Blow molding process with sheet interposed between mold and product being molded

A process for making a blow molded product which includes the steps of positioning a sheet having a surface pattern which is incompatible with a parison in a mold for blow molding; extruding a softened parison of a synthetic resin from a die head; holding the parison in the mold and blowing gas into the parison to be expanded to form a blow molded product with a surface feature of the sheet reproduced on the blow molded product; and separating the sheet and the blow molded product from each other. An apparatus used for the process is provided with a mold defining a cavity for blow molding, a die head for extruding a parison of a synthetic resin into said mold, a sheet positioned between the mold and the parison which is formed an effective surface thereon and incompatible with the material of the parison, a gas blowing device for blowing gas into the parison to form a blow molded product, a sheet supply device for supplying a sheet in the cavity of the mold, and a sheet takeup device for taking up the sheet from the mold after blow molding.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process and an apparatus for making a blow 
molded product, such as a hollow molding for an automobile. 
2. Description of the Prior Art 
Extrusion or injection molding has been a typical method for making a 
molding as a decorative or protective part of an automobile, but blow 
molding has also come to be employed for that purpose. 
FIG. 8 shows a molding made by a known blow molding process. It is a blow 
molded product 8 comprising a main body 81 having a hollow interior 82. 
The hollow interior 82 provides a shock absorbing function and also 
contributes to reducing the weight of the molding 8 as a whole. That is 
why the molding has come to be made by blow molding. 
FIGS. 9 and 10 show a known mold 9 which is used for making the blow molded 
product 8. The mold 9 comprises a core plate 91 and a cavity block 92 
which define a cavity therebetween when they are put together. The cavity 
is so shaped as to define the shape of the blow molded product 8. The core 
plate 91 has a gas blowing device 93 and a gas blowing needle 931 which 
face the cavity. The cavity block 92 has a cavity surface 920 provided 
with a plurality of air vent holes 924, and is also provided with an air 
passage 925 and an air vent pipe 926 to which the air vent holes 924 are 
connected by the air passage 925. The core plate 91 and the cavity block 
92 are supported on retainer plates 911 and 921, respectively. 
The blow molded product 8 is formed from a parison 80 of a softened 
synthetic resin which is extruded into the space between the core plate 91 
and the cavity block 92 which have been separated from each other. After 
the parison 80 has been extruded, the mold is closed. The blowing needle 
931 is pierced through the wall of the parison 80 into its hollow interior 
801, and air is blown through the needle 931 into the hollow interior 801 
to hold the outer surface of the parison 80 against the surface of the 
cavity. The parison 80 which has been expanded to the desired shape is 
cooled to yield the blow molded product 8 as shown in FIG. 8. 
When air has been blown into the parison 80, the air remaining in the 
cavity between the parison 80 and the cavity surface 920 is allowed to 
flow out through the air vent holes 924, the air passage 925 and the air 
vent pipe 926, whereby the parison 80 is brought into intimate contact 
with the cavity surface 920, as shown in FIG. 10. 
The cavity surface 920 is usually embossed to form an embossed or grained 
surface on the blow molded product 8, as shown in FIG. 10. The grained 
surface makes the blow molded product 8 look as if it were of leather, or 
gives it an ornamental feature. It has also been usual to coat the blow 
molded product with a paint to improve its appearance. 
The manufacture of blow molded products is described in, for example, 
Japanese Patent Application Laid-Open No. 206241/1984 and Japanese Utility 
Model Publication No. 22123/1988. 
There has been growing a demand for a blow molded product having a highly 
lustrous surface. There is known a process which employs a mold having a 
mirror finish on its cavity surface for making a blow molded product 
having a highly lustrous surface. The mold is preheated to a high 
temperature to heat and melt the surface of a parison, and is cooled to 
cool it. The heating and cooling of the parison, however, result in a 
longer molding time and therefore a lower degree of productivity. The 
apparatus which are required for heating and cooling the mold, and for 
controlling its temperature, necessarily add to the cost of manufacture. 
The mirror finish on the cavity surface calls for a great deal of labor 
and time. 
The mirror finish causes intimate contact between the softened surface of 
the parison and the cavity surface when the parison has been expanded. 
