Plastic tube and method and apparatus for manufacturing

A plastic tube and a method and apparatus for forming it in which a strip of relatively thin plastic sheet material is cut from a roll of the material and wound around a mandrel. Overlapping ends of the strip are sealed to form the tube. The edge of the strip is inwardly bent to form an inwardly extending lip. When used in the manufacture of a capacitor, the tube is placed in a can on top of the disc of insulating material and a capacitor roll is placed in the tube. The edge of the sheet material can be pre-creased so that when folded inwardly, darts of the material form the lip.

BACKGROUND OF THE INVENTION 
In the manufacture of electrical capacitors, layers of a metallic material 
separated by a layer of a dielectric material are wound in a roll. The 
capacitor roll is placed in a tube which is usually of paper, such as 
Kraft paper with a typical thickness of 0.015", and which is spiral wound. 
After the paper tube is formed, the periphery of one of its ends is bent 
inwardly to provide a mounting lip for the capacitor roll so it will not 
become displaced from the tube. The tube with the capacitor roll therein 
is placed within a metal outer housing, or can, with the lip end facing 
toward the bottom of the can. The capacitor is then impregnated with 
dielectric fluid and oil and the can is thereafter sealed. 
While a spiral wound paper tube is satisfactory, it is relatively costly 
and inconvenient to use. For example, different size tubes may require 
different manufacturing procedures. Also, if a large quantity of tubes 
have to be stored, substantial space may be required. Further, due to the 
thickness of the spiral wound paper forming the tube, the inwardly bent 
lip is rather bulky and increases the vertical height of the tube. This 
extra height creates difficulty during automatic assembly. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention relates to a tube made of sheet plastic material 
having particular usefulness in the manufacture of capacitors and a method 
and apparatus for making the tube. The plastic tube is easier to make than 
a paper tube and can be made on an in-house machine and on demand. 
The plastic material used in forming the tube of the invention is an 
elongated sheet and a roll of this sheet material is preferably used to 
provide continuous production of tubes. To form a tube, a strip of the 
plastic sheet material is rolled around a mandrel and cut from the roll. 
One edge of the strip overhangs the mandrel. The ends of the cut strip are 
overlapped and sealed together to form a closed body tube typically of 
cylindrical shape. The edge of the material overhanging the mandrel is 
folded or spun inwardly to form the inwardly extending lip for the bottom 
of the capacitor can. The folds forming the lip can be irregular in size 
and shape. Alternatively, the folds of the lip can be a continuous series 
of darts with overlapping layers of the sheet material forming the dart 
sealed to each other to create the tube lip. 
The lip which is formed is relatively flat and the tube is of simple 
construction and is cost effective. The tube is inserted into a metal 
capacitor can with the lip preferably resting on a separate disc of 
insulating material at the bottom of the can. The capacitor roll is then 
placed into the tube/can/disc sub-assembly. 
If desired, a piece of plastic material can be placed over and fastened to 
the tube lip. The fastening operations for the various parts of the tube 
are accomplished by, for example, ultrasonic welding, heat sealing or a 
suitable adhesive. 
OBJECTS OF THE INVENTION 
It is therefore an object of the invention to provide a tube of plastic 
material. 
A further object is to provide a tube of plastic material with an inwardly 
extending lip at one end and a method and apparatus for forming such tubes 
on a continuous basis. 
An additional object is to provide a novel method and apparatus for making 
a tube of plastic material suitable for holding a capacitor roll with one 
end of the tube having an inwardly extending lip formed by bending and 
folding an edge of the sheet material used in forming the tube. 
Another object is to provide a tube of plastic material in which a strip of 
plastic having an edge on which a series of creases are formed is cut from 
a roll of such material, the ends of the strip are overlapped and attached 
to form a cylinder and the strip edge with the creases inwardly bent to 
form a series of overlapping darts which are fastened to the underlying 
sheet material to form a lip for the finished tube. 
