Materials and Manufacting of the Fuel Tank Assembly (Part 50)
Material
The fuel tank assembly is made from High Density Polyethylene (HDPE). It is a type of polyethylene, which is inert and extremely resistant to fresh water, salt water and most water-based solutions. It is cheap and quite easy to mold and fabricate. It can also accept a wide range of colors, can be transparent, translucent or opaque, but is difficult to print on.
Other forms of polyethylene include Low Density Polyethylene (LDPE); which is usually used for film and packaging. LDPE has branched chains which don't pack well, making it less dense than water. Medium Density Polyethylene (MDPE) and HDPE have longer branches, less branched chains, making them stiffer and stronger and are usually used for containers and pipes.
In its pure form it is less resistant to organic solvents, but by converting its surface to a fluoro-polymer, by exposing it to fluorine gas, will overcome this weakness. When it has been treated in this way it can be used for petrol tanks and copes with oil, cleaning fluid, cosmetics and most other corrosive substances.
HDPE has a high melting point ranging from 125°C - 132°C meaning that even if the engine becomes quite hot the HDPE can withstand the heat. It's also a good thermal and electrical insulator, is quite strong, has a relatively low carbon footprint and can be recycled (category 2). It is quite cheap, for these reasons HDPE was used to make the fuel tank.
Some HDPE Material Characterics:
Melting point (°C) 125 - 132
Price (£/kg) 0.792 - 0.871
Tensile strength (MPa) 20.7 - 44.8
Compressive strength (MPa) 19.7 - 31.9
Hardness (HV) 5.4 - 8.7
Good thermal insulator
Good electrical insulator
Moldability 4 - 5
CO2 footprint (kg/kg) 1.95 - 2.16
Recyclable HDPE category 2
Manufacturing Processes
There are 2 possible manufacturing processes that can be used which are:
1. Blow Molding
Blow molding is the glass blowing technology which has been adapted/modified for polymers. There are different types of blow molding these include:
1. Extrusion blow molding - a tube is extruded and clamped with a hollow mandrel at one end. Hot air is forced under pressure through the mandrel, blowing the polymer against the mold walls where it cools and freezes.
2. Injection blow molding - a pre-form is injection molded over a mandrel and transferred to the blowing die. Air is injected under pressure through the mandrel blowing the polymer against the mold walls where it cools and freezes.
Injection blow molding gives better control over finished component weight and wall thickness than extrusion blow molding, with better precision in the unblown, injection molded neck areas for screw closures etc. Whereas, extrusion blow molding allows a variety hollow shapes to be formed, offering channels for air or liquid and is cheaper (because the die costs less), but tool costs are higher.
However, multi-layer injection blow molding is commonly used for components that need to be air tight and strong.
Blow molding is limited to thermoplastics, commonly PET (Polyethylene terephthalate), PC (Polycarbonate), HDPE, LDPE, PP (Polypropylene), ABS (Acrylonitrile butadiene styrene) and some PVC (Polyvinyl chloride).
Some Blow Molding Process Characteristics:
Tolerance (mm) 0.25 - 1
Roughness (μm) 0.2 - 1.6
Surface Roughness (A= v. smooth) A
Relative tooling cost medium
Relative equipment cost medium
Labour intensity low
Economice batch size 500 - 5e7
Capital cost (£) 3.09e3 - 1.03e4
Production rate (per hour) 30 - 300
Typical applications of blow molding are:
1. Injection blow molding; bottles and containers, particularly those with threaded closures.
2. Extrusion blow molding; containers, cases for tools and portable machinery, and large hollow structures such as car bumpers.