What should be done to plug the hole of the melt blown cloth die?

- May 07, 2020-

What should be done to plug the hole of the melt blown cloth die?

The traditional cleaning method (thermal vacuum furnace calcination + ultrasonic) takes about ten hours. Now the new dry ice cleaning method is used. The dry ice cleaning of melt blown mould spinneret has three advantages: 1. No damage to the mould; 2. Quick cleaning, cleaning a pair of mould for about 15 minutes; 3. Low use cost cleaning a set of mould for about 20 yuan. Time is money, efficiency is life!

Since the outbreak, as the core product of epidemic prevention, mask is also the most important strategic material. The shortage of raw materials once restricted the production of mask, and the core raw material of mask production is melt blown cloth.

As we all know, the protective mask is composed of three layers: inner (hygroscopic layer), middle (filter layer) and outer (water blocking layer). The most important one is the filter layer in the middle of the mask, that is, the melt blown cloth layer, which plays an important role in filtering bacteria and preventing the spread of bacteria. It is called the "heart layer" of the mask.


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Melt blown cloth is a kind of film made of polypropylene with high melt index and many interlaced fibers in random direction. The diameter of the fiber ranges from 0.5 to 10 microns, and the diameter of the fiber is about one thirtieth of that of the hair.


Detailed explanation of melt blown cloth production process

The melt blown cloth has good filtration, shielding, heat insulation and oil absorption, and its diameter is only one tenth of the diameter of the outer fiber of the mask. To have these characteristics, there are high requirements for materials, equipment and technology.

The raw material requirement of melt blown cloth is PP with high melt point, so that the material can achieve "melt blown"! The production of melt blown cloth in the middle of the industrial chain mainly goes through six processes: melt preparation - Filtration - metering - melt extrusion from the spinneret hole - melt fine flow drafting and cooling - meshing. The key part of the melt blown equipment is the melt blown die head and die.


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Production process flow chart ▲

Melting and spraying process ▲


Melting and spraying principle: the melting polypropylene, driven by pressure and air flow, forms tiny fibers, which are then sprayed onto the object and bonded into cloth.


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Spinneret head

Schematic diagram ▲


The figure above is the actual drawing of the spinneret, and the red circle is the spinneret that determines the quality of the melt blown cloth.


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The spinneret is the core part of the melt blown cloth equipment. There are many spinnerets on the die for the melt to pass through. Its production and processing are very important, which is related to the production efficiency and quality of the melt blown cloth. There are more than 300 ~ 2000 micropores in different die sizes. According to the different application level of the melt blown cloth, the micropore diameter also has the size difference.


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For N95 mask which is in short supply at present, its aperture should be 0.2mm or less. Imagine that it is very difficult to make so many tiny deep holes on the hard die steel! And these holes are not so simple holes, there are many nursing requirements.


But there are also problems. What should we do if such a small hole is blocked in production?

How to deal with die plug?

Impurities are inevitable in raw materials. When the impurities are large, it is inevitable to block the hole. Moreover, when the polymer is in a high temperature state, it is likely to decompose, and it will also coking after a long time. Moreover, nearly all the manufacturers of modified materials have invested in melt blown materials recently, and the quantity has been increased, which is hard to avoid roughness, so it is more likely to cause hole plugging!


So what if it's blocked?

First, remove the die head, clean it, and then find the spinneret, also known as spinneret. There are many holes on the spinneret, which is easy to identify.


The key point of dealing with the die head is to clean up the blocked holes of the spinneret. The traditional method of mold cleaning is mainly manual method, using wire brush, sandpaper and chemical cleaner to deal with, not only takes time and effort, but also may cause harm to human body. For larger or more complex molds, the difficulty is beyond imagination. With the development of technology, other methods have been developed.


1. Vacuum calcination

Generally, the melting point of polymers is below 300 ℃, so when the temperature is high, the polymers melt. If the temperature rises further, the polymer will decompose. If the vacuum pump is started again, the decomposed gas can be sucked away, and the decomposed residue can be washed away by the spray water.


Vacuum calciner ▲

The disadvantage of vacuum calcination is that it takes a long time, and the treatment is not clean when the pore carbon is heavy.


2. Ultrasonic cleaning method

The ultrasonic wave acts in the liquid, which makes the liquid move and produce a large number of tiny bubbles. When the sound pressure reaches a certain value, the bubbles increase rapidly, and then suddenly close. When the bubble is closed, shock wave will be generated, and thousands of atmospheric pressure will be generated around the bubble, which will destroy the insoluble dirt and make them disperse in the cleaning solution.


The disadvantages of ultrasonic cleaning method need to cooperate with special solvent, and the leakage of ultrasonic will cause certain bodily injury.


3. Dry ice cleaning

The dry ice cleaning method takes compressed air as the power and carrier, and takes the dry ice particles as the accelerated particles, and sprays them to the cleaned object surface through a special jet cleaning machine. It uses the momentum change (Δ MV), sublimation, melting and other energy of the solid dry ice particles in high-speed motion to change, so as to quickly freeze the dirt, oil dirt, residual impurities and so on on on the cleaned object surface, so as to condense  Embrittlement, peeling, and at the same time with the air flow clear.

Carbon dioxide solidifies into a solid at - 78.5 °, i.e. dry ice, which is crushed and dispersed in high-pressure air, and then accelerated through a high-speed nozzle to impact the cleaned surface. High pressure waves, plus dry ice particles in them, blow away the residue on the mold surface. The polymer becomes brittle at low temperature and shrinks at low temperature, separating from the mold. Dry ice particles sublimate from solid to gas during impact, expanding hundreds of times, further producing impact force, so as to achieve rapid and efficient cleaning effect.

The biggest advantage of dry ice cleaning method is high efficiency. Other methods take more than ten hours, and dry ice cleaning method can clean in 2-3 hours. Moreover, it is easy to operate and can be operated on site.

Of course, when the coking is very serious, it is possible to cooperate with many methods, even with the use of hammers, silver needles and other small tools, you can clean up according to the specific situation!