How to Analyze and Solve the Issues of Breakage and Difficult Drawing Caused by Uneven Carbon Black Dispersion in Black Masterbatch Production

After on-site observation, it was found that:

The material in the hopper doesn't flow down and often causes breakage. At first, I thought it was due to a lack of material, but when adding more, I found there was still plenty left in the hopper and had to manually push it down. This issue does not occur when the carbon black content is at 20% or 30%.

When feeding material, the strands keep breaking, and the discharge speed at the outlets is noticeably uneven, with the two middle outlets being faster (a total of 4 outlets).

Without on-site testing methods, it is directly used for production, and there is no easy way to determine the quality of the masterbatch.

The solution is as follows:

1. Increase the amount of EVA wax.

2. Lower the temperature by 10°C

3. Decrease or increase the rotational speed

The hopper is a component used for storing plastic raw materials. Some hoppers are equipped with heating and air-blowing devices to form a drying hopper. The shape of the hopper is generally a cylindrical upper section with a conical lower section. The cone angle of the conical surface varies depending on different particle sizes, particle shapes, friction coefficients between particles, and adhesion coefficients. Otherwise, it will either waste the storage capacity of the hopper or cause feeding difficulties, or even result in "bridging" or "ratholing" phenomena where the material does not discharge properly.

1. Definition of Bridging and Bridge Phenomenon

The diameter of the hopper near the extruder feed inlet gradually decreases and changes too quickly, causing the material to form a compacted solid, which will prevent the raw material from entering the extruder.

2. Causes of the Bridging Phenomenon

Due to the formation of an open bridge by plastic pellets at the small outlet of the cone, which supports the material above, this situation is more likely to occur with recycled materials that have larger and irregularly shaped pellets.

"Funnel flow occurs because the flowing particles are insufficient to pull adjacent particles together, which often happens when the size of plastic particles is relatively small."

Particularly when the filler itself is very light or the filler has a relatively large specific gravity, it is necessary to eliminate the bridging phenomenon or minimize the possibility of its occurrence.Before the powder enters the twin-screw extruder, mixed air needs to be separated. The vertical height between the feeding port and the screw should be minimized as much as possible. The powder should enter from the bottom of the screw, rather than being fed directly onto the top of the screw. Additionally, the screw design should ensure that air moves along the screw channel and is discharged in the venting section, while gases should not flow back to the feeding port for discharge.

Manufacturers generally do not differentiate between low density and high filling; as long as the product allows, they will add 1-2% white oil and mix at high speed for 2-3 minutes (but attention must be paid to the mixing time and method, ideally until the powder forms lumps). This practice essentially removes the air from the filler, and the filler is "compacted" in some way, effectively extracting the undesirable air.Once agglomeration occurs, the bulk density will increase accordingly, making the distance between the screw groove and the feed inlet, as well as the feeding direction, no longer significant issues.

3. Solutions to the Bridging Phenomenon

A common solution is to install a vibration device on the hopper or reduce the cone angle. If heat from the barrel is transferred to the hopper, causing the hopper temperature to become too high, the surface of the plastic pellets may soften or stick together in clumps, making it easier to form bridging or blockages.

Generally, adding calcium powder often leads to difficulties in discharging the material. This is mainly due to two reasons: first, the poor flowability of the calcium powder itself; and second, the external lubricants added during processing can also hinder the flow of the dry powder.

The solution is to ensure uniform mixing of light calcium and heavy calcium. Generally, heavy calcium is better than light calcium. Additionally, a vibrator can be added to the feeding hopper. From a formulation perspective, more internal lubricants need to be added.Among them, the vibrator is the most effective.

Since most of the conveying in the extruder relies on frictional drag, the temperature in the feeding zone is generally set between 120-160°C.If the temperature in this zone is too low, the resin cannot adhere to the inner wall of the barrel and merely idles with the rotation of the screw, resulting in a shortage of material in the melting zone. If the temperature in this zone is too high, the material will prematurely melt and plasticize, forming an annular melt film on the inner wall of the barrel, which acts as a lubricant and also leads to material shortage.

Another function of the feed section is to ensure that the compression ratio of the solid bed is sufficiently high, so that the material becomes dense enough to reach the state required for melting. Generally, after compression, the surface of granular or crushed flake materials tends to deform easily and adhere to the inner wall of the barrel, making them relatively easy to compress. Due to the greater fluidity of powder materials, their compression and melting will be delayed in both time and position compared to granular or flake materials.At the same time, due to the large void area of the powder material and its relatively low density, a large amount of air remains trapped within the material, isolating the solid bed layer.This will further slow down the melting process and perform exhaust work during the later ascent stage.

Due to the relatively high internal friction between thermoplastic elastomer particles and their relatively high viscosity, material shortage tends to occur during processing, leading to fluctuations in production yield and material degradation.Generally, the problem is solved by increasing the length-to-diameter ratio of the feeding section, forced cooling, and vibration feeding.

Some granulation raw materials are elastomers that have undergone one or multiple processing steps, and these materials contain varying amounts of plasticizers, fillers, resins, and other components.Although internal friction and viscosity are not as exaggerated as raw materials, the presence of oil makes feeding difficult. It is generally not recommended to crush such materials too finely.At the same time, it is necessary to increase the friction coefficient of the screw flight in the feeding section.

After mixing the pigment with the resin and melt granulating,Attention should be paid to the pre-dispersion of pigment powder ("grinding the powder"). Whether a pigment is dispersed during the post-treatment process largely depends on the dispersant used and the pre-dispersion during the preparation of the pigment.If a colorant is not predispersed, then in the melt mixing process, the base colorants configured with this colorant can lead to different colors appearing in various regions of the material due to uneven mixing ("mottling"). Therefore, if the blending manufacturer discovers mottling in the pigments you use during production, it may indicate that the colorant supplier is not very professional.

Masterbatch is pre-dispersed and mixed in the carrier, so theoretically there should be no issue of poor dispersion.However, it should be noted that carriers of the same color may exhibit different properties during processing.

If the MFR of the masterbatch and the MFR of the blending resin differ significantly (regardless of whether it is larger or smaller), it is entirely possible to cause color mixing.At the same time,If the masterbatch itself is too flexible and tough, making it difficult to be quickly crushed into fine masterbatch fragments during the mixing process, then even if the MFR compatibility between the masterbatch and the resin is good, it may still lead to color inconsistency (color streaks) in the final product.。