GB2303816A - Process for moulding a thermoplastics material by injection onto a rotating core - Google Patents

Description

2303816 Process for mouldina a thermoDlastic material by injection onto a

rotating core The present invention relates to a particular process for moulding a thermoplastic material by injec5 tion onto a rotating core.

The technique of injection onto a rotating core, known per se, is described especially in US Patent 3 907 952. it consists in injecting a molten thermoplastic material into a mould a part of which generally the middle part which is referred to as a core - is rotatable in relation to the other and is kept rotating during the introduction of the thermoplastic material into the mould. This technique strongly orients the thermoplastic material in the circumferential direction, and this makes it possible to compensate the predominantly axial orientation induced by the injection in the case of the mould being filled from one end, and to "obliterate" possible weld lines in the case of a side filling of the mould. It is thus possible to obtain hollow inj ec tion -moulded articles such as receptacles, sleeves, and the like, whose mechanical strength is greater than that of articles injection-moulded by means of a conventional technique of injection moulding, that is to say into a mould in which all the parts are immobile in relation to one another.

In the alternative forms of this technique which have been applied hitherto the mould exhibits specifically a symmetry of revolution in relation to its axis of rotation, and this permits only the manufacture of articles with symmetry of revolution, for example cylindrical sleeves or frustoconical receptacles such as tumblers. In these cases where the core exhibits a symmetry of revolution it is not difficult to understand that its rotation has a beneficial effect. In fact, during this rotation, the molten thermoplastic material "travels,' in a completely uniform manner in an annular channel - bounded by the stationary part of the mould and the core - whose width remains uniform over its entire periphery, as time passes, and this allows the is thermoplastic material, an well as any possible anisotropic fillers which it contains, to be oriented in a uniform manner, essentially circumferentially, before the material solidifies.

For these reasons the use of a core which does not have a symmetry of revolution, for example of a core of polygonal section, has never been envisaged because it was considered to result in the production of articles with deplorable mechanical properties. In fact, in the case of cores which do not exhibit a symmetry of revolution, the width of the annular channel in which the molten thermoplastic material travels fluctuates very greatly during the rotation of the core, owing to the very fact that the latter does not exhibit a symmetry of is revolution. In other words, the thermoplastic material is forced through a succession of alternately converging and diverging passages. It would therefore be expected that these fluctuations of the flow regime perturb unfavourably the flow of the thermoplastic material and of possible fillers which it contains, and give it an orientation which differs substantially from a circumferential orientation, which is ultimately detrimental to the article's strength.

In addition, especially in the case of injection moulding onto a rotating core of polygonal section, the circumferential strength of the articles thus obtained ought to be weakened since the ridges of the internal prismatic cavity thus created correspond to regions of reduced thickness. Because of this reduction in thickness and of the notch ef f ect which these ridges have on the wall of the article, it would be expected that the strength of the latter would be particularly low in these regions.

Surprisingly, however, it has been found that, in a process of injection moulding onto a rotating core, the use of a core which does not exhibit a symmetry of revolution is not only possible but in addition produces remarkable results. In addition, it goes without saying that the articles thus obtained, which include at least one cavity not exhibiting a symmetry of revolution, could not have been obtained by a process of injection moulding onto a conventional rotating core employing a core exhibiting a symmetry of revolution. 5 Consequently, the present invention relates to a process for moulding a thermoplastic material by injection onto a rotating core, which is characterized in that the core does not exhibit a symmetry of revolution. This process is particularly useful for the manufacture of articles which are at least partially hollow, including at least one cavity not exhibiting a symmetry of revolution (of a form corresponding to that of the core) and which have to withstand high mechanical stresses, for example where torsion or the resistance to the pressure of a fluid is involved.

This process makes it possible to manufacture articles which are topologically equivalent to a ring, comprising a cavity passing right through them, such as, for example, sleeves.

