Extruder extrusion process of common problems and solutions

Extrusion process common problems:
First, degradation
PVC
Is a thermosensitive plastic, light stability is also poor, under the action of heat and light, it is easy to take off HCl
Reactions, commonly known as degradation. The result of degradation is a decrease in the strength of plastic products, discoloration, a black line,
Severe lead to loss of use of the product value. Factors that affect the degradation of PVC polymer structure, polymer material
Volume, stable system, molding temperature and so on. According to experience, PVC profiles yellow mostly because the die appears at the die, the reason is that the die channel is not reasonable or poor local polishing within the runner, there is a lag area. The PVC profiles out of the yellow line mostly appear inside the barrel paste, the main reason is the sieve (or transition sets) between the dead ends, poor material flow. Yellow line in the PVC profile was a vertical straight line, then the slack at the die exit; if the yellow line is not straight, it is mainly in the transitional sets. Formulas and raw materials also appear yellow line, it should be mainly from the mechanical structure to find the reasons to find the occurrence
Decomposition of the starting point and be excluded. If you can not find the reason from the plastic machinery structure, you should consider the formula or
There are problems in the process. Measures to avoid degradation in the following areas:
(1) strict control of raw materials, technical indicators, to use qualified raw materials;
(2) to develop a reasonable extrusion molding process conditions, under the conditions of PVC material is not easy to degrade;
(3) extrusion molding equipment and mold should be well-structured, to eliminate equipment and material contact surface may exist
Of the dead or slit; flow path should be streamlined, suitable length; should improve the heating device to improve the temperature display
Set the sensitivity and cooling system efficiency.
Second, bending deformation
PVC
Profile bending deformation is a common problem in the extrusion process, the reasons are: uneven die exit;
Cooling stereotypes, material cooling is not sufficient, inconsistent after shrinking; equipment and other factors.
Concentricity and level of the extruder across the board is a prerequisite to solve the bending deformation of PVC profiles, so the concentricity and level of the extruder, die, formwork, water tanks, etc. should be corrected whenever the mold is changed. Among them, to ensure uniform die discharge is the key to solve the bending of PVC profiles, before starting the device should be carefully assembled die, the gap between the parts to be consistent, if the die is found uneven boot, should be based on parison bending deformation direction, corresponding Adjust the die temperature, such as adjustment is invalid, it should be appropriate to increase the degree of plasticization of the material. Adjust the mold adjustment of the vacuum and cooling system is necessary to resolve the deformation of PVC profiles should be enlarged profile to withstand the tensile stress on one side of the cooling water volume; the use of mechanical offset center approach to adjust, that is, while production, while adjusting Set the middle of the mold positioning bolt, according to the profile of the direction of micro-reverse micro-adjustment (the method should be careful, and the adjustment should not be too large). Pay attention to mold maintenance is a good preventive measure, should pay close attention to the quality of mold work, according to the actual situation at any time for mold maintenance and repair. By taking the above measures can eliminate bending deformation, to ensure the stability of the extruder, the normal production of high-quality PVC profiles.
Third, low temperature impact strength
Factors affecting the low temperature impact strength of PVC profiles formula, profile section structure, mold, degree of plasticization, test conditions.
(1) Recipe Currently, CPE is widely used as an impact modifier, in which CPE containing 36% of chlorine has no effect on PVC
The best modification, the amount of 8-12 parts by mass in general, low crystallinity and glass transition temperature, with a good bomb and PVC compatibility.
(2) section profile structure High-quality PVC profiles with good cross-sectional structure. Under normal circumstances, the structure of the small section is better than the structure of the large section, the location of the rib inside the section should be appropriate. Increase the thickness of the ribs, the ribs and the wall at the junction of arc transition, are helpful to improve the low temperature impact strength.
(3) mold
Mold impact on low temperature impact is mainly reflected in the melt pressure and cooling stress control. Once the recipe is confirmed,
Melt pressure is mainly related to die. The profiles coming out of the die experience different stress distributions through different cooling methods. Where the stress concentration PVC profiles low temperature impact strength is poor. PVC profiles are subject to quenching when prone to large stress, so stereotypes die cooling water flow path layout is critical, the water temperature is generally controlled at 14 ℃ -16 ℃, slow cooling mode is conducive to improving the low temperature impact strength PVC profiles. To ensure the best mold, regular cleaning of the die, to avoid prolonged production for the blockage of impurities and impurities, resulting in reducing the material, the support bar is too thin, the impact of low temperature impact strength. Periodic cleaning of the mold can be set to ensure stereotypes sufficient stereotypes of vacuum and water flow in order to ensure the profile is fully cooled during production to reduce defects and reduce internal stress.
pvc molding mold and the quality of plastic products

Core Tip: the use of molds The first section pvc molds the role and manufacturing requirements Since the extruder can produce a variety of shapes of plastic products, each shape and size of plastic products
The use of molds The first section pvc molding molds and manufacturing requirements
As the extruder can produce a variety of shapes of plastic products, plastic products of each shape and different specifications in the extrusion, there must be a set of special shape and size can be molded, so the extruder molding Plastic mold varieties will also be more, such as a variety of pipe, rod, profile, plate (film), film, wire, mesh and flat products such as molding with molds, have different sizes of special Molding mold. Molding mold design, manufacturing quality of plastic molded products have a significant impact on the quality, if the structure and size selection, design unreasonable, and sometimes can not even produce.