This makes it difficult to vent air from between the parison and the 
cavity surface. If the cavity surface is embossed, as shown at 920 in FIG. 
10, it may be relatively easy to vent such air, but it is often the case 
that the direct contact of the softened surface of the parison with the 
embossed cavity surface disables such air to escape. 
The incomplete removal of air results in the formation of concavities, or 
other defects in the surface of a blow molded product. This is the case 
with a product having an embossed surface, too, though the embossed 
surface may make any such defect less noticeable. 
The direct contact of the parison with the cavity surface of the mold 
presents a number of problems. Any defect that the cavity surface may have 
is reproduced in the surface of a blow molded product. The softened 
material in the surface of the parison protrudes into the air vent holes, 
as shown at 924 in FIG. 9, and forms small projections on the surface of a 
blow molded product. 
The known process as described above increases the manufacturing cost due 
to the process of heating and cooling the mold for a highly lustrous 
surface, and causes defects in the surface of a blow molded product 
resulted from incomplete removal of the air remained between the parison 
and the cavity surface. Those defects in the surface might occur when 
reproducing an embossed surface of the blow molded product. 
It has, therefore, been difficult to make a blow molded product having a 
highly lustrous surface presenting a good and pleasing appearance at a low 
cost. 
SUMMARY OF THE INVENTION 
Under these circumstances, it is an object of this invention to provide a 
process which can easily make a blow molded product having a highly 
lustrous surface, or a good and pleasing appearance at a low cost. 
It is another object of this invention to provide an apparatus which is 
suitable for use when the process of this invention is carried out. 
This invention provides a process for making a blow molded product 
comprising the steps of positioning a thin sheet having a surface pattern 
which is incompatible with a parison in a mold for blow molding; extruding 
a softened parison of a synthetic resin from a die head; holding the 
parison in the mold and blowing gas into the parison to be expanded to 
form a blow molded product with a surface feature of the sheet reproduced 
on the blow molded product; and separating the sheet and the blow molded 
product from each other. 
According to a salient feature of this invention, the thin sheet is 
positioned in the mold for blow molding, and the surface of the sheet is 
reproduced on the blow molded product, then the sheet and the blow molded 
product are separated from each other. That is, blow molding is carried 
out with the sheet positioned between the parison and cavity block of the 
mold. 
The sheet has an effective surface having an ornamental pattern, or 
appearance to be reproduced on the blow molded product. 
The sheet preferably has a thickness of 100 to 300 microns. If its 
thickness is smaller than 100 microns, the sheet is likely to contact the 
cavity surface so closely that air may be trapped between the sheet and 
the cavity surface and form a defective surface on the blow molded 
product. If its thickness exceeds 300 microns, the sheet may be too thick 
to fit the cavity surface and form on the blow molded product a desired 
surface contour defined by the cavity surface. 
The effective surface is determined according to desired surface 
characteristics. For example, such mold having a mirror finish or embossed 
pattern is employed for making a blow molded product with a highly 
lustrous surface and an embossed surface, respectively. 
The sheet is a film, sheet or strip of a material which is incompatible 
with the material of the parison. More specifically, the sheet may, for 
example, be a film of polyester, nylon, polytetrafluoroethylene (Teflon), 
polyvinyl fluoride, or an ethylene-fluoroethylene copolymer, or a 
laminated film of polyester and polyvinyl fluoride. A sheet of polyvinyl 
fluoride or Teflon can, for example, be used if the parison is of 
polypropylene, or a sheet of polyester or nylon if the parison is of 
polyvinyl chloride. Other combinations are, of course, possible. Moreover, 
the sheet is required to withstand the temperature of the parison which is 
a still hot and soft tubular body of a synthetic resin. 
The sheet has an effective surface having an ornamental pattern, or 
appearance to be reproduced on the blow molded product. The sheet is 
positioned near the cavity block of the mold so that, when the parison has 
been extruded into the mold, the sheet may stay between the parison and 
the cavity block, and so that its effective surface may face the parison. 
The sheet is separated at removing the blow molded product from the mold 
or after such removal. 
