Along with correct dimensional and mechanical properties, processors know that good color properties rank among the top factors in customer acceptance of a plastic product. Color is often an integral part of the identification and branding scheme of a product, and may, in some cases, be the determining factor in product selection by the user.
With so much value riding on color consistency and quality, it’s important to understand the process of coloring at the machine, typical coloring problems and their solutions, and the impact of coloring on the bottom line.
The Benefits of Coloring at the Machine
Compared with the cost of buying pre-colored resin or installing a large-capacity central blender, coloring at the machine can offer significant cost benefits, including lower material inventory costs and improved process flexibility.
How Coloring at the Machine Works
At-the-machine color addition usually involves a color feeder installed at the base of the material hopper on a processing machine.
Following installation and calibration, feeders meter colorant through a throat adapter, where it mixes with the mainstream of natural resin before plastication in the barrel of the machine.
3 Problems when Coloring at the Machine
There are three basic problem areas that occur with coloring at the machine:
- Delivering the right ratio of colorant to virgin material.
- Cleaning up and changing over from one color to another.
- Calculating and delivering the right ratio of colorant to a virgin/regrind mix.
Let’s take a look at each of these problem areas.
Deliver the Right Ratio to Color Virgin Material
Theoretically, coloring is a simple process. Colorant (or master-batch) manufacturers provide recommended colorant-to-resin mixing ratios for their products. For example, a particular color may be added at a ratio of 25:1, which is equal to 4% of the total material mix.
Getting this ratio right demands three things:
- Calibrating to ensure that your feeder can and does deliver the right amount of color
- Ensuring that your processing equipment is running smoothly
- Maintaining the correct ratios throughout the course of the job
The amount of colorant metered out can be controlled in two ways – by volume or by weight.
Until relatively recently, most color feeders have been volumetric. Colorant is usually fed by an auger screw, with the feed rate calculated based on the volume of colorant delivered with each turn of the screw. Calibration is accomplished by cycling the feeder at a particular speed, catching and weighing a series of feed samples, then comparing the sample weights to ensure that the auger is dispensing consistently.
Weight-based gravimetric feeding is a newer method that has been the preferred feeding method for polymer blenders for over 20 years. However, until a few years ago, it was prohibitively expensive in the simple feeders commonly used on individual plastics-processing machines.
Instead of relying on a fixed-volume auger to deliver a consistent weight, a gravimetric feeder utilizes a load cell. This cell continually registers the loss-in-weight of the colorant hopper and adjusts feed rate accordingly.
By comparing the delivery setting (i.e., grams/sec) in the control with the actual changes in hopper weight during the same period, gravimetric feeders can be essentially self-calibrating and self-regulating.
Table I shows how automatic calibration can save time and money.
Table 1. Automatic Calibration Saves Time and Money
(20 minutes/change x 3 changes)
(5 days/week x 50 weeks/year)
(60 minutes x 250 days)
|Billable Machine Time||$45/hr|
|Potential Savings with Automatic Calibration
(250 hours x $45/hr)
Signs of On-The-Machine Color Feeder Problems
So, what might indicate you have a problem with an on-the-machine feeder?
- Product is incorrectly or insufficiently colored.
- Costs for labor are higher than planned, usually due to feeder calibration, or maintenance/job changeover problems.
- Costs for colorant are higher than calculated.
The first step in problem-solving is to check and validate the colorant feed rate. Is the rate setting correct?
If it is, but product coloration is insufficient, a tempting “quick fix” would involve adjusting the colorant addition rate upward—from 4% to 5%, for example. While this may “solve” the immediate problem, it could mask other problems, since coloring consistency is also affected by:
- Material factors:
- Bulk density
- Pellet geometry and flow
- Feeder behavior (screw, motors, control)
- Small errors in measurement or calibration
Of these, measurement and calibration are the most common causes of problems. Calibration can take time — a volumetric feeder may require collection and individual weighing of up to 12 samples to predict the appropriate level of accuracy.
Even then, small variations in any of several factors — collection method, number of samples, rounding errors, bulk density of the colorant or masterbatch — can introduce additional errors into the volume-to-weight calculations that predict accurate colorant delivery in a volumetric feeder.
Figure 2 illustrates the inherent variability of the typical volumetric feeder. With such wide swings in output, processors often raise the setpoint to make sure that enough colorant is always delivered.
