GB2314040A - Plastics moulding arrangement with mixing - Google Patents

Abstract

A plastics moulding arrangement comprises a mould tool and a plastics injector (5,7) for injecting molten plastics material under pressure into the tool via an additive introduction means (1) which comprises (a) holding means (3) for receiving a charge of molten plastics material introduced therein by injector 5,7 (b) means (4) for causing the charge of material to flow relative to an additive inlet (18) within means 1 when the charge is not pressurised by the injector, and upon retraction of means (piston 4) means (20) for introducing into the flowing plastics material at a predetermined flow rate via the additive inlet an additive material for changing the appearance and/or physical properties of at least a portion of the moulded plastics material. Rotation of screw 7 causes molten plastics to enter bore 3 via a non-return valve 9 which forces piston 4 outwardly and draws piston 24 of means 1 outwardly whilst drawing in additive into bore 23 via reservoir 21. The nozzle 5 and means 1 are advanced until contacted with mould whereupon piston 24 retracts to cause plastics material in bore 3 to enter nozzle 5 via channel 16 and additive to flour into channel 16 from bore 23 via non-return valve 19 to allow mixing. Subsequent impelling of screw 7 drives mixed material via valve 9 into the mould via interconnected passages 10, 11, 13. A plurality of mould cavities may be used to mould plastics of different compositions.

Description

PLASTICS MOULDING SYSTEM Field of the Invention This invention relates to a plastics injection moulding system, and to an additive introducing device for a plastics mould tool.

Background to the Invention A problem encountered in plastics moulding using additive materials to change the properties of the basic plastics materials to be injected into the mould cavity is that of ensuring adequate mixing of the additive materials with the basic plastics materials before the materials reach the mould. One way of achieving this is to introduce the additive materials into the injector so that the screw causes mixing. However, some materials do not blend adequately when mixed in in this way, and certain materials cause changes which adversely affect the injection of the materials into the mould cavity, for example by causing an increase in the viscosity of the material, or raising its melting point.

Further, different-coloured plastics components are conventionally moulded separately using different mould tools and injectors, or by changing over the colour on a single machine. The first of these requires a high capital investment and a large factory area, while the second is time-consuming and labour-intensive.

It is known to injection mould multi-colour components on a family tool using a special injection machine with several injection units.

However, these machines are very expensive, and are only suitable for family tools and multi-colour tools.

It is also known to mould coloured articles by forming a coloured skin on the surface of the article and then filling the skin with the uncoloured plastics material, thus saving on the cost of colorant. Various techniques have been proposed to achieve this, for example using two separate injection units, one introducing an initial charge of coloured plastics material and the other introducing the bulk filler plastics material which causes the initial charge to be urged into the mould cavity, coating the surface and being filled by the bulk charge. In a refinement of this process, the initial charge is first injected into the front end of the second injection unit so that the injection of the main bulk charge drives the coloured material before it into the mould cavity.

While this process is cheaper than forming the whole bulk of the article with plastics material containing the relatively expensive colorants, it still requires the use of two injectors, and a colour change involves the cleaning of the injector introducing the coloured material.

Summary of the Invention The present invention provides a plastics moulding system, comprising a mould tool and a plastics injector for injecting molten plastics material under pressure into the mould tool via an additive introduction means, the introduction means comprising holding means for receiving a charge of molten plastics material introduced therein by the injector, means for causing the charge of material to flow through a restriction when the charge is not pressurised by the injector, thereby causing mixing to occur, and valve means permitting further material injected by the injector to displace at least part of the mixed charge into the mould tool.

Preferably, the system comprises means for introducing at a predetermined rate into the plastics material flowing in the restriction an additive material for changing the appearance and/or physical properties of at least a portion of the moulded plastics material.

A spring-biased piston is preferably arranged to cause the charge of plastics material to flow through the restriction, and a pump may be provided to introduce the dose of additive material, the piston and the pump both being driven by the motion of the injector nozzle towards the injection position before injection occurs. The additive inlet is suitably placed at a restriction in the cylinder in which the piston moves, the piston then causing the material to be forced through the restriction to ensure that the additive mixes with the plastics material.

The additive material will typically be a dye or pigment in liquid form, but it may alternatively be a material affecting the hardness, resilience, surface finish or other property of the moulded article. The additive material can be introduced in powder or even granule form, provided that it is readily dispersed into the plastics material. For example, the additive may be in the form of a solid master batch in fine granular form.

Additional mixing means, such as intersecting flow channels in the piston body, may be used to ensure that thorough mixing takes place before the plastics material enters the mould cavity.

With the system of the invention, the additive can be introduced before pressure is applied to the plastics material, and the dosing of the additive can be related to the timing of the cycle of operation of the injector and determined by the bore of the additive pump.