This invention provides an apparatus for making a blow molded product 
comprising a mold defining a cavity for blow molding, a die head for 
extruding a parison of a synthetic resin into the mold, a gas blowing 
device for blowing gas into the parison to form a blow molded product, a 
sheet supply device for supplying a sheet in the cavity of the mold, and a 
sheet takeup device for taking up the sheet from the mold after blow 
molding. 
As the aforementioned two devices, for example, a rotary sheet supplier 
winding the sheet around and sheet takeup device can be used, respectively 
as shown in FIG. 1. 
The sheet may be preliminary cut to the size needed for the mold surface. 
Then it is positioned and held by the mold for blow molding as shown in 
FIG. 6. 
The process and apparatus of this invention are useful for making a wide 
variety of blow molded products including moldings and bumpers for 
automobiles, bottles or containers, and edge protectors for desks. 
In a process of this invention, prior to molding, the sheet is positioned 
in the mold. It is so set to be faced on a part requiring a highly 
lustrous or an embossed surfaces. 
A parison is extruded from a die head into the mold to be held thereby. Gas 
is blown into the parison to form a blow molded product with a feature of 
the sheet surface reproduced thereon. Then the sheet is separated from the 
blow molded product, thereby obtaining the product having desired surface 
appearance such as a highly lustrous surface. 
The sheet keeps the parison from contacting the cavity surface, or the 
inner surface of the cavity block, and thereby makes it possible to 
overcome any of the problems which have resulted from their direct 
contact. The air which is present between the sheet and the cavity surface 
is easy to vent. Therefore, the blow molded product has a surface which is 
free from any defects, such as concavities, or unevenness produced by a 
non-uniform flow of the resin, which would occur if any air remained 
between the parison and the cavity surface. Moreover, the surface of the 
product is free from any projection formed by the protrusion of the 
parison material into air vent holes, since the parison is kept from 
contacting the cavity surface in which the air vent holes are open. 
Therefore, the blow molded product has a highly lustrous, or embossed, or 
otherwise ornamental surface produced by the effective surface of the 
sheet with which the parison has been brought into contact. 
This invention does not call for the heating of the mold to produce a 
highly lustrous surface on the blow molded product. It eliminates the 
necessity for any heating, cooling, or temperature controlling device and 
thereby enables a reduction in the manufacturing cost of the blow molded 
product. 
This invention is so constructed to position the sheet of a continuous 
strip between a sheet supply device and a sheet takeup device through the 
mold. After blow molding, the sheet is separated from the blow molded 
product to be rewound on the takeup device. By this a new portion of the 
sheet is unwound from the supply device to be automatically fed into the 
cavity, resulting in continuous supply and easy positioning of the sheet. 
Therefore, the process of this invention facilitates to reproduce a good 
and pleasing appearance on the blow molded product and to reduce the 
manufacturing cost. 
According to a preferred aspect of this invention, the sheet is a 
continuous strip which is easy to feed into the mold automatically to 
supply a new effective surface for each blow molding operation. This 
arrangement improves the efficiency of each blow molding operation and 
enables a further reduction in the cost of manufacture. 
Other features and advantages of this invention will become apparent from 
the following description and the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
A salient feature of this invention resides in the use of a sheet of an 
appropriate material which is positioned in a mold for keeping a parison 
from contacting a cavity surface, and for producing a desired ornamental 
pattern, or appearance on a blow molded product. 
EMBODIMENT 1 
The invention will now be described more specifically with reference to the 
drawings. Reference is first made to FIGS. 1 to 5 showing a process and an 
apparatus which embody this invention. 
This embodiment relates to a process for making a blow molded product 8 as 
shown in FIG. 8. The apparatus comprises a blow mold 9 defining a cavity, 
a die head 73 for extruding a parison 80 of a synthetic resin into the 
mold 9, and a device 93 for blowing gas into the parison 80. It further 
includes a sheet supply device 2 for supplying a sheet 1 into the mold 9, 
and a sheet takeup device 3 for collecting the sheet 1 from the mold 9. 
