mechanical Engineering, National Institute of Technology, Miyakonojo College473-1 Yoshio-cho, Miyakonojo City, Miyazaki Prefecture, 885-8567－ 357 －

Study on the Effects of Various Factors on Side Seal Strength

reliably manufacture satisfactory products to control the occurrence of these issues. This study
investigated factors such as the shape and material for the heat cutter used to cut and weld bags during
production, and investigated the conditions during operations to help improve the bag strength (side seal
strength) created by the side sealing method, and aims to optimize these factors as its ultimate objective.
This study includes strength tests for bags made by the side sealing method to examine therelationship between the breaking load and operating conditions. Finally, this study investigated methods
to increase side seal strength using numerical analysis3).

2.

Fundamental Items Related to Bag Manufacturing

2.1

The Side Sealing Method
Side sealing4) is one method of bag production that stacks 2 sheets of flat film such as• CPP (Cast Polypropylene film) and• OPP (Oriented Polypropylene film)
and cut/welds these sheets together at a specified width. CPP, OPP, and laminate are respectively,
un-stretched polypropylene, biaxial stretched polypropylene, and CPP and OPP joined together. Figure 1
shows an example of a bag made using side sealing production method. The long side of the rectangularbag is the seal.

sealed part
Fig.1

Example of a bag made using the side sealing production method

3. Practical Examination of Side Seal Strength
We investigated the cause and effect relationship between the manufacturing strength of side-sealed
bags and each operating condition. Each operating condition means the film material5) (CPP, OPP, or
laminate), thickness of materials (20～70 μm), preset heater temperature (430～440℃), shot number(cutting speed: 40～140 shot/min) and heat cutter material (stainless) .
3.1
MethodFive samples were collected from each of the 27 production machines and the collection date,manufacturing number, production machine number, preset heater temperature, shot number, material,
maker name, and material thickness were recorded.
i)Three of the five sheets were evenly distributed to make the specimen (Figure 2).
ii) The specimen had both ends fixed using the same method as the standard stretch test
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Vol.24No.6（2015）

and was stretched until it broke.
iii) The load at breaking point was designated as the breaking load. The breaking load
and its shape (Figure 3) were recorded on each occasion.
3.2

Results
The relationship between the breaking load and the operating conditions is shown in Fig.4–Fig.8.
Figure 2 shows the front and back of the bag referred to in the other figures. Also, the composite thicknessof the laminate used in Figure 6 is the sum of the thicknesses of both the CPP and OPP films that wereused to make it.

15 mm

Fig.2

Specimen

Fig.3

Schematic of break pattern

Fig.4

Relationship between breaking load andthickness in CPP film

Fig.5

Relationship between breaking load andthickness in OPP film

Fig.6

Relationship between breaking load
and thickness in laminate film

Fig.7

Relationship between breaking load andnumber of shot

(OPP film , thickness : 30μm,
preset heater temperature : 430℃)－ 359 －

Study on the Effects of Various Factors on Side Seal Strength

Fig.8

Relationship between breaking load andpreset heater temperature (OPP film,
thickness :30μm , number of shot :120shot/min)

4. Finite-Element Analysis of Two-Dimensional Non-steady Heat Conduction
This study examines the material and shape of the heat cutter in order to realize the stabilization thetemperature at the tip of the cutter near the preset temperature of the heater because this stabilization isconsidered necessary for improving side seal strength. This study method calculates a numerical value
based on a finite-element analysis of two-dimensional non-steady heat conduction6, 7) that is modeled on
actual operating conditions, then considers/examines effective shapes and materials for heat cutters basedon those results. Figure 9 shows analysis models and boundary conditions. A preset heater temperature
T=Tp (℃) is specified on the circumference. A heat pipe is known to have a high thermal conductivity of
the longitudinal direction. We assumed that the temperature diffusivity in the xy plane is at the same level
as copper. Further, when a heat cutter touches films, heat flow near the tip of heat cutter is modelled bygiving a contact part a high heat transfer coefficient (assumed value). In addition, at other surface of heat
cutter, convection to occur between external environment is considered.

