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1. Field of the Invention
The present invention concerns a supporting tube made from reinforced plastic for magnetic resonance scanners of the type having tracks for a patient bed and load-bearing shells formed on both sides, opening upwardly, with fastening members on the undersides of the shells.
2. Description of the Prior Art
Known supporting tubes of the above type are produced by winding resin-impregnated fiberglass threads (rovings) around a mandrel with an outer diameter conforming to the desired inner contour of the tube and, if applicable, are cured by heating. Moreover, it is known to manufacture the supporting tube in a multipart-casting mold with a reaction resin with subsequent hardening. The casting can ensue in a known manner under vacuum, or under vacuum with subsequent pressurization. In this production method, in principle a uniform wall thickness of the supporting tube is produced and a resin system plus additional reinforcement fibers or a resin system filled with filler have been used.
The use of a filler-free resin system is advantageous from a manufacturing point of view, but the use of such a filter-free resin system involves various disadvantages and risks.
The hardening phase has an associated temperature increase (exothermy) as well as with a volume shrinkage. Both effects can lead to mechanical stresses in the finished component, tears (cracks) or at least to shrinkage-dependent deformations and deviations of the outer contour from the desired contour. Particularly at risk are regions with greater wall thickness, i.e. larger material accumulations. These problems can be prevented by optimally complete filling of the hollow receptacle of the casting mold, with reinforcement materials such as glass fabric/felt (non-woven fiber) but these solutions involve high production and material expenditure. Such reinforcement, preferably with glass fabric or felt, also is absolutely necessary to achieve a sufficiently high rigidity.
Deformations and voids due to exothermy and reaction shrinking can be largely prevented by the use of a filler, for example silica powder, However, the rigidity of typical casting resins with approximately 65-weight percent silica powder is insufficient to achieve the necessary substantial freedom from deformations under load with respect to the predetermined contour of the supporting tube.
An object of the present invention is to provide a supporting tube of the initially described type that, with a simple design, allows a very high rigidity of the composite material to be achieved, such that in spite of the load-bearing that the supporting tube must accomplish in the region of the less stable, open end-side bearing shells, very few deformations occur under load.
This object is inventively by a supporting tube, including the guide rails and of the fastening members embedded at the bottom, Is manufactured from mold-cast plastic and that it is locally provided with cast-in reinforcements in the region of the load-bearing shells.
It has surprisingly been shown that the introduction of local thickenings in the region of the aprons or projections at the end-side, thus at the open part of the bearing shells, yields a very significant improvement in the flexural strength and leads to very few deformations under load, without—as previously needed—having to provide the entire supporting tube with a very high wall thickness throughout, This not only saves material but also reduces the weight.
In addition to the inventive local thickening in the region of the projections of the supporting tube, in an embodiment of the invention the casting resin can be provided with fillers in a known manner, whereby, in addition to, for example, silica powder or other fine-particle fillers such as fused quartz, aluminum oxide or aluminum hydroxide, these fillers can with particular advantage also contain particles of different particle sizes, and among these in particular larger spherical fillers such as glass beads. For example, these larger particles enable these fillers to be introduced into the mold before the pouring of the casting resin and therewith practically completely fill this mold before pouring the casting resin that need only be augmented with a powdery filler and thus has a correspondingly lower viscosity than if it were augmented with layer particles, In practice the presence of coarse-grained particles in accordance with the invention does not impair the flowability of the casting resin at all.
Moreover, the casting resin naturally can also contain fibrous reinforcement inserts, particularly in the form of felt or fabric mats made from mineral or glass fibers, aramid fibers or carbon fibers. These reinforcing mats initially are placed around the inner mandrel of the casting mold and are saturated by introduced casting resin after the sealing of the mold and, if applicable, the introduction of particulate fillers.
It has proven to be particularly advantageous to case the fillers and/or the fibrous reinforcement inserts with bonding agents adapted to the respective casting resin. For example, epoxy silane can be used given the use of epoxy resins.
The glass transition temperature of the casting resin is adapted to the working temperature range, meaning it is at least 10° to 20° C. above the upper working temperature.
The single FIGURE is a side view of an exemplary embodiment of an inventive RF supporting tube.
