FR2746696A1 - Plastic injection moulding using mould with spill cavities - Google Patents

Abstract

Method of plastic injection moulding comprising injection into a mould cavity (57) plastic material that is then displaced towards at least one spill cavity (60) under the action of a second material or fluid forced under pressure into the first material in such way that the second material or fluid is retained within the spill cavity without breaking through the first material skin.

Description

 The invention relates to the field of injection molding of plastic articles having different internal and external constitutions.

 Two illustrative examples of this type of part can be found in FR-A2 257 408 and in US-A-5 098 637 respectively.

 In the first case, it is a question of manufacturing parts constituted by the arrangement, one inside the other, of at least two plastics of different compositions.

 In the second case, it is a question of producing hollow parts, with an external plastic material, while the interior of the part will be hollow, the manufacturing process providing for this the injection of a fluid, which is preferably a pressurized gas.

Anyway, in either case, there is disclosed an injection molding process comprising the steps in which
a quantity of flowable plastic material (that is to say fluid) is injected into a molding cavity having a shape defining at least part of the part to be molded,
- Is made to move from the cavity to at least one annex tank communicating with this cavity, part of said amount of injected material, under the action of a load of at least a second fluid plastic material or a pressurized gas introduced into the cavity, inside said first injected plastic,
- the injected plastic is allowed to solidify, and
- we unmold the part.

 Whether for the production of a bi-material part, as in FR-A-2 257 408, or of a hollow part manufactured by gas injection, as in US-A5 098 637, it can be seen that the second plastic material or the "core" gas which is injected to push the outer "skin" plastic material towards the auxiliary reservoirs does not enter these reservoirs (called "spill cavities").

 Admittedly, in US-A-5 318 433 it is provided, for injecting a hollow part using gas under pressure, that gas is contained in these "auxiliary tanks", also called counterweights.

 However, unlike what has been indicated above, US-A-5 318 433 provides that, if gas is thus contained therein, it is because it has been directly injected into it, the nozzle (s) (s) of injection (s) provided for this purpose then opening directly into the (said) tank (s) annex (s).

The tanks or weights are then used to ensure a passage of the gas towards the main molding cavity, which is considered in this case as a drawback with regard to
- the quality of the hollow parts obtained, in particular if they are relatively complex parts,
- the possible control of a regular thickness of their wall inside the mold cavity, as well as the way of designing the mold and the injection devices which go with it.

It should also be noted that the company
SCHADE has already envisaged in a patent to ensure a displacement towards the weights of a part of a gas under pressure introduced inside the plastic material "of skin" injected into the main cavity of molding.

 However, this was only foreseen insofar as the gas which passed through these "overflow" auxiliary tanks then had to leave it, through the still fluid skin material, to then be recycled and injected again into the cavity. main molding, so that there is a closed circuit circulation of the gas during the molding of the hollow part.

 To obtain that the gas in question crosses the plastic material contained in the weights, at least two possibilities can be envisaged: either the "core" gas is injected at a pressure such that it pierces the still fluid plastic wall in the weights, or the wall of fluid material is deliberately and locally broken off in these weights, opposite an appropriate recirculation duct so that the gas can then escape.

 It is in this case considered that the fact of providing for a rupture, by annexed means or by the pressurized gas itself, in one or more place (s) of the plastic skin material contained in a "flyweight" overflow is problematic and can be a disadvantage.

 Indeed, the applicant, specialist in particular in the manufacture of injected plastic parts, considers that it is important to be able to use a range of injection of the gas under pressure (if such a gas is used) which does not require reaching necessarily a pressure of the gas such that it must ensure, often far from its point of injection, at the end of the weights, the piercing of the plastic "skin".

 In addition, it is considered that the second solution mentioned above consisting of a mechanical rupture by an annex means of this skin material inside the annex reservoir complicates the production of the mold and does not necessarily make it possible to work with the pressures the most suitable gas injection to obtain a skin material with the best thicknesses and surface appearance.

 Thus, the problem which the invention aims to solve is to ensure the best conditions for controlling either the fluid (gas) or the second plastic material which is injected into the heart of the first "skin" material as far as the auxiliary tanks. of the molding cavity, with the aim of improving the structural and appearance qualities of the finished part, by making the most of the presence of these additional overflow tanks, without causing any complication in the embodiments of the mold and of the manufacturing apparatus as a whole.

 The solution of the invention consists, in the context of the injection molding process presented above, in providing that the step of displacement of a part of the plastic material "of skin" towards the tank (s) ) annex (s) comprises a displacement, towards this (s) reservoir (s), of a part either of the fluid or of the second material still fluid "of heart", with a retention of this second material or of the fluid "of heart "in the tank (s), without the plastic or" skin "penetrating or piercing through the" heart "material or fluid in the tank.

