Compounding And Strand Pelletizing Line for Color Masterbatch

Compounding and Strand Pelletizing Line for Color Masterbatches

The main benefit for manufacturers by using co-rotating twin screw compoundinf extruders is the very good quality of dispersion. This is the basis for a high quality masterbatch. The two co-rotating screws that engage with each other produce a self-cleaning effect. This allows fast cleaning and consequently a more rapid change of product. Another advantage of co-rotating twin screw extruders is the modular design of the screws and barrels, which, depending on the process, enables a fast set-up and conversion of the processing unit.

PREMIX PROCESS

The process set-up shown here is suitable to produce both a masterbatch of mono pigments (SPC = Single Pigment Concentrate) as well as customized master batches of powder mixtures. All components are mixed in advance if the premix process is used. The quality of the color masterbatch is determined largely by the preceding mixing process. This is why this process step is extremely important: if the premix is not good, e.g. if agglomerates have formed, this fault cannot be remedied in the extrusion process.

In this, the most widespread production process, the color masterbatch extruder is fed with a premix of the polymer, pigment and dispersing agent through the main feed opening and the feeding barrel. Since volumetric metering is possible, this means not only a high operational reliability but also uncomplicated handling. Venting is carried out a few barrels further along so that the air introduced with the premix can escape. The screw geometry in the fusion zone should be chosen in such a way that not only is the complete mixture fused, but also the pigment is sufficiently wetted.

This is then followed by the actual dispersion zone. The masterbatch extruder can be adapted to various requirements by altering the number of barrels depending on the desired quality of the masterbatch. A masterbatch quality for injection molded parts can be produced with a short dispersion zone depending on the quality of the mixing. The highest quality demands, for example film or fiber products, on the other hand call for a long dispersion zone. The dispersion zone is followed by vacuum degassing and the discharge zone. A conventional pelletizer is used in the majority of cases.

ADVANTAGES OF THE PREMIX PROCESS

1.lower investment costs than for split-feed (use of gravimetric systems, no side stuffers) through the use of volumetric dosing

2.high pigment and additive loading possible

3.easier cleaning (only one dosing unit)

1. high speed mixer  2. solids conveyor  3. main feeder  4. twin screw extruder  5. water trough  6. air knife  7. pelletizer  8. silo

SPLIT-FEED PROCESS

Material feed through several dosing units

As an alternative to premix, both mono as a well as customized masterbatches can be produced by the split-feed process. The material flows are hereby separated.

Through a separation into two or more material flows, the split feed process allows processing of shear sensitive pigments (e.g. effect pigments) or pigmentswith a great tendency to compact (e.g. certain phathalo pigments) too. The energy needed to melt the polymer  is introduced directly into the polymer - without the pigment - in the split feed process. The pigment therefore experiences a lower physical stress.

With this version of color masterbatch production, the polymer, and possibly smaller amounts of additives, are introduced into the process through the main feed opening and then plasticized. It is important that the polymer is 100 % molten before feeding the pigment through one or two side stuffers. Only then can the subsequent wetting zone work. All components in the formulation are fed into the process by gravimetric feeders. Unlike in the premix process, wetting can be somewhat softer in the split feed process since the polymer is already molten at this stage. The masterbach melt is stabilized and discharged after distributive and dispersive mixing.

SPLIT FEED PROCESS WITH EFFECT PIGMENTS

Some effect pigments have a layer structure consisting of platelets with a high surface relative to the height of the particle. Since the surface layer of these pigments can be easily damaged and they can thus lose their pearl luster, flop or shimmering colors, they have to be incorporated into the molten polymer matrix very gently. The appropriate process is thus split feed. The melt mobilizes the platelets when they are added and largely prevents their destruction by the kneading and conveying elements of the screw. The viscosity of the melt dissipates the agglomerates and makes for an optimum development of the effect. Important: mixing here is distributive, not dispersive.

ADVANTAGES OF THE SPLIT FEED PROCESS

1.easy handling of the formulation via the color masterbatch extruder or feeder control system

2.no complicated premix steps facilitates the production of larger mono batches

3.allows the gentle incorporation of shear sensitive pigments

Application

High Concentration Color Masterbatches: PE, ABS, PS, EVA, PET, PP etc. with high concentration of pigments and other processing aids.

White Master Batch (TiO2 Master Batch) is a type of color master batch, and it is widely used in plastic products industry, almost covering the whole plastic products industry, such as all kinds of injection products, all kinds of film, sheet, pipe and other building materials industry.

Description of production process

The disadvantages of single stage twin screw white masterbatch extruder with water strand pelletizing process: The traditional production process compounded by a twin screw extruder , then through water cooling strand and cutting into pellets. The biggest disadvantage of single stage twin screw masterbatch extruder is that the twin screw extruder cannot bear high die pressure, so the strand extruded from the die is not dense, and the process of cooling traction process in the water trough absorbs higher moisture. Extra drying is required. On the other hand, a single-stage twin-screw die head pressure is too high, which leads to a large increase in motor current, and reduces the output.

In response to the above-mentioned process shortcomings, Chuangbo introduced a two-stage extruder + water ring granulation production process. The first stage uses a twin-screw extruder, the second stage uses a conical twin-screw extruder, and the granulation method is water ring granulation. The double-stage granulation process has the following advantages:

1.The first stage twin screw extruder with L/D=48~52:1 is high torque twin screw extruder. Long length screw is good for sufficient compounding and dispersion. With natrual venting port or vacuum degassing port, the volatiles in the melt are eliminated.

2.The second stage uses a conical twin screw extruder to compact the homogenized melt, cool and deliver to the die for granulation. The conical twin screw extruder is able to withstand higher die head pressure, resulting in more compacted pellets and increasing extrusion output. The dense granules can reduce the water absorption at the granulation stage while increasing the surface gloss.

3.Water ring granulation is provided. Water ring granulation can be applied to formulations with large differences in melt characteristics. For example, the color masterbatch in different proportions of TiO₂ content, CaCO₃ powder, and fluorescent whitening agent can be successfully pelletized. On the other hand, the water ring cutting have a shorter residence time in the water, and the moisture content after the dehydrator and the vibrating screen is less than 0.1%, and no additional drying is required.