ThreeBond Technical News
Issued January 1, 2024

103

UV-curing Gasket Molding Technology
for Use with Fuel Cells
Introduction
In recent times, the automotive industry has seen an accelerating shift away from fossil fuels
and toward alternative forms of energy with the aim of achieving carbon neutrality. Against thisbackdrop, fuel cells, which do not emit substances harmful to the environment during driving,have garnered particular attention as a new power source for next-generation automobiles. Gaskets
that can seal substances such as hydrogen gas and coolant fluid are indispensable in the use of fuelcells. With more varied applications and increased production as part of the progression towards
the widespread adoption of fuel cells, there is also a need for gasket molding processes that reduce
production takt times in order to enhance design freedom and reduce costs.
Conventional sealing methods include the UV-cured-in-place gasket (UV-CIPG) method, whichuses UV-curable gaskets that are applied and cured with UV light before assembly takes place.
These gaskets are described in ThreeBond Technical News No. 72. However, disadvantages ofthis method include the high level of production technology required to achieve consistent gasket
dimensions and the fact that there are restrictions on the cross-sectional shapes into which gaskets
can be molded.
In this article, the authors describe the light-curing mold method, a new molding technology that
resolves issues faced with conventional methods, and ThreeBond 3178B, a UV-curing gasket thatprovides the performance required for fuel cell-related applications. These innovative molding
technologies can be expected to make a major contribution to the widespread adoption of fuel cells
and the establishment of a sustainable automotive industry.
Hereafter, ThreeBond is abbreviated as TB.

Table of Contents
Introduction............................................................. 1
1. Overview of FCEVs and Gasket
Requirements..................................................... 2
2. Molding Methods for Use with FCEVs............... 3

3. TB3178B: UV-curing Gasket.............................. 5
4. Performance Improvement due to Gasket
Shape................................................................. 7
Closing.................................................................... 81

1.

Overview of FCEVs and Gasket
Requirements

1-1. FCEVs
Unlike automobiles with conventional engines, fuel cellelectric vehicles (FCEVs) are a type of automobile thatrun using motors powered by electricity produced usinghydrogen and oxygen as fuel.
Against the backdrop of the sustainable development
goals (SDGs) and other international initiatives aimed at
resolving social issues and building a sustainable future,
FCEVs’ qualities as vehicles that only emit water and do
not emit CO2 during driving mean they can be expected
to contribute to efforts to address climate change andother environmental issues by enabling the realization of
decarbonization.
1-2. Electricity Generation in FCEVs
Fuel cells generate electricity through a mechanism
comprising hydrogen, oxygen and cell stacks. As shownin Figure 1, cell stacks are stacks consisting of hundredsof cells. These cells consist of three layers—an electrolyte
membrane and two electrode plates (positive andnegative). They generate electricity through the chemical
reaction between hydrogen and oxygen.
1-3. Gasket Performance Requirements
When fuel cells generate electricity through the chemical
reaction between oxygen and hydrogen, oxygen, hydrogen
Enlarged View

and coolant flow between the cells. The coolant reduces theheat produced during electricity generation. The gaskets
between cells must have effective barrier qualities. One
reason for this is that hydrogen, in particular, has a low
molecular mass and can leak through even the slightest
gap. As a colorless and odorless flammable gas, when it
mixes with oxygen in the air, even small sparks can resultin fires.
Furthermore, because hundreds of cells are stacked
together, the gasket dimensions must be precise andgaskets require effective sealing properties that can adapt
to the expansion and contraction caused by the heatproduced during electricity generation.
In order to withstand the chemical reactions andtemperature changes that take place inside cell stacks,
gaskets must also have the durability to maintain a stablebarrier under low-temperature, high-temperature andhigh-pressure conditions. These gaskets are compressed
while being exposed to high temperatures, and must
maintain their repulsive force without failing.
Issues faced with conventional technologies using
thermoplastic resin include the significant thermal effecton cells when insert molding is used to apply the gaskets
and long processing times due to time needed for heating
and cooling.
To resolve these issues, ThreeBond began development
of UV-curing gaskets and related molding methods thatmeet the performance requirements for fuel cell-related
applications while using energy from UV light to enable
immediate curing at room temperature under normal
atmospheric pressure.

GasketSeparator
Stack
Cell

Electrode plate(positive)
Electrolyte membrane
Electrode plate (negative)

2

Separator

Fig. 1 Cell Stack Composition

2.

