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TPE Granulation Processing Technology and Matters Needing Attention

June 09,2022

1. How much does the TPE granulation method affect the product?

The main advantages of using pellets for molding are:
①Particles are more convenient to feed than powder, and no forced feeder is required
②The density of granular material is higher than that of powder material, and the product quality is good
③The content of volatile matter and air is less, and the product is not easy to produce bubbles
④Using functional masterbatch is easier to disperse than adding functional additives directly

2. What will have a greater impact on the product during TPE granulation?

1) Temperature

Temperature has a significant impact on the extrusion of TPE and the performance of the parison: it can reduce the melt viscosity, improve the melt fluidity, reduce the power consumption of the extruder, and appropriately increase the screw speed without affecting the mixing and plasticity of the material. It is beneficial to improve the strength and brightness of the final product, and it is beneficial to improve the transparency of the final product. However, if the melt temperature is too high, the extruded parison is prone to sag under its own weight, resulting in uneven longitudinal wall thickness of the parison. Therefore, the heating temperature of the extruder should be set according to this principle, that is, on the premise that the parison can be extruded smoothly and uniformly without overloading the transmission system, in order to ensure that the parison has high melting point Body strength, the lower heating temperature should be used as much as possible.

2) Melt pressure

The melt entering the machine head should be under even pressure. Appropriately increasing the pressure of the melt in the extruder can make the material evenly mixed and make the performance of the parison and the final product stable. In order to control the extrusion pressure of the melt, a melt pressure measuring instrument can be installed in the discharge section of the extruder.

3) Melt conveying speed

When the melt conveying speed is large, the extrusion volume of the extruder is large. Increasing the conveying speed of the melt can improve the self-weight sag of the parison and increase the wall thickness of the parison.
In addition to the extrusion process, the pelletizing process also has a great impact on the product. At present, it is mainly divided into two types: cold cutting and eager cutting.

1. Cold cutting method

1) Pull tab cold cut. The materials that have passed through the kneader or internal mixer are plasticized into pieces by the open mill and cooled and then cut into pellets.
2) Extruded and cold cut. The kneaded material is plasticized by an extruder, extruded into a sheet, and then air-cooled or naturally cooled, and then fed into a flat-plate pelletizer for pelletizing.
3) Squeeze and cut cold. Extrusion cold cutting is the most common granulation method of thermoplastic TPE, and the equipment and process are relatively simple. The material is plasticized into round bars by an extruder, and the round bars are air-cooled or water-cooled, and then cut into cylindrical granules by a pelletizer.

2. The eager method

1) Dry and eager. The rotating cutter is closely attached to the die plate of the machine head, and the extruded hot round strips are directly cut into pellets.
2) Underwater eagerness. It is a new technology of polyolefin TPE granulation, the machine head and cutter work in circulating warm water,
3) Eager in the air. The air cutting is similar to the dry cutting. In order to prevent the particles from sticking, the pellets are cooled by blowing cold air or spraying warm water in the ground cutting hood.

3. Common problems and solutions for the quality of TPE extruded pellets

1. The long strip is larger than the normal grain

Twice the length is considered a long bar. The main reasons are: starting to pull the strip or breaking the strip, the direction of the strip is not straight, the strip is not properly soaked in water, the cutter of the pelletizer is worn or has gaps, etc. Main solutions:

1) During normal production, adjust the direction of the strip entering the pelletizer and adjust the length of the immersion;
2) The extrusion process needs to be adjusted if the strip is seriously broken during production;
3) Pay attention to the inspection of the granulating motor knives, and replace the damaged knives in time;
4) Appropriate vibrating screen can be used to screen the long strips that appear.

2. Modified material continuous grain
Linking grains Two or more grains connected side by side are called linked grains. The main reasons for the continuous granulation of the modified material are that the strip is not divided or broken in time when driving the strip, the length of the immersion is insufficient, and the temperature of the machine head is too high. Main solutions:
1) When extruding the braid, use a nylon wheel with a suitable width to separate the sticky strips in time;
2) Adjust the appropriate cooling water immersion length to prevent overheating of the strips, which can reduce continuous grains;
3) Appropriately reduce the temperature of the machine head and adjust the temperature of the strip;
4) For the contiguous grains that appear, the vibrating screen with the appropriate mesh size can be used for sieving.

3. Centipede Strips

The cut surfaces of two or more pellets are not all cut, and they are called centipede strips when they are connected together. In fact, they belong to the category of connected pellets. The material is discharged from the feeding port, the length of the immersion is not enough, the cutter of the pelletizer is worn, or the gap between the fixed knife and the movable knife is too large, etc. Main solutions:

1) The operator should pull the strip at a constant speed to avoid the strip being too thick;
2) Adjust the appropriate extrusion process to avoid the material from the feeding port;
3) Adjust the appropriate cooling water immersion length to avoid overheating of the strip;
4) Adjust the gap between the fixed knife and the movable knife, and then replace it with a new one.

