Screw Introduction

 

Structure of injection screw and barrel:

 

 

Demag Universal Screw Dimensions

 

Feeding section L1
Function: Responsible for transportation and preheating of plastic; ensuring that melting begins at the end of feeding section and preheating to melting point.
Length: For materials with high solid-state specific heat capacity, high melting point, and slow heating, more heat is required to heat to melting point, so feeding section should be long.
Crystalline materials are the longest; amorphous materials are the second; heat-sensitive materials are the shortest.
Compression section L2 ----The narrower melting range of material, the shorter compression section.
·Function: Responsible for mixing, compression and pressurized exhaust of plastics; raw materials in this section have been almost completely melted, but may not be evenly mixed.
·Length: For non-crystalline plastics: compression section should be longer (gradual type), otherwise volume of screw channel space will decrease too fast and material volume will decrease slowly (reason is that amorphous plastics have no melting point and gradually soften and melt), which will cause excessive shearing. Cutting can cause material degradation or even blockage.
For crystalline plastics with low viscosity and good heat resistance, such as nylon: use short compression section length (abrupt type).
For high viscosity, poor heat resistance, low heat transfer, and high additives, long compression sections should be used.
Metering section L3
Function: All materials should be melted to ensure uniform temperature and uniform mixing.
·Length: The longer metering section, the better mixing effect; if metering section is too long, melt will stay for too long and cause thermal decomposition; if metering section is too short, temperature will be uneven.
Feeding tooth depth h1
·The deeper feed tooth depth, the greater conveying volume in feed area, but screw strength needs to be considered.
·Tooth depth of feeding section=(0.12-0.14)D
Measuring tooth depth h3
·The shallower measuring tooth depth is, the better mixing performance will be due to heat generated during plasticization. However, it is necessary to prevent plastic from burning, which is especially detrimental to heat-sensitive plastics.
·Measuring section tooth depth=(0.05~0.07)D
D↑, choose small K &D↓, slenderness ratio ↑, plastic with poor thermal stability, choose large K)
aspect ratio
If slenderness ratio is large, mixture will be uniform, but it will easily cause material degradation.
·Plastics with good thermal stability can use longer screws to improve mixing characteristics without worrying about degradation;
For plastics with poor thermal stability, a shorter screw or no thread at the end of screw can be used.
Compression ratio - Ideal compression ratio should satisfy gradient degree, that is, (h1-h3)/L2, and adapt to material melting rate, but melting rate cannot be calculated yet.
The higher compression ratio, the higher temperature rise generated during plasticization process of plastic in material tube, resulting in better mixing uniformity of plastic during plasticization, and relative discharge volume is greatly reduced.
High compression ratio is suitable for plastics that are not easy to melt, especially plastics with low viscosity and good thermal stability.
Low compression ratio is suitable for plastics that are easy to melt, especially high-viscosity, heat-sensitive plastics.
Screw speed
·Rotation speed of screw directly affects shear of plastic in spiral groove.
·Small screw has a shallow groove depth and absorbs heat quickly, which is enough to soften plastic during compression section. Friction heat between screw and material tube wall is low, which is suitable for high-speed rotation and increases plasticizing capacity.
·Large screws are not easy to rotate quickly to avoid uneven plasticization and excessive friction heat.
Pitch S, helix angle φ:
φ =πDtgφ Generally D=S, then =17°40'.
p has an impact on plasticizing ability. Generally speaking, the larger φ, the faster conveying speed. Therefore, φ of material will also change depending on shape of material. For powder materials, φ=25°, for cylindrical materials, φ=17°, and for square materials, φ=15°. However, different φs make processing difficult, so generally φ=17°40'.
Edge width e:
For materials with low viscosity, e should be as large as possible. If it is too small, leakage will occur, but if it is too large, it will increase power consumption and make it easy to overheat. e=(0.08~0.12)D.

(1) Ordinary barrel: nitrided steel (hardness: HV850-950); nitriding layer depth 0.5-0.7 mm
(2) Bimetallic barrel: (a) Nickel-based alloy surface hardening,
(b) Surface centrifugal casting 40% tungsten carbide lining, depth greater than 2 mm, hardness HRC55-62.
Service life is 4-5 times that of ordinary barrel.