However, this can be costly, as shown in Table 2:
Table 2. Over-Coloring is Costly
|Machine Throughput (Assumed)||150 lb/hr|
|Additive Target Percent (Assumed)||4%|
|Additive Used/Hour (4% x 150 lb/hr)||6 lb/hr|
|Additive Cost (Assumed)||$3|
|Additive Cost/Hr ($3 x 6 lb/hr)||$18|
|Productive Hours/Year (3 shifts x 5 days x 50 weeks)||6,000|
|Additive Cost/Year (6,000 hrs x $18/hr)||$108,000|
Cost of Over-Coloring
Actual % Dispensed
|10% Over Target||4.4%||$10,800|
|20% Over Target||4.8%||$21,600|
|30% Over Target||5.2%||$32,400|
|40% Over Target||5.6%||$43,200|
|50% Over Target||6.0%||$54,000|
Extremely high or low colorant delivery rates may also complicate the coloring process. Even when augers are properly calibrated, the volume they deliver typically varies from dispense to dispense based on turn speed, material size and geometry, and vibration.
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. It’s no surprise that tests show high variability in dispense consistency and per-dispense material consumption for volumetric feeders.
The behavior of a color feeder is also affected by its metering technology. A simple auger keeps colorant flowing, yet may dispense more in the first half of a rotation than the last. This behavior is known as pulsing or surging. While this 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.
Conair offers an alternative metering technology, the rotating dispensing cylinder, which evens out the flow of color particles as they flow out of the device. As a result, the dispense is delivered in an even stream−without surges.
Figure 3 compares the flow from an auger and a dispensing cylinder necessary to ensure a minimum 0.40 gm/shot dispense rate. The auger surges colorant for part of its rotation, then little or nothing for the rest.
The dispensing cylinder feeds colorant very consistently, making everything from feeder calibration to downstream mixing and melting more predictable and easier to manage. The setpoint (gm/shot) can be set lower without worrying unnecessarily about consistent coloration. A lower setpoint reduces coloring costs and increases profitability.
Considerations for Colorant Cleanup and Changeover from One Color to Another
In addition to the costs of colorant, feeder equipment, and quality, there are labor and material costs associated with changing from one color to another on a machine. Of course, the injection molding machine or extruder will need to be purged, but this activity is the same regardless of feeder type.
But processing-machine downtime and labor costs associated with feeder cleaning and changeover can vary significantly by feeder type and can be considerable.
Obviously, your color feeder should be designed for easy cleaning and color changeovers.
To simplify cleaning and reduce labor and downtime costs, look for:
- Easy disassembly and reassembly
- Easy access to key feeder components, with minimum disassembly
- Easy removal of leftover colorant, or changeover of the colorant hopper
Cleaning, Setting Up, and Calibrating Feeders
How often are colorants or other additives changed to meet job requirements in your plant? How long does each clean-out take? If the changes are required frequently, the minutes can add up quickly.
But savings can add up too. Thanks to continued advances in design, the latest Conair TrueFeed gravimetric feeders can be disassembled and cleaned in as little as one minute.
Once a color feeder is cleaned, how long does it take to set up and calibrate? How many adjustments are needed before color dispenses are consistent? How often is recalibration needed?
A microprocessor-controlled feeder allows faster setups, using stored color or job records, and provides continuous process updates.
How fast can this be done? This depends on the type of feeder. Assuming that inputs of process, rate and material information require equal time, how much time is needed for feeder calibration before or during the job?
While the answer varies for volumetric feeders, the answer for TrueFeed gravimetric feeders is easy: calibration time is essentially eliminated.
How to Calculate & Deliver the Right Ratio of Colorant to a Virgin/Regrind Mix
It can be a challenge to maintain consistent color when the ratio of regrind to virgin material changes.
Regrind can usually be assumed to have the same percentage of color as the finished product and, therefore, is similar to pre-colored material in that it does not require additional colorant.
The graph (Figure 4) can be used to determine how much colorant will be needed to color the virgin (natural) material into which the regrind is being mixed.
If you are running 10% regrind, for example, you only need 90% of the color you would have needed if you were running 100% natural resin. And, as shown in the graph, if you would normally add 3% colorant to 100% natural resin, you only need 2.7% colorant when running a 10%-regrind/90%-natural material mix.
Dual gravimetric feeders for both the regrind and color can be provided and with controls slaved together to accomplish this goal automatically.
The Importance of Feeding Equipment Accuracy
Though coloring at the machine should be a relatively simple and efficient process, there are more than a few things that can go wrong. Assuming that quality materials are used and the ratio of colorant to resin is correct, most coloring problems have more to do with the inherent quality and accuracy of the feeding equipment.
Volumetric feeders continue to offer lower acquisition costs and perform well in many coloring and additive feeding applications. Yet they can require a significant amount of labor time for setup, calibration, and recalibration. They also tend to cost more to operate in terms of coloring and additive materials because their dispense rates are not as accurate and processors tend to compensate by overfeeding.
Gravimetric feeders are becoming more competitive in price. Their self-calibration and self-regulation capabilities save setup time and virtually eliminate calibration and calibration-related errors and quality problems. Typically, they provide a higher level of predictability and operate with consistently lower colorant and additive costs due to their dispensing accuracy.