WO2019111086A1

WO2019111086A1 - Method of production of an object made of a thermoplastic polymer admixed with at least one gas at the supercritical state

Info

Publication number: WO2019111086A1
Application number: PCT/IB2018/059171
Authority: WO
Inventors: Antonio AFFINITA; Eric Deriche; Martial Sabatier
Original assignee: Sapa S.P.A.; Runipsys Europe S.A.S.
Priority date: 2017-12-04
Filing date: 2018-11-21
Publication date: 2019-06-13

Abstract

Method of production of an object made of a thermoplastic polymer admixed with at least one gat at the supercritical state, with a hot block (3) provided with hot mixing channels (4) having injectors (6-6 '-6 "-...) connected to a control unit (12), in turn connected to a gas tank (5); each injection nozzle (8-8'), located downstream of the hot mixing channels (4), having a third shutter (9-9') adapted to introduce a mixture of thermoplastic polymer and supercritical gas inside the interspace (13) of an injection-compression mold (20); said method consisting of: (A) extrusion of the polymer from the first shutter (2) of the feed duct (1) into the hot block (3); with the mold (20) in closed configuration; (B) injection of the supercritical gas from the injectors (6-6 '-6"-...) of the hot mixing channels (4) while the mold (20) is still in closed configuration; (C) distribution of the mixture to the injection nozzles (8-8'); (D) injection of the mixture inside the interspace (13) of the mold (20) while still closed configuration; (E) cooling of the mixture; opening of the mold and extraction of the object just produced and start of a new production cycle.

Description

“Method of production of an object made of a thermoplastic polymer admixed with at least one gas at the supercritical state”

Description

Field of the invention

The present invention relates to the field of mechanics and industry, in particular to the field of plastic components. More in detail, the plastic elements obtained according to the method of the present invention patent are suitable for use in the automotive industry.

Prior art

Most of the automotive industry’s research is now aimed at finding solutions to comply with international agreements and national pollution laws, which provide for increasingly lower admissible percentages of C0₂ as the years go by. This lowering of emissions can be achieved in two ways: the first is to optimize the operation of engines that, at least at present, does not seem to give sufficient results to comply with the regulations; the second is to reduce the weight of the vehicle and therefore of each component.

Therefore, for all car manufacturers, car lightening is an essential objective, or the manufacturers may incur economic sanctions. The directive 2009/29/EC issued by the EU, in fact, requires the reduction of pollutant emissions from vehicles. As 75% of fuel consumption is directly related to the weight of the car, a reduction of 100 kg would decrease consumption by 11 g of C0₂ per kilometer.

Among the methods to obtain such lightening, car manufacturers have various possibilities:

- the reduction of sheet thickness;

- the use of aluminum alloys in the body and in the chassis;

- the use of magnesium and titanium alloys, for example for connecting rods;

- the use of composite materials such as carbon fiber, glass fiber, vegetable fibers, etc.;

- the use of light components, for example in structural foam, of aluminum honeycomb structures, expanded polypropylene, etc.

A first object of the present invention, to lighten plastic parts and, with reference to the use of the present invention in the automotive field, to contribute to the reduction of vehicle emissions, is to produce plastic components for vehicles consisting of a mixture of a thermoplastic polymer and a gas at the supercritical state.

A second object of the invention, described in detail below, is to optimize the production process, considerably shortening the solidification times of the constituent material of said components.

There are some international inventions concerning the introduction of a gas into a thermoplastic polymer. The difficulty of this process lies in the homogeneous mixing of the two materials in order to obtain a correct distribution of the gas inside the plastic material. Patent US 2002024165, owned by Trexel Inc., describes a method suitable for the production of foams, in particular microcellular foams. The method comprises an element which reduces the backflow of the molten polymer inside the extruder while the polymeric material is injected into a mold. The method can be used in injection molding, blow molding, or other processing techniques that include injection or ejection cycles. In some embodiments, the method uses mixing screws.

The criticalities related to the method described above and to other patents in the same field are mainly due to the high cost of the machines, due to which the two elements are mixed and are injected into the mold.

The object of the present invention is therefore to propose a new, innovative and cost- effective method of mixing a supercritical gas in a thermoplastic polymer in the liquid state for the production of components by injecting the mixture into a mold. Description of the invention

According to the present invention, a new method is provided for the production of any element consisting of a thermoplastic material admixed with at least one supercritical gas.

