FR2758760A1 - Rapid process forming moulding with walls and foam core in same plastic - Google Patents

Abstract

In this new method, a plastic moulding is produced. This is hollow, comprising a rigid outer wall, internally-filled with plastic foam (B). Plastic is poured into the mould, which is closed and rotated in a hot oven. The temperature melts the plastic, forming the wall and a free internal space by a combination of centrifugation and even distribution of the plastic over the internal surfaces. Whilst the plastic is incompletely set, a mixture is introduced into the free space. This comprises finite quantities of expansion agent and the same plastic. Continuing rotation, the mixture foams, filling the free internal space completely.

Description

 The field of the invention relates to methods using rotational molding for the production of parts made of plastic material (s) having a rigid and hollow external wall filled with foam.

 In the field of production of hollow parts consisting of a rigid external wall filled with foam, various processes are already known.

A first known method comprises the following steps:
the rigid and hollow external wall of the part (which is generally called the "skin") is molded by conventional rotational molding (gravity deposition of the material on the internal wall of a mold),
- the part is removed from the mold after cooling and it is placed in a shaping device surrounding its external wall, which shaping is placed in a press,
- is injected, generally using a lance, in the form of a hot liquid, The expanding agent and the plastic with which it will react, so that they turn into foam (here called the "core") which will swell inside the wall, the role of the shaper being to prevent the latter, often not yet fully cured, from deforming by swelling in turn,
- the finished part is removed from the former.

 This first process requires a shaping device (a sort of formwork generally made of wood), a press to hold the shaping device around the part, and foaming equipment. It is also relatively long because of the resumption of the semi-finished product (waiting for the part to cool, placing it in the shaper, injecting the foam). In addition, this solution uses the general rotational molding process only to make the skin of the part, the foam (which may be totally different from the plastic skin material, for example a polyurethane foam in a polyethylene part) being subsequently injected. Finally, due to the fact that it is necessary to wait for the part to cool before removing it from the mold in order to place it in the former, the foam adheres very little or not at all inside the wall (peeling phenomenon of the foam), creating a part lacking in homogeneity resulting in a lack of rigidity.

 The second process uses "rotomoussage" and includes the main step of pouring at the same time into the mold in the form of powder (or fine granules), all the plastic (both that which will constitute the skin and that necessary when making the foam) and an expanding agent, and closing the mold to rotate it inside an oven brought to the chemical reaction temperature of the components.

 This solution is not ideal, in particular concerning the final state of the part after cooling. Indeed, by pouring at the same time the expanding agent and all the plastic material, the expanding agent will react from the start with a portion of the plastic starts to turn into foam, and it may not distribute itself evenly and homogeneously inside the skin. On the contrary, the part risks being almost entirely made up of foam, its density increasing appreciably from the heart to the skin due to the effect caused by rotational molding (displacement of gas bubbles inside the foam). As a result, the external surface of the skin may contain bubbles (porosities), which is detrimental to the quality of the part, to its cleanliness (the bubbles can collect dirt) and to its mechanical resistance (no skin to solid continuous structure really rigid).

The third solution is a variant of the previous one. It includes the following stages:
a) all the plastic material which will constitute the skin is poured into the molding cavity,
b) the rotational molding of said part is started in order to form this skin by introducing the mold into an oven brought to a temperature suitable for melting the plastic,
c) when the hollow external wall of the part has formed by centrifugation on the internal surface of the molding cavity but that that has not yet hardened, a mixture is poured (possibly from another tank) by an expanding agent and all the quantity of plastic material necessary for the production of the foam, (in general said plastic material is of the same nature as that of the skin)
d) rotational molding is continued, which is in fact rotational foaming here because its purpose is to distribute the foam throughout the hollow space located inside the external wall of the part, the foaming agent reacting chemically when hot the plastic with which it was mixed initially to create said foam.

 e) the rotation of the mold is stopped, it is removed from the oven, it is allowed to cool and the finished part is removed from it.

 This solution is not ideal because there is a large quantity of material (foaming agent + all the plastic material necessary for the production of the foam) which must be poured in step c), which implies a large reservoir, even involving a high inertia of the mold and a larger oven (for an identical size of mold). In addition, there is a high risk that the foaming agent and the plastic with which it is mixed react even before being poured into the molding cavity, since the tank containing them is also subjected to the environment at high temperature prevailing inside the oven.

 The present invention therefore proposes to solve at least part of the drawbacks of the prior methods known and summarized above, while proposing a less expensive, rapid process which makes it possible to obtain large parts very reliably.

