Basic knowledge of plastic injection molding process (1)

Basic knowledge of plastic injection molding process (1)

Text/Meite Polymer zhongming

First, physical state of polymer

Polymers can be classified into two types according to their crystallization properties, one with a tendency to crystallize and other without a tendency to crystallize. A polymer with a tendency to crystallize, as a rule, is not a 100% crystal, but a system in which crystalline and non-crystalline regions coexist. Whether it is a polymer that cannot crystallize, a polymer that can crystallize without crystallization, or an amorphous part of a crystallized polymer, there are four different physical aggregate states, namely three amorphous states (glassy state, highly elastic and viscous state) and a crystalline state. For amorphous polymers, under action of a certain external force, three physical states of polymer appear in different temperature ranges. At low temperature (or normal temperature), force between molecules is greater, and except for a few chains with some freedom, movement of long chain molecules is basically frozen, and entire polymer has a certain rigidity and hardness. but not brittle, but presents a glassy state at this time, as temperature rises, thermal kinetic energy continues to increase, and volume of polymer increases to a certain extent. Although entire macromolecular chain cannot move, chain segments in molecule have enough space for movement and displacement, therefore shape of molecules can straighten or twist, but their general arrangement in molecular structure remains unchanged, demonstrating a soft and elastic highly elastic state; if temperature continues to rise until entire macromolecular chain can move, it will begin to flow plastically and become a viscous state. The characteristics of each state are mainly manifested by morphological capacity (elongation or contraction). As shown in fig. 1-1.

Basic knowledge of plastic injection molding process (1)

Figure 1-1. Diagram of three states of thermoplastics at constant pressure

1 - amorphous plastic, 2 - crystalline plastic

Figure 1-1 shows that:

Tg is temperature at which a material changes from a glassy state to a highly elastic state, known as glass transition temperature;

Tf is temperature at which material changes from a highly elastic state to a viscous state, called viscous temperature;

When temperature rises to Td, polymer begins to decompose, and temperature at this time is called decomposition temperature, at a temperature below Tx, macromolecular chain is broken by a small external force, and this temperature is called embrittlement temperature. At a brittle temperature, polymer is in a brittle state, and material loses its consumer value. The change in three states of polymer is reversible: when temperature drops to Tf, polymer passes from viscous-liquid state to highly elastic state; when temperature drops to normal temperature, plastic goes into a glassy state. . In plastic injection molding, it is possible to obtain geometric shape and dimensional accuracy of required products, which consists in using characteristics of three-phase change of polymer at different temperatures.

Tg~Tf have high elasticity. It has two characteristics: (1) It can produce large deformation under small external force. The initial strain increases with increasing temperature and becomes constant after a certain limit. After external force is removed, it can return to its original shape. This kind of deformation is called high-elastic deformation. (2) High-elastic deformation does not occur instantly, but develops gradually over time. That is, highly elastic deformation is not like ordinary elastic deformation (such as a wire spring), which can return to its original shape immediately after external force is removed, but it will take some time to do so. Tf~Td – viscous state. In this temperature range, injection molding of plastics is carried out. The wider temperature range, easier molding process and less likely plastic will break. The narrower Tf~Td range, more difficult its formation and processing. In accordance with nature in which plastics exist in above three states with different heating temperatures, if plastics are to be processed into products, various processing methods suitable for them are required. For example, vitreous plastics can be processed by turning, milling, drilling, sawing, planing, etc.; highly elastic plastics - hot stamping, flexible, vacuum forming, etc.; viscous plastics - by extrusion, injection molding, calendering, etc. other processing methods. Strictly speaking, ternary transition temperature of a polymer is notis completely fixed. The transition of a polymer into three states is associated not only with temperature, but also with time of exposure and rate of application of force. Therefore, there are no strict temperature transition points Tg, Tf, and Td, but only temperature range dominated by one strain to another strain.

At same time, with an increase in molecular weight of a homologous polymer, region of transition from state of glass to state of a viscous liquid shifts to region of high temperatures. In addition, in polymers with rigid molecules, temperature range of highly elastic state is narrow.