A typical application of this method is in the automotive industry, for the manufacture of plastic components such as the engine cover or interior cabinets of the car, but this does not in any way limit the application of this method to any other industrial production process. More specifically, the present invention is applied in the production of components that require the use of a mold suitably configured to perform injection molding processes. Said mold, therefore, has a movable and a fixed component which can assume an open or closed configuration. The introduction of a supercritical gas or a mixture of gas at the supercritical state in the thermoplastic polymer of which an element consists has two advantageous consequences: firstly, the solidification of the plastic material inside the interspace of the mold is faster; secondly, the homogeneous distribution of supercritical gas in the plastic material involves the improvement of the mechanical performance of the manufactured element, even in cases of impacts or crushing.

It is important to maintain the supercritical condition of the gas at least until it is injected into the mold, to prevent the micro-bubbles of gas dispersed in the polymer from increasing too much in size and make the mixture unusable. This object is advantageously achieved by the present invention due to a series of technical devices described below.

The hot block used in the present invention, like the common hot chambers currently on the market, is provided with a heating system suitable for maintaining the temperature of the first thermoplastic polymer, and of the mixture of thermoplastic polymer and supercritical gas then, between 200°C and 320°C, preferably 270°C.

Upstream of the hot block there is a feed duct provided with a first shutter which, according to the pressure of the inlet polymer, blocks or at least partially allows the passage of the thermoplastic polymer at the liquid state inside the hot block.

Advantageously, inside the hot block, at least one distribution channel, preferably a plurality of distribution channels, direct the flow of thermoplastic polymer at the liquid state towards at least one distribution channel, preferably a corresponding plurality of hot mixing channels, where the addition of the supercritical gas to the thermoplastic polymer takes place.

One of the main advantages of the present invention lies in the positioning of said hot mixing channels, which are placed inside the hot block. This allows a significant reduction in gas leaks inside the mixture since, shortly after mixing, the injection into the mold occurs, thus minimizing the time in which the gas could detach from the polymer.

The introduction of the supercritical gas inside the polymer takes place due to at least one distribution channel, preferably a plurality of injectors placed along the hot mixing channels and connected to at least one control unit in turn connected to a selected gas tank. This control unit causes the supercritical gas to be admixed to reach the injectors at the desired pressure and temperature.

Each injector is of course provided with its own second shutter to allow the correct opening and closing sequence of said nozzles, which can occur sequentially or simultaneously for all the injectors, and the modulation of the supercritical gas flow.

The configuration of the hot mixing channels is advantageously such as to allow homogeneous dispersion of the supercritical gas inside said thermoplastic polymer at the liquid state.

As already mentioned, said hot mixing channels are placed, in the preferred embodiment of the present invention, immediately upstream of the injection nozzles, which downstream have a third shutter which allows or blocks the flow of the thermoplastic polymer mixture and supercritical gas inside the interspace of said mold. Each of said injection nozzles may have a perpendicular or oblique axis with respect to the plane of the mold.

As regards said third shutters of said injection nozzles, they can advantageously be provided with an electric or hydraulic adjustment device which, advantageously, allows adjustment of the position and/or speed of the opening of the third shutter itself. The greater or lesser opening of the latter is such as to allow the supercritical condition of the gas to be maintained and therefore the growth of the bubbles inside the polymer can take place only once the mixture has reached the interspace of the mold.

The production method object of the present invention consists of the following steps:

(A) first step of injecting the thermoplastic polymer at the liquid state through said supply duct, wherein the relative first shutter is opened at a predetermined pressure to cause a predetermined amount of said thermoplastic polymer to flow into the hot block. During the first injection step of the thermoplastic polymer at the liquid state, the underlying mold is in the second closed configuration, causing a pressure inside the hot block comprised between 500 bar and 1,200 bar, preferably 1,000 bar;

(B) second step of supercritical gas injection through the opening in sequence, or simultaneously, of the second shutters of said injectors so as to make a predetermined quantity of supercritical gas flow in the hot mixing channels during the passage of said thermoplastic polymer at the liquid state. In this step, due to the configuration of the hot mixing channels, the desired and planned mixing of the two components takes place. Also in this step, the mold downstream of the system is in closed configuration. For this reason, the introduction of the supercritical gas must occur at a pressure of between 500 bar and 1,200 bar;

(C) third step of distribution, in which the mixture consisting of the thermoplastic polymer at the liquid state and the supercritical gas is pushed inside the injection nozzles, while the mold is still in the closed configuration;