Molding Methods for Use with FCEVs

2-1. Conventional Molding Methods andRelated Issues
In general, when thermoplastic resin is molded to cells,
insert molding with a die is used to perform integral
molding on the cell and gasket.
As mentioned earlier in this article, ThreeBond has also
put forward the UV-CIPG method. With the UV-CIPG
method, simple equipment comprising an applicator anddispenser is used to apply UV-curing sealant to the seal
surface. The gasket is then molded through irradiation
with UV light.
A comparison of the two methods is shown in Table 1.

These conventional methods for molding gaskets to cells
presented issues in terms of production cost, quality andprocessing times.
2-2. New Molding Methods
With the aim of resolving the issues found withconventional methods, ThreeBond developed the lightcuring molding, which combines the simplicity of the
UV-CIPG method with high gasket dimension precisionsimilar to the insert molding method.
With this method, the gasket is molded using a transparent
shaping die with grooves in the required gasket shape.
An overview of the light-curing mold method is shownin Figure 2.

Table 1 Comparison of Conventional Methods
MethodUV-CIPG

Standard insert molding

Dimension precision

Average
Difficult to manage start and end point
dimensions

ExcellentHigh precision (mold shape is reproduced
accurately)

Equipment costs

Excellent
Low cost (simple equipment configuration)

Inadequate
High cost (die, injection equipment and heatsource required)

Running costs

Excellent
Low cost (cures immediately with UV light)

Inadequate
High cost due to heat energy

Suitability
for mass production

Average
Robot adjustment required for every
environmental change

Good
Mold shape is reproduced, reducing
variation during production

Freedom to change design

Excellent
Changes to applicator setup only

Inadequate
Redesign from prototype die

Gasket shape freedom

Inadequate
Dependent on liquid properties of sealant

Excellent
Even complex shapes are possible

Illustration

1. Application

2. Bonding

3. Curing

4. Removal

UV-curing gasket isdispensed into grooves inthe shaping die.

The workpiece and theshaping die to which thegasket was applied are
bonded together.

The gasket is cured
through irradiation with UVlight from UV-LEDs.

When the shaping die andworkpiece are torn awayfrom each other, a gasketof the desired shape ismolded to the workpiece.

Fig. 2 Light-curing Mold Method Overview

32-3. Benefits of Light-curing Mold Method

3. Reduced load on workpiece

Shaping dies similar to the example shown in Figure
3 are used. Gaskets accurately reproduce the shape ofthe grooves, making it easy to achieve high dimension
precision.

With insert molding, because heating and cooling form
part of the process, the thermal load on the workpiece isunavoidable. In addition, because high-pressure injection
is necessary to ensure that resin does not leave small
gaps when flowing into the die, a strong tightening force
between the die and the workpiece is required.

Additional benefits are as follows.

With the light-curing mold method, however, because thedie and workpiece are pasted together after application, astrong tightening force is not required. This reduces theload during molding.
4. Simplification of processing conditions

Fig. 3 Shaping Die

1. Improved process capabilities

With the UV-CIPG method, the gasket retains the same
shape it had when it was applied by the dispenser. To
prevent changes in shape due to the sealant’s flowability or
wettability on the workpiece, the sealant must have highviscosity and thixotropicity immediately after application.
Robots must also move slowly (approximately 30 mm/s)
in order to create moldings with uniform shapes, limiting
the processing time efficiencies that can be achieved.
With the light-curing mold method, by contrast, a lowviscosity, low-thixotropicity UV-curing gasket is used tofacilitate flow into the mold grooves, enabling robots tomove at higher speeds (approximately 150 mm/s).
2. Reduced load on gasket

During insert molding with thermoplastic resins, when
the resin flows into the channels, an orientation effect thatstretches the polymer molecules in the direction of flow
occurs at the boundaries between the still-molten resin
and the cool, solidified resin layer and the inner surface ofthe shaping die. Sheer stress can also occur between theflowing layer and the solidified layer, meaning that thetensile and bending strengths of molded items may differ
in the direction of flow and the vertical direction.
With the light-curing mold method, because the UVcuring gasket is cured immediately with energy from UVlight, flowing and solidified layers do not form and thisorientation effect does not occur. Gaskets with uniform
strength can therefore be molded.