From Figure 1-1 it can also be seen that changes in three states of amorphous polymers and crystalline polymers are significantly different. The former have obvious changes in three states, while highly elastic state of crystalline polymers is not obvious, at a temperature above melting point, it quickly melts and is in a viscous state. In addition, crystalline polymers are little deformed below melting temperature. That is, heat resistance is improved compared to amorphous polymers. For example, Tg of non-crystalline polystyrene obtained by general polymerization is 80°C, and melting point of crystalline polystyrene obtained by direction polymerization reaches 230-240°C, and its application area is much wider. It can be seen that amorphous plastics can only be used between embrittlement temperature and glass transition temperature, while crystalline plastics can be used between embrittlement temperature and melting temperature.

Secondly, rheology of polymer melt

The above briefly discussed three physical states of amorphous polymers at different temperatures, temperature range of molding process, and choice of appropriate molding method, but knowing this is not enough. We know that plastic injection molded products are often sold in a viscous state, because polymer in this state not only flows easily, but also easily deforms, which makes it very convenient to transport and shape. To understand rheological properties of plastics, people have done deep and painstaking research work and achieved certain results. The so-called rheology is a subject that studies properties of flow and deformation of materials under influence of stress, deformation and time. In plastic injection, results of rheology are decisive for processes, equipment and molds.

(1) Non-Newtonian flow

In plastic injection, you can use not only viscous polymers (polymer melts), but also polymer dispersions (pastes or solutions), which all belong to category of liquids. There are two forms of fluid flow: laminar and turbulent. Many years of practice have shown that flow of melts and polymer dispersions during molding is predominantly laminar. The flow and deformation of liquid are realized in conditionvoltage fluctuations. There are three main stresses: shear, tension and compression. Among them, shear stress is most important for plastic injection. According to behavior of liquids, liquids can be divided into two types: Newtonian liquids and non-Newtonian liquids, and non-Newtonian liquids in liquids Flows are very common, and There are generally three types of fluids that fall into this category: Bigna fluids, pseudoplastic fluids, and dilatant fluids.

Basic knowledge of plastic injection molding process (1)

Figure 1-2 Logarithmic coordinate chart

1 - expanding material, 2 - Newtonian material, 3 - pseudoplastic material

As shown in Figure 1-2, when slope is less than 1, it is a pseudoplastic material, when slope is greater than 1, it is an expanding material, when slope is 1, flow curve is at 45 degrees, which is a Newtonian material. The above exponential equation for behavior of a non-Newtonian fluid is an important content of rheology and is widely used in engineering.

(2) Factors affecting polymer flow

From above analysis, it can be concluded that viscosity is a measure of flowability of a polymer, which determines ease of molding. When viscosity is low, not only is operation more convenient, but performance can be increased and power can be reduced at same time. Therefore, it is always desirable during molding process to try to reduce viscosity of polymer without compromising quality of product. During molding process, shear stress, shear rate, temperature and pressure have greatest influence on viscosity of polymer.

1. Effect of Shear Rate on Polymer Viscosity

The effect of shear rate (shear stress) on polymer viscosity has been presented previously. It should be emphasized here that increasing shear stress or shear rate will reduce apparent viscosity of polymer, so that performance of Flow is greatly improved. This kind of plastic is called shear-sensitive polymer. During extrusion molding, its fluidity is mainly changed by changing speed of rotation of screw. Plastics such as polyoxymethylene and polyethylene fall into this category. In contrast, viscosity of some polymers is independent of shear rate. Increasing shear rate cannot effectively increase fluidity, but is temperature sensitive. If temperature is not enough during extrusion, screw speed will be increased blindly. which will lead to accidents with screw breakage and machine damage. This category includes plastics such as nylon, plexiglass, and polycarbonate.

2. Effect of Temperature on Polymer Viscosity

As temperature rises, mobility of chain segments will increase and interaction force between molecules will weaken, so fluidity of polymer will increase and apparent viscosity will decrease (see Fig. 1-3). Very clearly. On other hand, temperature dependence of apparent viscosity of different polymers is not same. On fig. 1-3 for polycarbonate, cellulose acetate and polymethyl methacrylate, apparent viscosity can decrease by an order of magnitude with a temperature increase of about 50°C, that is, flowability of this polymer is affected. Its sensitivity is extremely high, and its flowability can be significantly increased by slightly changing temperature during processing and molding. Generally speaking, greater rigidity of molecular chains of a polymer and greater force of attraction between moleculespolar chains, greater sensitivity of its apparent viscosity to temperature. For polyethylene and polyoxymethylene, situation is completely different: even with a temperature increase of 100°C, apparent viscosity does not drop by an order of magnitude. This type of polymer has a very low sensitivity to temperature, for example, its fluidity is increased only by increasing temperature, and range of temperature increase is in between.

