Detailed analysis and solution of internal stress in injection parts
The so-called stress refers to the force of the object in the unit area, which emphasizes the internal stress of the object; General objects under the action of external forces, its internal resistance to external forces will produce stress; In the case of the object without external force, the internal inherent stress is called internal stress, which is caused by the uneven plastic deformation of various parts of the object.
According to the range of internal stress, it can be divided into three categories:
(1) the first type of internal stress (macroscopic internal stress), that is, the internal stress in the macroscopic range caused by the uneven deformation of various parts of the material;
(2) the second type of internal stress (microscopic internal stress), that is, the internal stress between the grains or subgrains of the object (in nature, the vast majority of solid substances are crystals) caused by uneven deformation between the grains or subgrains;
(3) The third type of internal stress (lattice distortion stress), that is, due to lattice distortion, the internal stress caused by a part of the atom in the crystal deviates from its equilibrium position, which is the most important internal stress in the deformed object (destroyed object).
Plastic internal stress refers to a kind of internal stress caused by the orientation of macromolecular chains and cooling shrinkage during plastic melting processing.
The essence of the internal stress is the unbalanced conformation of the macromolecular chain formed in the melting process, this unbalanced conformation can not be immediately restored to the equilibrium conformation compatible with environmental conditions when cooling and curing, 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 plastic products, under appropriate conditions, This forced unstable conformation will be transformed into a free stable conformation, and potential energy will be transformed into kinetic energy and released.
When the forces and interlocking forces between large molecular chains cannot withstand this kinetic energy, the internal stress balance is destroyed, and plastic products will produce stress cracking and warping deformation.
Almost all plastic products will have varying degrees of internal stress, especially the internal stress of plastic injection products is more obvious. The existence of internal stress not only causes the warping 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 cause of internal stress and the way to eliminate internal stress, minimize the internal stress of plastic products, and make the residual internal stress on the plastic products as evenly distributed as possible, to avoid the phenomenon of stress concentration, so as to improve the mechanical thermal properties of plastic products.
Causes of plastic internal stress
There are many reasons for the internal stress, such as the plastic melt is subjected to strong shearing during processing, the orientation and crystallization in processing, the cooling rate of each part of the melt is extremely difficult to achieve uniformity, the melt plasticization is not uniform, and the product is difficult to release, which will lead to the generation of internal stress. According to the different causes of internal stress, internal stress can be divided into the following categories.
(1) Orientation internal stress
Orientation internal stress is a kind of internal stress caused by the freezing of the orientation conformation of macromolecular chains along the flow direction in the process of mold filling and pressure holding. The specific process of orientation stress generation is as follows: * The melt near the flow channel wall causes the viscosity of the outer melt to increase due to the fast cooling speed, so that the flow velocity of the melt in the center layer of the cavity is much higher than that of the surface layer, resulting in the shear stress between the internal layers of the melt and the orientation along the flow direction.
Oriented macromolecular chains frozen in plastic products also means that there is an unrelaxed reversible high elastic deformation, so the orientation stress is the internal force of the macromolecular chain from the oriented conformation to the non-oriented conformation. The orientation stress in plastic products can be reduced or eliminated by heat treatment.
The orientation internal stress distribution of plastic products is smaller and smaller from the surface to the inner layer of the product, and shows a parabolic change.
(2) Cooling internal stress
The cooling internal stress is a kind of internal stress caused by the non-uniform shrinkage of the plastic products during the melting process. Especially for thick-walled plastic products, the outer layer of plastic products first cools and solidifies, and the inner layer may still be a hot melt, so that the core layer 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 the internal cooling stress of plastic products is increasing from the surface to the inner layer of the product, and it 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, easy to form an uneven shrinkage of the internal stress.
In addition to the above two main kinds of internal stress, there are the following kinds of internal stress: For crystalline plastic products, the crystallization structure and crystallinity of various parts of the product will also produce internal stress. There are also configurational endogeny. The force and the internal stress of demoulding, but the proportion of the internal stress is very small.
Factors that affect the production of plastic internal stress:
(1) Rigidity of molecular chain
The greater the rigidity of the molecular chain, the higher the viscosity of the melt, and the poor activity of the polymer molecular chain, so for the reversible high elastic deformation recovery is poor, easy to produce residual internal stress, for example, some polymers containing benzene rings in the molecular chain, such as PC, PPO, PPS, etc., the corresponding product of the internal stress is larger.
(2) Polarity of the molecular chain
The greater the polarity of a molecular chain, the greater the attraction force between molecules, which increases the difficulty of moving between molecules, decreases the degree of restoring reversible elastic deformation, and leads to large residual internal stress. For example, some plastic varieties containing carbonyl, ester, cyanide and other polar groups in the molecular chain have large internal stresses in the corresponding products.