Fig.9

Analysis models and boundary conditions－ 360 －

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Vol.24No.6（2015）

4.1

Numerical Analysis based on Changes of Heat Cutter Material
We conducted an analysis of traditional stainless steel and die steel. The thermal conductivity foreach material is shown in Table 1. The parameters, with the exception of thermal conductivity, were kept
equal and the same shape and size were used (Figure 10(a)). Analysis were conducted for preset heatertemperatures of 350°C, 400°C and 450°C with a shot number of 100 shot/min.
Table 1

Thermal conductivity for each materialmaterial
thermal conductivity (W/m･K)(a) conventional shapeFig.10stainless15

die steel40(b) prototype shape

Shape and size of heat cutter (1/2 model) (units: mm)

4.2

Numerical Analysis based on Changes of Heat Cutter ShapeThe shape and size of the traditionally used heat cutter and the prototype heat cutter is shown in
Figure 10. The heat cutters’ physical properties (stainless) and operating conditions were kept equal andthe analysis was conducted at a preset heater temperature of 400°C and a shot number of 100 shot/min.
4.3

Results of Numerical Analysis Based on Changes of Heat Cutter Material
The results of numerical analysis based on changes of the heat cutter material (transfer of heat in
the tip of the heat cutter) are shown in Figures 11, 12 and 13.
The horizontal axis ‘time’ means the time passed since the start of operations (time passed since a heat
cutter in a stable state began cutting) in order to calculate operation time simulations. The figures only
show 0–20 s ; however, the actual numerical analysis calculated up to 60 s.

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Study on the Effects of Various Factors on Side Seal Strength

stainlessdie steelFig.11

Temperature of tip of heat cutter (preset heater temperature 350°C)

stainlessdie steelFig.12Temperature of tip of heat cutter (preset heater temperature 400°C)

stainlessdie steelFig.13Temperature of tip of heat cutter (preset heater temperature 450°C)－ 362 －

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No.6（2015）

Vol.24

conventional shape
prototype shapeFig.14

4.4

Transfer of heat in tip of heat cutter

Numerical Analysis based on Results of Changes of Heat Cutter ShapeFigure 14 shows the numerical analysis based on the results of changes of heat cutter shape shownin Figure 10.

5. Discussions
5.1
Discussions of Test Results
1) Figure 4 shows that the breaking load increases with an increase in the thickness of the CPP film. All
the CPP tests broke in the base material. This is because the film itself has a lower breaking load than theseal section. Therefore, we believe there is a proportional relationship between breaking load andthickness of a film.
2) Figure 5 shows that the breaking load tends to increase with an increase in the thickness of the OPPfilm but that it becomes almost fixed after a thickness of 30 μm. This is because the base material breaks
down at thicknesses less than 30 μm, however seal breakage occurs at 30μm and higher. As shown in
Figure 15, the seal width is not dependent on the film thickness and it could be considered that the
breaking load is roughly the same because the seal width is fixed.

0.25mm(a)30μm
Fig.150.25mm(b)40μm

0.25mm(c)50μm

Seal thickness for each OPP film thickness

3) As shown in Figure 6, we were able to confirm that increases in the composite thickness of laminate do
not affect the breaking load, which remains roughly the same. This information appears to coincide with

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Study on the Effects of Various Factors on Side Seal Strength

the OPP trend because the breaks all occurred in the seals. There is a slight rise compared to the OPP film,
which may be caused by the other film, CPP; however, confirmation of this requires further investigation.
4) Figure 7 shows that an increase in shot number is accompanied by a decrease in breaking load. This is a
result of a reduction in the contact time between the film and the heat cutter as the shot number increases,
which leads to an insufficient supply of heat during cutting/welding. Finally, an increase in the shot
number causes an increase in the number of contacts with the film and other materials (there is a rubberroll underneath the film), which contributes to a reduction of heat from the tip of the heat cutter and as aresult, causes insufficient joins.
5) There is a general trend of the breaking load increasing with an increase of preset heater temperature. A
similar trend is shown in Figure 8. Since the break pattern in OPP film with 30μm thickness occurs within
the seal, it can be concluded that an increase in the preset heater temperature improves the strength of the
seal.
5.2
Results of Two-Dimensional Non-steady Finite-Element Analysis
1) From Figures 11, 12, and, 13, die steel can be considered stable because it is more flexible to changes
in the temperature zone (the belt-shaped region, in the figures that forms as the maximum and minimum
temperatures, which generated as the temperature in the tip of the heat cutter fluctuates each cycle,changes over time). In addition, we enlarged the temperature transfer of the vicinity after 60 s, anddetermined the maximum and minimum temperatures, as well as their difference (dT) as shown in Figure
16. Each preset heater temperature is shown in a separate graph in Figure 17. Figure 17 shows that the die
steel has higher maximum and minimum temperatures and a smaller difference (dT) compared to stainlesssteel. The results also show that regardless of increasing the preset heater temperature, if stainless steel
comes in contact with the film, its temperature will lower to around 100°C, but the temperature of die
steel will not lower to that same degree. This shows that using materials with high thermal conductivity
for the heat cutter reduces the change in temperature in the tip of the heat cutter, which contributes tostable operation.
0.6