For this, the invention proposes a method for producing a plastic part constituted by a rigid and hollow external wall defining an empty internal space filled with foam, comprising the following steps:
a) a certain quantity of plastic material is poured into a mold cavity of a mold bounded by an internal surface,
b) the mold is closed and it is introduced into an oven brought to a temperature suitable for fluidizing said plastic material,
c) the mold is started to rotate in the oven in order to form the external wall of said part with an empty internal space, by gravity and essentially homogeneous distribution of the plastic material on the internal surface of the molding cavity
d) while the plastic material forming the wall is not yet completely cured, a mixture consisting of an expanding agent and another non-zero determined quantity is poured into the interior of the molding cavity of the same plastic material, so that foam can form in the internal space,
e) the mold is still rotated in the oven so that the mixture constituted by the expanding agent and said second quantity of plastic material is transformed into a foam filling substantially completely said internal space,
f) the rotation of the mold is stopped, the mold is removed from the oven, the part is allowed to cool and it is removed from the mold.

characterized in that:
in step a), the quantity of plastic material poured in at the start is measured so that it consists of a first quantity of plastic material intended to form the entire external wall of the part during step c), and a second quantity of plastic material intended at least in part to transform into foam during step d), and
in step d), the quantity of plastic material necessary for the production of the foam is measured for which represents the complementary portion of the second quantity of plastic material of step a) so that together, these two quantities constitute the total amount of plastic required to form all of the foam.

 Thus, the plastic material intended for the production of the foam is divided into two quantities: A portion is poured at the start with the quantity of plastic material necessary for the production of the entire wall, and the other additional portion is mixed with the expanding agent. The two portions of plastic material then react chemically hot with the foaming agent to transform into uniform foam completely filling the space defined inside the wall.

 In this way, the process is continuous (no recovery of semi-finished part after rotational molding) and faster, allows to obtain parts of very good quality (perfectly smooth external appearance of the surface of the wall, homogeneous distribution of foam for good rigidity). In addition, the mold serves as a conformator, which brings a clear saving of time and a reduction in manufacturing costs.

 According to a first consideration, in order to avoid the use of a single large reservoir (the latter being in any case significantly less bulky than in the solutions of the prior art), the expanding agent and the additional quantity of material plastic with which it is mixed will be stored in several separate small tanks before being introduced into the molding cavity. This advantageous solution also makes it possible, in particular for large parts or complex parts, to have several mix arrivals (expanding agent + additional quantity of plastic material) distributed over the mold. Preferably, the tanks will be arranged on the mold so as to balance the latter.

 In order to further reduce the size of said reservoirs, and to obtain a foam core the thickness of which can be controlled, the additional quantity of plastic material mixed with the foaming agent in step d) will be between 0 and 20% of the total amount of plastic material necessary for the production of said foam. In this way, in step a), we can already pour into the molding cavity the other portion (80% or more) of plastic material necessary for making the foam with the amount of plastic material which will constitute the skin. .

 In order to prevent part of the foaming agent from reacting with the additional quantity of plastic material with which it is mixed, each of the reservoirs can be kept at a temperature below that at which the chemical expansion reaction (foaming) occurs.

 In order to facilitate the introduction of the mixture into the molding cavity, it is possible, during the whole step d), to stop the rotation of the mold so that the reservoirs are above the mold in order to pour the mixture by gravity in the mold cavity.

 According to an additional consideration making it possible to avoid stopping the rotation of the mold during step d), the mixture can be injected under pressure into the molding cavity.

 In order to avoid, in order to pour the mixture, having to puncture part of the wall already formed in the molding cavity and to prevent a return of molten plastic material in the tanks, the mixture can be made to arrive by tubes. hollow ending inside the molding cavity.

The invention and its implementation will appear more clearly with the aid of the description which follows, given with reference to the appended drawings in which:
Figure 1 shows in three dimensions a finished part made with the method according to the present invention.

 Figure 2 is a sectional view of said part.

 Figures 3 to 6 show the different stages of the process for producing said part.

 In Figure 1, we see a part 1 produced using the method of the invention. This tank comprises a bottom 2 at the periphery of which stand lateral flanks 3, 4, 5 and 6 defining an upper opening 7. The bottom 2 and the lateral flanks 3,4,5 and 6 all consist of the same wall 8 continuous (making the entire periphery of the part) made of a rigid and solid plastic material A (a priori without cell or incomplete structure), such as polyethylene or polypropylene. In order to simplify the description, there will be distinguished in the wall 8 an internal "skin" 10 and an external "skin" 12, as can be seen more clearly in the sectional view of FIG. 2.

This wall 8 is hollow, that is to say that the internal skin 10 and the external skin 12 are separated by an empty space 15 intended to be filled with foam B. It can be noted that the shape of the part is here complex to show that the process works perfectly with this type of part, but we could also have represented a much simpler part, like a plastic sphere with a rigid and hollow external wall filled with foam.