It also makes it possible to manufacture articles comprising a cavity with only one orifice, for example cylindrical rods in which one end comprises a coaxial prismatic cavity running over only a portion of their length. Such articles may in particular be employed as transmission shafts, the prismatic cavity allowing the insertion of the end of another member of suitable shape, so as to be capable of transmitting a considerable torque without risk of slipping. Such articles may further be intended to contain a fluid under pressure and may be involved, for example, in the construction of storage containers for oil under pressure for braking systems.

It will be easily understood that the invention also extends to the manufacture of articles comprising more than one cavity, for example to the manufacture of transmission shafts which are bulky in their middle but which comprise a cavity of prismatic cross-section at each of their ends. To manufacture them it is necessary to employ a mould provided with a rotary core at each end. The invention further extends, for example, to the manufacture of a sheet comprising a number of orifices of hexagonal cross- section.

To manufacture such articles based on a thermoplastic material, the conventional solution is to resort to usual techniques such as injection moulding or extrusion. However, these techniques generally result in the material being oriented in a substantially lengthwise manner and the articles thus manufactured consequently exhibit a mediocre circumferential strength, and this means that they have to be greatly oversized where their lengthwise strength is concerned.

it is undoubtedly possible to envisage increasing the strength of such articles by incorporating anisotropic reinforcing materials in the thermoplastic material, such as fillers in the form of flakes or fibres, for example glass fibres. However, for the reasons set out above, when a thermoplastic material thus filled is injection-moulded or extruded, these fillers will be aligned in a lengthwise rather than circum- ferential manner, and the circumferential strength will be only very slightly improved. In this case these articles would also have to be considerably oversized where their lengthwise strength is concerned.

The thermoplastic material includes at least one thermoplastic polymer. The thermoplastic material preferably consists essentially of at least one thermoplastic polymer. Any thermoplastic polymer may be employed, especially vinyl chloride polymers, polyamides and polyolefins. Good results have been obtained when the thermoplastic material includes at least one polyolefin.

Among the polyolefins it is preferred to employ mono olefin polymers such as ethylene and/or propylene poly mers (including their copolymers additionally including one or several other monomers).

Good results have been obtained when the thermoplastic material includes at least one semicrystalline thermoplastic polymer. At least 50 mass% of the thermoplastic material preferably consists of one or several semicrystalline thermoplastic polymers. In a particularly preferred manner the thermoplastic material consists essentially of one or several semicrystalline thermoplastic polymers. Semicrystalline thermoplastic polymers are intended to denote thermoplastic polymers which are not amorphous. Examples of semicrystalline thermoplastic polymers are polyamides (in particular aromatic ones), polyphenylene sulphide, polyethylene and polypropylene.

Furthermore, it is is advantageous to employ thermoplastic polymers which crystallize rapidly, such as, for example, polyethylene (PE). If need be, a nucleating agent may be added to a thermoplastic polymer which, in its own right, might not exhibit a rapid crystallization. In addition, the thermoplastic material preferably exhibits properties which are generally required with a view to injection moulding, for example, a suitable viscosity, good demouldability, good behaviour under high shear, good heat stability and the like. 20 With regard to viscosity, it is very particularly preferred that the thermoplastic material should have a modulus of elasticity in shear Gn(7.5) higher than 0.15. Gn(7.5) denotes the normalized value of the modulus of elasticity in shear of the thermoplastic material, G(t) (as described by H.M. Laun in Rheologica Acta, vol. 17, No. 1 (Jan./Feb. 1978), pp. 1-15, in particular in equation [81), at 7.5 seconds and at a temperature 30C higher than the melting temperature of the thermoplastic material in question (T. as measured by DSC (differential scanning calorimetry) at a rate of 10 X per minute). More precisely, on the basis of measurements of the elastic and viscous moduli as a function of the excitation frequency (from 0.01 to 100 13-1), the generalized Maxwell modulus which comes closest thereto is deduced. This model then makes it possible to plot a curve representing the change of the modulus of elasticity in shear as a function of time, which is normalized so that Gn(t) = 100 for t = 0. In other words, G. (t) = 100 x G (t) 1G (0). The value of Gn (t) f or t = 7. 5 a is then read of f this curve. The Applicant Company has found that, among all the values of t at which the modulus of elasticity in shear (G,(t) can be evaluated, it is in the case of t = 7.5 a that it is possible to define the most reliable and most uniform criterion making it possible to characterize the thermoplastic materials which give the beet results when they are injection-moulded onto a rotating core.

