It is known to use liquid colourants to colour plastics material. There are many advantages to using a liquid colourant in favour of a colourant that is in solid form.

The colourant is generally added before or during blowing, moulding or extrusion of the plastics material. One of the prime concerns is to ensure that the colourant is dosed in the correct quantity per unit of plastics material so as to ensure that the finished product has the required colour density and opacity.

Traditionally liquid colourants have been dosed directly into the throat of moulding or extrusion apparatus so as to meet solid granular plastics resin just before the extrusion or moulding process commences. In certain applications gravimetric blenders are used to feed mixed plastics and colourant to the extrusion or moulding process. These blenders are able to control with greater accuracy the ratio of particulate plastics materials being mixed.

A gravimetric blender for weighing solid plastic resin and additives in batches and mixing the resulting material with a liquid colourant is described in US 6007236.

The blender comprises a frame with one or more hoppers for each of the solid granular materials to be blended. For example, one hopper may be filled with previously unprocessed"virgin"material, another with recycled"regrind"material (material that has previously been coloured and formed but has been discarded or is waste material from the forming process, such material being ground into particulate form). Another hopper may contain an additive. Below the hoppers there is disposed a weigh bin having a weight sensor. Each hopper is operated automatically to allow a controlled release of some of its contents into a single weigh bin until the desired amount by weight is present in the weigh bin. Liquid colourant is dispensed into the weigh bin via, for example, a peristaltic pump until the weigh bin detects the correct amount by weight. Once the correct weight of each material has been precisely metered one by one into the weigh bin, a piston-cylinder combination is operated to open the bin and allow the contents to fall into a mixing chamber below. Here the solid particulate material and the liquid colourant are mixed by an agitator before they are passed to moulding or extrusion apparatus.

The above described blender suffers from several disadvantages. First, small amounts of highly concentrated liquid colourants are difficult to weigh accurately particularly as the weigh pan of the blender is subjected to vibration from other parts of the apparatus. Secondly, there is a tendency for residual amounts of the liquid colourant to remain in the weigh pan after its contents have been emptied into the mixer. This residue is lost from the mixing process and results in inadequate colouring of the plastics material. Moreover, the residue affects the measurement of subsequent doses in that the tare weight of the weigh pan is inaccurate. Thirdly, contamination of the weigh pan in this way causes granules of the plastics resin to stick to the pan such that they are not dispensed into the mixer. In addition to absence from the mix, the stuck granules further affect the tare weight of the pan and can cause at least partial jamming of the outlet of the weigh pan.

It is an object of the present invention to obviate or mitigate the aforesaid disadvantages.

According to a first aspect of the present invention there is provided a method for dosing and mixing fluent colourant material and particulate plastics material, comprising the steps of : dispensing a batch of the particulate plastics material to a weighing station and recording the weight of the dispensed particulate material present; passing the weighed particulate material to mixing apparatus ; calculating the required volume of colourant material from the recorded weight of the dispensed particulate material; dosing the fluent colourant material by volume and dispensing the calculated volume of colourant directly into the mixing apparatus with the weighed particulate material; mixing the fluent colourant material with the particulate material in the mixing apparatus ; and passing the mixed colourant and particulate material to an outlet for dispensing to forming apparatus.

By dosing the colourant volumetrically and directly into the mixing chamber the inaccuracy associated with prior art systems is eliminated. The controllability of the method ensures that colour dispersion in the formed product is consistent.

The term"particulate"is used in the description and the claims to include material in powdered or granular form.

More than one source of particulate material may be dosed to the weighing station. For example, virgin plastics granular material may be dosed from one source, regrind form another source and additives from further sources.

Preferably the dosing of the fluent colourant material is performed under the control of a control device that includes means for calculating the required volume of the colourant based on the weight of material of that is sensed in the weigh receptacle and the data identifying the type of plastics material and fluent colourant.

The dosing of the colourant material is preferably delivered by a pump that is driven under the control of the control device, a signal being sent to drive the pump for a predetermined amount of time so as to deliver the calculated volume of colourant.

The pump may be a progressive cavity type pump and the signal sent by the control device controls the angle of rotation of a rotor of the pump.

The data identifying the type of plastics material and fluent colourant material is pre-programmed into the control device.

The mixing apparatus preferably has an opening that is closed for predetermined period during mixing before being opened so as to release the mixed contents to said outlet.

Preferably the level of the contents in the mixing apparatus is monitored and compared to a threshold value such that when the level falls below the threshold value the dosing of the particulate material into the weighing station is recommenced.

According to a second aspect of the present invention there is provided apparatus for dosing and mixing fluent colourant material and particulate plastics material, comprising: at least one container for particulate plastics material, the container having an outlet for dispensing the material ; a weighing station disposed below the, or each, hopper outlet, the station having a receptacle for weighing particulate plastics material dispensed from the, or each, hopper, the station having a weight sensor for sensing the weight of material in the receptacle; mixing apparatus adjacent to the weighing station and for mixing material received from the weigh station with fluent colourant material; a reservoir of fluent colourant material; a volumetric dosing pump in communication with the reservoir and the mixing apparatus so as to draw the colourant material from the reservoir and to dispense a predetermined volume of colourant into the mixing apparatus; means for releasing the mixed material from the mixing station to forming apparatus.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawing of a schematic representation of the apparatus of the present invention.

The exemplary dosing and mixing apparatus comprises a gravimetric blender 1 that receives and meters particulate plastics resin and mixes it with predetermined amounts of liquid colourant (the term"liquid"is used here to refer to material that is capable of flowing; it may in fact be part liquid and part solid). The mixed material is then released and conveyed to processing apparatus (not shown) such as an extruder, a moulder or any other kind of processing equipment for forming coloured plastics material.

