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Market development and new grades of DURACON® POM for automotive fuel system components


Market development and new grades of DURACON® POM for automotive fuel system components


1. Introduction

Polyacetal resin (hereafter abbreviated as "POM" representing Polyplastics product DURACON®) is used in large numbers of automotive fuel system components due to its superior mechanical properties, heat and fuel resistance, and good molding processability. Fuel system components is the general term for parts that operate connected to the fuel tank. Table 1 shows some specific component names, examples of resin materials used for them, and their most important characteristics. Here we will share the reasons why DURACON® POM is chosen for fuel system components and some specific components in which it is used, then introduce grades that target the latest fuel system components.


Table 1 : List of fuel system components

Component name

Main resin materials

Most important
characteristics

 Fuel tanks

 EVOH, HDPE, PA

 Impact resistance
 Gas barrier properties

 Fuel tubes

 PA, fluororesin

 Impact resistance
 Gas barrier properties
 Conductivity

 Fuel sending modules

 POM  Long-term durability
 Dimensional stability

 Module fixing nut

 POM  Long-term durability

 Caps

 POM, PA  Impact resistance
 Conductivity

 Filler necks

 PE, PA  Impact resistance
 Gas barrier properties
 Conductivity

 Valves

 PA, POM, PBT  Gas barrier properties
 Weldability


Illustration 1-1 : Fuel system components (all parts of a car)

Illustration 1-2 : Fuel system components (Fuel sending module)


2. Reasons why DURACON® POM is chosen for automotive fuel system components

(1) Initial mechanical properties

DURACON standard type (MFR※1 around 1:9) M90-44 is used in many fuel system components. This is because its basic initial characteristics such as tensile strength, flexural modulus, charpy impact strength, and deflection temperature under load have the mechanical properties and heat resistance required as a structural member.

Illustration 2-1 shows the SS curve (stress-strain curve) in tensile strength testing of
M90-44 at room temperature. Yield strain※2 is 8%, and with its superior spring elasticity it is suited for snap fits that bring about temporary warpage of several percent during fastening, which helps make assembly simpler.

High-flow type (MFR※1 around 1:14) grades are also widely used in Europe and some parts of the United States of America. Having taken moldability into consideration, Polyplastics also offers grade (H140-54C, details will be discussed in the latter half) for these purposes.


Illustration 2-1 : SS curve (stress-strain curve) in tensile strength testing

※1 MFR is a value indicating melt flowability of plastic materials

※2 Yield strain is strain around the maximum stress value of tensile strength

 

Reference) Snap fit design:

https://www.polyplastics.com/en/support/othe/snapfit_design/index.html



 

(2) Fuel resistance

 

Mechanical properties after two-fuel mixture (dummy fuel: Fuel C + methanol 15%) immersion is just as important as initial mechanical properties, and these are shown in illustrations 2-2 and 2-3.

Illustration 2-2 shows that retention of tensile strength after 5,000 hours of immersion is at least 80% after immersion in any type of fuel, and we can see that the value does not change over longer periods of time after 300 hours. This is because fuel swelling reaches saturation at around 300 hours as shown in illustration 2-3. It shows that physical properties do not degrade any further after reaching the saturated swollen state. Besides, POM is known to maintain a practical level (around 30 g/m2, 24 hours, atm @60℃, 1 mmt) of fuel permeability. Given these properties we can say that DURACON retains sufficient resistance over long hours of fuel immersion.

As for the amount of swelling and saturation and forecasted mechanical lifetimes of other two-fuel mixtures (for example, fuels mixed at arbitrary ratios such as regular gasoline/methanol) , please see "Lifetime forecasting technologies for fuel system components" in the link.


Illustration 2-2 : Retention of tensile strength after fuel immersion

Illustration 2-3 : Weight change after fuel immersion

(3) Weldability

DURACON products such as M90-44 are suitable for types of welding such as hot plate welding, ultrasonic welding, and laser welding, and one of the reasons POM is chosen is that each POM component is suitable for welding to form modules.

 

Aside from that, we believe that DURACON has been chosen for so long for automotive fuel system components which demand high reliability because it also offers excellent injection moldability (comparatively short molding cycle, good ejectability, etc.), and dimensional stability.



3. Automotive fuel system components in which DURACON® POM is recommended

(1) Module fixing nuts

This is a nut made from resin, which fixes fuel pump modules to the tank. Metal lock mechanisms are also used for the same purpose, but we recommend high-viscosity type DURACON which has superior creep resistance (long creep-fracture life, low creep deformation).