(D) fourth step of mixture injection, in which the third, preferably adjustable, shutters of said injection nozzles are opened in sequence, or simultaneously, allowing the desired amount of mixture to flow into the interspace of said mold, necessary for making the object to be produced. Also during this step, the mold is still in the second closed configuration. In a version of the present invention, during the fourth step of mixture injection, a jet of air at a predetermined pressure is introduced into the interspace of the mold, by means of suitable channels, so as to control the expansion of the supercritical gas inside of the mixture, to prevent the excessive growth of gas bubbles. These first four steps are substantially concomitant;

(E) fifth step of cooling of the produced object, to await complete solidification of the mixture inside the interspace of the mold, still in a closed configuration; (F) sixth step of extraction of the already solidified produced object by the mold taking the open configuration. After this step, a new production cycle can start. Depending on the construction needs, the complete filling of the hot block, before proceeding to the fifth step of closing the mold, takes place following a plurality of cycles including the first step of injection of the thermoplastic polymer, the second step of injection of the supercritical gas, the third step of distribution and the fourth injection step.

In a further version of the present invention, a single hot mixing channel is placed inside the hot block which feeds the injection nozzles through a plurality of distribution channels.

In a third embodiment, even more advantageous, on the other hand, the mixing of the gas with the polymer takes place directly inside the injection nozzles. This is due to the fact that the gas injectors are placed at the proximal end of the third shutters of the inlet nozzles themselves and in that said third shutters are made of a porous material, which therefore allows the gas to escape and be mixed with the polymer.

The advantages offered by the present invention are clear in the light of the above description and will be even clearer from the accompanying figures and the related detailed description.

Description of the figures

The invention will hereinafter be described in at least one preferred embodiment thereof by way of non-limiting example with the aid of the accompanying figures, in which:

- FIGURE 1 shows a diagram of the preferred embodiment of the present invention. The feed duct 1 is seen at the top which, through the first shutter 2, enters the thermoplastic polymer into the distribution channels Ί-T inside the hot block 3. From here, the polymer passes through the hot mixing channels 4, provided with injectors 6-6’-6” - ... connected to the relative control unit 12 and to the relative tank 5 of supercritical gas. Downstream, the injection nozzles 8-8’ are visible with the third shutters 9-9’ which modulate the escape of the mixture into the interspace 13 of the mold 20, consisting of the fixed component 10 and the movable component 11. - FIGURE 2 illustrates a second version of the invention in which a single hot mixing channel 4 injects the mixture into a plurality of distribution channels Ί-T which reach the injection nozzles 8-8’.

- FIGURE 3 shows the version of the present invention in which the injectors 6-6’-6” - ... are placed at the proximal end 9. a of the third shutters 9-9’ and the latter are made of porous material. Also in this figure, the control device 90 is visible which makes the relative third shutter 9 adjustable.

Detailed description of the invention

The present invention will now be described purely by way of non-limiting or binding example with the aid of the figures, which illustrate some embodiments relative to the present inventive concept.

With reference to FIG. 1, a preferred embodiment of the present invention is shown. In particular, the industrial machinery suitable for producing an object made of a thermoplastic material admixed with at least one supercritical gas is shown, having the shape of the interspace 13 of the mold 20.

The hot block 3 of the present invention is fed by a common feed duct 1 provided with a first shutter 2 which, under the required pressure, allows or reversibly, at least partially, blocks the flow of the thermoplastic polymer at the liquid state in a first injection step A. Once it has entered the hot block 3, the polymer remains in the molten state since said hot block 3 is provided with a heating system which maintains the temperature above the melting point, preferably in a neighborhood of 270°C. After passing through the distribution channels 7-7’, the thermoplastic polymer reaches the hot mixing channels 4 located upstream of each injection nozzle 8-8’.

During said first injection step of the thermoplastic polymer A, the mold 20 downstream of the machinery is in a closed configuration. This causes the injection pressure of the thermoplastic polymer at the liquid state to take place at a pressure of between 500 bar and 1,200 bar, preferably 1,000 bar. After said first injection step of the thermoplastic polymer A, then, a second step of injection of the supercritical gas B follows which results in the desired mixture. This takes place inside said hot mixing channels 4 which are provided with a plurality of injectors 6-6’-6” - ... arranged along the path traveled by the thermoplastic polymer. Each of said injectors 6-6’- 6” - ... has its own second shutter and is connected, upstream, with the gas control unit 12 which is fed by at least one tank 5 of the selected supercritical gas. Said control unit 12 is provided with a common system of heating elements adapted to heat said supercritical gas or mixture of gases at the supercritical state, up to a temperature ranging from l00°C to 320°C, preferably 250°C. The particular configuration of each hot mixing channel 4 is such as to allow the homogeneous dispersion of the supercritical gas inside the thermoplastic polymer, giving rise to the desired mixture. The positioning of the hot mixing channels 4 inside the hot block 3 will be such as to be as close as possible to the injection nozzles 8-8’ so as to avoid as far as possible the dispersions of supercritical gas.