Yet another object is to provide a method and apparatus for making a 
capacitor including a tube of plastic material suitable for holding the 
capacitor roll of metal and dielectric material with one end of the tube 
having an inwardly extending lip with the tube being placed in an outer 
metal can.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the sheet plastic material 10 of which the tube is 
made is preferably located on a supply roll 11. The material can be, for 
example, polypropylene, polyethylene teraphthalate, or any other suitable 
plastic sheet material which has a relatively high dielectric constant and 
can be heat sealed or ultrasonic welded. Since the sheet material is thin 
and flexible, it also can be folded or creased. In a typical application, 
the material is polypropylene 0.006" thick. This has approximately the 
same dielectric effect as cardboard 0.015" thick while having only about 
half the thickness. Such material is relatively flexible, even when formed 
as a tube. 
Along one edge 13 of the sheet 10, the material is preferably deformed with 
equally spaced upwardly facing creases 14 forming between two adjacent 
creases individual tabs 15 of the plastic material. The creases 14 are 
generally perpendicular to the sheet length direction and the tabs 15 are 
generally rectangular or are square. As the sheet material is fed for use 
from supply roll 11, the peaks of the creases point upwardly from the 
sheet top surface. A deformed area 17 can be formed on the sheet top 
surface to provide an easier fold for the creases at the start of a crease 
on the sheet. 
The material 10 can be formed, for example, by taking a roll of the raw 
plastic sheet material and re-rolling it onto the supply roll 11 while 
passing it through a mechanism (not shown) between the two rolls which 
makes the creases 14. The creasing mechanism can be, for example, another 
roller having a plurality of blades which engage under pressure and 
deform, but do not cut through, an edge of the plastic sheet material 10 
as it passes from the roll of raw material to the supply roll 11. The 
blades can be heated. 
As shown in FIGS. 5A and 5B, the material 10 from the roll 11 is supplied 
to a winding arbor, or mandrel, 20 which has a shape and diameter 
corresponding to the desired shape and diameter of the tube to be formed. 
The material is taken from the supply roll 11 and wound around the mandrel 
with the peak of the creases 14 facing outwardly away from the mandrel. A 
strip of material of sufficient length is wound around arbor 20 as it 
rotates to form overlapping strip ends 10a and 10b. A clamp 24 is actuated 
to hold the rolled strip to the arbor. The sheet 10 is then cut at the end 
10b of the strip by a knife blade 22 extending across the sheet's width. 
The tabs 15 and creases 14 along the strip edge 13 extend beyond the edge 
of the arbor. That is, the strip edge 13 does not lie on the surface of 
the arbor. The overlapping strip ends 10a and 10b are fastened together by 
a suitable device 25 which can be, for example, a heat sealer or an 
ultrasonic welder extending across the width of the strip. 
While the arbor is shown as being of cylindrical shape, it should be 
understood that it can be of any desired shape, e.g., elliptical. That is, 
a non-cylindrical tube can be formed. 
At this stage, the tube 29 is in a semi-finished state. It can be removed 
from the arbor 20 and thereafter placed on a rotating drum 30 having a 
solid end 32. A roller 27 is preferably provided at the end of the drum 30 
over which the creased strip edge 13 extends. Roller 27 has a stepped 
inner and outer diameter to engage the strip edge 13 at or near where it 
joins the material 10 and inwardly bends edge 13 over the drum end toward 
its center axis. The roller 27 inwardly folds the strip edge 13 along a 
series of lines 18, each of which lies below a respective crease 14 into a 
series of overlapping, generally triangular darts 16 as shown in FIGS. 4A 
and 4B. 
Each dart 16 has two layers of material of generally triangular shape. The 
lower layer has one leg on the bend line 18, the second the crease 14 and 
the third the outer edge of the sheet. The upper layer has one leg lying 
above the bend line, the second the crease 14 and the third the outer edge 
of the sheet. 
The lower layer of each dart 16 lies above a part of the sheet material of 
the next adjacent tab portion 15 on which the bend line 18 was made. If 
desired, the original sheet material also can have inward deformations for 
the bend lines 18. This is usually not necessary since the material is 
quite thin and can be easily folded about line 18. 
As seen in FIGS. 4A and 4B, the sheet edge 13 is now in the shape of 
inwardly bent overlapping darts 16. Another fastening device, such as a 
heat sealer or ultrasonic welder 34, is moved toward and engages and seals 
the overlapping layer of each dart to each other and to the underlying 
material of the next adjacent tab to form an inwardly extending lip 17 as 
seen in FIGS. 2 and 3. Thus, each sealed area has three layers of 
material. This increases the overall dielectric strength of the lip. 
