Method for covering with cylindrically shaped heat-shrinkable film

The present invention relates to a method for covering a substrate with a cylindrically shaped heat-shrinkable film, particularly to a method for covering a cylindrically shaped heat-shrinkable film so as to fit the contour of a substrate by conducting the heat treatment in multiple steps at different temperatures.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for covering a substrate with a 
cylindrically shaped heat-shrinkable film. 
2. Description of the Prior Art 
There have heretofore been used cylindrically shaped heat-shrinkable films 
for the purpose of protection of various articles. A cylindrically shaped 
heat-shrinkable film is a film in a shape of cylinder which can shrink by 
heating, and it is a general practice to apply a cylindrically shaped 
heat-shrinkable film over a certain article followed by heating, whereby 
there is formed a covering in close contact along the surface contour of 
the article. Thus, surface covering is possible with the use of a 
cylindrical heat-shrinkable film only by heating and within a short period 
of time, and hence cylindrically shaped heat-shrinkable films have 
frequently been used for various packaging purposes. However, when a 
cylindrically shaped heat-shrinkable film is utilized for covering of an 
article for which a precision is required, a cylindrically shaped 
heat-shrinkable film chosen is required to be as flawless as possible. 
Nevertheless, the heat-shrinkable films perpared according to the methods 
of prior art suffer from lines of flection formed thereon, which will 
being about deleterious influences on covering of an article for which a 
percision is required. For example, in case of surface covering on an 
electrophotographic photosensitive material, the image characteristics at 
the line of flection are extremely worsened to give rise to disorders of 
image or bad cleaning performance. 
Formation of a line of flection on a cylindrically shaped heat-shrinkable 
film of prior art is due to its production method, as explained now with 
reference to the drawing. FIG. 1 is a typical example of the production 
method of prior art. That is, a starting material resin 1 is fed into a 
hopper 6 and then melted by heating. The molten starting material resin 
progresses along the arrow 7 until it is extruded through an annular die 
portion 2. And, an inflating air is blown through a blow inlet for 
stretching of the extruded cylindrically shaped film. The staring material 
resin molten by heating is extruded through the annular die portion 2 and, 
while being wound up and drawn upwardly simultaneously with blowing of the 
inflating air to perform stretching operation, the cylindrically shaped 
heat-shrinkable film 8 is flexed through the roller 4 and wound up on the 
wind-up roller 5. Thus, the line of flection is formed in the course of 
winding up the fabricated cylindrically shaped heat-shrinkable film. 
Even when using a cylindrically shaped heat-shrinkable film free from such 
a line of flection, it is not easy to have the film shrunk fitted to the 
contour of a substrate. That is, in the state prior to heat shrinkage, a 
cylindrically shaped heat-shrinkable film covers the surface of a 
substrate. But not all the portions of the cylindrically shaped 
heat-shrinkable film contacts the surface of the substrate, that is, some 
portions are contacted while other portions remain uncontacted. The 
heat-shrinkable film at the portions contacted with the substrate will 
permit a considerable amount of heat to be dissipated into the substrate 
on heating and therefore undergoes heat shrinking at a slow rate. On the 
other hand, the heat-shrinkable film at the portions not contacted with 
the substrate will difficultly release the heat applied toward the 
substrate and therefore heat shrinking occurs rapidly. As the result, 
there takes place partially irregularity in the extent of heat shrinkage, 
and therefore, the film thickness of the portions shrunk earlier increases 
while that of the portions shrunk later is reduced, whereby no uniform 
heat shrinkage can be accomplished. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide a method for 
covering a substrate with a cylindrically shaped heat-shrinkable film with 
a line of flection, by which no inconvenient trace of the line of flection 
may remain on the shrunk film. 
Another object of the present invention is to provide a method for covering 
a cylindrically shaped heat-shrinkable film with or without a line of 
flection, by which the film can be heat shrunk uniformly. 
Still another object of the present invention is to provide a method for 
covering a cylindrically shaped heat-shrinkable film, by which entrapment 
of a heating medium between a substrate and a cylindrically shaped 
heat-shrinkable film can be avoided. 
