High Shear Processing Equipment

Technical Information High Shear Processing Equipment

1.About high shear processing equipment

High shear processing equipment is a machine to mix ingredients that do not mix naturally such as resin and resin (blend), resin and a filler (composite), etc. NHSS, our high shear machine, can mix these ingredients in micron (1/1000 mm) or even in nano (1/1000 μm) dimensions. 
 

2.Mixing

2 kinds of lquid at room temperature can be easily mixed by shaking the combined liquids. What about mixing 2 kinds of resin? For example, can we mix PET resin used for water bottle and polyethylene used for plastic bags? Both PET and polyethylene are solid at room temperature but melt when heated since these are thermoplastic resins. We can mix these 2 melted resins. NHSS, our high shear machine, is such a machine that can heat ingredients up to 400 ℃ and mix well. 
 

3.Our method to mix

Screw with internal feedback system is used to knead resin.
Molten resin in screw grooves is sent to screw tip as the screw revolves. When it reaches to the screw tip, it comes back to rear end of the screw by going through a hole inside the screw so that it goes through melting/kneading process again. Unlike regular screw, this special screw can repeat melting/kneading process as many times as required. It is called internal feedback screw as it returns the resin through the hole inside screw.
Screw has a hole at the tip.
 

4.Why internal feedback screw?

It is known that high shear stress needs to be given for a long period of time in order to mix molten resin well. Simply put, long-time, strong mixing is required. 
 
①Shear stress is associated with viscosity of resin and shearing speed. Resin viscosity varies with type and changes by temperature. Resin, as non-Newtonian fluid, decreases in viscosity when shearing is applied. Shearing speed is associated with screw rotation speed. If resin viscosity does not change when shear force is applied, it would be assumed that the faster the screw rotates, the larger shearing stress is caused. (Resin viscosity decreases when shearing force is applied.)
 
②In order to mix resins for a long time, it should stay in the barrel for a long time. Resins can stay longer in longer barrel. However, long screw is technically difficult to make. Rotation speed of our internal feedback screw can be increased up to 3000 rpm (5000 rpm as option). WIth this special screw, resins are kneaded as long as necessary by repeating kneading process.
 

5.Kneading section and plasticization section

With a typical twin-screw extruder, screw rotation is the same for all process; feeding, plasticization and kneading of resin. It is not possible to change rotation speed faster  only for kneading process. Our high shear processor has another screw for plasticizing process in addition to internal feedback screw for kneading process so that different speed setting for each process is possible. Our long-time experience in injection molding technology is used effectively for plasticization section.
 
Resin temperature in plasticization process affects viscosity of resin in kneading process. Different temperature setting for each process is possible for desired viscosity.
 
 

6.Infrared thermometer

We want to measure molten resin temperature during kneading process. Controlling temperature is important not only for high shear processor but also for twin-screw extruder. Even if thermocouple is in direct contact with molten resin, it cannot measure correct resin temperature because the resin is under pressure and in motion. Barrel temperature is not the same as resin temperature. The same is true for injectiom molding machine and twin-screw extruder. We put a thermocouple into a metal container and inserted it inside barrel trying to measure resin temperature. Temperature measured by the thermocouple seemed to be inaccurate after comparing with estimated temperature by viscosity of discharged resin.
 
We have been seeking better way to measure accurate resin temperature. We came up with an idea to use infrared thermometer. Although it could only waste the device, we gave it a try with spirit of nothing to lose .
 
Contrary to our expectation, the infrared thermometer captured resin temperature change that we had never seen. The thermometer didn't break, which was also beyond our expectation.
 
Infrared thermometer can read 100℃-400℃. Pressure resistance is up to 150Mpa. Installation is the same as  commercially available sensor. It can be used for other kinds of mixer to measure temperature of molten resin that is under pressure and in motion.
 
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7.What temperature is the infrared thermometer showing?

It is difficult to prove exactly where infrared thermometer is sensing.  This type of sensor is often considered to be unreliable due to lack of verification test.
 
