Plastic internal stress refers to an internal stress caused by the orientation of macromolecular chains and cooling contraction during plastic melting processing. The essence of the internal stress is the unbalanced conformation formed by the macromolecular chain during the melting process, the imbalanced conformation can not immediately return to the equilibrium conformation suitable for environmental conditions when cooling and solidifying, the essence of this unbalanced conformation is a reversible high elastic deformation, and the frozen high elastic deformation is usually stored in the form of potential energy in plastic products, under suitable conditions, this forced unstable conformation will be transformed into a free stable conformation, potential energy into kinetic energy and released. When the force and mutual entanglement force between the macromolecular chains cannot withstand this kinetic energy, the internal stress balance is destroyed, and the plastic products will produce stress cracking and warpage deformation.
Almost all plastic products will have internal stress to varying degrees, especially the internal stress of plastic injection products is more obvious. The existence of internal stress not only causes warpage deformation and cracking of plastic products during storage and use, but also affects the mechanical properties, optical properties, electrical properties and appearance quality of plastic products. To this end, it is necessary to find out the causes of internal stress and the method of eliminating internal stress, minimize the internal stress of plastic products, and make the residual internal stress distributed as evenly as possible on plastic products to avoid stress concentration phenomenon, so as to improve the mechanical 1 thermal and other properties of plastic products.
The cause of stress in plastics
There are many reasons for internal stress, such as the plastic melt is subjected to strong shear during processing, the orientation and crystallization in processing, the cooling rate of each part of the melt is extremely difficult to be uniform, the melt plasticization is uneven, and the product is difficult to demold, etc., which will cause internal stress. According to the different causes of internal stress, internal stress can be divided into the following categories.
(1) Orientation internal stress
Oriented internal stress is an internal stress generated by the freezing of the directional conformation of the macromolecular chain arranged along the flow direction during the flow filling and pressure holding feeding process of the plastic melt. The specific process of orientation stress generation is: * The melt near the runner wall increases the viscosity of the outer melt due to the fast cooling rate, and the flow velocity of the melt in the center layer of the cavity is much higher than the flow rate of the surface layer, resulting in shear stress between the inner layers of the melt, resulting in orientation along the flow direction. The freezing of the oriented macromolecular chain in the plastic product means that there is an unrelaxed reversible high elastic deformation, so the orientation stress is the internal force of the macromolecular chain from the orientation conformation force to the non-oriented conformation. By heat treatment, the orientation stress in plastic products can be reduced or eliminated.
The orientation internal stress distribution of plastic products is smaller and smaller from the surface layer to the inner layer of the product, and it changes in a parabolic pattern.
(2) Cooling internal stress
Cooling internal stress is a kind of internal stress caused by uneven shrinkage during cooling and shaping of plastic products during melting processing. Especially for thick-walled plastic products, the outer layer of plastic products first cools and solidifies and shrinks, and its inner layer may still be hot melt, which will limit the shrinkage of the surface layer, resulting in the core layer in a compressive stress state, and the surface layer in a tensile stress state.
The distribution of cooling internal stress of plastic products is larger and larger from the surface layer to the inner layer of the product, and also shows a parabolic change.
In addition, plastic products with metal inserts, due to the large difference in the thermal expansion coefficient of metal and plastic, are prone to form internal stresses with uniform shrinkage. In addition to the above two main internal stresses, there are also the following internal stresses: for crystalline plastic products, the crystalline structure and crystallinity of each part of the product are different. There is also a configuration within the application. Force and demolding internal stress, etc., but the proportion of internal stress is very small.
Factors that affect the generation of stress in plastics
(1) The rigidity of the molecular chain
The greater the rigidity of the molecular chain, the higher the viscosity of the melt, the poor activity of the polymer molecular chain, so the recovery of the reversible high elastic deformation is poor, and the residual internal stress port is easy to occur, for example, some polymers containing benzene rings in the molecular chain, such as PC, PPO, PPS, etc., the internal stress of the corresponding products is large.
(2) The polarity of the molecular chain
The greater the polarity of a molecular chain, the greater the mutual attraction force between molecules, which increases the difficulty of moving between molecules and reduces the degree of reversible elastic deformation, resulting in large residual internal stress. For example, some plastic varieties containing polar groups such as carbonyl groups, ester groups, and eye groups in the molecular chain have greater internal stress in their corresponding products.
(3) steric hindrance effect of substituent groups
The larger the volume of the macromolecular side group substituents, the hinder the free movement of the macromolecular chain, resulting in an increase in residual internal stress. For example, the volume of the phenyl group of the polystyrene substituent group is large, so the internal stress of the polystyrene product is large.
The order of internal stress magnitude for several common polymers is as follows:
PPO>PSF>PC>ABS>PA6>PP>HDPE
Reduction and dispersion of internal stress in plastics
(1) Raw material formula design
1) Select resins with large molecular weight and narrow molecular weight distribution
The larger the molecular weight of the polymer, the increase of the interchain force and entanglement degree of the macromolecule, and the stronger the stress cracking resistance of the product. The wider the molecular weight distribution of the polymer, the larger the low molecular weight component, and it is easy to form microscopic tears first, resulting in stress concentration and cracking of the product.
