(3) Polymer elasticity
Polymer melts are viscous and elastic under stress. The important role of viscous flow characteristics in molding processes has been briefly described above, and elastic properties are also of decisive importance in molding processes. Elasticity is ability of an object to resist an external force that causes deformation and return to its original shape after external force is removed. The essence of elastic deformation of polymer is bending and stretching of main chain of long-chain molecules, after stress is removed, restoration of this part of bending and stretching is necessary to overcome block of internal viscosity, and deformation is reversible. Moreover, this elastic recovery is not instantaneous, like solid objects (low-molecular or high-molecular crystals or vitreous bodies), but requires a certain period of time, which we call "relaxation time". On other hand, when a polymer melt is deformed by stress, its deformation energy is stored by melt, and after external stress is removed, deformation is restored. For example, expansion of melt during extrusion molding is so-called Barus effect. As shown in Figure 1-1, ratio of maximum diameter of extrudate after expansion to diameter of die is called expansion ratio, which largely depends on design of die. are flared, but also products of a special shape. The same applies to sheet products.
Figure 1-1 Die Expansion During Plastic Extrusion
do is internal diameter of extrusion head; df is diameter of extrudate after expansion; df/do - expansion factor
In molding process, main condition for determining whether melt is ductile or elastic deformation is: if deformation time is greater than "relaxation time", then ductile deformation is dominant. For example, when extruding rods of same material and at same temperature, if shear rate is 103 s-1 and shear stress is 3×105 Pa, then corresponding relaxation time is 2.5×10-3 s. If time for melt to pass through die is 20 s (i.e. elapsed deformation time), then it is clearly sticky
Sexual deformation predominates, and elastic deformation is an extremely small part. It should be noted that even weak elastic deformations can cause flow defects in melt.
The difference between elastic deformation in melt, shear or tension still depends on relaxation time. The specific method is to calculate shear and stretch relaxation time respectively according to process experienced by melt during molding. The one that dominates elastic deformation will be one with longer relaxation time.
(4) Polymer flow defects
Polymer melts often exhibit abnormal flow defects during molding process, ranging from haze, pitting, ripples and cracks on surface to severe deterioration in physical properties of product. These defects are related to factors such as molding process, product design, equipment, mold, resin type, etc. The main reasons are as follows.
- Slip on wall
When melt flows along pipe wall under high shear stress, layer of melt near pipe wall will be subject to intermittent flow, also known as slippage, which means that shear stress on wall is not constant along flow. path . A slippage process occurs on channel wall, first melt adheres to wall, then wall slips and, finally, melt shear flow occurs. Especially in extrusion molding process, sliding on wall at end of die is most significant, and weakening of slip is opposite to direction of melt, that is, sliding starts from end of die. which is our general extrusion process. Uneven expansion of molded article
Why. Once melt stick-slip occurs, it will also cause a zone of unstable flow, which is an important reason for formation of sharkskin on surface of product. In addition, degree of slippage is also related to type of polymer, lubricant in composition, and nature of pipe wall.
- Melting Fracture
During extrusion molding on surface of extruded product according toare coarse and periodic deformations, which often occur with an increase in extrusion speed. Ordinary helical cracks. This phenomenon is called destruction of melt. At present, reasons explaining destruction of melt are completely different. One explanation is due to “stick-slip” effect described above, another is due to elasticity of melt, and other is that shear rate or shear stress exceeds a critical value. After many years of practice, people have come to conclusion that melt failure is due to following factors: melt failure occurs only when shear stress or shear rate on pipe wall is higher than critical value, and critical value increases with length. The ratio of mold diameter and temperature A increases, and increases with a decrease in molecular weight of polymer and an increase in range of molecular weight distribution. The critical value of this shear stress is usually 105-106 Pa, with a streamlined structure, shear rate can increase by more than 10 times; melt failure is also related to mold material and has nothing to do with surface roughness; some polymers, such as high density polyethylene, do not experience melt breakdown above normal critical shear rate, so such polymers are possible at high processing speeds.
- Entry and exit effects (end effects)
When melt enters small pipe through large pipe or storage tank, pressure drop in initial section (see L in Figure 1-2) is relatively large. The reason is that as melt approaches small pipe from large pipe, it must deform to accommodate new and suitably compressible flows. Since polymer is elastic and has a certain resistance to deformation, it will consume corresponding energy, that is, it will consume a significant amount of pressure to complete deformation in this section of pipe in future. Secondly, speed of movement of each point of melt in large and small pipes is different, and a certain amount of energy (pressure drop) is spent on adjusting speed.
Figure 1-2 End effect principle
From figure (1-2) we know that when melt flows out of pipe, flow of material first contracts and then expands. This is due to fact that during flow of melt in pipe, speed of each point of section perpendicular to direction of flow of material is not same, and when it flows out of channel, its speed should be same, and diameter of material flow will decrease. Expansion after compression occurs due to elastic recovery. As a general rule, shorter pipe length, greater shear rate and greater expansion, which can vary from 30% to 100%.
3. Heating and cooling polymer
As we all know, polymers need to be heated and cooled during molding process. The difficulty of heating and cooling is determined by heat transfer rate of material, in other words, it is determined by thermal diffusivity of material.
(1) Characteristics of thermal diffusivity of polymers during molding
(1) At different temperatures, thermal diffusivity of various polymers varies slightly, usually less than 2 times;
(2) The thermal diffusivity from state of glass to state of a viscous liquid gradually decreases;
(3) Thermal diffusivity practically does not change over a wide range of temperatures in a viscous state;
(4) The thermal diffusivity of different polymers is not much different, even in viscous state, thermal conductivity rate is very small, so heating and cooling is not easy;
(5) Thermal diffusivity is proportional to polymer density.
(2) Factors affecting heating and cooling
Due to low rate of heat transfer of polymers during heating and cooling, you should pay attention to some points:
(1) The temperature difference between heating source and melt should not be too large, otherwise local temperature will be too high and polymer will degrade or decompose;
(2) The temperature difference between cooling medium and melt should not be too large, otherwise, due to too rapid cooling, internal stress will be generated on product, which will reduce bending strength, tensile strength and other characteristics. properties;
(3) Due to high viscosity of melt, it is necessary to fully utilize heat of friction that occurs during friction between molecules. This self-friction heat is useful in molding process and prevents melt from igniting;
(4) The polymer passes from state of glass to state of a viscous liquid. The crystalline type (for example, polyethylene) consumes more heat than the non-crystalline type (for example, polystyrene), i.e. more heat is required. exclude when cooling Lots of heat.