It is an important feature of the invention that the introduction of the additive can take place before injection into the mould cavity, so that the injection does not involve a significant pressure drop or other impedance to the flow of the material to be injected into the mould.

The invention also provides a method of plastics moulding in which a plastics material is injected into a mould cavity, characterised by introducing into the initial flow of plastics material into the mould cavity a dose of an additive material, whereby a surface layer of plastics material containing the additive material is formed in the cavity, and then continuing to inject the plastics material without the additive to form the core of the moulded article.

Another aspect of the invention provides a method of plastics moulding, comprising injecting plastics material from a single injector barrel into a plurality of mould cavities, characterised by introducing into the stream of material flowing into at least one of the mould cavities a dose of an additive material, whereby the moulding formed in said mould cavity is of a different composition from the mouldings formed in the other cavity or cavities.

The invention further provides an additive introducing device for a plastics mould tool, comprising an inlet engageable by a nozzle of a plastics injector, and outlet connectable to the injection inlet of the mould tool, and means between said inlet and outlet driven by motion of the injector nozzle against said inlet for introducing into a charge of plastics material a predetermined dose of an additive material.

By operating a number of separate pistons and additive pumps in the one additive introducing means, the invention will permit a single mould tool supplied by a conventional single injector machine to produce different coloured articles at the same time, for example key tops for keyboards, control knobs and the like.

The system of the invention is suitable for the moulding of components having a surface layer of a modified material and an inner core of the base unmodified material, or of a differently modified material. For example, the surface layer may contain colour, while the core may be simply the raw uncoloured plastics material. Alternatively, the core may be a blown or expanded plastics material, and/or the surface layer may contain glass for durability or a UV absorbing agent. An advantage of the system and method of the invention is that the surface layer is formed from the same material as the rest of the moulding, and there is no break in the flow which can produce an interface resulting in subsequent lamination in the moulded product; the change from the presence to the absence of additive, or from one additive to another, can be gradual, ensuring that no discontinuity arises.

The ability to control the content of separate layers in the mould without the need for separate injector barrels would be especially advantageous in the manufacture of larger components. In the system of the present invention, the additive is introduced into the raw plastics material at the last possible time before introduction into the mould cavity.

This is especially advantageous where the additive is subject to deterioration at elevated temperatures, for example some colorants, or where the additive is abrasive and therefore likely to cause damage to the injector, if added at that stage. Certain additives cause premature solidification of the plastics material, and delaying the addition of the additive ensures that the solidification occurs within the mould cavity, for example obviating the need for additional heating.

A further advantage is that a much greater dosage of additive, especially colorant, can be achieved in the system and method of the invention as compared with adding the colorant to the injector, where a phenomenon referred to as "screw slip" can occur when higher levels of colorant are added, especially when added in liquid form, the colorant acting as a lubricant between the injector screw and the bulk of the plastics material, preventing effective mixing from taking place. The level of additive at which this occurs in conventional systems is relatively low, limiting colour effects obtainable, and proving especially limiting where surface layer colour is required; higher concentrations of colour, for example, permit the surface layer to be thinner.

The cost savings achievable with the moulding system of the invention compared with conventional systems are substantial. In addition, improvements in the mechanical properties of the moulded articles may result from the ability to introduce modifying additives close to the mould cavity.

Brief Description of the Drawings In the drawings, which illustrate exemplary embodiments of the invention: Figure 1 is a diagrammatic sectional elevational view of a mixing device forming part of the system according to a first embodiment of the invention; Figure 2 is a diagrammatic sectional elevational view of an additive introduction device according to an alternative embodiment, at the first stage in the sequence of operations of the device; Figure 3 is a corresponding view to that of Figure 2, showing the injector nozzle and the device moved into engagement with the mould assembly in readiness for injection; Figure 4 illustrates the injection of plastics material into the mould cavity; and Figure 5 illustrates an alternative embodiment having two additive introduction means.

Detailed Description of the Illustrated Embodiments Referring first to Figure 1, the mixing device 1 comprises a body 2 having therein a cylindrical bore 3 in which is slidably mounted a piston 4 which extends from one end of the body 2. The device 1 is screwthreaded into the nozzle 5 of a plastics injector 6, of which only the leading end and the tip of the injector screw 7 are shown in the Figures for the sake of clarity. The construction of such plastics injectors is wellknown, however.