2) Select resin with low impurity content
The impurities in the polymer are the concentration of stress, which will reduce the original strength of the plastic, and the impurity content should be reduced to a minimum.
3) Blending modification
Resin prone to stress cracking can be blended with other suitable resins to reduce the presence of internal stress.
For example, if an appropriate amount of PS is mixed into the PC, the PS is dispersed in the PC continuous phase in the form of an approximate bead, which can make the internal stress dispersed along the spherical surface alleviate and prevent the crack propagation, so as to achieve the purpose of reducing the internal stress. For another example, mixed with an appropriate amount of PE in PC, the outer edge of PE chondrites can form a closed cavitation zone, and the internal stress can also be appropriately reduced.
4) Enhanced modification
Reinforcement modification with reinforced fibers can reduce the internal stress of the product, because the fiber is entangled with many macromolecular chains, thereby improving the stress cracking ability. For example, 30% GFPC is up to 6 times more resistant to stress cracking than pure PC.
5) Nucleation modification
The addition of suitable nucleating agents to crystalline plastics can form many small spherical crystals in its products, so that the internal stress is reduced and dispersed.
(2) Control of molding processing conditions
In the molding process of plastic products, all molding factors that can reduce the orientation of polymer molecules in the products can reduce the orientation stress; All the process conditions that can make the polymer in the product cool uniformly can reduce the cooling internal stress; Any processing method that helps to demold plastic products is conducive to reducing the internal stress of demolding.
There are mainly the following processing conditions that have a great influence on internal stress.
(1) Barrel temperature
Higher barrel temperature is conducive to the reduction of orientation stress, because at higher barrel temperature, melt plasticization is uniform, viscosity decreases, fluidity increases, and in the process of melt filling cavity, molecular orientation action is small, so the orientation stress is small. However, at lower barrel temperature, the melt viscosity is higher, there are more molecular orientations during the mold filling process, and the residual internal stress is larger after cooling and setting. However, the temperature of the barrel is too high is not good, too high is easy to cause insufficient cooling, easy to cause deformation during demolding, although the orientation stress decreases, but the cooling stress and demolding stress increase.
(2) Mold temperature
The temperature of the mold has a great influence on the orientation internal stress and the cooling internal stress. On the one hand, the mold temperature is too low, which will cause the cooling to accelerate, which is easy to make the cooling uneven and cause a large difference in shrinkage, thereby increasing the cooling internal stress; On the other hand, the mold temperature is too low, the melt enters the mold, the temperature drops faster, and the melt viscosity increases rapidly, resulting in filling the mold at high viscosity, and the degree of orientation stress is significantly increased.
The higher the mold temperature, the more conducive to the tight stacking of grains, the reduction or elimination of defects inside the crystal, thereby reducing the internal stress.
In addition, for plastic products of different thicknesses, the mold temperature requirements are different. For thick-walled products, the mold temperature should be appropriately higher.
Taking PC as an example, the relationship between the internal stress and the mold temperature is shown in Table 5-5.
(3) Injection pressure
The injection pressure is high, the shear force during the melt filling process is large, and the chance of orientation stress is also greater. Therefore, in order to reduce the orientation stress and eliminate the release stress, the injection pressure should be appropriately reduced.
Taking PC as an example, the relationship between the internal stress and the injection pressure is shown in Table 5-6.
(4) Holding pressure pressure
The influence of holding pressure pressure on the internal stress of plastic products is greater than the influence of injection pressure. In the holding stage, with the decrease of melt temperature, the viscosity of the melt increases rapidly, and if high pressure is applied, it will inevitably lead to the forced orientation of the molecular chain, thereby forming a greater orientation stress.
(5) Injection speed
The faster the injection speed, the easier it is to cause an increase in the orientation of the molecular chain, resulting in greater orientation stress. However, the injection speed is too low, and after the plastic melt enters the mold cavity, it may be layered successively to form melting marks, resulting in stress concentration lines, which is easy to produce stress cracking. Therefore, the injection speed should be moderate. It is best to use variable speed injection to end the filling with a gradual decrease in speed.
(6) Holding time
The longer the holding time, the shear effect of the plastic melt increases, resulting in greater elastic deformation and freezing more orientation stress. Therefore, the orientation stress increases significantly with the increase of holding time and the amount of feeding.
(7) Mold opening residual pressure
The injection pressure and holding time should be adjusted appropriately so that the residual pressure in the mold is close to atmospheric pressure when the mold is opened, so as to avoid greater internal stress of mold release.
(3) Heat treatment of plastic products
Heat treatment of plastic products refers to the method of staying molded products at a certain temperature for a certain period of time to eliminate internal stress. Heat treatment is the best way to eliminate orientation stress in plastic products.