On our high shear processing equipment, an infrared thermometer is attached aiming the hole on internal feedback screw, the hole that molten resin goes through to return to the screw rear end. Once we tried a smaller diameter hole of 0.5 mm, temperature the infrare thermometer showed was lower than expected. It seemed be wrong. We tried with another thermometer but the same result came up. We changed the hole diameter to 1.0 mm. With larger hole, temperature detection was successful. 
 
It turned out that the hole diameter needs to be larger than diameter of optical fiber used on infrared thermometer. Optical fiber of infrared thermometer is in contact with molten resin to detect temperature. Its diameter is 0.8 mm. If optical fiber is larger than the hole, it detects temperature of a part of screw, around the hole.
 
Another concern was accuracy of measured temperature. Our experiment was to check resin temperature change while heating barrel temperature was increased by 20℃ every 30 minutes.  30 minutes was considered to be long enough for heat to be transferred within resin. Barrel is heated by heater bands so the temperature increased quickly. Resin temperature increased 5 minutes after the barrel. Measurement error was ±1℃, which was wonderful.
 
 
 
 

8.Case study of nano dispersion

What is the basis for nano dispersion? As a machinery manufacturer, we have no means of resin analysis. Mr. Mizuno at The National Institute of Advanced Industrial Science and Technology, which we were working on the joint research with, once gave us advise. "We can say it is nano dispersion when transparent dispersion of polycarbonate and acrylic occurs", according to Mr. Mizuno. Making transparent dispersion became our goal since then and continued research and development. It took us 4 years of jurney to have our first transparent dispersion.
 
Our transparent dispersion of polycarbonate (PC) and acrylic (PMMA); 2 kinds of resin with different reflective index, was the first in history. It was told that mixed resins becomes transparent when resins are dispersed very finely, smaller than wavelength of light. Although one resin manufacturer told us that even textbook says color of PC and PMMA mixture is pearly white, it was a moment we proved otherwise. 
 
Characteristic of PC and PMMA blended material is;
・Dispersion is in 30 nm scale.
・Transparent.
・Has intermediate reflective index of PC and PMMA.
・Only 1 glass transition point (originally 2 from 2 kinds of resin).
・No phase separation occurs when melted again (maintains transparency).
 
We have 5 other cases of successful transparent dispersion (5 combinations using transparent material) although detail information cannot be disclosed due to confidentiality agreement with our clients.  We have conducted trials with about 100 clients for 300 days in total under confidentiality agreement. Above characteristics are all found in 5 other cases. Without adding compatibilizer or additives, only mechanical kneading is used to create transparent dispersion. Isn't it amazing?
 

9.Cooling method of high shear processor barrel

Our high shear processor injects melted resin into high shear section and kneads the resin using internal feedback screw. Kneading time by internal feedback screw can be up to 3,200 seconds but 60 seconds may be the maximum in reality. Long kneading time causes resin deteriorated. Controlling temperature is particularly important to avoid deteriotation since resin generates heat while kneaded.
 
Heat generated by resin transfers to the barrel, then cooling water is released into the circuit. This is a regular cooling method for other kind of mixer. With our high shear processor, heat generation by resin mainly happens during kneading process. However, when kneading process is only 5 to 10 seconds, it is too short time to increase barrel temperature even though temperature of resin inside is increasing.  Thus, it is difficult to control cooling water by detecting barrel temperature. 
 
Is motor workload associated with heat generation somehow? Is heat generation large when the motor workload is large? Can we control cooling water by checking motor? 
Motor workload = Motor revolution * Motor torque * index 
 
Motor workload is sampled every 1 second and valve opening frequency is calculated every 1 second in order to determin flow rate of cooling water. Barrel temperature control becomes possible in this way. 
 
Control barrel temperature without checking barrel temperature. It is reverse thinking
 

▼Feel free contact us

 Inquiry Form
TEL. +81-25-274-5130
NIIGATA MACHINE TECHNO Co., Ltd.
1300 Okayama, Higashi-ku, Niigata City, Niigata 950-0821
TEL: +81-25-274-5121
FAX: +81-25-271-5827

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