If desired, a portion of the heat sealer or ultrasonic welder fastening 
mechanism 34 can be located within the cylinder 30. The sealed overlapping 
darts form a lip 17 which is relatively stable. The lip 17 also is 
dielectrically sound since there are no cuts or holes which would cause 
dielectric leakage. The finished tube is then removed from the drum 30. 
If desired, the complete operation of forming and sealing lip 17 can be 
carried out on the arbor 20 by making the arbor with a solid end over 
which the tabs 15 can be bent and the darts 16 sealed together. Here the 
roller 27 can be located adjacent the end of arbor 20 and the arbor 
rotated relative to the roller. 
In another process for making the tube, the edge of the sheet material is 
not pre-creased along edge 13 to form the darts. In this process, the 
semi-finished tube 29 can be formed as described above. Here, while either 
on the arbor 20 the edge of the semi-finished tube 29 overhangs the arbor 
or else the semi-finished tube can be removed from the arbor and placed on 
the rotating drum 30 with the edge overhanging the drum solid end 32. 
The roller 27 is used to fold the overhanging edge inwardly to form the lip 
17. The folds are irregular. That is, the lip material is bunched, there 
are crinkles and folds of different shapes. Also, there is no regular 
pattern as would be expected when the uncreased overhanging edge of the 
cylindrical tube is folded inwardly. This can be tolerated when the tube 
with the capacitor roll therein is placed in the capacitor can. The weight 
of the capacitor roll and impregnant will somewhat flatten the irregular 
folds. The irregular folds resting on the disc on the can bottom will 
still provide adequate insulation between the capacitor roll and the can. 
If desired, the irregular folds forming the lip can be tacked together by 
heat sealing. 
After the tube is formed with the lip, either with the dart or with 
irregular folds, due to the thinness of the wall material, the tube is 
flexible and can be bent or even folded if desired. The lip is also 
somewhat flexible. 
During final assembly of the capacitor, a disc of cardboard or other 
similar material is placed on the bottom of the can (not shown) and the 
tube is inserted with the lip 17 facing the insulating disc. The rolled 
capacitor (not shown) is placed within the finished tube. The lip 17 of 
the tube with inserted capacitor rolls rests on the insulating disc (not 
shown). Thus, the end of the capacitor roll is fully insulated from the 
can bottom by the tube lip 17 and the disc at the bottom of the can and 
the roll insulated from the main part of the can by the plastic tube. 
FIG. 6 shows another embodiment of the invention wherein an end piece 40 of 
the same material as the sheet 10, e.g., polypropylene, is provided. End 
piece 40, which can be fully circular or annular, or even square, has a 
diameter or size such as to overlie the inwardly bent sealed darts 16. 
Piece 40 is located over the darts 16 or the irregular folds and is 
fastened thereto, such as by heat sealing or ultrasonic welding. The 
finished tube with end piece 40 then can be inserted into the capacitor 
can which also may or may not have a disc of insulating material at its 
bottom. 
The tube of the present invention has a number of advantages. As explained 
previously, it is thinner than a comparable tube of cardboard but yet has 
the same dielectric strength. The dielectric can be changed by varying the 
type of plastic material and/or its thickness. 
The lip 17, even with the folded and sealed darts, or the irregular folds, 
lies relatively flat since the plastic sheet material is relatively thin. 
The lip 17 is stable and provides a good mounting for the capacitor roll. 
The heat sealing or ultrasonic welding of the tabs or folds does not add 
to the lip thickness. Even with a lip formed with irregular folds the 
thickness of such folds is usually not more than twice the thickness of 
the sheet material and the weight of the capacitor roll tends to flatten 
out the irregular folds. This reduces the overall length of the tube as 
compared to one of a spirally wound paper. This makes assembly of the 
capacitor easier. 
The equipment for manufacturing the tube is relatively simple. The mandrel 
can be easily changed to produce various diameter tubes and different tube 
shapes. Also, there is good control of the tube diameter since it is wound 
over a mandrel or arbor. The width of the sheet and length of the mandrel 
can be changed to accommodate different tube lengths. 
A tube made with the plastic material in accordance with the invention is 
less expensive than a comparable paper tube. The construction is simple. 
The tube can have uses other than for a capacitor as would be provided by 
tubes of comparable construction with an inwardly extending lip on one 
end.