Apparatus and method for injection moulding of plastic parts of irregular shape, hollow or undercut form

The invention relates to a device and method of using the device and plastic injection molding processes. A mold is provided utilizing a core inside the injection cavity of the mold. The core is prior to insertion of the mold subjected to a sub-freezing atmosphere. The core is then placed in the mold cavity wherein is injected the plastic material to mold the plastic part substantially surrounding the core. A thin film of air is formed around the "core" at the time of the injection of the plastic material thereby providing a substantial increase in surface finish to the plastic molded part. The core is made from a material which when heated will melt prior to the melting of the plastic part so that the core and part are removed from the cavity and subjected to a heating step thereby enabling removal of the core from the part.

BACKGROUND OF THE INVENTION 
1. Scope of the Invention 
The present invention relates to a special process for carrying out 
injection molding of plastic parts which have hollow or irregularly shaped 
forms as well as undercut regions. The invention also concerns the core 
member which is used for carrying out the method. 2. Description of Prior 
Art 
Various methods using a destructable core for providing injection molded 
plastics parts which include hollow or undercut shapings are already 
known. 
The difficulty of carrying out injection molding of such parts essentially 
resides in the accuracy of molding. The surface finish of the cores used 
must be excellent, and the core must not be able to become deformed at the 
actual time of injection. 
One process which is known notably from French Pat. No. 1,366,921 consists 
of using cores in low melting point alloys. The disadvantages which are 
generally encountered when carrying out this process are as follows: 
Since the melting point of the plastic material injected into the mold is 
greater than the melting point of the metal or the alloy of which the core 
is fabricated, balancing of the temperatures of the injected material and 
of the core presents a very significant obstacle to the usage of such a 
process. 
The operation for recovering the core material by melting is a relatively 
slow process. It is necessary to reheat the complete core and injected 
plastic part combination at a temperature which must still be less than 
the melting point of the plastic material. 
SUMMARY OF THE INVENTION 
The invention has the objective of making it possible to provide, by the 
injection molding process parts in plastic material without the 
above-stated disadvantages being encountered. 
A method according to the invention for carrying out injection molding of 
parts in plastic material having hollow or irregular and undercut shapes 
by making use of a low melting point metal core, is characterized in that 
the core is cooled to a low temperature before injecting the plastic 
material into the mold. 
According to a further characteristic of the invention, the core is cooled 
to a low temperature as a result of its having spent a certain period of 
time in a freezing means. The temperature of the core at the actual time 
of the injection operation thus being of the order of -20.degree. C. to 
-40.degree. C., subjecting the core to a freezing temperature causes a 
thin film of air to form around the core at the actual time of injection 
which results in the plastic material being obtained with an improved 
surface finish. 
According to yet a further characteristic of the invention, a core pin 
having large dimensions and made from a metal which is a good conductor of 
heat is placed inside the fusible core. This core pin is placed in contact 
with the metal injection mold, and provides for good transfer of heat away 
from the core during injection. 
According to yet a further characteristic of the invention, the core is 
recovered after the injection operation by immersing the molded part and 
its core in a bath of boiling water or in a bath of hot liquid. The part 
and its core is then agitated until the latter has melted away. 
According to a further characteristic of the invention, the elimination of 
the core is enhanced by employing a centrifuging operation. This is 
particularly applicable in the case of parts which are axially 
symmetrical, for example, for turbine parts. 
A core which is employed to carry out the method according to the invention 
is characterized in that it is made up from a piece of metal of small 
thickness, which corresponds to the undercut shape of the part which is to 
be molded. The core is made from a low melting point alloy, arranged 
around a massive core pin having large dimensions and made from a metal 
which is a good conductor of heat, and has a relatively high melting 
point. 
According to yet a further characteristic of the invention, electrical 
resistances are incorporated in the core, so that, in order to cause the 
core to melt, all that is required is to connect these resistances to a 
source of electrical current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Using a known arrangement which is illustrated in FIG. 1, in order to carry 
out injection molding of a plastic part having hollow or irregular and 
undercut shapes, a core 1 having a low melting point is used which is 
arranged inside an injection cavity 2 of the mold 3. The plastic material 
is injected in the molten state into the space which is left around the 
core in order to obtain a molded part 4, shown in FIG. 2. In order to 
recover the core 1, it is melted by immersing the molded part 4 which has 
been molded around its core in a bath of boiling water, or alternatively 
in a bath of a hot liquid of an appropriate nature. The temperature of the 
bath is a function of the respective melting points of the core 1 and of 
the plastic material constituting the molded part 4. 
