Stress we usually refer to in injection molding refers to force on an object per unit area. It emphasizes internal force of object; when an object is subjected to external force, it will generate internal stress that resists external force; In the absence of external force, inherent internal stress is called internal stress, which is caused by uneven plastic deformation of various parts inside object.
According to scope of internal stress, it can be divided into three categories:
(1) First type of internal stress (macroscopic internal stress), that is, internal stress within macroscopic range caused by uneven deformation of various parts of material;
(2) Second type of internal stress (microscopic internal stress), that is, internal stress between grains or sub-grains produced by uneven deformation among grains or sub-grains of object(in nature, most solid substances are crystals);
(3) Third type of internal stress (lattice distortion stress), that is, internal stress caused by a part of atoms in crystal deviating from its equilibrium position due to lattice distortion, which is the most important internal stress in deformed objects (destroyed objects).

 

Schematic diagram of shear stress generation
Plastic internal stress refers to a kind of internal stress generated by orientation of macromolecular chains and cooling shrinkage during plastic melt processing.
Essence of internal stress is unbalanced conformation formed by macromolecular chain during melting process. This unbalanced conformation cannot immediately return to balanced conformation suitable for environmental conditions when it is cooled and solidified. Essence of this unbalanced conformation is a reversible high-elastic deformation, and frozen high-elastic deformation is usually stored in the form of potential energy in plastic products. Under suitable conditions, this forced unstable conformation will transform into a free stable conformation, potential energy converted into kinetic energy and released.
When force between macromolecular chains and entanglement force cannot withstand this kinetic energy, internal stress balance will be destroyed, plastic products will have stress cracking and warping deformation.
Almost all plastic products will have internal stress to varying degrees, especially internal stress of plastic injection products is more obvious. Existence of internal stress not only causes warping and cracking of plastic products during storage and use, but also affects mechanical properties, optical properties, electrical properties and appearance quality of plastic products.
To this end, it is necessary to find out cause of internal stress and method of eliminating internal stress, reduce stress inside plastic product to the greatest extent, and make residual internal stress distribute as evenly as possible on plastic product to avoid stress concentration, thereby improving mechanical and thermal properties of plastic product.

There are many reasons for generation of internal stress, such as strong shearing action of plastic melt during processing, orientation and crystallization of processing, extremely difficult cooling rate of each part of melt, uneven plasticization of melt, difficulty of demoulding product, will cause generation of internal stress. Depending on cause of internal stress, internal stress can be divided into following categories.

Orientation internal stress is a kind of internal stress generated by freezing of macromolecular chains aligned in direction of flow during process of flow filling and pressure maintaining of plastic melt.
Specific process of orientation stress generation is: melt near runner wall increases viscosity of outer melt due to fast cooling rate, so that flow velocity of melt in the center layer of cavity is much higher than flow velocity of surface layer, resulting in shear stress between inner layers of melt and orientation along flow direction.
Oriented macromolecular chains are frozen in plastic product, which means that there is unrelaxed reversible high-elastic deformation, so orientation stress is internal force of macromolecular chains trying to transition from oriented conformation to non-oriented conformation. Orientation stress in plastic products can be reduced or eliminated by heat treatment.
Orientation internal stress distribution of plastic products becomes smaller and smaller from surface layer to inner layer of product, and changes in a parabola.

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, outer layer of plastic product is first cooled and solidified to shrink, and inner layer may still be a hot melt, so that core layer will limit shrinkage of surface layer, resulting in core layer being in a state of compressive stress, while surface layer is in a state of tensile stress state.
Distribution of cooling internal stress of plastic products becomes larger and larger from surface layer to inner layer of product, and also changes in a parabola.
In addition, for plastic products with metal inserts, due to large difference in thermal expansion coefficients between metal and plastic, it is easy to form internal stress with uneven shrinkage.
In addition to above two main internal stresses, there are following types of internal stresses: For crystalline plastic products, internal stresses will also be generated by different crystal structures and crystallinity of each part of product. In addition, there are internal stresses of configuration and internal stresses of demoulding, etc., but proportions of internal stresses are very small.