According to the present invention, there is provided a method for covering 
with a cylindrically shaped heat-shrinkable film, which comprises the 
steps of: covering a substrate with a cylindrically shaped heat-shrinkable 
film; subjecting said cylindrically shaped heat-shrinkable film to a first 
shrinking at the portion corresponding to the lower end of said substrate 
at a temperature of the higher temperature zone in the heat-shrinkable 
temperature range of said film; subjecting said film, except for the 
portion corresponding to the upper end of said substrate, to a second 
shrinking at a temperature of the lower temperature zone in said 
temperature range; and subjecting said film wholly to a third shrinking at 
a temperature of the higher temperature zone in said temperature range, 
using a liquid heating medium for heating in the above steps of shrinking.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
According to the present invention, in the first place a substrate is 
covered with a cylindrically shaped heat-shrinkable film and the film is 
subjected first at the lower end of the substrate to heat shrinking, 
whereby entrapment of a heating medium between the substrate and the 
cylindrically shaped heat-shrinkable film is prevented. This shrinking is 
the first shrinking treatment which is performed at a temperature of the 
higher temperature zone of the heat-shrinkable temperature range of the 
cylindrically shaped heat-shrinkable film. By such a treatment, the 
cylindrically shaped heat-shrinkable film covering over the lower end of 
the substrate is abruptly shrunk to prevent entrapment of a heating 
medium. Subsequent to the first shrinking, the second shrinking treatment 
and the third shrinking treatment are carried out. By performing the third 
shrinking treatment at the higher temperature zone of the heat-shrinkable 
temperature range in addition to the second shrinking treatment at the 
lower temperature zone of said temperature range, there can be 
accomplished a covering free from any inconvenient trace of line of 
flection remaining on the cylindrically shaped heat-shrinkable film. That 
is, by the second shrinking treatment at said lower temperature zone, heat 
shrinking proceeds slowly, whereby the degree of shrinking is not so 
different from place to place (even if there may be portions contacted 
with a substrate and portions not contacted, the difference between both 
portions does not become so much). Through heat shrinkage in the next step 
at said high temperature zone, the whole cylindrically shaped 
heat-shrinkable film is completely shrunk to accomplish a uniformly 
covered state. It is particularly effective to employ a heating 
temperature for the third shrinking treatment which is higher by 
15.degree. C. or more than the minimum temperature of heat-shrinkable 
temperatures. 
Particularly in the present invention, a liquid is used as the heating 
medium, whereby the heat content of the heating medium can be increased to 
enable lowering of shrinking temperature and shortening of shrinking 
treatment time. Since the shrinking treatment can be done within a short 
time, the effect of heat on a substrate can be decreased. 
The first, second and third shrinking treatments may be conducted under the 
conditions, which may differ depending on the cylindrically shaped 
heat-shrinkable film employed, but the second shrinking treatment may be 
conducted generally at a temperature of 70.degree. to 90.degree. C., 
particularly 75.degree. to 90.degree. C., for a heating time which may 
sufficiently be 3 minutes or less. At the time of completion of the second 
shrinking treatment, shrinking appears to be completed apparently to have 
formed a uniform film, but there remains yet the trace of flection line, 
which will clearly appear on produced image in case of an 
electrophotographic photosensitive material. 
The first and the third shrinking treatments may be preferably conducted at 
a temperature of 85.degree. C. to 120.degree. C., particularly 85.degree. 
to 100.degree. C. As the heating time, 2 min. or less is sufficient. On 
the cylindrically shaped heat-shrunk film after the third shrinking, there 
is still observed a trace of flection line, which trace, however, will not 
appear as an inconvenient trace. It is not easy to have such a trace of 
flection line completely disappeared during heat shrinking. The third 
shrinking may be conducted after the second shrinking by elevating the 
temperature either continuously or stepwise. 
The temperature difference in heating between the first shrinking or the 
third shrinking and the second shrinking may perferably 10.degree. C. or 
higher. 
If shrinking is effected at a temperature near the third shrinking 
temperature without performing the second shrinking, abrupt shrinking will 
occur momentarily to entrap the air and therefore no complete shrinking 
can be expected. 
As the substrate to be used in the present invention, there may be employed 
various articles, but a typical example of such materials is a substrate 
for image holding member to be used in electrophotography. 
As an image holding member for electrophotography on which electrostatic 
images or toner images are formed, there is an image holding member which 
is called as an electrophotographic photosensitive material having a 
photoconductive layer on a support, and also an image holding member 
having no photoconductive layer. Generally, it is constituted of a support 
and an image holding layer provided thereon. Accordingly, in case of an 
electrophotographic photosensitive material, the substrate is a support 
having formed a photoconductive layer on its surface, while in case of an 
image holding member having no photoconductive member, the substrate is a 
support. In both cases, a cylindrically shaped heat shrunk film is used as 
an insulating layer. 
