Screw arrangement and process settings

1. Segmentation and function of screws

(1) The screw is generally divided into five sections: conveying section, melting section, mixing section, exhaust section, and homogenization section.

  • Conveying section, conveying materials to prevent overflow.
  • Melting section, which allows the material to fully melt and homogenize through heat transfer and frictional shearing.
  • The mixing section further refines and unifies the size of material components, forming an ideal structure with distribution and dispersion mixing functions.
  • The exhaust section discharges impurities such as water vapor and low molecular weight substances.
  • Homogenization (metering) section, conveying and pressurization, establishing a certain pressure to achieve a certain density of materials at the mold mouth, while further mixing, ultimately achieving the goal of smooth extrusion granulation.

(2) Distribution and dispersion mixing

  • Distribution mixing is used to divide and recombine the melt, resulting in uniform spatial distribution of each component. It is mainly achieved through displacement flow under stress effects such as separation, stretching (alternating compression and expansion), twisting, and reorientation of fluid activity.
  • Dispersion mixing, which breaks the components into particles or achieves the required size of incompatible two component dispersed phases, mainly achieved by shear pressure and tensile stress.

2. Conveying element, threaded representation: for example, for a ‘56/56’ conveying block, the previous ‘56’ indicates the lead of 56mm, and the subsequent ‘56’ indicates the length of 56mm. Large lead refers to the pitch of screw is 1.5D~2D. Small lead refers to the pitch of screw is 0.4D.

Its usage pattern: As the lead increases, the screw extrusion amount increases, the material residence time decreases, and the mixing effect decreases.

  • In situations where large lead threads are used and conveying is the main focus, it is beneficial to increase production. Thermally sensitive polymers that shorten residence time and reduce degradation. At the exhaust point, shallow grooves can also be used to increase the surface area which is beneficial for exhaust and volatilization.
  • In situations where medium lead threads are used and mixing is the main approach, there is a combination of gradually shrinking working sections for conveying and pressurization.
  • In situations where small lead threads are used, which are generally gradually reduced in combination, used in the conveying section and homogenization metering section to increase pressure and improve melting. Improving the degree of mixing and extrusion stability.

3. Mixing components, including two major categories: ‘K’ series and ‘M’series.

‘K’ series: such as K45/5/56, belonging to shear blocks.  ‘K’ refers to sheet-like shear blocks. ‘45’ refers to the angle at which the pieces are assembled. ‘5’ refers to a total of 5 pieces. ‘56’ refers to the length of 56MM, and the screw edge width is 56/5=112mm). Its parameters:

  1. Direction, there are forward and reverse. Reverse, which hinders the transportation of materials, prolongs time, increases filling pressure and greatly improves mixing efficiency.
  2. Angles are generally divided into 30 °, 45 °, 60 °, and 90 °, and their functions and effects are as follows: When in the forward direction, increasing the staggered angle will reduce the conveying capacity, prolong the residence time and improve the mixing effect, but the more prone it is to leakage. For distribution mixing and dispersion mixing, distribution mixing becomes more effective with increasing angle. Dispersion mixing is best at an angle of 45 °, followed by 30 °, and worst at 60 °. When reversing, increasing the angle will reduce the effective limitation of the polymer, but it is more prone to leakage.
  3. The width of the screw edge is generally 7mm, 11mm, 11.2mm, 14mm, 19mm, etc. This is one of the most important parameters for measuring the shear size and mixing size. The larger the width, the smaller the shear and mixing. The smaller the width, the smaller the shear, and the greater the mixing. For distribution mixing and dispersion mixing, the effectiveness of distribution mixing decreases with increasing width, while the effectiveness of dispersion mixing increases with increasing width. The smaller the width, the greater the ratio of axial effective flow rate to radial effective flow rate of the material.
  4. Number of heads, generally single head, double head, or triple head. Its effect:
  • When in the forward direction, the fewer the heads, the greater the extrusion conveying capacity and torque, and the better the mixing characteristics, but the less shear action.
  • When in reverse, the fewer the heads, the smaller the extrusion conveying capacity, and the better the mixing characteristics.
  • Two head threads can be used for extrusion molding, with uniform heating and good self-cleaning performance (commonly used).
  • Triple thread, able to flexibly select the pressure and temperature distribution of materials at the machine corner, stable fiber addition, good exhaust surface renewal effect, but low production.

M series: tooth shaped, mainly used to stir up the material flow and accelerate material homogenization. The more teeth there are, the stronger the mixture. However, attention should be paid to the destructive nature of high shear during use.

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