The greater rigidity of molecular chain, the higher melt viscosity, and the poor mobility of polymer molecular chain, so recovery of reversible high elastic deformation is poor, and it is easy to generate residual internal stress. For example, some polymers containing benzene rings in molecular chain, such as PC, PPO, PPS, etc., internal stress of corresponding products is relatively large.

The greater polarity of a molecular chain, the greater force of mutual attraction between molecules, which increases difficulty of inter-molecular movement and reduces degree of recovery of reversible elastic deformation, resulting in large residual internal stress. For example, some plastic varieties containing polar groups such as carbonyl, ester, and cyano groups in their molecular chains have relatively large internal stress in their corresponding products.

The larger volume of macromolecular side substituent group, the greater hindrance to free movement of macromolecular chain and increase of residual internal stress. For example, phenyl group of polystyrene substituent group has a large volume, so internal stress of polystyrene products is relatively large.
Order of internal stress of several common polymers is as follows:
PPO>PSF>PC>ABS>PA6>PP>HDPE

 

The larger molecular weight of polymer, the greater force between macromolecular chains and degree of entanglement, and the stronger stress cracking resistance of product; the wider molecular weight distribution of polymer, the larger low molecular weight component, it is easy to form microscopic tears first, resulting in stress concentration and cracking of product.

Impurities in polymer are concentration of stress, which will reduce original strength of plastic, and impurity content should be reduced to a minimum.

Resins prone to stress cracking can be blended with other suitable resins to reduce degree of internal stress.
For example, mixing an appropriate amount of PS in PC, PS is dispersed in continuous phase of PC in the form of beads, which can make internal stress disperse along spherical surface and prevent crack from expanding, so as to achieve purpose of reducing internal stress. For another example, if an appropriate amount of PE is mixed into PC, outer edge of PE spheres can form a closed cavitation zone, and can also appropriately reduce internal stress.

Reinforcement modification with reinforcing fibers can reduce internal stress of product, because fibers entangle many macromolecular chains, thereby improving stress cracking ability. For example, stress cracking resistance of 30% GFPC is 6 times higher than that of pure PC.

Adding a suitable nucleating agent to crystalline plastics can form many small spherulites in its products, reducing and dispersing internal stress.

In molding process of plastic products, any molding factor that can reduce molecular orientation of polymer in product can reduce orientation stress; any process conditions that can make polymer in product cool evenly can reduce cooling internal stress; Processing methods for demoulding of plastic products are all conducive to reducing internal stress of demoulding.
Processing conditions that have a greater impact on internal stress are mainly as follows:

① Cylinder temperature

A higher barrel temperature is conducive to reduction of orientation stress, because at a higher barrel temperature, melt is plasticized uniformly, viscosity decreases, and fluidity increases. When melt fills cavity, molecular orientation effect is small, so orientation stress is small. At a lower barrel temperature, melt viscosity is higher, molecular orientation is more during mold filling process, residual internal stress after cooling and setting is larger.
However, it is not good if barrel temperature is too high. If it is too high, it will easily cause insufficient cooling, and it will easily cause deformation during demoulding. Although orientation stress decreases, cooling stress and demoulding stress will increase instead.

Temperature of mold has a great influence on orientation internal stress and cooling internal stress. On the one hand, if mold temperature is too low, cooling will be accelerated, and cooling will be uneven, which will cause a large difference in shrinkage, thereby increasing cooling internal stress;
On the other hand, if mold temperature is too low, after melt enters mold, temperature drops faster, and viscosity of melt increases rapidly, resulting in filling of mold under high viscosity, and degree of orientation stress is significantly increased.
Mold temperature has a great influence on crystallization of plastics. The higher mold temperature, the more compact crystal grains will be packed, and defects inside crystal will be reduced or eliminated, thereby reducing internal stress.
In addition, for plastic products of different thicknesses, mold temperature requirements are different. For thick-walled products, mold temperature should be appropriately higher.

③ Injection pressure

Injection pressure is high, shear force on melt during mold filling process is large, and chance of orientation stress is also large. Therefore, in order to reduce orientation stress and eliminate release stress, injection pressure should be appropriately reduced. .

