FIELD OF THE ART
This invention relates to the injection orientation blow molding of
molded articles such as receptacles, bottles and the like formed of materials
comprising synthetic resins such as polypropylene, polyamide and the like.
BACKGROUND OF THE INVENTION
From EP 0 144 196 A3, there is known a process, where in the injection
orientation blow molding, a lip portion of an injection-molded preform with a
bottom is held and fixed while other portions are subjected to orientation blow
The bottom which becomes a body after blow molding, is gradually reduced
in thickness from its center toward its peripheral edge, being adjacent to a lip
In this case, if the resin used as material for the molded article
is a synthetic resin which is liable to become milky such as polypropylene, polyamide
and the like, the orientation blow molded portion becomes transparent because
of biaxial orientation but a portion from a lip portion 21 to a shoulder 22 of
a molded article 20 remains to be milky as shown in FIG. 3.
Most of the lower edge of the milky portion 23 appear to be uneven
in the shoulder 22, and for this reason, the external appearance of the article
is impaired to reduce the value as the goods by half.
In view of the foregoing, the present inventor has repeatedly made
various studies on the cause of generation of the milky portion 23 produced in
the shoulder 22 of the molded article, as a consequence of which the inventor
has found that this resulted from insufficient orientation of the lower side of
the lip portion, and if a portion below said portion is sufficiently oriented,
the milky portion 23 merely appears at the lip portion 21.
SUMMARY OF THE INVENTION
It is an object of this invention to solve the above-described problems
with respect to molded articles subjected to injection orientation blow molding
by use of resins as materials which are liable to become milky such as polypropylene,
polyamide and the like.
For achieving the above-described object, the present invention is
featurized by injection molding a preform with a bottom in which a wall thickness
of a lower portion of a peripheral wall of a lip portion is made thicker than
other oriented portions by a shoulder provided internally thereof, and holding
and fixing a lip portion of said preform to orientation blow mold a portion below
BRIEF DESCRIPTION OF THE DRAWINGS
BEST MODE FOR EMBODYING THE INVENTION
- FIGS. 1 and 2 show the injection orientation blow molding method according
to this invention; in which FIG. 1 is a sectional view of a preform indicated by
dash-dotted contour lines, and FIG. 2 is likewise a sectional view of a molded
- FIG. 3 is a longitudinal sectional view of a half portion of a molded article
obtained by a conventional molding method.
First, as shown in FIG. 1, a preform 3 with a bottom having a shoulder
2 provided at a lower part and internally of a lip portion 1 is injection molded,
and a wall thickness of a lower portion 4 of a peripheral wall of the lip portion
is made thicker than that of the lip portion 1 and a bottom 6 by the presence
of said shoulder 2.
Formation of the shoulder 2 can be easily carried out by providing
a shoulder 10 around the lower portion of a core mold 9 for forming a cavity as
well as a lip mold 7 and an injection mold 8.
Subsequently, as shown in FIG. 2, the lip portion 1 is held and fixed
at a position of the shoulder 2 by inserting a blow core 11, the lower side of
the lip portion and the bottom 6 are oriented by a rod 13 from the shoulder 2
portion to the cavity bottom of a blow mold 12, and air is blown to fully inflate
the cavity to mold the preform 3 into a hollow molded article 14.
In the above-described orientation, a portion that may be most easily
stretched is a thick portion of the lower portion 4 of the peripheral wall of the
lip portion formed to be thicker than other portions.
Among the portions to be oriented, the lower portion 4 of the peripheral
wall of the lip portion formed to be thicker than other portions has much more
heat-retaining amount than other portions so that even if the surface temperature
during the progress of the orientation blow molding is lowered, the loss of heat
is smaller than the case where the wall thickness of said portion is made to be
the same as that of other portions, and the portion with the shoulder 2 as a boundary
is well stretched.
As the result, the portion below the shoulder 2 is biaxially oriented
by the orientation blow molding, and those other than the lip portion 1 become
transparent. The milky portion is definitely separated from the transparent portion
with the lower side of the lip portion where the shoulder 2 was present as a boundary
as shown, and the lower edge of the milky portion is not extended to and above
While in the above-described embodiment, the boundary between the
milky portion and the transparent portion is in the form of a straight line, if
the wall thickness portion of the shoulder 2 is formed to be wavy, the boundary
between said two portions becomes a wave shape. Accordingly, in the present invention,
the shape of the shoulder 2 is not limited to the straight line as shown.
As described above, in the present invention, the wall thickness
of the lower portion of the peripheral wall of the lip portion is formed to be
thicker than other oriented portions by the provision of the shoulder provided
internally so that the heat retaining amount of the lower portion of the peripheral
wall of the lip portion is increased to make orientation easy to thereby prevent
the milky shoulder from being formed due to non-orientation. Therefore, even if
polypropylene or polyamide is used as material, receptacles or the like in which
only the lip portion becomes milky can be molded, and in addition, molded articles
excellent in external appearance such that a milky portion and a transparent portion
are defined with a straight line as a boundary can be easily molded. Therefore,
the molded articles are very advantageous as receptacles and are very effective
in industry and extensively used.