Introduction of the relationship between injection

Introduction to the relationship between plastic injection injection injection screw and plasticization quality

functions of plastic injection screw:

feeding, conveying, compression, melting, exhaust, homogenization

important geometric dimensions of screw:

screw diameter, feeding section, compression section, metering section, feeding tooth depth, metering tooth depth

Introduction to important geometric dimensions of screw:

screw diameter (d) is related to the required injection volume

injection volume = 1/4 π × D2 × (ejection stroke) × 0.85

generally speaking, D2 is inversely proportional to the maximum ejection pressure

d, the greater the ejection rate; Q ＝1.29D2HmNr × 60/1000 (kg/hr)

feeding section

is responsible for the transportation, extrusion and preheating of plastics

it should be ensured that the melting starts at the end of the feeding section and is preheated to the melting point

solid specific heat ↑, melting point ↑, latent heat ↑, heating to the melting point requires more heat, the feeding section should be long solid-state heat conductivity ↓, heat transfer is slow, the temperature rise in the plastic center is slow, the feeding section should be long preheating ↑, and the feeding section can be short

the longest crystalline material (such as POM, PA); Amorphous materials take the second place (such as PS and PU); The thermal sensitivity is the shortest (e.g. PVC)

compression section

is responsible for the mixing, compression and pressurized exhaust of plastics. The raw materials passing through this section should have been almost melted, but they may not be evenly mixed

in this area, the plastic gradually melts, and the volume of the screw groove must be reduced accordingly, otherwise the material pressure is not solid, the heat transfer is slow, and the exhaust is poor

for amorphous plastics, the compression section should be longer, otherwise if the screw groove volume decreases rapidly and the material volume does not decrease, blockage will occur

crystalline plastics are not all crystalline in fact (e.g. pe:40 ~ 90% crystallinity, ldpe:65% crystallinity), so the compression section has a trend of lengthening at present

generally accounts for 25% of the working length of the screw

nylon (crystalline material) 2-3 turns, accounting for about 15% of the working length of the screw

high viscosity, fire resistance, low conductivity, high additives, accounting for 40% - 50% of the working length of the screw

pvc can use the working length of 100% of the screw to avoid intense shear heat

metering section

theoretically, from the beginning of the metering section, all the materials should be melted, but at least the metering section = 4D, so as to ensure uniform temperature and mixing

if the metering section is long, the mixing effect is good; If the metering section is too long, it is easy to make the melt stay too long and produce thermal decomposition; Too short is easy to make the temperature uneven

generally accounts for 20~25% of the working length of the screw

pvc is heat sensitive and should not stay too long to avoid thermal decomposition (do not measure section)

feed tooth depth, metering tooth depth

the deeper the feed tooth depth, the greater the conveying capacity in the feed area, but the screw strength should be considered

the shallower the measuring tooth depth is, the higher the heating and mixing performance index of plasticization is, but it is necessary to prevent the plastic from charring. (if the measuring tooth depth is too shallow, the shear heat ↑, autogenous heat ↑, and the temperature rise is too high, which is particularly detrimental to heat sensitive plastics.)

measured tooth depth = Kd = (0.03 ~ 0.07) d d ↑, K choose the smaller; D ↓, slenderness ratio ↑, plastic with poor thermal stability, choose larger K

main factors affecting plasticizing quality:

slenderness ratio, compression ratio, back pressure, screw speed, electrothermal temperature setting

slenderness ratio

slenderness ratio = working length of screw/diameter of screw

if the slenderness ratio is large, it is easy to eat evenly, but it is easy to overdo it

plastics with better thermal stability can use longer screws to improve mixing without worrying about scorching; For plastics with poor thermal stability, a shorter screw or screw tail without thread can be used

considering the plastic characteristics, the general slenderness ratio is as follows:

compression ratio

compression ratio = feed tooth depth/measured tooth depth

considering the compressibility of the material, filling degree, reflux, product compactness, heat transfer and exhaust

an appropriate compression ratio can increase the density of plastic, make the combination between molecules more compact, help to reduce the intake of air, reduce the temperature rise caused by pressure, and affect the difference in output. An inappropriate compression ratio will destroy the physical properties of plastic

