FIELD OF THE INVENTION
The present invention relates to a method of molding a product including
the step of cooling the product while the product is in the mold while at the same
time cooling the mold. The invention is particularly used for the formation of molded
BACKGROUND OF THE INVENTION
Plastic pipe is generally formed by extruding plastic into a mold
tunnel. The pipe is formed by either vacuum forming or blow molding. After the pipe
has been shaped, it is typically cooled by cooling of the mold blocks. As the pipe
continues down the mold tunnel the cooling becomes less and less efficient because
the pipe shrinks producing an air gap between the mold blocks and the pipe. Accordingly,
the pipe is insulated from and has difficulty in giving heat off to the mold blocks
as the pipe continues down the mold tunnel.
US 4,319,872 discloses an apparatus for producing corrugated thermoplastic
tubing with mold blocks cooperating in pairs. The walls of the mold blocks have
troughs for forming the corrugations which have passages which can be connected
to a suction source to help in forming the corrugations. The passages can also be
connected to a pressure source after cooling and setting of the tubing in order
to urge it out of the mold.
US 4,226,580 describes a similar apparatus in which the mold blocks
include an inner liner of a porous material and first and second passages. A cooling
fluid can be fed through the first passages and the porous liner to the second passages
in order to cool the mold blocks.
SUMMARY OF THE INVENTION
The present invention provides methods as defined in claims 1 and
2 for molding a plastic product with novel means for cooling of both the mold and
the product. The invention is particularly applicable to the molding of a plastic
pipe. of the mold through an outlet at a location remote from the inlet.
In the case of a plastic pipe, which is formed in a rounded mold tunnel,
the cooling gas is introduced through mold block sections on one side of the mold
tunnel and is moved circumferentially around the pipe.
The method of the present invention can be used in combination with
interior cooling to further increase the cooling effect on the product.
BRIEF DESCRIPTION OF THE DRAWINGS
The above as well as other advantages and features of the present
invention will be described in greater detail according to the preferred embodiments
of the present invention in which;
DETAILED DESCRIPTION ACCORDING TO THE PREFERRED EMBODIMENTS OF THE PRESENT
- Figure 1 is a side view of a plastic pipe molding apparatus according to a preferred
embodiment of the present invention;
- Figure 2 is a top perspective view looking down on a pair of mold block sections
from the apparatus of Figure 1;
- Figure 3 is a sectional view through the mold tunnel of the apparatus of Figure
1 showing pipe being vacuum formed according to one preferred embodiment of the
- Figure 3a is a sectional view through the mold tunnel of the apparatus of Figure
1 with the pipe being blow formed according to another preferred embodiment of the
- Figure 4 is a sectional view through the mold tunnel of the apparatus of Figure
1 showing initial cooling of the pipe;
- Figure 5 is a sectional view through the mold tunnel of the apparatus of Figure
1 showing final cooling of the pipe;
- Figure 6 is a sectional view through a mold tunnel of a further preferred mold
Figure 1 shows a pipe molding apparatus generally indicated at 1.
In this apparatus, molten plastic is fed from an extruder 2 to a mold tunnel downstream
of the extruder. Pipe indicated at P emerges from the downstream end of the mold
The mold tunnel is formed by an upper track of mold block sections
3 and a lower track of mold block sections 4. The mold block sections meet with
one another to define the mold tunnel generally indicated at 5.
Figure 2 shows a pair of side by side mold block sections 4 from the
lower track of mold block sections. As will be seen, these mold block sections have
an interior face generally indicated at 7 which defines the external shape of the
pipe P. In this particular embodiment, the interior face of the mold block sections
has alternating lands and grooves to produce a ribbed shaping of the pipe.
Mold block sections 4 are particularly designed for vacuum forming
of the pipe in that they are provided with small vacuum slits 9 at each trough in
the mold block face. For vacuum forming of the pipe, suction is drawn through these
vacuum slits which pulls the molten plastic onto the faces of the mold block sections.
These vacuum slits may be intermittent or they may be continuous around the semicircular
face of each mold block section. Figure 2 shows two slits in each trough of each
mold block section, however there may only be one slit or more than the two slits
shown for each trough.
Mold block sections 3 in the upper track of mold block sections have
an identical construction.
Figure 3 shows the vacuum forming of the pipe as it occurs in an upstream
region of mold tunnel 5. Here it will be seen that the mold block sections 3 and
4, both of which are mounted to mold block carriers 2, are closed tightly with one
another in the mold tunnel. A source of vacuum 11 which as shown in Figure 3 is
located externally of the mold block section is in direct communication with the
small slits 9 in the upper mold block section at discrete locations 16, 17 and 18
through passages 13, 14 and 15 respectively. These passages, as shown, go directly
through the wall of the upper mold block section. A similar arrangement is found
in the lower mold block section 4 where a source of vacuum 19 again outside of the
mold tunnel is in connection with the vacuum slits 9 opening at the face of the
lower mold block section through passages 21, 22 and 23 having separate access regions
25, 26 and 27 respectively with the vacuum slits.