In Figure 2, a piece of a sidewall 3 is shown in section. We see that this sidewall 3 is constituted by an inner skin 10 and an outer skin 12 made of plastic A, said skins being separated by a space 15 filled with foam B. At the end of the process, the wall 8 comprises a quantity Q2 of plastic and foam quantity
Q5 of the same plastic material A, the latter having expanded (presence of gas bubbles in its constitution due to the expanding agent). The use of the method obliges, as will be seen in the following figures, to use the same plastic material for the wall and for the foam B. For the part shown, it is in particular polyethylene.

 FIG. 3 describes the first step of the method according to the invention.

A mold 21 is thus seen defining a molding cavity 20 constituted by an internal surface 22 at the bottom of which a quantity Q1 of plastic material A is poured in the form of granules or powder. This quantity
Q1 is in fact divided into a first portion Q2 intended to produce the rigid and hollow external wall 8 (see FIG. 2), and a second portion Q3 which constitutes a significant proportion, for example at least 80%, of the total quantity Q5 of plastic to transform into foam
B (see Figure 2). So we have the following relation:
Q1 = Q2 + Q3
The first operation therefore consists of pouring a quantity Q1 of plastic material into the mold (step a), closing it (step b) and rotating it (step c) at an average speed of between 2 and 15 revolutions per minute. along at least one axis of rotation, and preferably two orthogonal axes, inside an oven, which will be brought to a known temperature depending on the material to be melted (approximately 290 "C for polyethylene) adapted to make the quantity Q1 of plastic material A poured in. In this figure 1, we also see tanks 25, preferably 5 in number distributed at the corners and in the center of the mold 21 and on the bottom side 2 of the part in order to balance the mold 21. These tanks 25 contain a mixture M consisting of an expanding agent C, which can be purchased commercially, for example under the reference BOREALIS 80-05, and by a quantity Q4 of plastic material A. They are also provided of a peripheral zone c insulated 28 to maintain them at a temperature lower than that at which the foaming reaction takes place (typically 290 "C for a polyethylene foam), and by means of obturation 27, for example pneumatic, of a channel 30 terminating in the molding cavity 20. The sealing means 27 are here in the closed position, so that the mixture M remains confined in each tank 25.

 FIG. 4 describes an intermediate step in the process for producing the final part 1, the mold being still rotating in the oven at the same speed as previously and being kept at the temperature already mentioned (approximately 290 "C.). It can be seen that the hollow external wall 8 has developed in contact with the entire periphery of the internal wall 22 of the molding cavity 20, over a relatively constant thickness E which depends substantially on the quantity Q2 of plastic material A initially poured in. The plastic material is pressed by gravity against the internal wall 22 of the molding cavity 20 due to the rotational molding The interior of the part 1 is therefore hollow, the space 15 between the internal skin 10 and the external skin 12 serving in the step Preferably, in order to reduce the cycle time, which is between 30 and 50 minutes, the quantity Q3 of plastic material A is not melted (it remains substantially in the form of the powder or granules poured in step a) so that it can be mixed with the quantity Q4 of material poured in the next step. We can therefore begin, without removing the part from the mold and the mold from the oven but possibly by stopping the rotation of the mold, to pour a mixture M consisting of an expanding agent C and an amount Q4 of plastic material.

This quantity Q4 corresponds to the additional quantity (between 0 to 20%) of the total quantity Q5 of plastic material necessary to make the foam B. We therefore have the following relationships:
Q5 = Q3 + Q4 OsQ4 <20% of Q5
The mixture M, consisting of the expanding agent C and the quantity
Q4 of plastic material A is preferably poured through the channels 30 of each reservoir 25 by opening the closure means 27.

It is therefore obvious from the above that it is essential to respect the times at which the expanding agent C is poured and the quantity Q4 of plastic material A into the molding cavity 20. In fact, if it is poured too late , on the one hand the expanding agent C risks reacting with the quantity Q4 of plastic material with which it is mixed because the tanks are heated by the oven, and on the other hand there will no longer be enough melted plastic material inside the molding cavity 20 to react with the foaming agent C because all this plastic material (quantity Q1) will have transformed into a wall 8 completely hardened. Conversely, if the mixture M is poured too early, the plastic poured at the start (quantity Q1) will not yet be melted and the foaming agent will react as well with the quantity Q4 as with the quantity
Q1, creating a wall almost entirely of C foam (incomplete structure with undesirable porosities).