The particulate plastics material is contained in hoppers 2,3. A first hopper 2 contains virgin plastics resin 4 in granulated form and, adjacent thereto, a second hopper 3 contains regrind resin material 5 in granulated form. Additional hoppers may be provided for other ingredients such as additives.

Each of the hoppers 2,3 is supported by a frame 6 over a weigh chamber 7 which in turn is suspended by the frame 6 over a mixing chamber 8. The granulated material 4,5 is dispensed from the hoppers 2,3 into the weigh chamber 7 by activation of outlet valves 9,10 at the base of the hoppers 2,3. Release of the weighed particulate material from the weigh chamber 7 into the mixing chamber 8 is controlled by activation of a further outlet valve 11 at the base of the weigh chamber 7.

The liquid colourant material 12 is stored in a reservoir 13 and fed directly to the mixing chamber 8 of the gravimetric blender 1 via a liquid dosing pump 14.

The mixing chamber 8 has an outlet 15 that feeds the mixed colourant and plastics material to a buffer hopper 16. A plastics processing or forming machine (not shown) connected downstream of the blender draws the mixed material from the buffer hopper 16. The downstream processing may be, for example, injection, compression or rotational moulding or extrusion.

The dosing and mixing apparatus is operated under the control of a program run by a microprocessor of a computer. Instruction data relating to the colour parameters of the end product, the particular colourant, and the resin material being used is initially loaded into the computer ("the controller") by an operator. The controller then activates the apparatus and generates signals to control the status of the valves 9,10, 11 and the pump 14 whilst at the same time receiving and processing information and data signals from transducers associated with the apparatus (as described below).

The outlet valves 9,10 of the hoppers 2,3 may be in the form of shutters or sliding doors that are controlled by an actuator in response to signals sent by the controller.

The weigh chamber 7 comprises a receptacle 20 for receipt of particulate material that falls under gravity from the hoppers 2,3 when the outlet valves 9,10 are open. The receptacle 20 is mounted on load cell transducers 21 that, in use, send signals representative of the magnitude of the load in the receptacle 20 to the controller. The controller ensures that the signals are received continuously during use of the apparatus and takes into account the predetermined tare weight of the receptacle 20.

The liquid colourant 12 is drawn from the reservoir 13 to. an inlet 14b of the pump 14 through a feed tube 22. The pump 14 may, for example, be of the progressive cavity type whereby rotation of a rotor is driven by a stepper motor 14a under the control signals sent by the controller. Rotation of the rotor moves the liquid in the pump progressively from the inlet 14b to an outlet 23. From there the liquid is conveyed to the mixing chamber 8 via a delivery tube 24 and a nozzle 25. The pump 14 is calibrated accurately so as to ensure that a consistent volume of colourant is delivered for a given angular rotation of the rotor. This volumetric delivery eliminates the requirement for feedback signals from transducers associated with the weigh chamber 7.

The mixing chamber 8 contains a motor driven agitator 30 having a plurality of radially extending blades 31. The agitator 30 is rotatable by the motor (not shown) so as to agitate the contents of the chamber in order to mix them together. The closeable outlet valve 15 at the base of the mixing chamber 8 remains closed during mixing but is opened in response to a signal sent from the controller. A level sensor 32 on a wall of the mixing chamber 8 detects the level of the contents in the chamber and sends signals to the controller to be compared to a pre-selected threshold.

Once the controller has been provided with the instruction data, it calculates the desired weight of virgin and (if required) regrind plastics resin, additives and colourant for the particular batch. The hopper outlet valves 9,10 are then opened sequentially for a predetermined period to allow the particulate plastics material or additive to pour into the weigh receptacle. After each particulate material has poured for the predetermined time period the respective hopper valve closes and the resulting addition to the contents of weight receptacle is weighed. The signals generated by the load cell transducers 21 are passed to the controller whereupon the actual weight is compared to a predetermined desired weight and a correction to the time period for the opening of the respective hopper valve is calculated for the subsequent batch. The combined weight of the virgin plastics, regrind plastics and any additives (which may fluctuate for each cycle of the apparatus) is used by the controller to calculate the required volume of colourant to be dispensed.

When the weighing operation for each material is complete the controller generates a signal to open the outlet valve 11 of the weigh chamber 7 so that the contents pour into the mixing chamber 8. At the same time the controller issues a signal to the motor 14a driving the pump 14. The rotor of the pump 14 is driven by the motor 14a through a predetermined angle so as to deliver the requisite volume of colourant as calculated by the controller into the mixing chamber. 8 The agitator 30 is then activated by the controller to mix the ingredients in the mixing chamber 8 for a predetermined time period whilst its outlet remains closed.

Thereafter, the controller issues a signal to open the outlet 15 of the mixing chamber 8 so as to allow the mixed contents to fall into the buffer hopper 16. When the level sensor 32 detects that the level of the mix in the mixing chamber 8 has fallen below the pre-selected threshold an appropriate signal is sent to the controller and the cycle is repeated so as to prepare a fresh batch. The outlet to the mixing chamber is closed by the controller during the pouring of materials into the mixing chamber, the volumetric dosing of the liquid colourant and the subsequent mixing period.

Dosing of the liquid colourant 12 directly into the mixing chamber 8 eliminates the problems caused by colourant residue in the weigh receptacle 7. Using a volumetric delivery system eliminates the requirement for the colourant to be weighed. The result is that a more accurate method for blending particulate material with a fluent material such as liquid colourant is provided.

It will be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims. For example, the liquid colourant dosing pump may be of any suitable design to delivery liquid colourant volumetrically into the mixing chamber. In particular, a peristaltic pump may be used in place of a progressive cavity pump.

Additionally, more than one pump may be used to deliver different colourants simultaneously to the mixing chamber thus allowing a wider range of colours to be obtained during the operation.