Molding die manufacturing and technical requirements are as follows.
(1) forming steel mold manufacturing requirements
① good heat resistance, small deformation in the high temperature working environment.
② strong wear resistance.
③ anti-corrosion, long time in a corrosive gas melt environment, not easy to be corroded, rust oxidation.
④ Machining performance is good.
⑤ heat treatment when the oxide layer is thin, small deformation.
⑥ for the manufacture of die lip and blocking the regulation of the alloy steel, in addition to the above conditions should also have better flexibility.
Die commonly used in the manufacture of steel parts are: 45,40 Cr, 65 Mn, T8A, TlOA, 2Cr13, 3Cr13 and 38CrMoAIA and so on.
(2) different raw materials should be used when molding plastic products more suitable for the molding of the mold structure. Such as the production of PVC resin pipe molding, should be used straight die structure; PE or PP resin extrusion film, should be selected with a spiral groove mandrel, extrusion melt flow direction into a rectangular shape of the mold structure.
(3) Molding mold body, the melt circulation cavity should be streamlined, different diameter section of the transition should be arc-line connection, that is, the melt flow through the segment can not have dead ends or stagnant.
(4) The cross section of the melt flow cavity in the mold body to the melt molding die should be gradually reduced in cross section, with a certain compression ratio, so as to eliminate the melt bond stitch caused by the shunt cone support and make the product compact. The compression ratio refers to the cross-flow cone bracket cross-sectional area of several cross-sectional area of the die outlet and mold flow cross-sectional area ratio. According to the molding products with different raw materials, the general compression ratio in the range of 3 to 10.
(5) mold parts should be as small as possible, compact structure, symmetrical shape of the parts to facilitate the installation and removal and heater layout.
(6) the mold parts to have sufficient work intensity. The main parts should be quenched and tempered, the die face to have a higher hardness, the general hardness of not less than 40 ~ 45HRC.
Section second use of mold
Molding molds are generally made of high quality alloy steel, machining accuracy is relatively high, so the mold manufacturing costs. Operators pay attention to the rational use of mold, mold can extend the application time, which reduces the cost of plastic products, improve the economic benefits of extruder production. The use of forming molds in production should pay attention to the following matters.
① forming mold parts in the assembly before the removal of burrs, dirt, clean the parts before they can be assembled.
② When assembling or disassembling the molds, each piece should be handled gently and gently. During the disassembly or assembly process, it is forbidden to hit each working face of the mold with a hand hammer to avoid hammer hammering on the working face and affect the product quality.
③ molding mold installation order (straight through the mold tube as an example): first to install the mold body and barrel connection flange, and then the mold body, taper cone, mandrel, jacket and other types of body parts, and finally install the die, Gland and sizing sets and other parts.
④ mold connection bolts and adjusting screws, the installation should be first coated with a layer of copper sulfide or silicone oil, painted in the threaded parts to facilitate the removal of high temperature mold parts. Tighten the connection bolts, diagonal nuts should be tightened at the same time, to prevent the parts due to uneven force caused by the connector can not rely on the two planes can make the mold work leakage.
⑤ When the mold needs to be adjusted in the work (to adjust the uneven thickness of the pipe products), should first loosen the thin-walled tube wall thickness of the screw, and then tight tube thickness thicker side of the adjustment screw to avoid damage to the mold parts and Screw off the adjustment screw.
⑥ clean production at the material at the mouth of the viscous material, should be used bamboo or copper knife, shovel scraper cleaning tools, not allowed to use hard steel knife scraping, shovel mold mouth at the residue to prevent scratches the mold Face, affecting product quality.
⑦ repair mold burrs or scratches the mouth, not allowed to scratch the file, should be used fine stone or fine sand cloth for dressing.
⑧ For temporary use of the mold, the remnants of the mold to remove the clean, coated with a layer of anti-rust oil, then the parts together; seal the mold into and out of the mouth, stored in dry and ventilated place; Die allowed heavy objects, to prevent deformation caused by heavy mold over time.
The third section pvc mold and the relationship between the quality of plastic products
Die structure, shape, temperature and melt channel finish, choose reasonable or not, the quality of the extruded products have a direct impact. When the product after the formation of the following quality problems, if the barrel, screw plastic parts of the quality of the impact of the product has been ruled out, then the working conditions from the mold to find out the factors that may affect product quality.
① the surface of the product with a melt line forming mold body cavity melt flow rate compression ratio is small.
② longitudinal groove on the surface of the product stereotypes die at the metal mold burrs, scratches or sticky residue.
③ product geometry error size Large mold body temperature control instability or uneven temperature of the mold body parts; mandrel, mold stereotypes size length is not enough.
④ deformation after cooling the mold body compression ratio is small or enough product structure design unreasonable.
⑤ products matte mold body temperature is low