According to the invented method, the core is brought to a low temperature 
before carrying out the injection operation, by, for example, placing the 
core in a freezing means so as to lower its temperature down to 
approximately -20.degree. C. to -40.degree. C. The method results in the 
following advantages: 
The negative heat energy stored in the core prevents any softening of it 
while the core is in contact with the molten plastic material, thereby 
insuring that the exact geometry of the molded part will be preserved. It 
should be noted that the molten plastic material is injected at pressures 
which may reach 500 bars to 1000 bars, or even higher in the injection 
cylinder. Experience has shown that a thin film of air forms around the 
cold core at the time of injecting the plastic material. This thin film of 
air provides the molded part 4 with a good surface finish at the area of 
contact with the core 1. 
By varying the composition of the alloy used for manufacturing the low 
melting point core, it is clearly possible to select a predetermined 
melting point. A high melting point would bring about the risk of damage 
being caused to the plastic molded part while a melting point which is too 
low could lead to deformation of the core during the injection operation. 
Such deformation is clearly harmful to the geometrical quality of the part 
molded around the core. The alloy is consequently selected on the basis of 
the plastic material used in the injection process. 
According to the invented method, it is also possible to use a core 5, as 
shown in FIG. 4, which is made up from a metal part, of small thickness, 
formed around a thick core pin 6 of large dimensions. The core pin 6 is 
made from an alloy which is a good conductor of heat, such as an aluminum 
alloy, and is itself in contact with the mold 3 thereby insuring that 
there is a good heat flow towards the mold. In this alternatively, the 
core pin 6 provides the mechanical support for the core. 
According to yet a further alternative embodiment of the invention, the 
mold 3 may be a non-ferrous, non-cuprous alloy, and is preferably based on 
aluminum, as for example of the "AU4G" type, which provides the following 
advantages: 
The mold is thus of good mechanical strength, the rigidity of the mold 
being a significant aspect of the molding process, bearing in mind the 
high injection pressures. 
An aluminum alloy is easy to machine; 
has a good coefficient of thermal conduction; and 
such an alloy does not adhere to the molten metal of the core, fusible 
metals do adhere onto ferrous or copper-based metals. 
In order to decrease the time necessary for recovering the core by a 
melting process, the molded part and its core are agitated in the bath of 
hot liquid or boiling water, using alternating movements, or any other 
convenient movements. 
Also, electrical resistances 7 as shown in FIG. 5 may be incorporated into 
the core. In this case, in order to cause the core to melt, all that is 
needed is to connect these resistances to a source of electrical current. 
The use of alternative embodiments which might suggest themselves to those 
skilled in the art would not lead to a departure from the scope of the 
present invention, such non-limiting variations being: 
More rapid methods for melting the core may be used by distributing 
electrical resistances in the core or by using systems operating on 
induction, microwaves, Foucault currents, or simply by constructing the 
composite assembly in such a way as to allow a better flow of heat towards 
the core, as by conduction. This would notably make it possible to employ 
alloys having a higher melting point, either when this is necessary as a 
result of the type of plastic material used, or simply for the purpose of 
reducing the time duration of the operation for recovering the core. 
With the objective in mind of still obtaining more rapid melting of the 
core, a system of the "heat exchanger" type may be utilized by inserting 
either a hollow core pin into the core, a finned tube, a tortuous passage, 
or any other hollow body into the core, through which it is possible to 
cause a hot fluid to circulate. 
In order to economize on the fusible alloy, non-fusible material inserts 
could be inserted into the core which have low thermal inertia and are 
able to be extracted from the plastic material part through orifices 
formed in the latter. Such inserts may, for example, be provided by balls, 
or by metal plates. 
In order to speed up the rate of manufacture, the core can be brought to a 
low temperature by immersing it in an enclosure containing a liquified 
gas. 
When the melting point of the alloy used for providing the fusible part of 
the core is greater than 100.degree. C., the core is advantageously 
recovered by immersing it in a bath of hot oil. While the invention has 
been described in connection with the preferred embodiments, it is to be 
understood that this description is not intended to be limited with 
respect to those embodiments. On the contrary, it is intended to cover all 
alternatives, modifications and equivalents, as may be included in the 
spirit and scope of the invention as defined by the appended claims.