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Tech Focus: Hot Runners Two shells, one material

| Author / Editor: Dorothee Quitter / Steffen Donath

Injection moulded motorcycle tank made of blow moulded polyamide.

This motorcycle tank is a joint development by BMW Motorrad and Röchling Automotive Italia S.r.l. in cooperation with Lanxess.
This motorcycle tank is a joint development by BMW Motorrad and Röchling Automotive Italia S.r.l. in cooperation with Lanxess.
(Source: BMW Motorrad)

Compared to tank constructions made of metal sheets, the injection-moulded single-material solution made of polyamide 6 offers great integration potential with varying wall thicknesses.

Compared to tank constructions made of metal sheets, the injection-moulded single-material solution made of polyamide 6 offers great integration potential with varying wall thicknesses.

Materials for motorcycle tanks must meet high requirements. For example, they must only allow very small amounts of fuel to diffuse through the tank walls. The materials must have high toughness and break resistance, especially in crashes, offer great freedom of shaping for the often complex tank geometries and should be economical to process.

BMW’s F 900 R and F 900 XR motorcycle models have succeeded in meeting all these requirements with a single material. The tanks on both machines are made of Durethan BC550Z 900116 DUSXBL from Lanxess. The non-reinforced and impact-resistant modified polyamide 6 is injection moulded into two half-shells that are joined together to form the tank by hot plate welding.

“With our material, the strict permeation requirements of the US Environmental Protection Agency (EPA) can be met without any problems. Since it is also very easy to process by injection moulding, even complex geometries can be produced economically. And when subjected to impact loads, the extremely impact-resistant material finally shows its strengths,” explains Dr. Klaus Küsters, Business Development Manager Blow Molding in Lanxess’ High Performance Materials (HPM) business unit. The tanks are a joint development by BMW Motorrad and Röchling Automotive Italia S.r.l. in cooperation with Lanxess.

Strict EPA limits adhered to

The mono-material solution with polyamide 6 has several advantages over the tank materials previously used. Compared with metals such as sheet steel and aluminium, the freedom of design and the potential for integration are much greater. Cost-intensive forming, punching and joining steps are also eliminated. Corrosion is also not an issue when using polymers.

Compared with other polymers such as high-density polyethylene (HDPE), Durethan BC550Z 900116 DUSXBL has much lower fuel permeation. Fluorination can further reduce permeation in HDPE tanks. However, the associated costs and the aggressive properties of the fluorine argue against this additional manufacturing step. Compared to multilayer systems, which contain a highly polar layer, usually of ethylene vinyl alcohol copolymer, as a permeation barrier, the single-material solution with polyamide 6 is significantly less expensive. If the wall thickness is designed accordingly, the permeation values of the single-component solution are also several times lower than the limit value of 1.5 g/m²∙pro day required by the EPA in the EPA 40 CFR guideline.

Reproducible tight welds

Durethan BC550Z 900116 DUSXBL was developed for blow moulding hollow parts and has been used successfully for many years in the series production of fuel tanks. As a result of the increasing number of functions — such as ASC (Automatic Stability Control), for example — the installation space for fuel tanks is becoming increasingly rugged and complex. These geometries can hardly be implemented in the blow moulding process. For this reason, two halves of the current BMW motorcycle tanks are injection moulded and then welded together to form the tank. “Not only can more complex geometries be mapped in the injection moulding process, but the wall thickness distribution can also be precisely controlled, which in turn allows a more precise design of the tanks with CAE tools,” explains Küsters. The welding properties of the polyamide 6 are essential for the reliable production of the tanks: "The material is characterized by very good weldability. Our customers have a wide process window at their disposal in order to produce tight weld seams in a reproducible manner. A weld seam factor of 1 can be achieved without any problems," says Frank Krause, plastics welding expert at Lanxess.

High crash safety

Another special feature of the material solution is its very high impact strength. This means that the tank bladder remains intact even in the event of a crash, and no fuel can escape. Pendulum impact tests on aged tanks, for example, do not lead to leaks at either -20 °C or 60 °C.

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