Also in the second step of supercritical gas injection B, the downstream mold 20 is in closed configuration, allowing the introduction of supercritical gas only at a pressure of between 500 bar and 1,200 bar.

At this point, the mixture just obtained is pushed inside the injection nozzles 8-8’ in a third distribution step C, while the mold 20 is still in the closed configuration.

A fourth step of injection of the mixture D follows, in which the sequential or simultaneous opening of the third shutters 9-9’ of the injection nozzles 8-8’ causes the controlled escape of the mixture of thermoplastic polymer and supercritical gas to the interior of the interspace 13 of the mold 20 which is still in the closed configuration.

In the preferred embodiment, said third shutters 9-9’ are made adjustable by control devices 90 which can be both electric and hydraulic and which have the function of only partially opening said third shutters 9-9’ to keep the inner pressure of the hot block 3 constant and therefore the supercritical state of the gas, preventing an excessive growth of gas micro bubbles dispersed in the polymer.

Again in order to control the expansion of said supercritical gas micro-bubbles, once the mixture has been injected into the mold 20, air can be blown through special channels at a predetermined pressure which controls the expansion of the mixture inside the interspace 13 and prevents the excessive growth of bubbles of supercritical gas.

A fifth cooling step E is then carried out, that is, waiting for the complete solidification of the material inside the interspace 13 of the mold 20.

Finally, by bringing the mold 20 into the open configuration, it is possible to extract the object just created in a sixth extraction step F of the produced object, after which a new production cycle can begin.

With reference to FIG. 2, a possible variant is shown to be made to the hot block 3 of the machinery for making an object made of a thermoplastic material admixed with a supercritical gas. In particular, the injection of thermoplastic polymer at the liquid state from the feed duct 1 takes place directly inside a hot mixing channel 4, as previously described. Here, the mixing of the two components takes place and, subsequently, a plurality of distribution channels Ί-T supplies the mixture to the injection nozzles 8-8’.

A third embodiment is shown in FIG. 3 and provides for the provision of the injectors 6-6’- 6” - ..., at the proximal end 9. a of said third shutters 9-9’. In this case, the material of construction of the third shutter 9-9’ is of the porous type, thus determining the addition of the supercritical gas to the thermoplastic polymer in the proximity of the mold 20.

Finally, it is clear that modifications, additions or variants may be made to the invention described thus far which are obvious to a man skilled in the art, without departing from the scope of protection that is provided by the appended claims.

Claims

1. Method of production of an object made of a thermoplastic polymer admixed with at least one gas at the supercritical state, characterized in that it uses:

- a hot block (3), comprising:

o a heating system adapted to maintain the temperature of said thermoplastic polymer comprised between 200°C and 320°C, preferably 270°C; o at least one primary feed duct (1) equipped with at least one first shutter (2) adapted to adjust, by at least partially stopping or releasing the flow of said thermoplastic polymer at the liquid state;

o at least one hot mixing channel (4) fed by means of the primary feed duct (1); o at least one distribution channel (7-7’), fed by said hot mixing channel (4) and reaching at least one injection nozzle (8-8’);

o at least one injector (6-6’-6”-...) of the supercritical gas connected to at least one control unit (12), provided with a common system of heating elements adapted to heat said supercritical gas, up to a temperature comprised between l00°C and 320°C, preferably 250°C; said supercritical gas coming from a common gas tank (5); each of said nozzles (6-6’-6”-...) being provided with a second shutter adapted to modulate the flow of the supercritical gas admixed within said hot mixing channel (4); said hot mixing channel (4) being organized in proximity to the injection nozzle (8-8’) in order to allow the uniform dispersion of the supercritical gas fed inside said thermoplastic polymer at the liquid state;

o each injection nozzle (8-8’) being provided with relative third shutter (9-9’) adapted to introduce a predetermined quantity of said mixture of thermoplastic polymer at the liquid state and of said supercritical gas, inside the interspace (13) of a common mold (20);

said mold (20) comprising a fixed component (10) and a movable component (11) adapted to assume, in a reversible manner, a first open configuration and a second closed configuration; said fixed component (10) and said movable component (11) defining an interspace (13) fed by said injection nozzles (8-8’);

said production method comprising the following steps:

(A) first step of injection of the thermoplastic polymer at the liquid state, in which said first shutter (2) of said feed duct (1) is opened at a predetermined pressure, in a manner such that a predetermined quantity of said thermoplastic polymer flows within said hot block (3); said mold (20) being arranged in said second closed configuration, so as to determine a pressure within the hot block (3) comprised between 500 bar and 1200 bar, preferably 1000 bar;

(B) second step of introduction of the supercritical gas, in which each second shutter of the injectors (6-6’-6”-...) of said hot mixing channels (4) is opened and a predetermined quantity of supercritical gas flows within said hot mixing channels (4) during the passage of the thermoplastic polymer at the liquid state; said mold (20) being arranged in the second closed configuration, so as to determine a pressure within the hot block (3) comprised between 500 bar and 1200 bar;

(C) third step of distribution, in which the mixture constituted by the thermoplastic polymer at the liquid state and the supercritical gas is thrust within each injection nozzle (8-8’); the mold (20) still being situated in the second closed configuration, so as to determine a pressure within the hot block (3) comprised between 500 bar and 1200 bar;

(D) fourth step of injection of the mixture, in which each third shutter (9-9’) is opened, allowing the desired quantity of said mixture necessary for making the object to be produced to exit into the interspace (13) of the mold (20); the mold (20) still being situated in the second closed configuration, so as to determine a pressure within the hot block (3) comprised between 500 bar and 1200 bar;

(E) a fifth step of cooling of the produced object, in which the mold (20) is maintained in said second closed configuration up to the solidification of the mixture of thermoplastic polymer and supercritical gas, said movable component (11) exerting a preset pressure on said fixed component (10) of said mold (20) comprised between 500 bar and 1200 bar, preferably 1000 bar;

(F) a sixth step of extraction of the produced object, in which said mold (20) assumes the first open configuration.

2. Method according to claim 1, characterized in that the position and/or the speed of each third shutter (9-9’) is adjustable, each injection nozzle (8-8’) being connected to a control device (90), hydraulic or electric, adapted to modulate the opening of each adjustable third shutter (9-9’) so as to maintain the pressure of the thermoplastic polymer within the hot block (3) such to preserve the desired supercritical condition of the gas.

3. Method according to any one of the preceding claims, characterized in that the complete filling of the hot block (3) occurs following a plurality of cycles including said first step of injection of the thermoplastic polymer (A), said second step of introduction of supercritical gas (B), said third step of distribution (C) and said fourth step of injection of the mixture (D) which are substantially simultaneous with each other.

4. Method according to any one of the preceding claims, characterized in that the interspace (13) of said mold (20), at least when arranged in said second closed configuration, is hermetically sealed.

5. Method according to the preceding claim 4, characterized in that during the fourth injection step of the mixture (D), air is introduced within the interspace (13) of the mold (20), by means of suitable channels, at a predetermined pressure such to control the pressure of the thermoplastic polymer and, consequently, the development of micro- bubbles in the supercritical gas inside the mixture.

6. Method according to any one of the preceding claims, characterized in that said hot block (3) is provided with a hot mixing channel (4), adapted to feed a plurality of injection nozzles (8-8’).

7. Method according to any one of the preceding claims, characterized in that each injector (6-6’ -6”-...) of gas is arranged at the proximal end (9. a) of each third shutter (9- 9’) and in that each third shutter (9-9’) is made of a porous material adapted to allow the mixing of the supercritical gas within the thermoplastic polymer at the liquid state directly inside each injection nozzle (8-8’).

8. Method according to any one of the preceding claims, characterized in that each injector (6-6’-6”-...) of gas is arranged both at a hot mixing channel (4), and at the proximal end (9. a) of a third shutter (9-9’) and in that each third shutter (9-9’) is made of a porous material adapted to allow the mixing of the supercritical gas within the thermoplastic polymer at the liquid state directly inside each injection nozzle (8-8’).

9. Method according to any one of the preceding claims, characterized in that each injection nozzle (8-8’) is arranged in an oblique manner with respect to the plane of the mold (20).