Basic knowledge of plastic injection molding process (1)

Fig. 1-3 Relationship between melt viscosity and temperature of some plastics

1 - polycarbonate (4 MPa), 2 - polyethylene (4 MPa), 3 - polyoxymethylene,

4 - polymethyl methacrylate, 5 - cellulose acetate, (4 MPa), 6 - nylon (1 MPa)

3. Effect of pressure on polymer viscosity

Because viscosity depends on force between molecules, when a liquid is under pressure, distance between molecules is reduced, so viscosity of liquid increases accordingly. The compressibility of low molecular weight liquids is very limited, which cannot be said of high molecular weight melts, and molding pressure of polymer melt is very high. For example, extrusion molding pressure is generally 10-50MPa, injection molding pressure reaches 30-300MPa, so their compressibility is significant. It is known from experiments that when a polymer melt is under pressure, its viscosity will increase, and in some cases apparent viscosity may increase by an order of magnitude. Sometimes it happens that same polymer can be molded at normal pressure, but when pressure is increased, it is not easy to mold it or productivity drops, and even plastic becomes immobile, like a solid body, and of course it is impossible to mold it. . It should also be noted that flow characteristics of same polymer melt at same pressure will be different if size of equipment used for molding is different, because shear stress can still be different despite same pressure. . The influence of pressure on viscosity of polymer dispersion is basically same as that of polymer melt, but since a portion of dispersion has a low molecular weight, degree of influence is relatively small. In addition, dispersion is not under high pressure during molding, so this effect is rarely taken into account.

4. Influence of molecular weight on polymer viscosity

Polymer flow is result of relative displacement between polymer chains and polymer chains, so molecular weight has an obvious effect on polymer flow. As molecular weight increases, fluidity decreases. This is because larger molecular weight, less likely relative displacement movement will be. From a shaping point of view, it is necessary to properly reduce molecular weight without affecting basic properties of product. Because molecular weight is too high, molding temperature will be higher, which is not good for production. In addition, different uses and different processing methods have different molecular weight requirements. For example, for extrusion of rigid thin-walled PVC profiles, SG6 resin with a lower molecular weight and an average polymerization degree of 730-870 is used, and for extrusion of soft PVC profiles with a higher molecular weight and an average polymerization degree of 1100.-1240, SG3 type resin is suitable. Generally speaking, casting process under yesThe process requires a relatively low polymer molecular weight, and extrusion molding process requires a relatively high polymer molecular weight.

5. Effect of Additives to Plastics on Polymer Viscosity

In order to improve processability and performance, some polymers are added with a small amount of plastic additives during molding. For example, addition of a plasticizer can weaken intermolecular strength of polymer, which is beneficial for movement of chain segment, glass transition temperature is reduced, and fluidity is increased, which is beneficial to molding process. For those polymers in which viscous flow temperature and decomposition temperature are very close, it is necessary to add a certain amount of stabilizer during molding process, which also has a certain effect on viscosity, and degree of influence of different stabilizers is different. In order to reduce cost of plastics, improve light resistance, thermal insulation and other properties, a certain amount of filler is added to them, which does not affect quality of product, which will reduce fluidity of polymer.

6. Polymer viscosity changes over time

When applied shear stress remains constant, apparent viscosity of polymer at constant temperature will gradually increase or decrease depending on duration of applied stress, and stop after reaching a certain value. The mechanism explaining this behavior of flow cannot be analyzed quantitatively due to lack of theoretical studies. In absence of stress, rheology should not change with time after completion of polymer melting process. But in fact, melts of many polymers gradually change over time, and viscosity of polymer melts gradually decreases over time due to thermal decomposition. In actual use, on one hand, a polymer with a higher molecular weight should be selected in order to improve overall physical properties of product, on other hand, highest viscosity limit should be taken into account, and molding above this limit will be difficult. To reduce viscosity, thermal decomposition at viscous temperature should not be carried out to extent that this will affect quality of resulting product.

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