4

Setting the molding conditions is an important aspect ofinsert molding. Several parameters must be considered,including the temperature of the melted resin and die, theinjection speed and the pressure. These conditions are set
based on experience and fluid analysis, and the process istime and labor intensive. For the resin to flow efficiently
through the channels, heating-induced changes in resin
viscosity and changes in temperature during injection
must be calculated. The injection speed and pressure
also affect the temperature, which further complicatesthe calculation conditions, increasing the time needed toascertain the optimal conditions.
The shaping die must also be designed with precision. The
position and number of inlets and outlets are importantfactors in ensuring that resin flows into the entirety of thechannels. Design work and repeated prototyping basedon experience and fluid analysis take time and increase
costs.
In addition, experience and highly-accurate analysis are
required to reduce stress in the molded object caused byorientation effects.
With the light-curing mold method, application is
performed using grooves in a transparent shaping die,which means that the complex fluid flow calculations
required for insert molding can be omitted and the moldcan have a simpler structure. As a result, managing theamount of discharge from the dispenser is sufficient whensetting the conditions.
In addition, because molding is performed undernormal atmospheric pressure and at room temperature,quantitative conditions can be set using sensors during
the bonding and removal processes, making process
management even easier. Through these characteristics,
this method simplifies processing conditions.

3.

TB3178B: UV-curing Gasket

Table 3 Barrier Properties Comparison(TB3178B and Silicone Resin)

3-1. Features
TB3178B is a polyisobutylene-based non-solvent UVcuring gasket. Its characteristics are described below.

Test item

Unit

TB3178B

Siliconeresin(1) Curable in a short time with irradiation with lightwavelengths from UV to visible light (wavelengths of
200 to 405 nm)

Hydrogen gas
permeability
factormol·m/(m²·s·Pa)

8.6 × 10 -15

1.8 × 10 -13

Moisturepermeability

g·mm/(m²·24h)

80.5

545.8

(2) Excellent hydrogen gas and humidity barrier
properties(3) Rubber elasticity maintained over a wide temperature
range (-40°C to 120°C)

Hydrogen gas permeability - Test conditions: 23°C, t = 1.0 mm
Moisture permeability - Test conditions: 85°C x 85%RH,
t = 1.0 mm(4) Small compression set and excellent sealing properties

3-4. Rubber Elasticity(5) Excellent chemical resistance

A dynamic mechanical analysis (DMA) and thetemperature retraction (TR) of TB3178B are shown in
Figures 4 and 5 respectively.

3-2. Properties and Characteristic Values
The properties and characteristic values of TB3178B are
shown in Table 2. To ensure suitability for the light-curingmold method, the viscosity and thixotropicity (structuralviscosity ratio) have been adjusted such that the productflows easily into grooves during application.
3-3. Barrier Properties
The barrier properties of TB3178B are shown in Table
3. Polyisobutylene, which is the main component of
TB3178B, has a C-C chain in its backbone, and the
distance between molecules is shorter than with thesiloxane chains found in silicone. Due to its low freevolume, it is flexible yet has excellent hydrogen gas andmoisture barrier properties.

TB3178B has a glass transition temperature of -43°C
and can retain rubber elasticity even in low temperaturebands of approximately -40°C.
3-5. Compression Set
The compression set of TB3178B is shown in Figure 6.
Compression sets are used to evaluate the distortion of agasket after durability testing. If the strain increases, therepulsive force during compression decreases, making it
difficult for sealing properties to be maintained. TB3178B
has a small compression set, meaning it can maintain its
sealing properties for a long time.

Table 2 TB3178B Properties and Characteristic Values

Characteristic value

Property

Test item

Unit

TB3178B

Testing Method

Remarks

Appearance (color)

–

White

3TS-2100-020–

Viscosity

Pa·s

45

3TS-4200-001

25°C, shear rate: 1.0 (s-1)

Structural viscosity ratio–

1.9

3TS-4200-001

25°C, shear rate: 10 (s-1)/1.0 (s-1)

Specific gravity

–

1.00

3TS-2500-002

25°C, liquid specific gravity

Hardness–40

3TS-2B00-010

Durometer A

Tensile strength

MPa

2.4

3TS-4190-001

No. 3 dumbbell

Elongation%380

3TS-4190-001

No. 3 dumbbell

Thick film curability

mm

6.2

3TS-3160-001

ø32mm

Storage modulus

Pa

4.9 × 106

3TS-4730-001

1 Hz, 25°C

Glass transition temperature

°C

-43

3TS-4730-001

1 Hz, tanδ peak value

Hydrogen gas permeability
factormol·m/(m2·s·Pa)

8.6 × 10 -15

JIS K7126-1

23°C

Moisture permeability

g·mm/(m2·24h)80.5

JIS K7129-2

85°C × 85%RH* Curing conditions: 405 nm LED, illuminance: 500 mW/cm , UV dose: 40 kJ/m
2

2

5

The chemical resistance of TB3178B is shown in Figures
7 to 10. Solid polymer fuel cells fitted to items such as
automobiles have operating temperatures of 70°C to
120°C, and contact between sulfonic acid groups fromthe solid polymer membrane and water vapor generated
during operation result in an acidic environment.
Consequently, fuel cell gaskets require heat resistance,water resistance, coolant resistance, and acid resistance.

