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PU for electronics protection  #9 (91), 2010

Nowadays there is a great variety of man-made materials designed for protection of sensitive electronic and electric components. Thanks to a new technology it becomes possible to combine processes for the production electronic components and protective housings. The only material required to do so is polyurethane which allows finished products to meet various requirements in terms of mechanical and thermal properties.

Gerd Viertel, an expert in polyurethane encapsulation of electronic components at BaySystems, Bayer MaterialScience AG
Daniel Luthi, managing director at Isotherm

A share of electrical and electronical units in computer, mechanic engineering and automotive products increases constantly. Patchplugs, cables, connectors and circuits become more and more sophisticated and in the light of miniaturization trend their dimensions are progressively reduced. Their geometry is also frequently very complex as it is defined by their installation space. At the same time reliability requirements for such components are tough. For example patchplugs contacts must be protected from corrosion and moisture. Moveable cables isolation layer must withstand friction without damages and cable shafts situated in engine compartment must withstand temperatures up to 130°С maintaining their mechanical properties.
In order to meet such diversified requirements including ones for sealing, thermal, chemical and abrasion resistance, electrical encapsulation quality and dimensional stability, electrical and electronic units are "packed" in various housings made of rubber, nitrite, polyester, polyamide, polypropylene or PVC. Therefore unit manufacturer has to make sense of man-made material processing and invest into corresponding technologies as well as logistic systems while maintaining production process time frame. The production costs are consequently increased. Frequently electronic components are so tiny and intricate that manufacturer is not capable to create precise and cost effective thermoplastic housing for them on its own.

RIM and PUR

It is considerably easier and cheaper to exchange the diversity of "packaging" with an only man-made material which meets every requirement and can be produced with simple technological process. Such task was set by Denmark-based BaySystems Polyurethan-Systemhaus a member of Bayer MaterialScience AG and Isotherm AG in Uetendorf, Switzerland. Both partners chose universal materials which has been used in production of electronics housing for a long time – polyurethane.
The companies developed a cost-effective technology which enables both the housing and the protection to be produced in a single step. This technique represents an immediate extension of well-proved Reaction Injection Molding (RIM) technology and is based on polyurethane systems from the Baydur E and Bayflex E ranges from Bayer MaterialScience. Isotherm was focused mainly on developing production plant and mixing unit and Bayer MaterialScience was engaged in material properties optimization. Leverkusen-based man-made materials producer offers its customers tailor-made formulations Baydur®   E and Bayflex® E fitted for new technologies requirements. The service is provided through BaySystems Systemhaus global network.
Polyurethane systems usually can be formulated precisely to meet individual requirements thanks to adequate selection of liquid or low viscosity substances. Materials can be produced in various forms from soft, solid elastomers to semi-rigid and impact-resistant integral skin foam. Soft PU formulations applied to achieve soft and muggy surfaces do not need require plasticizers. It allows to avoid eventual increase of surface fragility due to plasticizer evaporating.

Complex geometry – gentle production.

Compact elastomer systems and semisolid foam plastics are most commonly selected for housing production and simultaneous part molding according to the new method. In order to produce compact part housing, cable shaft or battery chamber electronic components are placed in molding tool. Polyurethane material is then homogenized and injected into molds at low pressures of up to 6 bar. Therewith due to low mixture viscosity (2500 MPa•s or less) there is virtually no shearing force which otherwise could assign load to components. Here, the material reacts rapidly to produce polyurethane featuring no volume shrinkage. Finished parts can usually be removed from the mold in less than two minutes.
Curing takes place at a maximum temperature of 120 °C which does not introduce damaging effect to electronic parts which are frequently sensitive to high temperatures. Large protuberances on walls does not represent a problem anymore. The can be applied for production fully integrated reinforcing ribs, ventilation slits and spring locks. This contributes to lowering of production costs as mounting components made of other materials such as clips, plate seals, guides or screws can be also placed in molding tool and integrated into part's structure.

Plants and mixing heads

The new technology makes it possible to fill even the smallest spaces via, in some cases, very narrow flow paths. For this purpose machines with low discharge rates of less than 10 ccm/s  required for adequate air discharge are used.
Isotherm specializing in production technology made required corrections of RIM technology and offers special plunger dosing devices as well as special mixing heads with ceramic nozzles which are designed for minimal material dosages. Precise dosing ensure mold filling with no turbulence and even if dosing device is filled by 60 percent maximal mixture quality is ensured. High filling level required for effective ignition protection or reliable heat discharge from parts subjected to high temperature loads.

For housing production and tiny electronic units molding Isotherm has developed high-pressure plunger dosing device with proportional venting technology which ensures exceptional precision in PU dosing. The dosing device is capable to make shots of as little as 2 g are with 10 ccm/s throughput/

Low tools costs
An enterprise applying RIM technologies for PU processing and special materials meeting various requirements has a range of advantages. The processor saves costs for logistics, lowering processing temperatures and simplifying consecutive mounting which is represented by only integrating required functions into produced part. Furthermore cycle times are reduced contributing to processor's benefits.
A special advantage is represented by lowering investment costs for production plants. For example low pressure in molding tool specific to RIM technology allows to apply cheaper molds made of man-made steel and aluminum instead of expensive steel. Particularly this advantage increases flexibility in production of middle and small-sized parts compared to thermoplastics processing.

Applications
The new technology has already proved its efficiency for example in production of braided cords which are laid from vehicles engine to passenger compartment through splitter wall. When such cords are molded with polyurethane it ensures improved sealing of transitions from engine to passenger compartment, protect contact plates from air, water and damages due to corrosion.
The technology is also well-proven in protection of screw or molded patchplugs from corrosion. Polyurethane is a cost effective alternative to hot molding due to lower costs of raw materials. The most spectacular example of new technology's advantages is production of automotive cables which are shaped with placing in foamed polyurethane. All patchplug contacts are sealed and all metal and plastic mounting elements are shaped simultaneously.
Production of chargers for example for mobile phones, shaving tackles and laptops is one more prospective application area.  Instead of producing two thermoplastics parts all contents may be included in polyurethane block. This results in monolithic housing with hardly fixed and stress-protected components. Thanks to high thermal conductivity and flame resistance of polyurethane, all molded parts are 20-30 percent cheaper than thermoplastic ones.


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