Nice and hollow
Presentation by Jochen Schmid,
Processing Technology, Engineering Plastics,
BASF Aktiengesellschaft, Ludwigshafen, Germany
The water-injection technique (WIT) can be employed to manufacture complex
thermoplastic hollow parts highly efficiently by means of injection molding.
The WIT is a refinement of the gas-injection technique or gas-internal pressure
technique (GIT), which is already well established on the market. Like the GIT,
the water-injection technique removes material from the interior of the
component, resulting in a weight reduction, fewer visible sink marks, less
warpage, reduced clamping force and greater design freedom. Major advantages in
comparison to GIT are the significantly shorter cooling time – which reduces
the cycle time by 25% to 40% – uniform residual wall thicknesses over the
entire length of the channel and, last but not least, the possibility of
creating molded parts having larger cross sections. In the WIT, the cavity of
the mold is first filled with melt and the internal melt volume is then
expelled by water before the melt solidifies. If the part geometry allows this,
preference is given to the compound backpressure method in which water forces
the plastic material back into the plasticizing unit.
BASF research project involving WIT types
The numerous conceivable applications were the reason for BASF to conduct an
extensive research project on the possibilities of using and optimizing the
WIT. Towards this end, the installation and mold technology as well as the
injector, among other things, had to be modified. The objective was to get to
know the process very precisely so as to intelligently optimize BASF’s plastics
for use in this process. It turned out that the WIT in its conventional form is
not suitable for all engineering plastics. With a modified WIT method, however,
most non-reinforced as well as filled plastics can be processed. Here, air or
nitrogen acts like a thermal buffer, thus preventing an abrupt hardening of the
melt upon contact with the water. The advantages of the WIT have only become
truly accessible with this process variant, which has now become
well-established on the market. For instance, the WIT can be used to create
channel diameters that would have been possible with the GIT – if at all – only
with highly fluctuating residual wall thicknesses. Even components that were
never attempted with the GIT can now be created in this manner. Fundamentally
speaking, there are two types of WIT applications: components that carry media,
in other words, functional channels that serve to transport fluids, and
structural components in which the channel increases the stiffness of the
component or else serves to avoid surface flaws. The former need to have a
smooth channel surface with favorable flow properties, they have to be
chemically resistant and to exhibit high strength when exposed to media.
Moreover, in the case of reinforced materials, it is also important that
leaching does not expose any fibers. Functional channels are found in the case
of coolant channels but also in cylinder head covers that have lubricant lines
or in continuous flow heaters. In contrast, when it comes to structural
components, it is important to have a good exterior surface, very high
stiffness, strength and suitability for coating. Examples of applications are
handles, pedals, mirror frames or roof rails as well as seat shells. Both types
of channels can be installed in tubular as well as in flat components.
Products for the WIT
BASF’s product portfolio already contains several standard types that are very
well-suited for processing by means of the WIT. One example is Ultramid®
A3WGM53 for applications exposed to hot oil. On the other hand, merely
optimizing the process technology of the WIT does not attain the desired
objectives for all engineering plastics. This is why a hydrolysis-resistant PA
66 type (Ultramid® A3HG6 WIT) has been developed within the scope of BASF’s
research project especially for processing by means of the WIT. This material
also meets the requirements for a smooth channel surface with favorable flow
properties, it exhibits high resistance to media, it is chemically resistant
and prevents leaching. During the development of the material, it was possible
to precisely work out the interaction between the chemical structure and the
processing properties as well as the characteristic values of the material that
are crucial for processing by means of the WIT and for the quality of the WIT
components. Building upon this knowledge, the BASF experts studied and
optimized other products. In this process, they obtained excellent results for
the PA 6 type Ultramid® B3G10 SI (SI stands for surface improved) as well as
for the new polyester (PBT) Ultradur® B 4040 G10 WIT, reinforced with 50
percent fiberglass and modified specifically for the WIT. Both types are
suitable for structural components that have to meet high mechanical
requirements while also exhibiting an excellent exterior surface.
First applications
BASF’s first WIT application is the mirror bracket for the DAF XF105, with a
shot weight of 290 grams. Since 2005, this component has been produced in
series with Ultramid® B3WG6 GP, a PA reinforced with 30% fiberglass. Together
with a renowned tier 1 company, BASF has developed a cooling water pipe made of
the new Ultramid® A3HG6 WIT. The first samples obtained with an experimental
mold showed excellent results and the product passed the working-life tests
under exposure to temperature and media on a hydrodynamic test bench. The
component is scheduled to go into serial production in August. The feasibility
of a roof rail made completely of plastic was studied together with the Decoma
company of the Magna Group. Following good, reproducible results in terms of
channel formation and surface quality, Decoma manufactured roof rails out of
Ultramid® B3G10 SI and Ultradur® B 4040 G10 WIT that passed the mechanical
component tests, especially the crash tests. Future WIT applications could be,
for example, bus seats or automobile seat backrests, areas where the stiffness
would be raised by means of the hollow cross sections without using ribs.
As usual, with this new method, BASF applications technicians are once again
lending a helping hand to processors and design engineers in the configuration
and design of their components. At the present time, BASF is providing support
to several projects by computing the maximum expulsion pressure at a given
geometry and by making recommendations regarding the design of the mold such
as, for instance, the position of the cavities as well as the diameter of the
feed channel and of the overflow. Even though a complete solution for the
simulation of the process is not yet available, further experiments using WIT
simulation in comparison to actual occurrences are planned in order to fill
this gap in the near future.
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New products for the water-injection technique (WIT)
The water-injection technique (WIT) is a special type of injection molding
for plastics with which hollow parts such as cooling water pipes and door
handles can be produced efficiently. But the WIT also makes special demands of
the materials, which is why BASF – in addition to several of its standard types
– is also offering engineering plastics that have been specially optimized for
the WIT. These include Ultramid® A3HG6 WIT and Ultradur® B 4040 G10 WIT.
Lower right side: The mirror bracket of the exterior mirror in the DAF XF105
truck has been serially produced with Ultramid® B3WG6 GP since 2005. Center:
Feasibility study conducted by the Decoma company of the Magna Group for an
automobile roof rail measuring almost two meters in length and made of the new
Ultradur® B 4040 G10 WIT. An analogous part made of Ultramid B3G10 SI has
likewise passed the crash tests. Upper left side: Both structural components
are made by means of the WIT and display very constant wall thicknesses.
Foto: BASF, 2007 - The press photos are only available cost-free to
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