Colouring polymers at the machine can offer significant cost benefits, including lower material inventory costs and improved process flexibility when compared with the cost of buying pre coloured polymer or installing a large-capacity central blender to premix polymer and colour concentrate. While colouring at the machine should be relatively simple and efficient, several things can go wrong. As such, it is essential to understand the process of colouring at the machine, typical colouring problems and their solutions, and the impact of colouring on the bottom line. The three basic problem areas with on-machine colouring are:
Delivery of the right ratio of colourant to virgin resin
Calculation and delivery of the right ratio of colourant to a virgin/regrind material mix
Clean-up and changeover from one colour to another

Typically, colouring at the machine involves a colour feeder installed at the base of the main material hopper on the processing machine. Once installed and calibrated, the feeder meters colourant through a throat adapter where it mixes with the main stream of natural polymer before plastication in the barrel of the machine. Assuming the colourant ratio is correct, and there is no problem with the quality of the colourant itself, most colouring problems at the machine have to do with the quality and accuracy of the feeding equipment.
The amount of colourant metered out can be controlled by volume or by weight. Until relatively recently, most colour feeders had been volumetric, whereby the colourant was fed by an auger screw, with the feed rate calculated based on the volume of the colourant passing through each turn of the screw. Such a feeder is calibrated by cycling the feeder at a certain speed, catching and weighing a series of samples, and comparing the sample weights to ensure the auger is dispensing consistently.

Although volumetric feeders have lower prices than gravimetric types and perform well in many colouring and additive feeding applications, they require a significant amount of time and labor for setup, calibration and recalibration. Additionally, they tend to have higher operating costs as their dispensing rates are not as accurate as volumetric metering, which often leads processors to compensate by overfeeding.

Weight-based or gravimetric feeding has now become the preferred method for compounders, yet this technology has been prohibitively expensive. Such feeders utilize a load cell that continually registers the loss in weight of the colourant hopper and adjusts the feeder control to maintain the target feed rate. By comparing the delivery setting (grams/sec) with the actual changes in hopper weight during the same period, these devices are essentially self-calibrating and self-regulating. Due to their dispensing accuracy, they typically provide a higher level of predictability and operate with consistently lower colourant and additive costs. The recent introduction of gravimetric units starting at about US$3500 (less than 25% more than a conventional volumetric feeder), is making this technology much more attractive for the average processor.

Adding colour to virgin polymer is a simpler situation than if coloured regrind is also present. Colourant and/or masterbatch manufacturers provide recommended colourant-to-polymer mixing ratios. However, getting the right ratio demands calibration of the feeder to ensure that it delivers the right amount of colour, smooth running of equipment and maintaing correct ratios throughout the course of the job.
It is necessary to ensure that colourant feed-rate setting is correct. Many times, when very little colour gets into the product, a tempting quick fix is to adjust the colourant ratio upwards. However, this could mask other problems that affect colouring consistency, such as the type and bulk density of the material, pellet geometry and flow, feeder behavior (screw, motor, control) and small errors in measurement or calibration.
Of all the above, measurement and calibration errors are the most common. Variations in collection method, number of samples, rounding errors and bulk density of the colourant masterbatch can introduce additional error into the turn-volume/weight calculations that predict accurate colourant delivery in a volumetric feeder.
Extremely high or low colourant delivery rates may also complicate the colouring process. Even when the feed augers are properly calibrated, the volume they deliver typically varies from dispense to dispense based on turn speed, colourant particle size and geometry, and vibrations from the processing machine. While these variances typically average out and don�t affect quality on routine jobs, they can be very difficult to manage without the continuous self-monitoring and self-calibration functions of gravimetric technology. Tests show significantly higher variability in dispensing consistency and per-dispense material consumption for volumetric feeders compared with gravimetric feeders.
The behavior of a feeder is also affected by its metering technology. A simple auger may keep colourant flowing, but may dispense more in the first half of the rotation than the last. While this pulsing or surging behavior would not affect the total amount of material dispensed over long periods of time, it could complicate the process of sample collection and make accurate calibration more difficult.

There are alternative metering technologies, such as the rotating metering tube, which dispenses colourant in an even stream without surges. Figure 2 compares the flow from an auger and a metering tube necessary to ensure a minimum 0.40 gm/shot dispense rate. The auger surges colourant for part of its rotation, then delivers little or nothing for the rest. However, the dispensing tube meters out colourant very consistently, making everything from feeder calibration to downstream mixing and melting more predictable.

While boosting the colourant level until colour consistency is achieved is one to way fix a colouring problem, there is no precise way to determine how much extra colourant is really needed to make the difference. In addition, the cost of that extra colourant adds up quickly. Using the earlier example, an adjustment from a recommended ratio of 4% to 5% to solve colour-variation issues represents a 25% increase in colourant usage level and cost.
Maintaining consistent colour when there are changes in the quantity of virgin and regrind material can be a challenge. Regrind can usually be assumed to have the same percentage of colour as the finished product and therefore is similar to pre coloured material in that it does not require additional masterbatch. Dual gravimetric feeders for regrind and colour can be provided with controls slaved together to accomplish this goal automatically.
The amount of processing-machine downtime and labor costs associated with feeder cleaning and changeover can be considerable and can vary significantly by feeder type. To simplify cleaning and to minimize labor and downtime costs, look for these features in a feeder: easy disassembly and reassembly, easy access to key feeder components with minimum disassembly, and easy removal of leftover colourant or interchange of the colourant hoppers.
(Courtsey :PTOnline)