For injection molded parts with large rigidity and high glass transition temperature of polymer molecular chains; For parts with large wall thicknesses and metal inserts; Parts with wide operating temperature range and high dimensional accuracy requirements; Parts with large internal stress and not easy to self-eliminate and machining parts must be heat treated.
Heat treatment of parts can transform polymer molecules from unbalanced conformation to equilibrium conformation, so that the forcible freezing in unstable elastic deformation obtains energy and carries out thermal relaxation, thereby reducing or basically eliminating internal stress. The heat treatment temperature is often used 10~20 °C higher than the use temperature of the part or 5~10 °C lower than the heat deflection temperature. The heat treatment time depends on the type of plastic, the thickness of the part, the heat treatment temperature and the injection molding conditions. General thickness of parts, heat treatment 1~2 hours can be, with the increase of the thickness of the parts, heat treatment time should be appropriately extended. Increasing the heat treatment temperature and extending the heat treatment time have similar effects, but the effect of temperature is more pronounced.
The heat treatment method is to put the parts into liquid media such as water, glycerin, mineral oil, ethylene glycol and liquid paraffin, or heat them to a specified temperature in an air circulation oven, stay at this temperature for a certain period of time, and then slowly cool to room temperature. Experiments show that heat treatment is carried out immediately after demolding, which has a more obvious effect on reducing internal stress and improving the performance of parts. In addition, increasing the mold temperature, extending the cooling time of the parts in the mold, and performing thermal insulation treatment after demolding have similar effects to heat treatment.
Although heat treatment is one of the effective ways to reduce the internal stress of the part, heat treatment can usually only reduce the internal stress to the range allowed by the use conditions of the part, and it is difficult to completely eliminate the internal stress. When the PC parts are heat treated for a long time, the PC molecular chain may be orderly rearranged, or even crystallified, thereby reducing the impact toughness and reducing the notch impact strength. Therefore, heat treatment should not be regarded as the only measure to reduce the internal stress of the part.
(4) Design of plastic products
(1) The shape and size of plastic products
In the specific design of plastic products, in order to effectively disperse internal stress, the principle should be followed: the shape of the product should be as continuous as possible, avoiding sharp angles, right angles, notches and sudden expansion or contraction.
For the edge of plastic products, the corner should be designed as rounded corners, in which the radius of the inner fillet should be greater than 70% of the thickness of the thin in the adjacent two walls; The radius of the outer fillet is determined according to the shape of the product.
For parts with large differences in wall thickness, cooling internal stress and orientation internal stress are easy to occur due to different cooling rates. Therefore, parts should be designed with as uniform a wall thickness as possible, and if uneven wall thickness is necessary, a gradual transition of wall thickness difference should be carried out.
(2) Reasonable design of metal inserts
The thermal expansion coefficient of plastic and metal is 5~10 times different, so when the plastic products with metal inserts are cooled, the degree of shrinkage formed by the two is different, because the shrinkage of the plastic is relatively large and tightly hold the metal insert, and the inner layer of the plastic around the insert is subjected to compressive stress, and the outer layer is subjected to tensile stress, resulting in stress concentration phenomenon.
When setting up juice inserts, the following points should be paid attention to to help reduce or eliminate internal stress.
a. Choose plastic parts as inserts as much as possible.
b. As far as possible, choose metal materials with small differences in the thermal expansion coefficient of plastics as insert materials, such as aluminum, aluminum alloy and copper.
c. Apply a layer of rubber or polyurethane elastic buffer layer on the metal insert, and ensure that the coating layer does not melt during molding, which can reduce the shrinkage difference between the two.
d. Surface degreasing treatment of metal inserts can prevent stress cracking of grease-accelerated products.
e. Metal inserts undergo appropriate preheat treatment.
f. The thickness of the plastic around the metal insert should be sufficient. For example, if the outer diameter of the insert is D and the plastic thickness around the insert is h, the plastic thickness is ≥ 0.8D for aluminum inserts; for copper inserts, the plastic thickness is h≥0.9 D.
g. Metal inserts should be designed in a sleek shape, preferably with a delicate knurling pattern.
(3) The design of the upper hole of plastic products
The shape, number of holes and position of holes on plastic products will have a great influence on the degree of internal stress concentration.
In order to avoid stress cracking, do not open prismatic, rectangular, square or polygonal holes in plastic products. Circular holes should be opened as much as possible, of which the oval hole has the best effect, and the long axis of the oval hole should be parallel to the direction of external force. If a round hole is opened, a process round hole of equal diameter can be added, and the center connection line of the adjacent two round holes is parallel to the direction of external force, so that it can be
to achieve a similar effect to an elliptical hole; Another method is to create symmetrical slots around the round holes to disperse the internal stresses.
(5) Design of plastic molds
When designing plastic molds, the pouring system and cooling system have a greater impact on the internal stress of plastic products, and the following points should be paid attention to in the specific design.