(3) steric effect of substituting groups
The larger the volume of the macromolecular side group substituents, the obstruction of the free movement of the macromolecular chain leads to the increase of residual internal stress. For example, the phenyl group of the polystyrene substitution group is larger, so the internal stress of the polystyrene product is larger.
The order of the internal stresses of several common polymers is as follows:
PPO>PSF>PC>ABS>PA6>PP>HDPE
Reduction and dispersion of plastic internal stress:
(1) Raw material formula design
1. Select trees with large molecular weight and narrow molecular weight distribution
The greater the molecular weight of the polymer, the greater the interchain force and the degree of entanglement, 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, which is easy to form microscopic tearing first, resulting in stress concentration and cracking of the product.
2. Select a resin with low impurity content
The impurities in the polymer are the concentration of stress, and will reduce the original strength of the plastic, and the impurity content should be reduced to a minimum.
3. Blending modification
The resin prone to stress cracking can be blended with other suitable resins to reduce the degree of internal stress.
For example, the PC mixed with an appropriate amount of PS, PS is approximately granular dispersed in the PC continuous phase, the internal stress can be dispersed along the sphere to alleviate and prevent crack growth, so as to achieve the purpose of reducing the internal stress. For example, in the PC mixed with an appropriate amount of PE, PE spheroidal epitaxy can form a closed cavitation area, and can also appropriately reduce the internal stress.
4. Enhancement modification
Reinforcement modification with reinforced fiber can reduce the internal stress of the product, because the fiber entangles many large molecular chains, thereby increasing the stress cracking ability. For example, 30%GFPC is six times more resistant to stress cracking than pure PC.
5. Nucleation modification
By adding appropriate nucleating agents to crystalline plastics, many small spherulites can be formed in their products, so that the internal stress is reduced and dispersed.
(2) Control of molding processing conditions
In the forming process of plastic products, all the forming factors that can reduce the orientation of polymer molecules in the product can reduce the orientation stress; All the process conditions that can make the polymer uniformly cool in the product can reduce the internal cooling stress; All processing methods that are conducive to the demoulding of plastic products are conducive to reducing the internal stress of the demoulding.
The processing conditions that have a greater impact on the internal stress are mainly as follows:
① cylinder temperature
A higher barrel temperature is conducive to the reduction of orientation stress, because at a higher barrel temperature, the melt plasticizes uniformly, the viscosity decreases, and the fluidity increases. In the process of the melt filling the cavity, the molecular orientation effect is small, so the orientation stress is small. At lower barrel temperature, the melt viscosity is higher, the molecular orientation is more during mold filling, and the residual internal stress after cooling is larger.
However, the cylinder temperature is too high is not good, too high easy to cause insufficient cooling, easy to cause deformation when demoulding, although the orientation stress decreases, but the cooling stress and demoulding stress increase.
② Mold temperature
The mold temperature has a great influence on the internal stress of orientation and cooling. On the one hand, the mold temperature is too low, will cause cooling speed, 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, after the melt enters the mold, the temperature drops faster, and the melt viscosity increases rapidly, resulting in the mold filling under high viscosity, and the degree of orientation stress is significantly increased.
Mold temperature has a great influence on plastic crystallization, the higher the mold temperature, the more conducive to the compact grain stacking, the reduction or elimination of defects in the crystal, thereby reducing the internal stress.
In addition, for plastic products of different thickness, the mold temperature requirements are different. For thick wall products, the mold temperature should be appropriately high.
③ Injection pressure
The injection pressure is high, the shear force in the melt filling process is large, and the chance of orientation stress is also large. Therefore, in order to reduce the orientation stress and eliminate the release stress, the injection pressure should be properly reduced. .
④ Maintain pressure
The influence of holding pressure on the internal stress of plastic products is greater than that of injection pressure. In the pressure holding stage, with the decrease of melt temperature, melt viscosity increases rapidly. At this time, if high pressure is applied, it will inevitably lead to forced orientation of molecular chains, thus forming greater orientation stress.
⑤ Injection speed
The faster the injection speed, the easier it is to increase the orientation degree of the molecular chain, resulting in greater orientation stress. However, the injection speed is too low, the plastic melt into the mold cavity, may stratify and form a melting mark, produce a stress concentration line, easy to produce stress cracking. Therefore, the injection speed should be moderate. It is best to use variable speed injection and finish filling the mold at a gradually decreasing speed.
⑥ Pressure holding time
The longer the holding time, the greater the shear effect of the plastic melt, resulting in greater elastic deformation, freezing more orientation stress. Therefore, the orientation stress increases significantly with the extension of pressure holding time and the increase of feeding amount.
⑦ Mold residual pressure
The injection pressure and the holding time should be properly adjusted so that the residual pressure in the mold is close to the atmospheric pressure when the mold is opened, so as to avoid greater internal stress in the mold.