the higher the compression ratio, the higher the temperature rise of the plastic in the plasticization process in the feed pipe, the better the mixing uniformity of the plastic in the gelation, and the relative discharge is greatly reduced

high compression ratio is suitable for non fusible plastics, especially those with low melting viscosity and thermal stability

low compression ratio is suitable for fusible plastics, especially high melting viscosity. The oil pump adopts low-noise linear conjugate internal gear pump and heat sensitive plastics

back pressure

increasing the back pressure can increase the work done by the screw on the molten resin, eliminate unmelted plastic particles, increase the density and uniformity of raw materials in the tube, and reduce the injection pressure and warpage

back pressure is used to improve the temperature of the material pipe. According to the global regional market, its effect is the most significant

the back pressure is too large, and the plastic with high heat sensitivity is easy to decompose; For low viscosity plastics, there may be 'rhinorrhea' phenomenon

if the back pressure is too small, the finished product may have bubbles

screw speed

the rotation speed of the screw directly affects the shear of plastic in the spiral groove

the small screw has a shallow groove and absorbs heat quickly, which is enough to make the plastic soften in the compression section. The friction heat between the screw and the tube wall is low, which is suitable for high-speed rotation and increases the plasticizing capacity

large screws are not easy to rotate quickly, so as to avoid uneven plasticization and excessive friction heat

for plastic with high heat sensitivity, if the rotation speed of the glue injection screw is too large, the plastic will be easily decomposed

generally, screws of various sizes have a certain speed range. Generally, the speed of 100 ~ 150 rpm is too low to melt plastic; If it is too high, the plastic will be scorched

at present, the maximum surface velocity is limited to 1m/sec, which is less than 0.5m/s for shear sensitive materials

the electric heating temperature setting

melts the chilled resin retained in the feed pipe and the screw to facilitate the rotation of the screw and provide part of the heat required for the resin to obtain melting

the setting is 5 ~ 10 ℃ lower than the melt temperature (part of it is provided by friction heat energy)

the adjustment of nozzle temperature can also be used to control runny nose, solidification (plug), wire drawing and other problems

general temperature control of Crystalline Plastics:

1000~1500

note 1: the above are based on non reinforced plastics without glass fiber

note 2: the melt temperature in the pipe is usually higher than the temperature controlled outside the pipe, which is indicated by the discharge temperature from the nozzle

selection principle of material pipe group

key points to consider

ratio of three sections of feeding section (a), compression section (b), metering section (c), compression ratio (CR), metering tooth depth (HM), slenderness ratio (l/d), number of teeth (n)

selection principle

for better mixing effect, adopt the design of large slenderness length ratio, large number of teeth, large compression ratio and shallow metering tooth depth

(such as PA, PE, PP, POM). For those who want to prevent excessive fire, adopt the design with small slender ratio, small number of teeth, long compression section, small compression ratio and deep measured tooth depth (such as PC, PMMA, rigid PVC, glass fiber or fireproof material). For high plasticization rate, adopt the design with small compression ratio and deep measured tooth depth (such as ABS, 475 material)

design of glue passing head group

the former needs to use rougher abrasive to ensure that the polishing process has a larger polishing rate. A good check valve should have:

(1) fast check speed ability

(2) complete check ability - to maintain the minimum plastic reflux

(3) the material flow is smooth - there is no dead corner to avoid local shear heat and plastic deterioration

(4) wear resistance and corrosion resistance

(5) it can be used for a variety of plastics

(6) competitive price

design focus of glue head group:

1 appropriate cross-sectional area design to form a certain ratio with screw diameter and measured tooth depth

2 stroke of check valve - the most important factor affecting the check speed

3 appropriate cross-sectional area design - affects the check time

4 stainless steel shell with appropriate sectional area; 2) The heating furnace is conveniently installed in the machine extrusion room and has enough space; 3) Size limitation of heating furnace: length/width/height is 250 ~ 400mm/150 ~ 300mm/150 ~ 300mm; 4) The inner diameter of furnace ≥ 80mm ★ 5) the length of soaking band ≥ 50mm (the temperature fluctuation in soaking band ≤± 5 ℃ ratio - determines the force on the screw during plasticization