Electrophotographic photosensitive materials will assume various 
constitutions in order to obtain desired characteristics, or depending on 
the electrophotographic processes to be applied. As a typical example of 
an electrophotographic photosensitive material, there is a photosensitive 
material composed of a support, a photoconductive layer overlying the 
support and an insulating layer overlying the photoconductive layer, which 
has widely been employed. An insulating layer is provided for various 
purposes such as protection of the photoconductive layer, improvement in 
mechanical strength of the photosensitive material, improvement in dark 
decay characteristics or application for a specific electrophotographic 
process. Typical examples of such photosensitive materials having 
insulating layers or electrophotographic processes using such 
photosensitive materials are disclosed in, for example, U.S. Pat. No. 
2,860,048, Japanese Patent Publication Nos. 16429/1966, 15446/1963, 
3713/1971, 23910/1967, 24748/1968, 19747/1967 and 4121/1961. 
For an electrophotographic photosensitive material, a desirable 
electrophotographic process is applied to form an electrostatic image 
thereon, and the electrostatic image is developed for visualization. 
A typical constitution of an image holding member having no photoconductive 
layer has an insulating layer as an image holding layer. Some of the 
representative uses of such an image holding member are exemplified below: 
(1) As disclosed in, for example, Japanese Patent Publication Nos. 
7115/1957, 8204/1957 and 1559/1968, for the purpose of improvement of 
repeated use characteristics of an electrophotographic photosensitive 
material, an electrostatic image formed on an electrophotographic 
photosensitive material is transferred onto an image holding member having 
no photoconductive member and then developed to form a toner image 
followed by transfer of the resulting toner image onto a recording medium. 
An image holding member to be used in such an electrophotographic process. 
(2) Alternatively, as another electrophotographic process for forming an 
electrostatic image on an image holding member having no photoconductive 
layer corresponding to the electrostatic image formed on an 
electrophotographic photosensitive material, as disclosed in, for example, 
Japanese Patent Publication Nos. 30320/1970 and 5063/1973, and Japanese 
Laid-Open Patent Application No. 341/1976, there may be mentioned a 
process in which an electrostatic image is formed according to a desirable 
electrophotographic process on a screen-like electrophotographic 
photosensitive material having a number of minute openings, and corona 
charging treatment is applied to an image holding member having no 
photoconductive member through the electrostatic image thereby to modulate 
the ion stream of corona and form the electrostatic image on the image 
holding member having no photoconductive member, followed by toner 
development of the electrostatic image and then transferring the resulting 
toner image onto a recording medium to form the final image. 
The characteristics of such an insulating layer of the image holding member 
will have noticeable effects on the image quality of the image formed, and 
for this reason it is effective to produce a good insulating layer by 
using a heat shrunk film. 
As cylindrically shaped heat-shrinkable films to be used in the present 
invention, there may be conveniently employed heat-shrinkable films made 
of a material such as polyvinyl chloride, polypropylene, polyester, 
polystyrene, polyvinylidene chloride, polyethylene, chlorinated rubber, 
and the like. As a film which is excellent in surface smoothness and 
mechanical strength, being consequently excellent in durability, cleaning 
characteristic and corona resistance, and further excellent in humidity 
resistance and weathering resistance, there may be mentioned 
heat-shrinkable poly (ethyleneterephthalate) films. 
As a liquid to be used as a heating medium, there may be preferably 
employed a liquid having a relatively high boiling point, for example, 
water, polyethyleneglycol, glycerine, liquid paraffins, silicone oils, 
etc. 
Example 1 
To 100 parts (by weight) of CdS powders, there were added 14 parts of a 
solvent-soluble vinyl chloride-vinyl acetate copolymer (trade name: VMCH; 
produced by UCC) and 30 parts of methyl ethyl ketone solvent. After the 
mixture was subjected repeatedly three times to roll milling through a 
80.mu. gap, it was adjusted to a viscosity of 300 cps and coated on an 
aluminum cylinder of 80.phi..times.300 mm by a dip coating method, 
following by drying at 80.degree. C. for 20 minutes, to provide a 
photoconductive layer of 40.mu. thick. 
