Molded plastics are often a blend of two or more materials. Colors (masterbatch) and other additives are often mixed/blended with the raw plastic material prior to the molding process in molding plants. Different types of blenders and dosing units are used in this process, some with marginal and others with better accuracy. Gravimetric batch- blenders and volumetric feeders installed directly at the throat of the injection molding machine are the most common. Gravimetric batch-blenders are applied either as remote blenders or on the machine feed throat. Remote and central blenders are often used to feed multiple machines with a blend of virgin resins, regrind material, masterbatch and other additives. The remote blenders are also used to feed a single processing machine to avoid transmitting vibrations from the processing machine to the blender aiming for high blending accuracy and longer life. They are also applied when direct installation of a batch-blender on the feed throat of the processing machine is not practical. The blend is then conveyed from the floor standing blender to one or more processing machines by vacuum loaders, where considerable separation of the masterbatch/additives from the virgin resin/s (due to different specific gravities, bulk densities and granule size and free-flowing properties) takes place. Separation is also typical to batch blenders that are mounted at the throat of the processing machine mainly due to mixing of materials of very different nature and bulk. The product quality varies and the color grade changes from cycle to cycle when the blend separates. It is easy for the machine operator to notice the difference between the color grades, and this usually results in increasing the percentage of masterbatch to achieve a uniform color. The down side of this is the waste of expensive Masterbatch.
Volumetric feeder meters the masterbatch or additive into the main flow of material. The metered material is drawn into and distributed in the core of the main material � there is no mixing equipment and thus no risk of separation. However, unlike gravimetric systems, the volumetric dosing device does not possess scale to record or control dosing quantities. The volumetric device has to be calibrated against the material. This has to be done for set-up and for switches of material and batches, as well as for quality testing. Evaluation and calibration take time and tie up workers. Since continual referencing is not performed, changes that occur between test intervals due to variation in bulk density, particles geometry, or free-flowing properties escape detection. Pumping liquid dies (ink or color) into the material flow on the feed throat of a processing machine is a messy method and rarely used in the plastics industry.
Scrap is molded but undesired parts of a molded plastic product that are normally trimmed off and separated from the product after the molding process. Scrap is very often produced in the majority of plastics molding processes. Depending on the molding process, the mold construction and the shape of the product, scrap may weigh more than the finished product. Scrap is often recycled by being collected, reground and blended (mixed) with virgin resin in certain percentages to reduce material cost and to help save the environment. The mixing ratio of recycled plastics and virgin resins varies, depending on the type of material and the desired product quality. High accuracy in the mixing ratio is normally not required. Simple proportional vacuum loaders with marginal accuracy are often used to blend virgin resins with regrind and feed it to the process. Reblending both virgin and regrind materials when more accuracy is required and the more sophisticated gravimetric blenders are applied only when high accuracy is necessary.
It is often necessary to blend two or more different types of plastics in order to manufacture certain products with specific mechanical properties. More accuracy is normally required and, therefore, volumetric or gravimetric blenders are used to supply the processing machine with the special blended in smaller quantities with virgin resin or with a blend of virgin resin and regrind material in volumetric orgravimetric batch-blenders prior to the molding process.
A major share of plastic products is manufactured in different colors. When high production volumes in a specific color are required, the molder may elect to purchase the main virgin material in the desired color from the resin supplier. It is more economical to blend colors (color dosing) with main materials in molding plants prior to the molding process when low production volumes are required or low production volumes in different colors are desired.
Solid color grains (masterbatch) are the most common materials used in the plastics industry � pumping liquid dies (ink or color) is a messy and rarely used method. Masterbatch is blended in very small amounts (usually lower than 4% and often below 1%) with virgin resins in blending units prior to the molding process. A higher ratio of masterbatch normally does not show negative effects in the product quality but masterbatch is expensive.
Ideal blending system depends on high accuracy and blend homogeneity. Proportional loaders, volumetric blenders and remote blending systems are applicable when both low accuracy and less homogeneity are acceptable.
The ideal blending system comprises as:
•   Continuous feeding method.
•   Installed directly on the feed throat of the processing machine, and consisting of one or more.
Gravimetric blenders
•   Metered material is drawn into and distributed in the core of the main material � there is no mixing equipment and thus no risk of separation.
The accuracy and reliability of the ideal gravimetric blender depend on:
•   Accuracy of the load cells
•   Isolation of vibrations transmitted from the processing machine to the blender (noise isolation).
•   Mechanical design of the blender.
•   Control system and software.
Gravimetric blenders can have blending problems. To ensure problems free blending first is to carefully select the correct blenders for an application that is being used. Plastic products are required to have a wide variety of special properties, including surface aesthetics, which means more and more additives and specialty colorants. Increased demand for recycling puts more pressure on the blending because of different materials has with different and unusual flow characteristics. Wood-plastic composites have introduced a whole new set of requirements for blenders that can accurately blend pellets, powders, flakes, and wood flour at very high throughputs.Today�s gravimetric blenders can blend from 2 to 12 components with nominal throughput capacities up to 5-6 MT/hour. Generally blenders work well to their full capacities if pellets of different materials have similar bulk densities. Most processors today are blending components with varied densities and characteristics. One of the most common problems encountered with blenders is that some regrinds or additives do not flow properly through the bin to the mixing chamber. This makes it imperative that the blender be fitted with proper bridge breaking and dispensing mechanisms to suit such �difficult� materials. Blender manufacturers have developed different dispensing mechanisms for hard-to-flow materials.
After acquiring the correct blender some practical measures are required to ensure that no blending problems occur. Dispense valves are pneumatically actuated, so make sure that there is a good supply of clean, dry air at the pressure recommended by the manufacturer. Check should be done to ensure that the air-cylinder operation works properly. Check solenoid operation by cycling several times through the controller, since manual actuators bypass the solenoid. A failed load cell typically causes overfilling of bin.
The latest blender controls make troubleshooting much easier. Most blenders have a microprocessor controller with simple thumbwheel entry of component percentages. Alarms, diagnostics, and set-up procedures are displayed as alphanumerical codes, which you have to look up in the manual to make sense of them.
However, in recent years, processors have demanded more sophisticated controls that often include a user-friendlier interface with two-line, 40-character vacuum fluorescent display or a full touch screen.
(Source: www.colorsave1000.com, ptonline.com)