This RF supporting tube 1, Including the non-visible inner guide rails as well as under-embedded fastening members 2 that serve for the fastening in the magnetic resonance scanner, is produced by casting from a plastic that can contain various fillers and fiber reinforcements. The provision of thickened wall regions 3 is inventively significant in the region of load-bearing shells 4. Underside protrusions that act as insertion guides in the mounting of the supporting tube are indicated at 5.
The inventive, only local increase of the wall thickness in tho region of the aprons or open bearing shells 4 yields nearly the same reduction of the deformation under load as an end-to-end reinforcement of the wall thickness of the supporting tube, which would, however, lead to an excessive increase of the tube weight. Moreover, an increase of the wall thickness in the middle RF-sensitive region is avoided by this local reinforcement only in the region of the open load-bearing shells 4.
The content of filler that can be added to a casting resin is limited because the flow behavior and therewith the processing capability is drastically worsened with increasing filler content. An optimally high filler content, however, is desirable since the rigidity of the casting resin molding material is significantly increased by a higher filler content. Given use of mixtures made from fillers with different particle sizes—or even the use of mixtures of spherical fillers such as glass globes—the filler content can be significantly increased with relatively low processing viscosity.
An optimal rigidity with predetermined geometry is achieved when the largest portion of the hollow mold to be cast is filled with glass fabric/felt and is subsequently surrounded with a filler-containing casting resin. The glass fabric/felt can exhibit respectively different glass contents in the longitudinal direction and in the circumferential direction; preferably the glass content is higher in the circumferential direction. The glass fabric/felt has a relatively coarse meshed, whereby a mesh width>1 mm, still better >2 mm, should be used. Both resin-impregnated and hardened and dry glass fabric/felt can be used in principle.
A production-related optimized variant is to wrap the inner mandrel with glass fabric or glass felt up to a thickness that corresponds at most to the lowest wall thickness of the tube. The remaining free space to form the desired contour is filled by the filler-containing casting resin.
A further possibility already mentioned for reinforcement of the supporting tube 1 with its open load-bearing shells 4 with regions 3 with larger wall thickness is to fill the hollow space of the mold with coarse-grained particles, if applicable using vibrations in order to subsequently fill the intervening spaces (interstices) with casting resin, preferably filler-containing casting resin with silica powder or the like, and thus to bind the individual particles with one another.
The particle diameter depends on the wall thickness of the hollow space to be filled: it should be >2 mm in order not to let the flow resistance become too high; the maximum diameter should be between 30%-40% of the wall thickness; for a wall thickness of, for example, 10 mm, particles with a diameter of 2-4 mm are advantageous. The use of spherical particles is preferred, and thereby the use of particles with optimally uniform diameter, especially the use of glass beads.
It is surprising that a complete saturation and very high rigidity of the composite material can be achieved by a special combination of a filler-containing casting resin and a glass felt, This is all the more notable because similar rigidity has heretofore been achieved only with significantly more elaborate production methods (RTM, DP-RTM, wet winding method).
In addition to the examples 1 and 2 that correspond to the prior art, the following table shows in the exemplary embodiments 3 and 4 that using the casting methods according to the examples 3 and 4 (which are significantly simpler relative to the winding method according to example 1) an equally good or even significantly better rigidity is achieved, and in particular the rigidity is significantly better than is achieved with simple vacuum casting without the local reinforcements in the region of the open load-bearing shells.
|Example Nr.||Production method||Resin system||Reinforcement||[N/mm2]|
|1||Winding of resin-||Epoxy resin,||77.2 weight||18200|
|(=prior art)||saturated rovings||filler-free||percent glass|
|around a mandrel||fiber; winding|
|2||Vacuum casting||Epoxy resin,||Without||12700|
|(=prior art)||64 weight|
|3||Insertion of glass||Epoxy resin,||Glass fabric:||17300|
|fabric + vacuum||64 weight||Pre-|
|casting||percent silica||impregnated 9|
|percent in x-|
|4||Insertion of glass||Epoxy resin,||Glass felt:||x-direction:|
|fabric + vacuum||64 weight||11.4 volume||21400|
|casting||percent silica||percent (x-||y-direction:|
Although modifications and changes may be suggested by those skilled in the art, It is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.