Process and apparatus for bending thin-wall plastic tubing

Details Process and apparatus for bending thin-wall plastic tubing Process and apparatus for bending thin-wall plastic tubing

1999-06-04

2001-10-30

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

Including hydrostatic or liquid pressure

C072S201000, C072S202000, C072S342600, C072S342940, C072S369000, C264S322000, C264S339000, C425S363000, C425S392000

Reexamination Certificate

active

06309588

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to bend forming of tubular members, and more particularly refers to processes and apparatus for bending thin-wall plastic tubing for use in automotive fluid conduit systems.
BACKGROUND OF THE INVENTION
In order to use thin wall, monowall, or multiple layer wall plastic tubing (i.e., wherein the ratio of OD: total or cumulative Wall Thickness>4:1) in an automobile fluid handling system, it generally becomes necessary to bend and shape such tubing to fit the application. Such tubing may be of monowall thermoplastic construction, or multiple (e.g., two through seven) layer tubing wherein all layers are of thermoplastic material or wherein some layers are of metallic material and the remainder of plastic material. As with any thermoplastic tube, the addition of heat is necessary to not only allow the tube to bend to the required shape but also to set the tube in this shape so that it remains in the bent shape after the bending is completed. This bend forming process has proved to be a difficult task because of the nature of the plastic tube to kink or deform at the bend, which at the minimum can cause unwanted turbulence of liquid passing through the tube. Such deformations also cause weaknesses in the tube which would not be acceptable for the end use from a performance standpoint.
Bend forming the tube only in the required areas requires a knowledge of how the bending stress is distributed in a tube as it is being bent. This stress can be simplified for this explanation to be shown as axial stresses acting along a given zone of the walls of the tube in either a compressive or tensile state. Tensile stresses acting in the walls of the tube stretch the tube so as to lengthen it, and compressive stresses compress the walls of the tube so as to shorten it. Most of the deformation problems associated with bending the tube occur because of the compressive stresses that occur on the inside of the bend radius. As the tube is bent around a bending die, material in the tube must flow either in a compressive or tensile state to conform with the physical bending forces that are being exerted on the tube. This material flow is what allows the tube to eventually conform to the shape of the bending die. The tube bending stresses resolve into compressive stresses acting on the inside radius of the tube during bending that make the material on the inside radius of the bend want to push together. However, this is physically impossible because the material will not naturally form a thicker wall while maintaining the shape of the original tube. It will instead flow in what is referred to as “kinks” or unwanted deformation, actually pushing or folding the tube wall material up and down away from the bend die.
OBJECTS OF THE INVENTION
Accordingly, among the objects of the present invention are to provide a new and improved method or process, and a new and improved apparatus for performing the same, which enables thin wall (both monowall and multiple layer wall) plastic tubing to be bent successfully while preventing the aforementioned unwanted deformations in the tube wall at the bend area of the tube.
Another object is to provide a new and improved method and apparatus of the aforementioned character which is economical to respectively practice and construct, efficient and reliable in operation and adaptable for forming bends in thin wall plastic tubing through a wide range of bend angles.
SUMMARY OF THE INVENTION
In general, and by way of summary description and not by way of limitation the process and apparatus of the invention accomplishes the foregoing as well as other objects by utilizing a combination of (1) pressure inside the tube being bent to maintain the shape of the tube during the bending process and (2) applying heat only in the location of the tube which will be stretched. Combining these two steps allows the tubing to be bend formed to the desired bend radius while maintaining the original cross-sectional shape of the tubing. Hence, to sum up, the invention involves the application of heat only in the tensile stress areas of the tube during bending and the application of pressure to the inside of the tube during the same, in order to yield the aforementioned desired results. Various versions of this process have been developed which will be described in this application.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional objects, features and advantages of the present invention will become apparent from the following detailed description, appended claims and accompanying drawings wherein:
FIG. 1
is a sketch to diagrammatically illustrate certain features of the invention and associated analysis involved in preparation of preliminary steps of performing the process of the invention.
FIG. 2
is a schematic diagram illustrating diagrammatically the reinforcement forces exerted by fluid pressure applied to the inside of the tube during the bending process.
FIGS. 3
,
4
and
5
are simplified schematic illustrations of a first embodiment process and apparatus for performing the same, and respectively illustrating three successive steps of this embodiment.
FIGS. 6
,
7
,
8
and
9
are simplified schematic illustrations of a second embodiment process and apparatus for performing the same, and respectively illustrating four successive steps of this embodiment.
FIGS. 10
,
11
,
12
and
13
are simplified schematic illustrations of a third embodiment process and apparatus for performing the same, and respectively illustrating four successive steps of this embodiment.
FIGS. 14
,
15
and
16
are simplified schematic illustrations of a fourth embodiment process and apparatus for performing the same, and respectively illustrating three successive steps of this embodiment.
FIGS. 17 and 18
are respectively side and end elevational views of an exemplary bending die form roll, utilizable in the apparatus and methods of FIGS.
3
-
16
.


REFERENCES:
patent: 4097573 (1978-06-01), Parmann
patent: 58-51112 (1983-03-01), None

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