Influence of holding pressure on internal stress of plastic products is greater than that of injection pressure. In pressure holding stage, as temperature of melt decreases, viscosity of melt increases rapidly. If a high pressure is applied at this time, it will inevitably lead to forced orientation of molecular chain, thereby forming a greater orientation stress.

⑤ Injection speed

The faster injection speed, the easier it is to cause degree of orientation of molecular chain to increase, thereby causing greater orientation stress. However, if injection speed is too low, after plastic melt enters mold cavity, it may be layered successively to form melting marks, resulting in stress concentration lines, which are prone to stress cracking. Therefore, injection speed should be moderate. It is best to use variable speed injection and end mold filling at a gradual decrease in speed.

⑥ Pressure holding time

The longer holding time, the shear action of plastic melt will be increased, resulting in greater elastic deformation and freezing of more orientation stress. Therefore, orientation stress increases significantly with extension of holding time and increase of feeding amount.

Injection pressure and holding time should be properly adjusted so that residual pressure in mold is close to atmospheric pressure when mold is opened, so as to avoid greater internal stress of demoulding.

 

Heat treatment of plastic products refers to method of eliminating internal stress by keeping molded products at a certain temperature for a period of time. Heat treatment is the best way to eliminate orientation stress in plastic products.
For injection molded parts with high polymer molecular chain rigidity and high glass transition temperature; for parts with large wall thickness and metal inserts; for parts with wide temperature range and high dimensional accuracy requirements; Parts with large internal stress that are not easy to eliminate themselves and parts that have been machined must be heat treated.
Heat treatment of workpiece can change polymer molecule from an unbalanced conformation to an equilibrium conformation, so that forced-frozen high-elastic deformation obtains energy and undergoes thermal relaxation, thereby reducing or basically eliminating internal stress. Heat treatment temperature often used is 10-20 ℃ higher than service temperature of part or 5-10 ℃ lower than heat distortion temperature.
Heat treatment time depends on type of plastic, thickness of part, heat treatment temperature and injection molding conditions. For workpieces of general thickness, heat treatment is sufficient for 1-2 hours. As thickness of workpiece increases, heat treatment time should be extended appropriately. Increasing heat treatment temperature and prolonging heat treatment time have similar effects, but effect of temperature is more obvious.
Heat treatment method is to put parts into liquid media such as water, glycerin, mineral oil, ethylene glycol and liquid paraffin, or put them in an air circulation oven to heat to a specified temperature, stay at this temperature for a certain period of time, then slowly cool to room temperature. Experiments have shown that effect of reducing internal stress and improving performance of parts is more obvious if parts are heat treated immediately after demoulding.
In addition, increasing mold temperature, prolonging cooling time of parts in mold, and performing heat preservation after demoulding have similar effects to heat treatment.
Although heat treatment is one of effective ways to reduce internal stress of parts, heat treatment usually can only reduce internal stress to range allowed by use conditions of part, and it is difficult to completely eliminate internal stress. When PC parts are heat treated for a long time, PC molecular chains may be rearranged in an orderly manner, or even crystallized, thereby reducing impact toughness and reducing notched impact strength. Therefore, heat treatment should not be used as only measure to reduce internal stress of part.

When designing plastic products, in order to effectively disperse internal stress, principle should be followed: shape of product should be as continuous as possible, and sharp angles, right angles, gaps, and sudden expansion or reduction should be avoided.
Edges of plastic products should be rounded, and radius of inner fillet should be greater than 70% of thickness of thinner of two adjacent walls; radius of outer fillet should be determined according to shape of product.
For parts with a large difference in wall thickness, cooling internal stress and orientation internal stress are likely to occur due to different cooling rates. Therefore, it should be designed to have a uniform wall thickness as much as possible. If wall thickness must be uneven, a gradual transition of wall thickness difference must be carried out.