This modulus of elasticity in shear is preferably higher than 0.2, particularly preferably higher than 0.3 and ideally higher than O.S. Furthermore, it is preferred that the value of Gn(7.5) should be lower than 10 and very particularly lower than 5.

In addition to at least one thermoplastic polymer, the thermoplastic material which is injectionmoulded may optionally include at least one filler. Any known filler can be employed. Examples of fillers that can be employed, given without any limitation being. mplied, are talc, calcium carbonate and mica. it is preferred to employ anisotropic fillers, for example in the form of flakes or fibres. The use of fibres is advantageous where mechanical properties are concerned. Examples of fibres which may be mentioned are glass and carbon fibres and polymeric fibres such as aramid fibres. It is preferred that the filler should include glass fibres. The improvement in the results is especially remarkable when the content of the filler (s) exceeds 10 % and in particular exceeds 20 %, relative to the total weight of the thermoplastic material and of the filler(s).

Finally, the thermoplastic material may also optionally contain one or several conventional additives such as pigments, antioxidants, stabilizers, flame retardants and the like.

For reasons of simplicity it will be considered that the mould comprises a single rotary core, without any limitation being implied thereby. For the same reason it will be considered that the central part of the mould - the core - is rotary while its outer part is stationary (it will be referred to as nstationary mould,'). However, this arrangement does not imply any limitation and it would equally well be possible to employ an apparatus in which only the external part of the mould, or alternatively both parts of the mould, would be rotatable at different speeds.

The internal surface of the stationary mould and the surface of the core are not necessarily coaxial nor even parallel an a whole to the axis of rotation of the core. In other words, the axis of rotation of the core does not necessarily correspond to the possible axis of symmetry of the core or of the stationary mould.

Lengthwise, that is to say when moving parallel to its axis of rotation, the core may exhibit a variable is cross-section. The lengthwise variation in its section must, however, be such that it allows the core to be extracted after the article is manufactured. The use of a dismantlable or similar core, however, allows the process of the invention to be successfully applied to the manufacture of articles whose central cavity has undercuts or other nonuniformities preventing the extraction of a conventional core.

The indication according to which the core does not exhibit a symmetry of revolution means that its surface does not consist exclusively of circles whose centres are aligned on a straight line, each being contained in a plane perpendicular to this straight line. If the distance measured perpendicularly to the axis of rotation of the core, separating the said axis from the surface of the core, is called a ncore radiusn, the indication according to which the core does not exhibit a symmetry of revolution expresses the fact that the core radius is not circumferentially constant in at least a portion of the core. Thus, for example, in the case of a core whose cross- section is in the shape of a regular polygon rotating about its axis of symmetry, the core radius is high in line with the apexes of the polygon and low in the middle of its sides. if r.i. and r... are the minimum and maximum radii of the core, on its periphery, in the same plane which is perpendicular to the axis of rotation of the core, the process of the invention gives particularly good results when the ratio r../r.in is higher than 1.1 and very particularly higher than 1. 3 in 5 at least a portion of the core.

There is nothing against one or several parts of the core exhibiting a symmetry of revolution, provided that at least a portion is not such. To give an example, the core may consist of a cylindrical body extended by a square-based parallelepiped which would be coaxial with it.