As can be seen in FIG. 5, once the mixture M has been poured into the mold cavity 20 between the inner skin 10 and the outer skin 12 during solidification, if the temperature prevailing inside the cavity molding is sufficient (substantially equal to 290 "C), then the expanding agent C and the quantities Q4 (poured with the expanding agent C) and Q3 (poured in the first step illustrated in FIG. 3) of plastic material
A react with said expanding agent C to start creating the foam
B by chemical reaction and release of nitrogen. It can be noted that, while the expanding agent C and the quantity Q4 of plastic material A rise in temperature in the mold cavity, the quantity Q3 of plastic material
A paid at the start begins to melt. This solution makes it possible to avoid having to wait for all the quantity Q3 of plastic material A to melt before pouring the mixture M. During this step, the shutter means 27 will return to the closed position to prevent foam B from come into the tanks.

 FIG. 6 is the final step in the process for producing the part 1. All the expanding agent C reacted with the quantities Q3 and Q4 of plastic material A melted to transform into foam B, and the wall 8 is almost solidified . A part is therefore obtained having a rigid wall 8 which is smooth on the outside, the internal space 15 initially present between the internal skin 10 and the external skin 12 being filled with foam B. If the plastic material A poured at the start was polyethylene, then the heart will be a polyethylene foam. Other thermoplastic materials can be used, in particular polypropylene or polystyrene, the conditions of temperature and proportion of plastic material / foaming agent then being to be adapted accordingly.

 The invention thus described is in no way limited to the embodiments described. Thus, it is possible to envisage other proportions for the plastic material intended to transform into foam between the portion Q3 already poured with the plastic material intended to form the wall and that Q4 mixed with the expanding agent, without however exceeding 50 % of plastic A mixed with the foaming agent C.

 It is also possible to envisage other modes for introducing the mixture M into the molding cavity, in particular if it is not desired to stop the rotation of the mold. In this case, provision may be made to inject this mixture M under pressure via one or more nozzles connected to one or more tanks and controlled by a control system.

Claims (8)

Claims  1. Method for producing a plastic part (1) comprising a rigid and hollow external wall (8) defining an empty internal space (15) filled with foam (B), comprising the following steps:  a) a certain quantity Q1 of a plastic material (A) is poured into a molding cavity (20) bounded by an internal surface (22) of a mold (21),  b) the mold (21) is closed and it is introduced into an oven brought to a temperature suitable for fluidizing said plastic material (A),  c) the mold (21) is started to rotate in the oven in order to form the external wall (8) of said part (1) with its internal space (15) empty, by centrifugation and essentially homogeneous distribution of the plastic material ( A) on the internal surface (22) of the molding cavity (20),  d) while the plastic material (A) forming the external wall (8) is not yet completely cured, poured into the interior of the molding cavity (20) into said empty internal space (15), a mixture (M) consisting of an expanding agent (C) and another non-zero quantity Q4 determined of the same plastic material (A), so that foam (B) can form in said internal space (15 ),  e) the mold (21) is still rotated in the oven, so that the mixture (M) constituted by the expanding agent (C) and said quantity Q4 of plastic material (A) is transformed into a foam (B) substantially filling said internal space (15),  f) the rotation of the mold (21) is stopped, the mold (21) is removed from the oven, the part (1) is allowed to cool and it is removed from the mold,  characterized in that:  in step a), the quantity Q i of plastic material (A) is metered so that it consists of a first quantity Q 2 intended to form the entire hollow outer wall (8) of the part (1) during from step c), and a second quantity Q3 intended at least in part to transform into foam (B) during step d), and  in step d), the quantity Q4 of plastic material (A) necessary for the production of the foam (B) is measured so that it represents the complementary portion of the second quantity Q3 of step a) so that together these two quantities Q3 and Q4 constitute a quantity Q5 of plastic material (A) required to form all the foam (B).  2. Method according to claim 1, characterized in that, in step d), the quantity Q4 of plastic material (A) mixed with the foaming agent (C) is between 0 and 20% of the quantity Q5 of plastic material (A) necessary for producing the foam (B).  3. Method according to any one of the preceding claims, characterized in that the expanding agent (C) and the quantity Q3 of plastic material (A) with which it is mixed are stored in several reservoirs (25) communicating with the cavity molding (20).  4. Method according to any one of the preceding claims, characterized in that, during all of step d), the rotation of the mold (21) is stopped so that the reservoirs (25) are located above it to pour the mixture (M) by gravity into the molding cavity (22).  5. Method according to any one of claims 1 to 3, characterized in that, during step d), the mixture (M) is injected under pressure into the molding cavity (20).  6. Method according to any one of claims 3 to 5, characterized in that said reservoirs (25) are maintained at a temperature below that at which the chemical expansion reaction between the expanding agent (C) and the quantity Q4 of plastic material (A) occurs.  7. Method according to any one of claims 3 to 6, characterized in that, in step d), the pouring of the mixture (NI) into the internal space (15) is carried out using hollow tubes (30) connected to the reservoirs (25) and penetrating inside the mold cavity (20).  8. Method according to any one of claims 3 to 7, characterized in that the said tanks (25) are distributed so as to balance the mold (21).