Molding and processing characteristics of the most

2022-08-24
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Molding processing characteristics of commonly used plastics

first, introduction

the production of plastic products mainly consists of raw material preparation, molding, post-processing, machining, modification, assembly and other processes. Generally, we are involved in the first two processes. Molding refers to the process of making various forms of plastics (powder, granule, solution or dispersion) into products or accessories with required shapes. The molding process generally includes three stages: plasticization, shaping and curing of raw materials. The essence of polymer molding processing is a process of determining the structure of polymer and obtaining certain properties. The structure mentioned here mainly refers to the aggregated structure of polymer, that is, crystallization, orientation and other special microstructures, such as branching, crosslinking, continuous transfer, etc. These directly affect the performance of products, so the important task of polymer molding is to meet the shape and appearance requirements of products and the service performance of products through the constitutive process. And processing performance. The whole system of polymer molding includes determining the reasonable raw material formula, selecting the appropriate molding method, determining the reasonable process conditions, and putting forward reasonable requirements for the process equipment, so as to obtain the products with the best performance

second, the properties of plastics

generally speaking, the molding and processing of plastics should be considered. First, the properties of plastics are closely related to the properties of plastics. The properties of plastics mainly include:

1, physical properties: such as density, glass transition temperature, melting temperature, degradation temperature, water absorption, permeability, crystallinity,

2, mechanical properties: tension, compression, impact, bending strength, touch, elongation at break, Resistance to long-term stress cracking, etc

3, molding processability, but there are significant differences between the two: rheological properties, heat conductivity, crystallization properties, macromolecular orientation properties, degradation crosslinking properties

4, electrical properties: relative dielectric constant, surface and volume resistance, dielectric loss, breakdown voltage, etc. 5. Chemical corrosion resistance: resistance to acids, alkalis, organic solvents, etc

6, thermal performance: thermal deformation temperature, long-term service temperature

7, environmental aging resistance

and point 3: molding performance is our key consideration. In the processing process, we are concerned about the hardness, crystallinity, melt fluidity, ease of flow filling, shrinkage, decomposition, degradation, intersection, etc

⑴. Rheological properties

the rheological properties of polymers are the most basic characteristics in the process of polymer molding, which mainly refer to the elastic, plastic and viscous deformation behaviors of plastics under the action of stress, and the relationship between these behaviors and the structure, properties, temperature, pressure, mode of action, action time, etc. of plastics. It directly affects the selection of polymer materials, processing conditions, processing equipment, forming molds, product quality, etc

⑵. Heat conductivity

generally, plastic is heated first, then cooled and shaped. The essence of heating and cooling is the heat conduction of plastics. During the heat transfer of plastic, the possible temperature gradient is limited. Because plastic is a poor conductor of heat, the temperature difference between the heat source and the heated plastic is too large, which will lead to local excessive temperature and plastic degradation. Plastic can't be cooled too fast. If the temperature difference between the cooling medium and the melt is too large, it will lead to large internal stress in the product. Cause product deformation. The viscosity of plastic melt is very high, so it is difficult to conduct convective heat transfer, and shear friction heat will be generated in the flow process, which will raise the melt temperature and sometimes cause the product surface to be scorched. Many plastics are semi crystalline. When heated and melted, they have phase transition, thus absorbing more heat. When cooled, the crystallinity is closely related to the cooling rate

⑶. Crystallinity

generally, plastics are divided into two categories: one is crystalline polymers, and the other is amorphous polymers. The crystallization of plastics is imperfect, and the crystallization rate, crystallinity and crystal structure are affected by many factors. In molding processing, it is very important to control the crystallization conditions of plastics by adjusting process parameters and selecting appropriate molding processing equipment, so as to obtain excellent products

⑷. Orientation properties of macromolecules

in the process of plastic molding and processing, it is inevitable that there will be different degrees of orientation. Generally, there are two kinds of orientation processes, one is the flow orientation of macromolecules in polymer melt, chain segment, re stretching or shear flow, along the flow direction. The other is that when the polymer is stretched by an external force, the macromolecules, segments or microcrystals are stretched and oriented along the direction of the force

third, molding process characteristics

we are mainly involved in the processing of granular materials, and its process performance characteristics mainly include:

1 Shrinkage plastic products produced with powder or granular materials are usually formed in the mold under high temperature and melting state. When the products are cooled to room temperature, their size will shrink. In order to ensure the accuracy of the product size, the appropriate magnification factor should be determined in combination with the shrinkage when specifying the size of the mold cavity. Generally, products with too large shrinkage are prone to warpage and cracking

2. Fluidity, the ability of plastic to fill the entire cavity under heat and pressure is called fluidity

3. Water, plastic often contains more or less water. When there is too much water in the plastic, its fluidity will increase, the molding cycle will increase, the shrinkage of the product will increase, the product will be porous and prone to warpage, and the surface will be wavy and dull, which will reduce the mechanical and dielectric properties of the product

Fourth, let's discuss the commonly used plastics (HDPE, PP, PET):