Fig.16

Method to determine maximum temperature, minimum temperature, and dT－ 364 －

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Vol.24No.6（2015）

dT
dT

Fig.17

Temperature of a neighboring heat cutter tip after 60 s

2) Figure 14 shows that the prototype shape can be considered stable because the changes in thetemperature zone are more flexible. In addition, Figure 18 shows that dT decreases as the maximum andminimum temperature increase, which results in improvements in the characteristics related totemperature stability in the tip of the heat cutter.
Thus, the prototype shape showed favorable results; however, the prototype shape used in this study
was designed based on the improvement areas, below.
Plan A: The shaded section (1) in Figure 19 removes heat from the heater and obstructs heattransfer to the tip of the heat cutter.
Plan B: By reducing the length (L) represented by the thick line in Figure 19, namely the heat pipe,
reducing distance between the tips of the heat cutter (heater – distance between tips of theheat cutter) improves temperature recovery of the heat cutter.

0.6

199.5℃
230.2℃
conventional shape
prototype shapeFig.18Temperature of a neighboring heat cutter tip after 60 s

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Study on the Effects of Various Factors on Side Seal Strength

Fig.19 Improvement areas forthe conventional shape(a) plan A
Fig.20(b) plan B

Shape and size of heat cutter (1/2 model)

In order to examine the type of effect the two improvement areas have on the temperature in the tipof the heat cutter, analysis was conducted on only Plan A with prototype shape A (Fig.20 (a)), and on only
Plan B with prototype shape B (Fig.20 (b)).
The results are shown in Figures 21 and 22.
conventional shapeplan A

conventional shapeplan B
0.6

0.6

250.1℃

183.4℃
230.2℃

230.2℃

Fig.21 Temperature of a neighboring heat
cutter tip after 60 s

Fig.22Temperature of a neighboring heat
cutter tip after 60 s

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In Figure 21, the maximum temperature of prototype shape A is relatively unchanged but theminimum temperature is approximately 50°C higher. This is because part (1) on Figure 19 was removed,
which reduced wasteful diffusion of heat and suppressed a decrease in temperature in the tip of the heat
cutter.
In Figure 22 the minimum temperature of prototype shape B is relatively unchanged but the
maximum temperature is approximately 20°C higher. This is because the length (L) in Figure 19 was
shortened, which reduced the amount of time taken for the heat to reach the tip of the heat cutter andincreased the speed of temperature recovery in the in the tip of the heat cutter.

6.

Conclusion

This paper covers a practical investigation of side seal strength and a numerical analytical study on
the shape and material used in heat cutters with the objective of improving the strength (side seal
strength) of bags manufactured using the side sealing method. The main conclusion remarks are as
follows:
1) The breaking load of CPP film increases with an increase of thickness.
2) The breaking load of OPP film increases with an increase of thickness until 30 μm,
after which the breaking load remains mostly steady.
3) The breaking load of laminate film remains mostly steady with an increase of composite
thickness but there is a possible trend of a slight increase. Further research required.
4) The breaking load decreases as shot number increases.
5) The breaking load increases with an increase in preset heater temperature.
6) Confirmed that thermal conductivity greatly influences the temperature in the tip of the heat
cutter by changing the thermal conductivity and conducting a finite-element analysis on theheat cutter’s two-dimensional non-steady heat conduction. In addition, the use of materials withhigh thermal conductivity in heat cutters reduces temperature change in the tip of the heat cutter,
which contributes to stabilizing operations.
7) Conducted a thermal conductivity analysis and refined future prototype selections by studying
heat cutter forms that have the possibility to improve the temperature in the tip of the heat
cutter.

Acknowledgements
We would like to sincerely thank Kouichi NAGAMORI from Yunidopack Co. Ltd. for, upon thecollection experiment data collection and essay writing, offered us a place for our various experiments
and for providing much guidance and information in related research.

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3) Doi T., Nishizaka T. and Shiraiwa H., Effects of a design for heat cutter on the Strength of a Side Seal

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Study on the Effects of Various Factors on Side Seal Strength

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（Received 20March 2015）（Accepted 1October 2015）－ 368 －