(2) Fuel sending modules

These components integrate the function of supplying fuel from the fuel tank to the engine, and typically are modules (integration of functions into one component) that include a fuel pump, fuel sender, fuel filter, and pressure regulator. DURACOM and DURAFIDE® PPS are used in many of these components.


(3) Caps

   

This is a cap for filler necks. For its components, M90-44 or
M25-44 that are imparted with balanced mechanical properties and long-term properties are recommended in order to maintain tightening force retention upon repeated operation , and high sliding grades of DURACON are also recommended for stable sliding.

Recently, the use of capless fuel systems which can supply fuel by opening the fuel lid and inserting a nozzle has also started to be employed.

For the capless systems, in addition to the grades of DURACON used in the existing cap systems, the grades of DURAFIDE are also recommended for the components that require higher rigidity.


(4) Valves

Valves such as ORVR (Onboard Refueling Vapor Recovery) valves and cut-off valves are connected to fuel tanks. ORVR valves stop fuel vapor from being released into the air through the filler neck during refueling. Cut-off valves stop fuel from flowing out of the fuel tank when vehicles overturn. For both of these parts we recommend DURACON standard grade (M90-44) which has good mechanical properties in addition to superior assemblability (snap fit, weldability).

Recent trends in the fuel system components addressed here include fuel pump modules with integrated controllers, and brushless fuel pumps. These are all technologies that reduce power consumption, and Polyplastics is also working to offer materials and develop technologies that contribute to this cause.



4. Introducing our newest grades

Next we will introduce our newest materials. (Table 2) They are all high-flowability grades with an emphasis on being easy to mold, and have high rigidity for a higher level of freedom in design. Each grade also has a different function according to its purpose.


(1) High-flow high-rigidity grade H140-54C

High-flowability type (MFR around 14) POM which suits a wide range of molding conditions (cylinder conditions, injection pressure) is used in Europe. This is a grade that has high flow and high rigidity. The aim of going to higher rigidity is to allow products to be thinner, and higher flow suppresses increase in peak pressure that occurs during injection molding due to thin-walling. Using this grade makes smaller parts and thinner walls possible.


(2) Electrically conductive grade (developed material)

Materials imparted with conductivity are used in components that surround fuel as a measure against static electricity. Typically, when resin is imparted with conductivity it often produces an impact on other characteristics, resulting in less flowability and toughness. However, this grade has successfully imparted conductivity while maintaining the benefits that POM resin has to offer.


(3) Acid-resistant grade (material under development)

We are currently developing a grade with acid-resistance on top of high flow and high rigidity. As shown in Illustration 3-1, due to its reaction to acid degradation POM has the characteristic of being vulnerable to strong acids. Among the components of fuel systems, ORVR valves, cut-off valves, and fuel pump module flange parts attached to the outside of fuel tanks can be damaged by the acidity in automotive cleaning liquids, acid rain, and other such substances. We move ahead with this development for the purpose of significantly improving acid resistance which is a weakness of POM. For reference, we will show the results of our acid resistance test below.

 

Evaluation methodology for acid resistance test (Illustration 3-2)

(1) Apply a constant strain to an ISO tensile test piece (4 mmt)

(2) Apply acid to the entire test piece

(3) Check the surface of the test piece after 24 hours, and if there is no cracking then it

      clears one cycle (Illustration 3-3)

(4) Repeat the cycle in (2) -> (3), and the result is measured as the number of cycles it

      takes for cracking to occur



Illustration 3-1 : Reaction to acid degradation in POM

Illustration 3-2 : Acid resistance evaluation methodology

Illustration 3-3 : Surface state after acid resistance evaluation
(left: fractured item, right: non-fractured item)

Table 2 : Physical properties for each new grade

 

High strength

Conductivity

Standard

H140-54C

Developed material

M90-44

 MFR

g/10min

14

11

9

 Tensile strength

MPa

72

69

 62

 Nominal tensile
 strain at break

%

34

11

35

 Tensile modulus

MPa

3,050

8,000

2,700

 Charpy impact strength

kJ/m2

6

4

6

 DTUL

101

-

95

 Volume resistivity

Ω-cm

-

2 × 103

1 × 103


5. In conclusion

Although the elimination of fossil fuels in automotive power is a sought-after goal, fuel efficiency improvements should enable us to continue using some of our existing power systems in the future. Polyplastics will continue developing new materials and spreading new technologies to reduce size and weight of fuel system components, and improve their functionality.





 Technical data sheet is available online:

  ◆M90-44   ◆H140-54C

 For inquiries about our technologies
 and materials, please contact us via:

  ◆Polyplastics Group Representative
  ◆WEB Inquiry


25th June 2019