Thermal expansion coefficients of plastics and metals differ by 5 to 10 times. Therefore, when plastic products with metal inserts are cooled, shrinkage of two forms is different. Because shrinkage of plastic is relatively large, it tightly hugs metal inserts. Surrounding plastic inner layer is subjected to compressive stress, while outer layer is subjected to tensile stress, resulting in stress concentration.
When designing inserts, following points should be noted to help reduce or eliminate internal stress.
a. Choose plastic parts as inserts as much as possible.
b. As far as possible, choose a metal material with a small difference in thermal expansion coefficient from plastic as insert material, such as aluminum, aluminum alloy and copper.
c. Coat a layer of rubber or polyurethane elastic buffer layer on metal insert, and ensure that coating layer does not melt during molding, which can reduce shrinkage difference between the two.
d. Degreasing surface of metal insert can prevent oil from accelerating stress cracking of product.
e. Proper preheating of metal inserts.
f. Thickness of plastic around metal inserts should be sufficient. For example, if outer diameter of insert is D and thickness of plastic around insert is h, then thickness of plastic for aluminum insert is h≥0.8D; for copper insert, thickness of plastic is h≥0.9D.
g. Metal inserts should be designed in a smooth shape, preferably with delicate knurling.

Shape, number and position of holes on plastic products will have a great influence on degree of internal stress concentration.
To avoid stress cracking, do not create prismatic, rectangular, square or polygonal holes in plastic products. Circular holes should be set up as much as possible, and effect of elliptical holes is the best, and long axis of elliptical holes should be parallel to direction of external force.
If a round hole is opened, it is possible to add a round hole of equal diameter, and make center connecting line of two adjacent round holes parallel to direction of external force, so that effect similar to that of elliptical hole can be obtained; there is another method, that is, open symmetrical slots around circular hole to disperse internal stress.

 

When designing plastic molds, gating system and cooling system have a great influence on internal stress of plastic products, and following points should be paid attention to in specific design.

① Gate size

An excessively large gate will require a longer time for maintaining pressure and feeding, and feeding flow during cooling process will definitely freeze more orientation stress, especially when filling cold material, it will cause a lot of internal stress near gate.
Appropriately reducing size of gate can shorten time for holding pressure and filling material, and reduce pressure in mold when gate is sealed, thereby reducing orientation stress. But too small gate will prolong filling time, resulting in shortage of products.

② Gate position

Position of gate determines flow condition, flow distance and flow direction of plastic melt in mold cavity.When gate is located at part with the largest wall thickness of product, injection pressure, holding pressure and holding time can be appropriately reduced, which is beneficial to reduce orientation stress. When gate is located in a thin-walled part, it is advisable to increase wall thickness at gate appropriately to reduce orientation stress near gate.
The longer melt flow distance in cavity, the greater probability of orientation stress. For this reason, for plastic parts with thick walls, long processes and large areas, multiple gates should be properly distributed, which can effectively reduce orientation stress and prevent warping deformation.
In addition, since area near gate is prone to internal stress, it can be designed as an ear-protected gate near gate, so that internal stress is generated in the ear, and ear with a large internal stress is removed after demoulding, which can reduce internal stress in plastic product.

③ Runner design

Designing short and thick runners can reduce pressure loss and temperature drop of melt, correspondingly reduce injection pressure and cooling speed, thereby reducing orientation stress and cooling pressure.

Distribution of cooling water channels should be reasonable, so that parts near gate, away from gate area, thick wall, and thin wall can be cooled uniformly and slowly, thereby reducing internal stress.

⑤ Design of ejection system

It is necessary to design an appropriate demoulding taper, a higher core finish and a larger ejection area to prevent forced demoulding from causing demoulding stress. Method of checking stress of plastic parts is mainly solvent immersion method. Dip it in glacial acetic acid for 30s, and dry it in air. Whitish part is place where stress is concentrated. When stress is high, plastic will crack, and more cracks indicate more stress. It can also be soaked for 2rain, cracks will be deeper and more obvious.
Instead of dipping in glacial acetic acid, a 1:1 mixture of methyl ethyl ketone and acetone can be used for 15 seconds.
Method of stress relief is heating, that is, baking at 65-70℃ for 4 hours. Small pieces can be soaked in 25% acetone aqueous solution for 30rain to relieve stress. When stress is too high, neither method is effective, and part cannot be plated.