(1) Material: high-performance alloy steel or ion nitrided alloy steel (standard nitriding time 72 hours) Example: 38CrMoAl, 34CrAINi7.
(2) Surface treatment: Surface chrome plating hardening (applicable to materials PC, PVC, etc.).
HRC55; Chrome plating layer depth: 0.05-0.1mm; Surface roughness: Ra0.4um (mirror grade 6)
(3) Screw straightness: 0.015mm; Screw radial clearance: 0.002-0.005D Check valve and barrel clearance:
(4) Bimetallic screw:
(a) Spray nickel-based alloy on screw edge
(b) Surface high-pressure and high-speed spraying 80% tungsten carbide. Depth greater than 2mm, hardness HRC55-62.

1. Set a reasonable processing temperature range, especially for reinforced plastics containing glass fiber or mineral powder, barrel processing temperature should be controlled to be close to this temperature range. Granular plastics enter barrel from hopper and will first reach feeding section. Dry friction will inevitably occur in feeding section. When these plastics are not heated enough and melt unevenly, it is easy to cause increased wear on inner wall of barrel and surface of screw. Similarly, in compression section and homogenization section, if melting state of plastic is disordered and uneven, it will also cause increased wear.
2. Speed should be adjusted properly. Because some plastics are added with reinforcing agents, such as glass fiber, minerals or other fillers. Friction of these substances on metal materials is often much greater than that of molten plastics. When injecting these plastics, if a high speed is used, while increasing shear force on plastic, reinforcement will also produce more torn fibers accordingly. Torn fibers contain sharp ends, which greatly increase wear force. When inorganic minerals slide at high speed on metal surface, their scraping effect is also not small. Therefore, speed should not be adjusted too high.
3. To detect and remove debris in plastic. A magnet rack must be installed, feeding must be strictly managed and monitored.
4. Moisture contained in plastic has a certain effect on wear of screw surface.
(1) If plastic is not completely dehydrated before injection molding, residual moisture will enter compression section of screw, forming "steam particles" with high temperature and pressure that are previously melted and mixed in molten plastic. As screw advances during injection process, from homogenization section to screw head, these "steam particles" are decompressed and expanded during injection process, like tiny impurity hard particles, causing friction and damage to wall surface.
(2) For some types of plastics, under high temperature and high pressure, moisture may become a catalyst that promotes cracking of plastics, producing harmful impurities that can corrode metal surface. Therefore, drying work before plastic injection molding is not only directly related to quality of product, but also affects working life of screw.
5. Also pay attention to concentricity of screw and barrel!
6. Influence of additives: Flame retardants will decompose corrosive gases during processing. Since temperature in feeding section is low, it is easy to accumulate gas, so it is easy to corrode.
Screw wear performance
Feeding section wear is mainly manifested in: slow feeding and difficulty in feeding, and too long pre-plasticization time.
Compression section wear is mainly manifested in: pre-plasticization time is too long.
Homogenization section wear is mainly manifested in: material backflow, screw forward rotation, unstable back pressure, and too long pre-plasticization time.

Wear of feeding section is manifested in: slow feeding and difficulty in feeding, and pre-molding time is too long.
Wear of compression section is manifested in: pre-molding time is too long.
Wear of homogenizing section is manifested in: material backflow, screw forward rotation, unstable back pressure, and too long pre-molding time.
Wear of screw head and check valve: unstable back pressure, long pre-molding time, black spots and other symptoms.
Of three sections of screw, homogenizing section (metering section) is generally the most severely worn and the fastest worn.
Black spots on product are caused by many factors. From perspective of screw, there are:
(1) Screw wear;
(2) Screw head and check valve wear;
(3) Coating and nitride layer fall off;
(4) Check valve storage.
Scorching and yellowing: From perspective of screw, there are:
(1) Screw compression ratio does not match plastic performance
(2) Screw wear (or coating fall off).
Comparison between PC/ABS special screw and general screw

 

Comparison of PA, PBT special screw and general screw

 

Comparison of screw dimensions for different materials

 

1. Length of check ring is generally about same as outer diameter of screw.
2. Angle between check ring and valve seat cone is generally 30 degrees. If contact surface between check ring and sealing ring is small, the angle can be larger, but not more than 45 degrees.
3. Displacement of check ring varies according to size of screw diameter and depends on fluidity of material. The poorer fluidity, the greater displacement.