 If inside said plastic "skin", it is planned to inject not a fluid but (at least) a second plastic (such as a more viscous material, or a foaming material), an additional characteristic of the invention provides several injection points for the second core material, at least some of these points being located near the corresponding auxiliary tanks, so as to promote the penetration of said "core" material.

 If it is a gas under pressure, and more generally a fluid, which is introduced inside the plastic material "of skin" still fluid, one will use separate injection nozzles for the gas and the plastic material "of skin ", the gas being, according to a complementary characteristic of the invention, preferably injected into a zone of the cavity remote from that where the nozzle (s) for injecting material communicates (s) with this same cavity .

Two more specific embodiments will now be given, in relation to the appended drawings, in which
FIG. 1 illustrates a multi-material injection system making it possible to inject the core material up to the interior of the auxiliary tanks,
FIG. 2 is a plan view of the movable part of the mold used according to FIG. 1, and
FIG. 3 shows the diagrammatic embodiment of a molding machine in the hypothesis of manufacturing a hollow part, with gas injection at the heart ".

 Firstly, in FIG. 1, it can be seen that the molding assembly illustrated comprises a fixed plate 3 against which the fixed part 5 of a mold 7 is arranged, as well as a movable plate 9 against which is arranged l other movable part 11 of the mold.

 Two separate plastic supply systems 13 and 15 will make it possible to inject the external plastic material "of skin" and the internal plastic material "of heart" respectively.

 Each feed system comprises a plasticizing screw 17,18 operated by a motor (not shown) and supplied with plastic material from a hopper (not shown).

 Channels 19, 21 bring each material to the molding cavity 23 which can be seen in more detail in FIG. 2.

 Dispensing valves shown diagrammatically at 25 control the passage of the corresponding fluid plastic.

 For more detail on the realization in particular of the valves, one can refer to FR-A2 257 408. In a conventional manner, these are valves movable in translation in housings, so that when a fluid material is injected into the pipe where the valve is placed, the pressure of the material pushes the valve out of its seat, thus allowing the flow of the material. And when the injection pressure of the material in the pipe stops and that another material is injected in a second pipe, it is the first valve which closes and the corresponding second valve which opens. It is thus possible to control the injection of each of the "heart" and "skin" materials.

 To maintain the materials in their fluid state, a heating device is provided, for example electric or by circulation of a hot fluid (water for example).

 Valves placed just after the injection screws also control the material arrival, the corresponding valve opening when one of the screws begins to rotate, while it will close automatically when the rotation of the screw goes to cease.

 In FIG. 1, we will also note the arrangement distant from one another of the ends 19a, 2la for connecting the material supply channels in the main molding cavity 23 of the part to be produced, and, in particular, the proximity of the end 21a of each of the injection channels of the "core" material supposed to come from the screw 18, with respect to the overflow auxiliary tanks identified at 27 in FIG. 2 with which the cavity 23 communicates.

 In this FIG. 2, it can moreover clearly be noted the presence of several (in this case four) additional tanks communicating with the molding cavity of the part P to be molded, the end 21a of communication with the cavity of the channels d 'brought from said matter "heart" having been shown schematically by mixed lines.

 Thanks to such an arrangement, the "core" plastic material 30 coming from the screw 18 will be able to penetrate into the additional tanks, without however passing through these tanks, the skin plastic 40 in the process of cooling.

 According to a preferred implementation, one will inject into the cavity 23 first one or more charge (s) of the first plastic material intended to form the skin, and this by all or part of the injection channels.

 Next, a (second) quantity of a second plastic material will be injected inside this first quantity of material, so that this material preferably opens into the mold cavity at least through the end 21a of the channels 21 .

 The plastic skin material "(the least viscous) will for example be a polyolefinic thermoplastic elastomer (TPO) or based on polyurethane (TPU), while the" core "material (the most viscous) may be polypropylene or" 1 'ABS ", respectively.

 With such an implementation, the core material will be completely embedded, without piercing other than at the injection end 21a, inside the skin material, this both in the molding cavity and in the reservoirs overflow annexes 27, the core material pushing, when injected, the skin material against the walls of the molding cavity as well as inside the annex reservoirs, where it also accompanies it, as this is clearly shown in Figure 2.

 The substantially constant thickness of the skin material can be seen in the molding cavity, as in fact in the counterweights 27.

 Advantageously, the injection pressure of the "core" material will be less than or equal to the injection pressure of the "skin" material.

 Let us now take a look at Figure 3 and in the case of making a hollow part, with core injection "of a pressurized fluid, in this case a gas.

 In this FIG. 3, the reference 51 shows diagrammatically the nose of the machine for injecting the plastic material and 53 the casting sleeve which will allow, with the casting conduit 55, to bring the molten fluid plastic material into the cavity. molding 57 whose shape essentially corresponds to that of the finished part.