In a given cross-section of the core the variations in the core radius are preferably distributed uniformly over its periphery. According to an advantageous alternative form at least a portion of the core has a polygonal cross-section, in particular a square cross-section. In the particular case of a core of square cross-section rotated about its axis of symmetry, the ratio r1max/r.in is V2. An advantageous alternative form consists in giving the core a noncircular cross-section, but without any sharp angles, for example an oval or elliptical section, or else the shape of a polygon with rounded corners.

When the core has an axis of symmetry, the latter preferably coincides with its axis of rotation.

The stationary mould may have an internal crosssection of any shape, for example circular or polygonal, optionally concentric with the axis of rotation of the core. Lengthwise, this cross-section may be uniform or may vary. When the stationary mould has an axis of symmetry, the latter preferably coincides with the axis of rotation of the core.

Because of the highly nonuniform cross-section of the core in the circumferential direction (marked vari- ations in its radius), care must be taken that, during the rotation of the core, the thermoplastic material is not entrained as a whole, slipping over the walls of the stationary mould, which would run the risk of making a sufficient orientation of the thermoplastic material impossible. With a view to avoiding such slipping or at least of reducing it to acceptable values, it is consequently desirable that at least some regions of the stationary mould should have a rough surface or should comprise geometrical irregularities. Such arrangements do not generally have to be made if the stationary mould does not exhibit a symmetry of revolution: thus, if the stationary mould has, for example, a square cross-section over at least a portion of its length, no slip is, in principle, to be feared, even if its surface is relatively smooth. If, on the other hand, the stationary mould is of cylindrical or conical shape and if the axis of rotation of the core coincides with (or is close to) the axis of symmetry of the stationary mould, it is prudent to provide its surface with irregularities such as rough regions or else lengthwise ribs or grooves.

In some cases, for example when the thermoplastic material employed exhibits good mechanical strength as a melt and/or sets rapidly, the injection sprue, that is to say the portion of thermoplastic material remaining stagnant in the injection channel after the mould has been filled, suffices to prevent the thermoplastic material from being entrained as a whole by the rotation of the core in the mould.

The speed of rotation of the core is advantageously such that the average shear rate to which the thermoplastic material is subjected is at least 10 s-1, preferably higher than 20 s-1. It is furthermore generally such that this rate is not more than 60 a-1, preferably lower than 40 s-1. In the case of articles whose thickness is of the order of 1 to 5 mm the period of rotation is generally from 5 to 120 s, preferably from 10 to 80 s. This period is related to the nature of the thermoplastic material and in particular to its solidifi- cation time, which increases approximately as the square of the thickness of the article. It also depends on the degree to which it is desired to "obliterate" a possible predominantly axial initial orientation of the thermoplastic material by orienting the material circumferentially. By way of example, a longer core rotation period is generally advantageous in the case of articles which must exhibit a circumferential strength that is markedly higher than the axial strength.

The core may be rotated before or after the end of the filling stage (that is to say the instant when the mould is completely filled with thermoplastic material); it in desirable, however, that its rotation should continue for at least a proportion of the dwell stage, which follows it, during which the thermoplastic material is kept under pressure until it completely solidifies. The rotation of the core is generally stopped before the complete solidification of the thermoplastic material; it is preferably stopped before the end of the dwell stage, at an instant when at least a portion of the thermoplastic material has not yet solidified. Good results have been obtained when the rotation of the core intervenes only during the dwell stage.

The mechanical resistance, especially to torsion or to the pressure of a fluid, of the articles obtained by means of the process of the invention is excellent, to such a degree that it is not only advantageous to employ this process for manufacturing articles in respect of which it has been decided that they must comprise a cavity not exhibiting a symmetry of revolution, but that it is even advantageous to employ a core which is not symmetrical in revolution, if the shape of the cavity of the articles can be freely chosen, with a view to improving their mechanical properties when they are manufactured by injection moulding onto a rotating core.