Figure 3 shows that when vacuum is applied through both the upper
and the lower vacuum source, the plastic is pulled onto the interior surfaces of
the two mated mold block sections to shape the pipe P.
Figure 3a, while using the same mold block sections 3 and 4 shows
that the pipe can also be blow molded from air pressure applied internally of the
pipe as shown by the arrows in Figure 3a. This air pressure pushes the molten plastic
outwardly onto the mold block faces.
Regardless of the method of forming the pipe, i.e. either by vacuum
forming or by blow molding of the pipe, the pipe is cooled using openings in the
mold block faces and in this case by using the vacuum slits 9 through the mold block
sections. After the pipe has been shaped and as it continues down the mold tunnel
where the plastic starts to set to hold the shape of the pipe, a cooling gas is
introduced from one of the mold block sections into a gap between the mold tunnel
and the pipe. Eventually, this cooling gas is drawn out of the mold tunnel through
an outlet remote from the location where the gas is introduced to the mold tunnel.
More particularly, as indicated by the arrows in Figure 4, cooling
gas is introduced to the upper mold block section 3. In the event that the pipe
has been vacuum formed, the vacuum while being continued in the upstream region
of the mold tunnel is replaced by a flow of cooling gas into the mold tunnel in
the more downstream region of the tunnel after the pipe has been shaped and sufficiently
set to hold its shape. Passage 11 which now becomes a pathway for the cooling gas.
This cooling gas, which may be in a number of different forms including outside
ambient air or air which is passed through a cooling device, is forced along the
passages 13, 14 and 15 and in this case through the vacuum slits 9 in the upper
mold block section. These slits now become inlets into the mold tunnel for directing
the cooling gas onto the exterior surface of the pipe P.
Figure 5 shows that the pipe, with some initial cooling, shrinks away
from the interior surface of the mold sections creating a gap G between the outside
surface of the pipe and the interior face of mold tunnel. This allows the cooling
gas to be forced circumferentially around the pipe as indicated by the arrows in
gap G to the vacuum slits in the lower mold block section where the gas is then
drawn out of the mold tunnel. Therefore, the slits in the lower mold block section
which continue to be subjected to vacuum, now become gas outlets from the mold tunnel.
It is not necessary to introduce cooling gas to the mold tunnel in
order to produce some initial shrinkage of the pipe away from the mold tunnel wall.
This occurs naturally as a result of normal cooling of the pipe. Therefore, the
cooling gas is introduced after the natural pipe shrinkage which still produces
the gap G allowing the cooling gas to be fed into the mold tunnel from one of the
mold block sections and around the pipe to the other mold block section.
The cooling gas has two benefits. Firstly, it provides faster cooling
and setting up of the pipe. Secondly, it reduces the temperature of the mold blocks
which results in greater ability of the mold blocks to absorb heat from the pipe
which again allows the pipe to cool and set faster than normal.
One of the advantages of using the vacuum slits for both the introduction
and the discharge of the cooling gas, preferably cooling air, is that the vacuum
slits whether they be continuous or intermittent form paths for the cooling air
completely through the main body of the mold block sections. Therefore, the cooling
air not only runs around and along the interior surfaces of but additionally penetrates
into the mold block sections. This substantially enhances cooling the mold block
sections. This in turn increases the ability of the mold blocks to cool the pipe
particularly after the mold block sections are returned along their endless loops
in a much cooler state than normal to the upstream end of the mold tunnel where
they are in direct contact with and have the greatest cooling effect on the plastic
as it is extruded into the mold tunnel.
Figure 6 shows a slightly different mold tunnel design comprising
a pair of mold block sections 21 and 25 provided with passages 23 and 27 respectively.
These passages first provide vacuum paths during formation of the pipe P and later
act as cooling gas inlet and outlet passages. They feed into the interior of the
mold tunnel along the parting faces of the mold block sections where they meet as
indicated at 29 rather than through holes provided directly in the walls of the
mold block sections.
If desired, for faster cooling of the pipe in the mold tunnel, a cooling
medium such as a cooling plug or even cooling gas can be introduced interiorly of
the pipe simultaneously with the exterior cooling of the pipe.
In another embodiment of the invention a cooling gas is introduced
at the interior face of the mold tunnel, e.g. through the slits in the mold blocks,
along the parting faces of the mold block sections, or through other inlets provided
in the mold block faces. After cooling both the product and the mold blocks the
gas is discharged through the downstream end of the mold tunnel which is open for
releasing the pipe from the mold tunnel as earlier described with respect to Figure
1 of the drawings.