4. Black spots exist on the surface or inside of particles with black spot particles

The main reasons for black spots are: unclean pumping screw and casting; black spots in raw materials; screw carbonization; environmental pollution; artificial introduction. Main solutions:

1) During the process of converting the product to extract the screw, if it is not cleaned up, the screw can be re-extracted; the cleaning point: “three ports”, the screen plate behind the filter screen and the large head;
2) Strengthen the inspection of incoming materials, especially the phenomenon of black spots and impurities in filled mineral powder or flame retardants; for example, Z30S or T30S grains containing sawdust in the package, low-grade filled mineral powder or flame retardants; these are all It cannot be used to produce modified plastics with strict black spot requirements;
3) Pay attention to control and adjust the barrel temperature and the residence time of the material in the barrel to prevent the processing temperature from being too high; that is, design the corresponding extrusion process according to the formula of each product.
4) It is necessary to pay attention to the surrounding environmental sanitation and keep the ground moist, and it is necessary to do a good job of isolation and protection between machines that produce products of different colors.
5) Auxiliary equipment ofextrusion equipmentsuch as water tank, pelletizer, etc. should be cleaned regularly;
6) Every time the screen is changed, the residual material on the mobile screen and the template must be cleaned up to prevent carbonization for too long;
7) Note that the falling strips are strictly prohibited from entering the pelletizer to prevent dust from being brought in;
8) Anti-plastic static electricity, static electricity is easy to absorb dust.

5. Discoloration

The discoloration is manifested in the sudden change of the product color during the production process, and the obvious color difference occurs. The main reasons for the discoloration are uneven mixing or incorrect mixing; the cleaning machine is not clean; the temperature is too high; the shearing is too strong; grain. Main solutions:
1) Strictly follow the mixing process to ensure the uniformity and correctness of the mixing;
2) After the cleaning is completed, a special person will check to ensure that the cleaning is clean;
3) Control the extrusion temperature to prevent the toner from decomposing due to excessive temperature;
4) Select the appropriate screw combination to avoid too strong shearing;
5) Make sure that there are no impurities in the back admixture.

6. There are two or more vacuum eyes or foaming phenomenon on the cut surface of the vacuum bad pellets.

The main reasons for poor vacuum are insufficient vacuum, vacuum return, bad vacuum system, poor sealing, etc. Main solutions:

1) Regularly check and maintain the vacuum pump and vacuum pressure gauge;
2) Clean up the vacuum tank, vacuum chamber and vacuum pipeline in time;
3) Appropriately control the flow of circulating water of the vacuum pump;
4) Solve the vacuum return material and ensure that the vacuum cover is well sealed.

7. Iron filings or other metal particles have iron filings or other metal particles on the surface or in the middle of the plastic pellets.
The main reasons are screw wear, broken moving knife, iron filings or other metal particles in the raw material. Main solutions:

1) Check whether the screw, moving knife, vibrating screen, etc. are damaged;
2) Check whether there are corresponding metal particles in the raw materials;
3) For bad products, a strong magnetic or other metal separator can be used to screen out the metal particles;
4) Non-enhanced cars add filters.

8. Poor plasticization: There are refractory substances in the particle section, or the surface is black and toad skin-like.

The reasons for poor plasticization are too low heating temperature, impurities in raw materials, too high feeding speed, too weak screw shear, and too much lubricant in the formula. Main solutions:

1) Control the temperature according to the process regulations, and appropriately increase the temperature when poor plasticization is found;
2) Check whether there are impurities insoluble in the raw materials or in the process;
3) Appropriately control the feeding speed to increase the heating and plasticizing time of the plastic in the screw;
4) Select a screw combination with moderate shearing capacity;
5) Properly adjust the formula.

9. The water content is too high. From the surface of the strip, you can see shiny water, or feel the moisture of the material by hand, or drip water from the bottom of the pelletizer, etc.

The reasons for the high water content are that the strips are soaked in water too much, the strips are not cut in time when driving, the speed of the pelletizer is too fast, the wind force of the air knife is too small, and the distance between the pelletizer and the air knife is too close. Main solutions:

1) Properly adjust the length of the material strip soaked in water;
2) When pulling strips, the strips are cut into strips immediately after entering the pelletizer;
3) Appropriately adjust the speed of the pelletizer, and at the same time, the height of the monolithic bar support;
4) Ensure that the wind power of the air knife is strong enough, and use two air knives if necessary;
5) Properly adjust the distance between the pelletizer and the air knife.

10. After the shredded material is diced by the granulator, many tiny shredded particles are produced.

Due to the static electricity of plastics, the main reasons why they are adsorbed at the feed opening of the vibrating screen are the excessive water soaking of the strips, the wear of the cutter of the pelletizer, and the too much filler and the strips are too hard. Main solutions:

1) Appropriately adjust the amount of water soaked in the strips;
2) Replace the pelletizing tool with a new one;
3) Properly adjust the formula to reduce the distance between the pelletizer and the air knife.

11. The leakage material at the carbonization die is coked into a black substance, and granular scorch appears on the surface of the plastic.

The main reasons for carbonization are that the heating temperature is too high, causing melt scorch; the screw is used for a long time without cleaning, and the scorch is accumulated; the shutdown time is too long, and the machine head and screw are not cleaned. Main solutions:

1) Always check whether the heating system is normal, if there is a problem with the heating system, solve it in time;
2) Regularly pump the screw or clean the machine head, and clean it thoroughly;
3) The screen plate, filter screen and die head should be cleaned in time and clean to prevent carbonization. If carbonization is found, the machine head and screw should be cleaned immediately. Clean and prevent carbonization. If carbonization is found, the head and screw should be cleaned immediately.

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