The mold 9 comprises a core plate 91 for forming the bottom of a blow 
molded product 8, and a cavity block 92 for forming its outer surface, as 
is the case with any known mold. The core plate 91 and the cavity block 92 
are supported on retainer plates 911 and 921, respectively, and hydraulic 
cylinders 916 and 926 are connected to the retainer plates 911 and 921, 
respectively, for moving the core plate 91 and the cavity block 92, 
respectively, to open and close the mold 9. A pair of parison pinchers 917 
and 927 facing each other and each comprising a spring are fastened to the 
lower ends of the core plate 91 and the cavity block 92, respectively. 
The die head 73 is situated above the mold 9. A molding machine 72 is 
connected to the die head 73 for melting a synthetic resin feedstock 800 
and supplying the softened resin to the die head 73. The die head 73 is 
provided with a device not shown, but used for supplying compressed air 
into the tubular parison 80 before the blowing of gas thereinto. 
A pair of tension rolls 21 are provided near the sheet supply device 2, 
another pair of tension rolls 22 above the cavity block 92, and still 
another pair of tension rolls 32 below the cavity block 92. The tension 
rolls 21, 22 and 32 maintain a satisfactory amount of tension on the sheet 
1 extending from the sheet supply device 2 to the sheet takeup device 3 
through the mold 9. The sheet 1 in the mold 9 is so positioned that, when 
the parison 80 has been extruded into the mold 9, the sheet 1 may stay 
between the parison 80 and the cavity block 92. 
The sheet 1 has a mirror surface for producing a highly lustrous surface on 
the blow molded product, and is of a material which is incompatible with 
the material of the parison 80. 
The gas blowing device 93 is provided in the core plate 91. The cavity 
block 92 is provided with air vent holes 924, an air passage 925, and an 
air vent pipe 926, as shown in FIG. 2. The cavity block 92 has an embossed 
or wrinkled cavity surface 920, as shown in FIG. 3. The wrinkled cavity 
surface 920 enables the effective removal of air from between the sheet 1 
and the cavity surface 920, as will hereinafter be described in further 
detail. 
Referring now to the process, the sheet 1 is positioned in the mold 9, 
while it is open, as shown in FIG. 1. The parison 80 is extruded from the 
die head 73 into the mold 9 between the sheet 1 and the core plate 91. The 
hollow interior 801 of the parison 80 is supplied with compressed air 
through the die head 73. The hydraulic cylinders 916 and 926 are actuated 
to move the core plate 91 and the cavity block 92 toward each other to 
close the mold 9, whereupon the parison 80 is held between the core plate 
91 and the cavity block 92 near the top and bottom of the mold 9 and the 
lower end of the parison 80 is held between the parison pinchers 917 and 
927, as shown in FIG. 2. 
The gas blowing device 93 has a needle 931. The needle 931 is pierced 
through the wall of the parison 80, and compressed air is blown into the 
hollow interior 801 of the parison 80 through the needle 931 to expand the 
parison 80. The expanded parison 80 has its outer peripheral surface 
brought into contact with the inner surface of the core plate 91 and the 
sheet 1, and the sheet 1 is in turn brought into contact with the cavity 
surface 920 of the cavity block 92, whereby the features of the inner 
surface of the core plate 91 and of the effective surface of the sheet 1 
are reproduced on the outer peripheral surface of the parison 80. 
When the parison 80 has been expanded, the air confined between the sheet 1 
and the cavity surface 920 is vented through the air vent holes 924, the 
air passage 925 and the air vent pipe 926. The sheet 1 and the finely 
rugged, or wrinkled cavity surface 920 has therebetween clearances which 
facilitate the escape of air into the air vent holes 924. On the other 
hand, the concavities in the rugged cavity surface 920 holds air which 
forms a cushion for the sheet 1 and thereby prevents the ruggedness of the 
cavity surface 920 from being reproduced on the parison 80 through the 
sheet 1. 
After the expanded parison 80 has been allowed to cool and solidify to 
yield a blow molded product 8, the mold 9 is opened, and the blow molded 
product 8 is removed from the mold 9, while the sheet 1 is separated from 
the product 8, as shown in FIG. 5. The used portion of the sheet 1 which 
has been separated from the product 8 is rewound on the sheet takeup 
device 3, while a new portion of the sheet 1 is fed from the sheet supply 
device 2 into the mold 9 to make the mold 9 ready for the subsequent blow 
molding operation, as shown in FIG. 1. 
