METHOD AND DEVICE FOR THE PRODUCTION OF ARTICLES HAVING A NON-CELLULAR SMOOTH SKIN AND A CELLULAR CORE FROM POLYMER MATERIALS

METHODE ET APPAREILLAGE POUR LA PRODUCTION A PARTIR DE COMPOSES POLYMERIQUES D'ARTICLES A CENTRE CELLULAIRE RECOUVERTS D'UNE MEMBRANE LISSE NON CELLULAIRE

English Abstract

ABSTRACT OF THE DISCLOSURE:
A method for the production of articles having a non-
cellular smooth skin and a cellular core from polymer materials
which contain blowing agents, by injection molding. The method
of the invention comprises heating the polymer material to a
temperature lower than the decomposition temperature of the
blowing agent as the polymer material flows through an injection
nozzle and into a mold cavity to form a non-cellular skin,
abruptly increasing the temperature of the flowing polymer melt
to above the decomposition temperature of the blowing agent to
cause the polymer to foam and to form a cellular core and
thereafter reducing the temperature of the flowing polymer melt
to below the decomposition temperature of the blowing agent to
complete the non-cellular skin in the area of the injection nozzle.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. A method for the production of articles having a
non-cellular smooth skin and a cellular core from polymer
materials which contain blowing agents, by injection molding,
which comprises heating the polymer material to a temperature
lower than the decomposition temperature of the blowing agent
as the polymer material flows through an injection nozzle and
into a mold cavity to form a non-cellular skin, abruptly increasing
the temperature of the flowing polymer melt to above the
decomposition temperature of the blowing agent to cause the
polymer to foam and to form a cellular core and thereafter
reducing the temperature of the flowing polymer melt to below the
decomposition temperature of the blowing agent to complete the
non-cellular skin in the area of the injection nozzle.
2. In an injection molding device for the production
of articles having a non-cellular smooth skin and a cellular core
from polymer materials which contain blowing agents, an injection
nozzle having along its axis an electric heating element which
consists of a resistive wire connected to a supply and control
block, said resistive wire extending substantially over the entire
length of the nozzle.

Description

0S387;2
This invention relates to a method and a device for the
production of articles with a non-cellular smooth skin and a
cellular core from polymer materials, which contain blowing agents,
by injection molding.
Several methods are known for the production of such
articles with a smooth skin. In one of them, a preformed deep-
drawn foil is placed in the mold cavity to form the skin before
filling it with expanding polymer melt. In anothèr method, the
mold cavity is completely filled under high pressure with polymer
10 melt, and after the formation of a non-cellular skin the mold is -
opened up at such a distànce as to crea-te an additional volume
for the foaming of the material in the interior. However,the
application of these methods is limited only to articles of
specific shapes.
Other methods are known, in which a material which
does not contain a blowing agent is used for the formation of the
non-cellular skin, while a material without a blowing agent is ~ -
used for the cellular core. A general disadvantage of these
methods is the necessity of additional injection and distributing
units, as well as of additional mechanisms to drive them.
Another method is known, in which a gas pressure is
produced in advance in the mold cavity in order to prevent any
foaming of the polymer melt when it fills the mold. This method
provides a possibility to control the process of structure
formation and to obtain articles with a non-cellular skin of
uniform thickness. To achieve this, it is necessary to add to ;
the injection molding machines a aistributing unit and a vertical
cylinder with piston, which are provided with driving and
heating devices.
It is therefore a general object of the invention to
avoid the disadvantages of the known methods and devices and
to provide a method and device for the production of articles
having a non-cellular smooth skin and a cellular core from
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polymer materials containing blowing agents, by injection molding.
The method of the invention comprises heating the
polymer material to a temperature lower than the decomposition
temperature of the blowing agent as the polymer material flows
through an injection nozzle and into a mold cavity to form a
non-cellular skin, abruptly increasing the temperature of the
flowing polymer melt to above the decomposition temperature
of the blowing agent to cause the polymer to foam and to form a
cellular core and thereafter reducing the temperature of the
flowing polymer melt to below the decomposition temperature of
the blowing agent to complete the non-cellular skin in the area
of the injection nozzle.
The invention also provides an injection molding device
suitable for carrying out the above method, wherein the injection
nozzle has along its axis an electric heatihg element which t
consists of a resistive wire connected to a supply and control
block; the resistive wire extends substantially over the entire
length of the nozzle.
For a better understanding of the invention, reference
shall be made to the accompanying drawing in which there is
illustrated a preferred embodiment of the invention. The drawing
shows a diagrammatical cross-section through an injection nozzle.
The nozzle illustrated consists of two components 1 and
5, which are connected by the nut 4 and are electrically insulated
by means of the rings 3. In the axis of the nozzle hole, there
is an electric heating element 2 in the form of a resistive wire. -
One end of this element is welded to the component 1, while the
other end is welded to the component 5. The switching-on and
switching-off of the heating element is performed by the supply
30' and control block 7. The nozzle can be attached to the end of
the plastificating-injection unit 6 of the injection molding
machine.
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When utilizing such injection nozzle, the process of the
invention is as follows: ~
Polymer melt, heated to a temperature lower than the ;
decomposition temperature of the blowing agent is injected into -
the mold cavity. After a predetermined quantity of material that
is necessary for the formation of the non-cellular skin has been
introduced into the mold, the electric heating element 2 in the
nozzle is then switched-on/ thus abruptly increasing the
temperature of the polymer melt which is flowing through to above
the decomposition temperature of the blowing agent. This latter
material fills up the core portion Gf the molded article. This `
heating continues until a predetermined quantity of foamed material
which is necessary for the formation of the cellular core has been
introduced into the mold. At the end of the filling, the electric
heating element 2 is switched-off in order to reduce the temperature
so that the last portion of material cannot foam, in order to
obtain a non-cellular surface in the area of the sprue.
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