The bore 3 communicates with a cavity 3a in the body which opens in turn into the interior of the injector nozzle through a passageway 8 provided with a non-return valve 9 only permitting flow from the nozzle 5 to the bore 3. The piston 4 is provided with a first passageway 10 therein extending from the face of the piston within the bore 3 to a position on the cylindrical surface of the piston, and a second passageway 11, extending from the surface of the piston adjacent to where the first passageway opens to the exposed front face 12 of the piston. The two passageways 10 and 11 can only communicate one with the other when the piston 4 is at a position within the bore 3 where a recess 13 is aligned with the openings of both passageways in the cylindrical surface of the piston 4, corresponding to the fully retracted position of the piston. A secondary channel 16 bypasses the passageway 8 to provide an alternative communication between the nozzle 5 and the bore 3.

In use, the injector screw rotates to introduce a charge of plastics material under pressure into the bore 3 via the passageway 8 and the non-return valve 9. This causes outward displacement of the piston 4 to the position shown in the Figure. As the material passes through the passageway 8 (and the secondary channel 16), it is caused to mix together so that any separate components present in the injector are blended. The injector is then advanced towards the mould tool (not shown) and when the piston 4 contacts the tool it is depressed, urging the charge of material back into the cavity 3a via the secondary channel 16 (the non-return valve 9 closes). The screw is in turn urged backwards slightly by the flow of plastics material. As the material passes through the relatively small secondary channel 16, further mixing of the components occurs. The injector can then inject under pressure, displacing the mixed charge through the passageway 8, and then through the passageways 10 and 11 into the mould tool. The passageway 8 causes relatively little pressure drop as injection takes place.

Referring to Figure 2, the device shown in Figure 1 can be modified for the introduction of an additive material into the plastics material between the injector and the mould tool, affording a number of advantages over the introduction of additives into the injector itself, as hereinbefore explained. The components in common between the different Figures are given the same reference numerals for the sake of clarity, and are not described again. The secondary channel 16 has an additive inlet 18 containing a non-return valve 19. The inlet 18 leads to an additive pump 20 supplied from an additive reservoir 21. The additive pump 20 consists of a block 22 containing a narrow bore 23 extending parallel to the bore 3. A piston 24 slides in the bore 23 and is linked to the piston 4 by means of a yoke 25 so as to be driven by motion of the piston 4. The reservoir 21 communicates with the narrow bore 23 via an inlet 26.

The body 2 may be provided with heating elements 27 to ensure that the plastics material is maintained at the correct temperature.

The sequence of operation of the system will be described with reference to Figures 2, 3 and 4 in turn. In Figure 2, the previous moulding operation has just been completed and the moulded article has been ejected from the opened mould cavity. The injector nozzle 5, and with it the additive introduction device 1, has been withdrawn from the mould assembly, and an initial charge of molten plastics material is introduced into the bore 3 by the injector through the passageway 8 and its nonreturn valve 9. The pressure of the injector resulting from rotation of the screw 7 (the piston moves back while the screw 7 rotates and extrudes plastics material forward into the chamber in front of the piston) causes the piston 4 to be driven to its outermost position, in turn drawing the piston 24 in the additive pump 20 outwardly. The non-return valve 19 closes under the pressure of the plastics material, and the movement of the piston 24 draws in a charge of the additive material from the reservoir 21, or allows it to flow into the bore 23 under gravity.

The nozzle 5, and with it the additive introduction device 1, are then advanced so that the free end of the piston 4 comes into contact with the inlet to the mould. Further movement of the nozzle causes the piston 4 to be driven rearwardly in its bore 3, and with it at the same time the piston 24 in the additive pump, as shown in Figure 3. This has the effect of driving the charge of plastics material in the bore 3 through the secondary channel 16 back into the leading end of the nozzle 5, since the non-return valve 9 prevents flow through the passageway 8, and at the same time pumping into the flow of plastics material through the secondary channel 16 a continuous metered stream of the additive from the pump 20. Since the injector is not pressurised at this stage, there is no significant pressure to be overcome, and so the resultant small increase in volume due to the introduction of the additive is readily accommodated, especially since the injector screw is pushed back making space for the plastics material propelled by the movement of the piston 4.

Since the secondary channel 16 is of relatively small cross-section, the flow through it assists in the mixing of the additive into the charge of the plastics material. The result is an initial charge of mixed plastics material and additive in the front end of the nozzle 5, ahead of the main charge of material free from additive, contained within the injector body.

Figure 4 shows the injection stroke of the screw 7, driving the initial charge back through the passageway 8, causing further mixing, and then through the passageways 10 and 11 to be injected into the mould cavity. The initial charge tends to cover the surfaces of the cavity, so that the following main charge of plastics material with no additive, which drives the initial charge before it, fills the bulk of the cavity, leaving a surface layer of material with additive, which, as explained hereinbefore, may be a colorant or some other material changing the physical (including optical) and/or chemical properties of the surface layer of the moulding.