The thus prepared sample was covered with a polyester heat-shrinkable tube 
of 90 mm in diameter (produced by Hoechst; heat-shrinkable temperature: 
70.degree. C. or higher) and the lower end (non-image region) of the drum 
was subjected to heat treatment with hot water of 95.degree. C. for 20 
seconds. Then, the drum was dipped vertically into a low temperature hot 
water of 80.degree. C. for 30 seconds with the upper end (non-image 
portion) being left undipped, and thereafter the whole region including 
the upper end was subjected to heat treatment again with hot water of 
95.degree. C. for 15 seconds. 
On the resultant photosensitive material, there was formed a latent image 
by primary .sym. charging, secondary AC discharging simultaneously with 
image exposure and whole surface irradiation, and then development, 
transfer and cleaning steps were practiced repeatedly. Using such an 
electrophotographic process durability and image formation were tested, 
whereby no image irregularity due to line of flection or entrapment of the 
air was observed and there was observed no change in potential 
characteristics by the heat treatments at all. In this example, when the 
whole body was subjected once to the heat shrinkage only by the heat 
treatment at 95.degree. C., the resultant photosensitive material, when 
subjected to similar image formation, gave the result that the image 
density at the flection line of the heat shrunk tube was lowered to give a 
white line. 
Example 2 
According to the same procedure as described in Example 1, there was 
provided a photoconductive layer of 40.mu. which on a 80.phi..times.300 mm 
aluminum cylinder. 
The resultent sample was covered with a heat-shrinkable film made of a 
polyvinyl chloride of 85 mm in diameter with a thickness of 25.mu. 
(shrinkage: 20%; heat-shrinkable temperature: 65.degree. C. or higher) and 
the lower end (non-image region) of the drum was subjected to heat 
treatment with hot water of 90.degree. C. for 30 seconds. Then, the drum 
was dipped vertically into a low temperature hot water of 75.degree. C. 
for 50 seconds with the upper end (non-image region) being left undipped, 
and thereafter the whole region including the upper end was subjected to 
heat treatment again with hot water of 85.degree. C. for 30 seconds. 
When durability and image formation were tested for the thus prepared 
photosensitive material based on the same electrophotographic process as 
in Example 1, there was observed no image irregularity by flection line, 
shrinkage irregularity or entrapment of the air, and there was also no 
deterioration in potential characteristics by the heat treatments at all. 
Example 3 
According to the same procedure as described in Example 1, there was 
provided a photoconductive layer of 40.mu. thick on a 80.phi..times.300 mm 
aluminum cylinder. 
The resultant sample was covered with a heat-shrinkable tube made of a 
polyester of 85 mm in diameter (film thickness: 25.mu.; shrinkage: 25%; 
heat-shrinkable temperature: 70.degree. C. or higher) and the lower end 
(non-image region) of the drum was subjected to heat treatment with 
polyethylene glycol (polymerization degree: 4000) of 100.degree. C. for 30 
seconds. Then the drum was dipped vertically into a polyethylene glycol 
(polymerization degree: 4000) of 80.degree. C. for 30 seconds with the 
upper end (non-image region) being left undipped, and thereafter the whole 
region including the upper end was subjected to heat treatment again with 
polyethylene glycol of 100.degree. C. (polymerization degree: 4000) for 20 
seconds. After cooling to room temperature, the polyethylene glycol 
adhered on the surface was removed by washing with water to provide a 
photosensitive material. 
When durability and image formation were tested for the thus prepared 
photosensitive material based on the same electrophotographic process as 
in Example 1, there was observed no image irregularity by flection line, 
shrinkage irregularity or entrapment of the air, and there was also no 
deterioration in potential characteristics by the heat treatments at all. 
Comparative example 1 
In Examples 1, 2 and 3, the first shrinking step was omitted and 
photosensitive materials were prepared directly according to only the 
second and third shrinking steps. 
In any case, the heating medium penetrated into the photoconductive layer 
to destroy the characteristics of the photoconductive layer and there was 
observed image irregularity. 
Comparative example 2 
In Examples 1, 2 and 3, photosensitive materials were prepared according to 
only the first and second shrinking steps, by omitting the third shrinking 
step. 
In any case, the line of flection had a height which was very high, and the 
photosensitive material obtained could not stand repeated uses due to 
incomplete shrinkage. The heights of respective lines of flection are 
shown below. 
______________________________________ 
In case of only 
In case of first, 
first and second 
second and third 
Example shrinking steps 
shrinking steps 
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1 15.mu. 1.0.mu. 
2 20.mu. 1.0.mu. 
3 12.mu. 0.5.mu. 
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