 On a wall of this mold cavity is also connected a channel 59 for the introduction into this cavity of a fluid, which in this case will preferably be nitrogen supplied by a source (not shown).

 The molding cavity 57 is also in fluid communication with the annex tank 60 "overflow", through a communication segment 61.

 Of course, more than one reservoir 60 and therefore more than one segment 61 can be provided for the same cavity 57.

 With such a device very schematically shown and a more complete description of which can, if necessary, be found in US-A-5 098 637, the process for obtaining the hollow part to be produced will be as follows.

 First of all, preferably the whole quantity of plastic material necessary for the manufacture of the part will be injected towards the cavity 57 from the injection nozzle 51.

 Under this injection pressure, the plastic material will distribute itself inside the cavity 57, occupying most of it, and a priori all of this cavity.

 Shortly before the end of this injection period of the plastic material, or after the end of this injection (that is to say after the injection pressure of the material has decreased), the pressurized gas coming from the (or) channel (channels) 59 will be blown into the interior of the cavity 57, through the still fluid plastic.

 It will be noted that for this, and in a conventional manner, the outlet end 59a of the nozzle 59 extends to the interior of the cavity 57.

 Note also that unlike what is provided in EP-A-0 321 117 or in US-A-5 318 433, the pressurized fluid from the nozzle 59 is not used to constrain or distribute the plastic resin at least partially on the interior surfaces of the molding cavity, but to cause the displacement of part of said plastic material from the cavity 57 towards the annexed tank (s) 60.

 It should also be noted that, in accordance with the invention, the injection of gas under pressure into the heart of the still fluid plastic material already distributed inside the cavity 57 is such that part of the gas will accompany the material in the (the) auxiliary tank (s) 60, as shown in Figure 3.

 As already indicated, a control of the injection conditions of the plastic material and of the gas will, once again in accordance with the invention, prevent any piercing by the gas of the plastic material contained in the auxiliary tanks 60, thus promoting the manufacturing hollow parts of very good structural and aesthetic qualities.

 For this purpose, it is advised that the injection pressure of the "core" gas is lower than the injection pressure of the "skin" plastic.

 And of course, the wall of the weights 60 is devoid of any mobile system that may be provided for ventilation or internal degassing of the part being cooled and of the molding cavity.

 Also to note in FIG. 3, the situation of the gas injection nozzle 59 which is distant both from the material injection nozzle 51 and from the channel 62 for the arrival of this plastic material in the molding cavity 57.

 Of course, and in a conventional manner, as already indicated by a number of patents, after the plastic material hollow at the core "and intended to constitute the part marked 63 in FIG. 3 has been able to cool, ventilation by restoring the ambient pressure to the heart The room will be gassed for this purpose, gas can be returned via the channel (s) 59, for example by re-aspiration.

 It will then only remain to unmold the part by opening the mold.

Claims (5)

 1 - Process for injection molding of a plastic part, comprising the steps in which  - a quantity of flowable plastic is injected into a molding cavity (23, 57) having a shape defining at least part of the part to be molded,  - Is made to move from the cavity to at least one annex reservoir (27, 60) communicating with said cavity, part of said amount of injected material, under the action of a charge of a pressurized fluid or d '' at least one second plastic material introduced into said cavity inside the first quantity of plastic plastic injected,  - the injected plastic is allowed to solidify, and  - The part is removed from the mold, characterized in that the step of moving part of said first quantity of injected material to the auxiliary reservoir (27, 60) comprises moving part of the second material to this auxiliary reservoir or of the fluid introduced inside said first quantity of injected material, with a retention of said fluid or of the second material in the adjoining tank (s), without the second material or the fluid passing through it the first material.  2 - Process according to claim 1, characterized in that the injection pressure of the fluid or of the second material is lower than that of the first quantity of material.  3 - Method according to claim 1 or claim 2, characterized in that the step of moving towards the adjoining reservoir part of said first quantity of material injected comprises the introduction, directly into the molding cavity (23, 57) and at the heart of this first quantity of material, of said second plastic material which is introduced into said cavity, in at least one zone of the latter close to the annexed tank (s) (27, 60 ), to promote the penetration of this second material into the annexed tank (s).  4 - The method of claim 1 or claim 2, wherein the step of moving to the auxiliary tank (27, 60) of a portion of said first amount of injected plastic material comprises  - introducing a pressurized gas into the mold cavity,  - injecting said first quantity of plastic material through at least one first injection nozzle (51) which communicates with the molding cavity (57),  - And the injection of said gas into this same cavity, through at least a second nozzle (59) in a zone remote from that where said first nozzle communicates with the cavity.  5 - Method according to claim 4, characterized in that the gas under pressure is introduced into the heart of said first quantity of material contained in the cavity, after the end of the step of injecting this said first quantity of material, and the pressure to introduce the gas into the cavity is lower than the injection pressure of said first quantity of plastic material.