(3) Heat treatment of plastic products
Heat treatment of plastic products refers to the method of eliminating internal stress by staying the molded product at a certain temperature for a period of time. Heat treatment is the best way to eliminate the orientation stress in plastic products.
For high polymer molecular chain rigidity, high glass transition temperature injection parts; Parts with large wall thickness and metal inserts; Parts with a wide temperature range and high dimensional accuracy requirements; The parts with large internal stress and not easy to self-dissipate and the parts that have been machined must be heat treated.
Heat treatment can make the polymer molecules change from the unbalanced conformation to the equilibrium conformation, so that the energy obtained by the forced freezing in the unstable high elastic deformation can be thermal relaxation, thus reducing or basically eliminating the internal stress. The heat treatment temperature commonly used is 10~20℃ higher than the use temperature of the parts or 5~10℃ lower than the thermal deformation 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. The general thickness of the parts, heat treatment 1 to 2 hours can be, with the increase of the thickness of the parts, the heat treatment time should be appropriately extended. Increasing heat treatment temperature and extending heat treatment time have similar effects, but the effect of temperature is more obvious.
The heat treatment method is to put the parts into the liquid medium such as water, glycerin, mineral oil, ethylene glycol and liquid paraffin, or into the air circulation oven to heat to the specified temperature, and stay at the temperature for a certain time, and then slowly cool to room temperature. The experiment shows that heat treatment immediately after demoulding has more obvious effect on reducing the internal stress and improving the performance of the parts. In addition, increasing the mold temperature, extending the cooling time of the parts in the mold, and thermal insulation treatment after demoulding have similar effects to heat treatment.
Although heat treatment is one of the effective ways to reduce the internal stress of the parts, heat treatment can usually only reduce the internal stress to the range allowed by the conditions of use of the parts, 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 and even crystallized, thereby reducing the impact toughness and the notched impact strength. Therefore, heat treatment should not be used as the only measure to reduce the internal stress of the parts.
(4) Design of plastic products
① Shape and size of plastic products
In the specific design of plastic products, in order to effectively disperse the internal stress, the principle should be followed: the shape of the product should be as continuous as possible to avoid acute angles, right angles, gaps and sudden expansion or contraction.
The edges of plastic products should be designed into rounded corners, of which the inner rounded radius should be greater than 70% of the thickness of the thin in the adjacent two walls; The radius of the outer corner is determined according to the shape of the product.
For the parts with large difference in wall thickness, the internal stress of cooling and the internal stress of orientation are easy to occur because of the different cooling rates. Therefore, the wall thickness should be designed to be as uniform as possible, if the wall thickness must be uneven, then the gradual transition of the wall thickness difference should be carried out.
② Reasonable design of metal inserts
The thermal expansion coefficient of plastic and metal is 5 to 10 times different, so the plastic products with metal inserts are cooled, the contraction degree of the two is different, because the contraction of the plastic is relatively large and tightly cling to the metal inserts, the inner layer of the plastic around the inserts is stressed, and the outer layer is subjected to tensile stress, resulting in stress concentration.
In the specific design of inserts, the following points should be paid attention to to help reduce or eliminate internal stress.
a. Choose plastic parts as inserts as possible.
b. As far as possible, select metal materials with a small difference in the thermal expansion coefficient of plastic as insert materials, such as aluminum, aluminum alloy and copper.
c. The metal insert is coated with a layer of rubber or polyurethane elastic buffer layer, and ensure that the coating layer does not melt during molding, which can reduce the shrinkage difference between the two.
d. Surface degreasing of metal inserts can prevent stress cracking of products accelerated by grease.
e. Appropriate preheating of metal inserts.
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 thickness of the plastic around the insert is h, the plastic thickness of the aluminum insert is h≥0.8D; For copper inserts, plastic thickness h≥ 0.9D.
g. Metal inserts should be designed in a smooth shape, preferably with delicate knurling.
③ Design of holes on plastic products
The shape, number 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 on plastic products. Circular holes should be opened as far as possible, of which the oval hole is the best effect, and the long axis of the oval hole should be parallel to the direction of external force. If the round hole is opened, the process round hole of equal diameter can be opened, 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 the oval hole; Another method is to open symmetrical slots around the round holes to disperse the internal stress.
(5) Design of plastic mold
In the design of plastic mold, the pouring system and cooling system have a great impact on the internal stress of plastic products, and the following points should be paid attention to in the specific design.
① Gate size
Too large gate will need a long time to hold the pressure and fill the material, and the feed flow in the cooling process will definitely freeze more orientation stress, especially when filling the cold material, it will cause great internal stress near the gate.
Properly reducing the size of the gate can shorten the time of holding pressure and feeding material, reduce the pressure in the mold when the gate is sealed, and thus reduce the orientation stress. But too small a gate will