Nonlimiting examples of articles based on thermoplastic material that can be manufactured by the process of the invention which may be mentioned are:

a solid transmission shaft comprising a coaxial cavity of hexagonal section at both of its ends; a disc pierced in its centre with an orifice of rectangular section, allowing a rod of the same section, acting as a shaft for rotation, to be inserted therein; is - 11 a plate including a number of square-section cavities allowing it to be secured to a support provided with a corresponding number of squaresection stems; a sleeve whose external surface might have a circular section and the internal surface an octagonal section.

By way of illustration, Figure 1 shows in perspective an article (1) produced in accordance with the process of the invention, the external surface of which is cylindrical and comprises, over a part of its length, starting from one of its ends, a cavity (2) of square cross-section. This article has been manufactured by injection moulding a thermoplastic material in a station- is ary cylindrical mould in which a rotary core of square cross-section was placed, whose axis of rotation (3) was parallel to that of the stationary mould but nevertheless distinct from the latter. In the case shown it will be noted that the axis of rotation of the core (3) coincided with its axis of symmetry. In addition, the smallest (r.:,.) and the largest (r.,.) core radius has been shown in Figure 1.

The examples which follow illustrate the invention without any limitation being implied. Examples 1R and 4R are given by way of comparison, the others being in accordance with the invention. Ex@Mles Various thermoplastic materials were injectionmoulded on a rotating core by employing an injection 30 press of Engel 250 T type fitted with a screw of 55 mm diameter, so as to manufacture externally cylindrical rods with an external diameter of 32 mm and a length of 105 mm, comprising over their whole length an internal coaxial cavity of square cross-section (of 20 mm side).

The thermoplastic material was injected from one end of the mould.

Examples 1R to 3: Effect of the period of rotation of the core on the torsional strenqth The thermoplastic material employed for injection 12 moulding of the rods was an aromatic polyamide (Ixefo 1022 from Solvay). The square-section cavity allowed a torque to be applied to the rods, by introducing into each end of this cavity a square-based (20 inm side) steel parallelepiped protruding 27.5 mm inside the cavity, and thus to measure their torsional strength (torque resulting in the breakage). These rods were inj ec tion -moulded by making the core rotate for different periods at a constant rate of rotation of 60 revolutions /minute. The duration of the mould filling stage was 4 s and that of the dwell stage 30 s (holding the hydraulic pressure at 40 bars).

is Example Period of rotation Torsional strength (0) (Nm) 1R 0 150 2 10 250 3 20 225 Examples 4R to 10: Effect of the rotation period on the bursting strengt High density polyethylene (Eltexo B3002 from 20 Solvay) was processed in the same operating conditions as above. Here, too, the core was rotated for different periods at a constant rate of rotation of 60 revolutions/minute. The bursting pressure of the rods thus obtained was measured after their ends had been closed by 25 stoppers joined by steel rods so as to take up the axial stresses.

Example Period of rotation Bursting pressure (B) (bars) 4R 0 55 5 60 6 10 60 7 is 60 8 20 70 9 30 110 60 90 is

Claims (7)

1. Process for moulding a thermoplastic material by injection onto a rotating core, characterized in that the core does not have a symmetry of revolution.
2. 2. Process according to Claim 1, in which the thermoplastic material includes at least one polyolefin.
3. 3. Process according to either of the preceding claims, in which the thermoplastic material includes at least one semicrystalline thermoplastic polymer.
4. 4. Process according to one of the preceding claims, in which the thermoplastic material includes at least one filler.
5. 5. Process according to the preceding claim, in which the filler includes glass fibres.
6. 6. Process according to one of the preceding claims, in which the ratio rm&x/rmin (in which r... and rmin denote the maximilift and minimum radii of the core, respectively, on its periphery, in the same plane which is perpendicular to its axis of rotation) is higher than 1.1 in at least a part of the core.
7. 7. Process according to one of the preceding claims, in which at least a part of the core has a polygonal cross-section.