LPE has regular molecular space arrangement and small chain links, which is the internal condition for it to be a crystalline polymer. Due to different polymerization methods, there are differences in the length and number of branch chains on the PE main chain. HDPE is a nearly linear molecule (there are 5 methyl groups and 0.5 ethyl groups per 1000 carbon atoms on the molecular chain on average). The molecular chain can be neatly arranged and stacked tightly, which is easy to form a crystalline structure. Therefore, it has high density (0.94-0.96/g,/cc), high crystallinity (80%-95%), melting temperature (126-136 ℃), and relatively high hardness, strength, rigidity and toughness. It also has excellent heat resistance (softening temperature 124-127 ℃, continuous heat resistance temperature 120 ℃, decomposition temperature 350 ℃), good solvent resistance and steam permeability resistance. Generally, the crystallinity decreases with the increase of temperature. When HDPE is quenched, the crystallinity can be reduced by 40%, thereby changing its performance. Melt flow rate refers to the amount of molten material extruded within a certain time under specified conditions. It reflects the size of melt viscosity and is a measure of processing fluidity. For HDPE, due to its large molecular weight (relative molecular weight from hundreds of thousands to hundreds of thousands), high density and high viscosity, the melt flow rate is low (0.1-4/g/10min), which is mostly expressed by specific viscosity. HDPE has a very low water absorption (0.03%), so it does not need to be dried during processing. The extrusion temperature of HDPE is 165-260 ℃ and the pressure is 35-140mpa. During injection, the molding temperature is 180-250 ℃, the mold temperature is 50-70 ℃, and the injection pressure is 80-100 MPa. The linear shrinkage of molding is 2% - 5%

2PP. Polypropylene and polyethylene belong to polyolefin, which must have some commonalities. They are linear polymer compounds, almost without double bond structure. It is still a saturated aliphatic hydrocarbon long-chain polymer and is also a nonpolar crystalline polymer. Especially in terms of swelling property, it is very similar in terms of hydraulic properties (resistance to, acid, alkali, organic solvents, absorption and expansion of water, etc.) and electrical properties when the buffer is rotated to the appropriate position. However, there are some differences between PP and PE in the main chain structure. There are methyl groups alternately on the carbon atoms of PP main chain. The existence of methyl groups makes the main chain slightly stiff, and also reduces the molecular symmetry. The activation of carbon atoms makes PP more sensitive to oxygen and more unstable. Overheating will lead to the breakage of PP molecular chain rather than gluing. (usually add compound antioxidant and antioxidant). The relative molecular weight of PP for general industrial use is higher than that of PE, ranging from 220000 to 700000. Another important factor affecting the properties of PP is isotacticity. That is, the order of methyl groups in the molecular chain. (random PP is an amorphous viscous substance, which cannot crystallize and has no strength.)

based on the above factors, isotactic PP has higher strength, rigidity and hardness than PE under normal conditions. The thermal oxidation stability is worse than that of PE. The heat resistance of PP is higher than that of PE. The melting temperature of general industrial PP (isotacticity 90%--95%) is 165--170 ℃, the long-term use temperature can be 100--120 ℃, and the thermal deformation temperature/℃ (1.86mpa) is 56--67 ℃. PP (2401 Yanshan brand) has a melt flow rate of 2-3.5/g/10min, which is larger than PE and has good fluidity. When processing, it is easier to fill the mold. Due to the high melting temperature of PP, it is more sensitive to thermal oxidation -, so the heating time in the molding process should be shortened as far as possible. During PP injection molding, the injection temperature is 200-230 ℃, the mold temperature is 20-60 ℃, the injection pressure is 70-100 ℃, the density of PP is 0.9-0.91g/cc, the water absorption is 0.02-0.03% (during processing, there is no need to dry), and the molding shrinkage is 1.0-2.5%. In general, during the injection molding process of HDPE, the 1st reviewer also stipulates that if the plastic part is not satisfied, its temperature, pressure and holding time can be appropriately increased. On the contrary, if the plastic part has flying edges, the above parameters will be reduced accordingly. Because PP polypropylene is sensitive to temperature, pressure regulation is usually the main method

3 pet polyethylene terephthalate

① PET resin molecules contain ester groups, which make it have certain water absorption (water absorption is greater than 0.08%), and the performance of the product changes little after water absorption. However, when the particles are melted after moisture absorption, even a small amount of moisture will lead to resin degradation, molecular chain fracture and strength reduction. Therefore, it must be dried before forming. It is recommended to dry at 130 ℃ --150 ℃ for 3 to 5 hours and 150--170 ℃ for 2 hours. After drying, it should be placed in the dryer for standby to prevent water absorption again

② at the 2016 German plastics industry exhibition - k2016, pet is a crystalline polymer with an obvious melting point (258 ℃). After melting, it has good fluidity, but the processable temperature range is narrow (270-290 ℃). When the temperature exceeds 304 ℃, the resin degrades and the product blackens

③ the shrinkage of pet molding is 1.5-2%. (end)

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