The process as hereinabove described was employed for blow molding, for 
example, a parison of a polypropylene resin by using, for example, a sheet 
of polytetrafluoroethylene (Teflon) having a thickness of 250 microns. 
There was obtained a blow molded product having a highly lustrous surface 
produced by the smooth mirror surface of the sheet. 
In the above described process, the sheet 1 is positioned between the 
cavity surface 920 and the parison 80 to keep them from contacting each 
other for blow molding. The air remained between the sheet 1 and the 
cavity surface 920 can be easily vented outside through the air vent holes 
924. Particularly the cavity surface 920 is so formed to be finely rugged 
or wrinkled as shown in FIG. 3, thereby forming a clearance between the 
sheet 1 and the cavity surface 920, helping the air vent. 
The air trapped in the concavities of the cavity surface 920 provides a 
shock absorbing function for the sheet 1, by which the ruggedness of the 
cavity surface 920 is not reproduced on the surface of the blow molded 
product 8. 
Therefore the blow molded product 8 has a surface which is free from any 
defects, such as concavities, or unevenness produced by a non-uniform flow 
of the resin. The blow molded product 8 has a highly lustrous, or 
embossed, or otherwise ornamental surface produced by the effective 
surface (film surface) of the sheet 1 with which the parison 80 has been 
brought into contact. 
This invention requires no process step of preheating the mold for a highly 
lustrous surface as is the case with any known process, resulting in 
manufacturing cost saving. 
The above-described apparatus is so constructed to stretch the sheet 1 
between the sheet supply device 2 and the sheet takeup device 3 to be 
subsequently fed after blow molding, resulting in easy positioning and 
supply of the sheet 1. 
EMBODIMENT 2 
Attention is now drawn to FIGS. 6 and 7 showing modified forms of process 
and apparatus which embody this invention. In this embodiment, the 
apparatus includes a cavity block 92 having an even cavity surface 922 and 
the process is carried out by employing a sheet 10 having two rugged or 
embossed surfaces 101 and 102, of which 101 is the effective surface with 
which a parison 80 is brought into contact. 
The sheet 10 is a discrete film of nylon having a thickness of 300 microns 
and knurled to form the rugged surfaces 101 and 102. The grooves forming 
each rugged surface have a depth of about 150 microns. The rugged surface 
101 of the sheet 10 forms a feature to be reproduced. 
The apparatus further includes a pair of L-shaped clamping members 15 
supported rotatably on the cavity block 92 at the upper and lower ends 
928, respectively, thereof. Each clamping member 15 has a base portion 151 
attached rotatably to the cavity block 92, and a free toothed portion 152 
which is rotatable to hold the sheet 10 fast to the upper or lower end 928 
of the cavity block 92. 
The sheet 10 is a discrete film instead of being a continuous strip. It is 
placed in the mold 9, and fastened to the cavity block 92 by the clamping 
members 15. After the parison 80 is extruded into the mold 9, the mold 9 
is closed, and the parison 80 is expanded, whereby the embossed surface 
101 of the sheet 10 is reproduced on the parison 80. The air which is 
trapped between the sheet 10 and the even cavity surface 922 is easily 
vented through the grooves in the rear surface 102 of the sheet 10 and 
through air vent holes 924. The expanded parison 80 yields a blow molded 
product 8 having an embossed surface produced by the effective surface 101 
of the sheet 10, and free from any such defect as has hereinbefore been 
pointed out. 
After the molded product is removed from the mold 9, the clamping members 
15 are opened to release the sheet 10, and a new sheet is positioned in 
the mold 9 to make it ready for a new blow molding operation. 
The blow molded product 8 obtained from this embodiment has an embossed 
surface reproduced by the rugged surface 101 of the sheet 10. The air 
remained between the cavity surface 922 and the sheet 10 can be easily 
vented, which causes no defects on the surface of the blow molded product, 
such as concavities, or unevenness produced by a non-uniform flow of the 
resin frequently occurred when using the known apparatus. The blow molded 
product 8, thus, has a good and pleasing embossed surface. 
The process and apparatus under description are otherwise substantially 
identical to those which have been described with reference to FIGS. 1 to 
4.