It will be appreciated that the device of the invention may readily be modified to provide two or more separate additive introduction devices working with the same injector. These may be made entirely separate, or may simply be defined by divisions within the one device to keep the materials with different additives separate. Such materials may be introduced into different parts of the same cavity, or into separate cavities to permit manufacture of different coloured articles, for example, simultaneously.

Figure 5 shows an example of the device of the invention having two additive introduction devices in parallel within the same body 40.

The body has two bores 41 therein, in each of which a separate piston 42 is slidably mounted. Each bore 41 communicates with a respective chamber 43 in the end of the body opening into the interior of the injector nozzle. All the other components described with reference to Figures 1, 2 and 3 are also duplicated, to operate in parallel.

In use, the pistons 42 move in the manner described hereinbefore with reference to Figures 1-3, introducing plastics material into separate mould cavities within the same mould tool, different additives being introduced by the separate additive introduction means. For example, different coloured articles can be moulded at the same time and in the same mould tool. By varying the sizes of the additive pumps, the quantity of additive material introduced in the two pumps can be changed to suit the nature of the additives; for example different pigments may require different concentrations to achieve satisfactory colouring to the same intensity.

It will be appreciated that, since the additive pump can be mounted externally of the additive introducing device, the additive may, if desired, readily be kept cool to avoid thermal degradation, for example by providing around the body of the pump a cooling jacket supplied with cold water.

Claims (17)

1. A plastics moulding system, comprising a mould tool and a plastics injector for injecting molten plastics material under pressure into the mould tool via a mixing device, the mixing device comprising holding means for receiving a charge of molten plastics material introduced therein by the plastics injector, means for causing the charge of material to flow through a restriction when the charge is not pressurised by the injector, thereby causing mixing to occur, and valve means permitting further material injected by the injector to displace at least part of the mixed charge into the mould tool.

2. A system according to Claim 1, comprising means for introducing at a predetermined rate into the plastics material flowing in the restriction an additive material for changing the appearance and/or physical properties of at least a portion of the moulded plastics material.

3. A system according to Claim 2, wherein the additive material is a dye or pigment.

4. A system according to Claim 2, wherein the additive is a material affecting the hardness, resilience, surface finish or other property of the moulded article

5. A system according to Claim 3 or 4, wherein the additive is in liquid form.

6. A system according to Claim 3 or 4, wherein the additive is in solid particulate or powdered form.

7. A system according to any preceding claim, wherein the means for causing flow of the charge comprise a piston driven by the motion of the injector nozzle as it is moved towards the injection position thereof before injection occurs.

8. A system according to Claim 7, further comprising a pump for introducing the additive material, the pump being driven with the piston.

9. An additive introducing device for a plastics mould tool, comprising an inlet engageable by or connectable to a nozzle of a plastics injector, and an outlet connectable to or engage able by the injection inlet of the mould tool, the device also comprising: holding means for holding a charge of molten plastics material introduced therein by the injector, whereby injection of each charge into the holding means displaces at least part of the previously-held charge into the mould cavity; means for causing the charge of material to flow relative to an additive inlet within the additive introduction means when the charge is not pressurised by the injector, and means for introducing into the flowing plastics material at a predetermined flow rate via the additive inlet, and contemporaneously with the flow of the plastics material, an additive material for changing the appearance and/or physical properties of the moulded plastics material.

10. A plastics moulding system, substantially as described with reference to, or as shown in, the drawings.

11. A method of plastics moulding in which a plastics material is injected into a mould cavity, characterised by introducing into the initial flow of plastics material into the mould cavity a dose of an additive material, whereby a surface layer of plastics material containing the additive material is formed in the cavity, and then continuing to inject the plastics material without the additive to form the core of the moulded article.

12. A method according to Claim 11, wherein the additive material is a colorant.

13. A method of plastics moulding, comprising injecting plastics material from a single injector barrel into a plurality of mould cavities, characterised by introducing into the stream of material flowing into at least one of the mould cavities a dose of an additive material, whereby the moulding formed in said mould cavity is of a different composition from the mouldings formed in the other cavity or cavities.

14. A method according to Claim 13, comprising introducing into the stream of material flowing into each of the cavities a separate dose of an additive material, whereby all the mouldings are of different composition from each other.

15. A method according to Claim 13 or 14, wherein the or each additive is a colorant material, whereby the mouldings are of different colours.

16. A method according to any of Claims 13 to 15, characterised by introducing the dose of additive material into the initial flow of plastics material into the mould cavity, whereby a surface layer of plastics material containing the additive material is formed in the cavity, and then continuing to inject the plastics material without the additive to form the core of the moulded article.

17. A method of plastics moulding, substantially as described with reference to the drawings.