Hardness (A)

3-6. Chemical Resistance

120°C environment

Under these conditions, TB3178B can maintain stablerubber characteristics for a long time.

(h)Hardness (A)tanδ (-)

E’, E’’ (Pa)Fig. 7 Heat Resistance (Hardness)

Pure water, 95°C environment(h)Temperature (°C)

Fig. 8. Water Resistance (Hardness)

Shrinkage (%)

Hardness (A)Fig. 4 TB3178B DMA

Coolant, 95°C environment(h)Temperature (°C)

Fig. 5 TB3178B TR

Hardness (A)Compression set (%)

120°C heat resistance
Water resistance at 80°C

H2SO4 aqueous solution (pH=2),
95°C environment(h)Fig. 6 TB3178B Compression Set

6Fig. 9 Coolant Resistance (Hardness)(h)Fig. 10 Acid Resistance (Hardness)

4.

Performance Improvement due toGasket ShapeThe cross-sectional shape of gaskets molded with the UVCIPG method are dependent on the fluid characteristics ofthe gasket, wettability with the workpiece and dispenser
performance. Differences in the cross-sectional shape ofgaskets can lead to changes in performance. The lightcuring mold method enables the molding of gaskets witha consistent shape and optimization of the cross-sectional
shape, making it possible to stabilize and improve
performance.
Accordingly, the next section describes the sealing
characteristics of four typical cross-sectional shapes at
low compression.
4-1. Sealing Properties: Test Method

4-2. Sealing Properties: Evaluation Content• Cross-sectional shape of gasket under evaluation(Figure 12)• Groove shape: 65 mm square, four corners of R10(Figure 13)

Semicircle

Triangle

SquareTrapezoid

Fig. 12 Cross-sectional Shape of Gasket(Height: 1mm)

• Gasket: TB3178B• Test temperature: 25°C; 50%RH• Adherend: A5052• Seal surface: A5052• Gasket compression ratio: 20% compression• Curing conditions: 405 nm LED conveyor, 40 kJ/m2• Pressure medium: Dry air (maximum: 800 kPa)
• Pressurizing conditions: 50 kPa/30 sFig. 13 Shaping Die Used for Evaluation• Leak threshold: Air leak of 15 mL or more in 1 sec• Data: Median (n= 3)

Gasket for evaluation

Flange after completion

The sealing properties of each cross-sectional shape at
20% compression are shown in Figure 14. A two-folddifference depending on the cross-sectional shape wasobserved. This is due to differences in the repulsive force
during compression and the contact area with the sealinterface. In compression seals, higher repulsive forces
generally mean a higher sealing pressure. Based on the
results of the cross-sectional shapes that were evaluated,
it was found that increasing the repulsive force under low
compression conditions and expanding the contact area
were effective in improving sealing properties.

Sealing properties (kPa)

Fig. 11 Flange for Sealing Property Test

4-3. Results

Semicircle

Triangle

SquareTrapezoid

Fig. 14 Sealing Properties by Cross-sectional Shape at 20% Compression

7

Closing
TB3178B’s characteristics make it suitable for the harsh environments found inside fuel cells, and the light-curing moldmethod is a flexible method combining the features of the UV-CIPG and insert molding methods. As a result, thesetechnologies have the potential for widespread applications.
ThreeBond will continue to contribute to industrial development by focusing on further product and technology
development that goes beyond existing technologies.
<References>
1) Watanabe, Kuniyuki et al. Technical News No.72, ThreeBond (2009)2) Medium-term Forecast for Plastic Machinery Demand, The Japan Society of Industrial Machinery Manufacturers
3) Plastic Molding Technologies, Shiguma Shuppan

ThreeBond Co., Ltd. R&D Group
Automotive Market Department I, Product Development Division I
Nao Suzuki
Equipment & Machine Development Department, Application Equipment Division
Takuma Kado

● ThreeBond is a trademark or registered trademark of ThreeBond Holdings Co., Ltd.

ThreeBond Co., Ltd.
8

4-3-3 Minamiosawa, Hachioji,
Tokyo 192-0398
Tel: +81-42-670-5333