This article can solve most of your questions about internal stress of injection molding products

Time:2020-02-19 13:42:25 / Popularity: / Source:

Plastic products tend to generate internal stress during injection molding process. If they are not eliminated, subsequent spraying will be bad. This article will focus on following core issues.
1. How does internal stress occur? What factors are it related to?
2. How to detect internal stress, what is intuitive and convenient method?
3. How to eliminate internal stress, is there a convenient and fast way to operate?
4. What are effects of internal stress on spray? What are other risks of internal stress? Is there any advantage?
internal stress 
The so-called stress refers to force received by an object in a unit area, it emphasizes force inside object. Under action of external force, general object will generate stress against external force. When object is not subjected to external force, internal fixed force is called internal stress, which is caused by uneven plastic deformation of various parts inside object. According to range of internal stress, it can be divided into three categories:
(1) First type of internal stress (macro internal stress), that is internal stress in macroscopic range caused by uneven deformation of various parts of material;
(2) Second type of internal stress (microscopic internal stress), that is each of objects Internal stress between grains or subgrains produced by uneven deformation between grains or subgrains (naturally, most solid materials are crystals);
(3) Third type of internal stress (lattice distortion stress), which is internal stress caused by lattice distortion, which causes some atoms in crystal to deviate from its equilibrium position. It is the most important internal stress in a deformed object (destroyed object).
Plastic internal stress refers to an intrinsic stress caused by factors such as orientation and cooling shrinkage of macromolecular chains during melt processing of plastics. Essence of internal stress is unbalanced conformation formed by macromolecular chain during melt processing. This unbalanced conformation cannot be restored to equilibrium conformation suitable for environmental conditions when cooled and solidified. Essence of this unbalanced conformation is a reversible high elastic deformation, while frozen high elastic deformation is 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 will be converted into kinetic energy and released. When force between macromolecular chains and intertwining force cannot withstand such kinetic energy, internal stress balance is destroyed, and plastic products are subject to stress cracking and warping deformation.
Almost all plastic injection products have internal stresses to varying degrees, especially internal stress of plastic injection products is more obvious. Existence of internal stress not only causes warpage deformation and cracking of plastic injection products during storage and use, but also affects mechanical properties, optical properties, electrical properties and appearance quality of plastic injection products. Therefore, it is necessary to find out cause of internal stress and method of eliminating internal stress, minimize stress inside plastic product, distribute residual internal stress as evenly as possible on plastic product to avoid stress concentration, thereby improve mechanical and thermal properties of plastic injection products.

Causes of internal stress in plastics

There are many reasons for internal stress. For example, plastic melt is subjected to strong shearing during processing, orientation and crystallization existing in the processing, cooling rate of each melt part is extremely difficult to be uniform, plasticization of melt is uneven, and product is difficult to demold. All these will cause generation of internal stress. Internal stress can be divided into following categories depending on the cause of internal stress.

(1) Oriented internal stress

Orientation internal stress is an internal stress generated by macromolecular chain being aligned in flow direction during flow filling and pressure maintaining feeding process. Specific process of orientation stress generation is as follows: Melt of near-flow channel wall increases viscosity of outer layer melt due to rapid cooling rate, so that flow rate of melt in the center layer of cavity is much higher than surface layer flow velocity, resulting in shear stress between inner layer of melt and orientation in flow direction. Freezing of oriented macromolecular chain in plastic article means that there is an unrelaxed reversible high elastic deformation, so orientation stress is internal force of macromolecular chain transitioning from oriented conformational force diagram to non-oriented conformation. By means of heat treatment, orientation stress in plastic article can be reduced or eliminated. Orientation of internal stress of plastic article is smaller and smaller from surface layer to inner layer of article, and changes in a parabola.

(2) cooling internal stress

Cooling internal stress is an internal stress generated by uneven shrinkage of plastic product during melt processing due to cooling setting. Especially for thick-walled plastic injection products, outer layer of plastic product first cools, solidifies and shrinks, inner layer may still be a hot melt, so that core layer will restrict shrinkage of surface layer, causing core layer to be under compressive stress while surface layer is under tensile stress. Distribution of cooling internal stress in plastic products is increasing from surface layer to inner layer of product, and also changes in a parabola. In addition, plastic injection products with metal inserts are likely to form uniform internal stresses due to large difference in thermal expansion coefficient between metal and plastic. In addition to above two main internal stresses, there are also several internal stresses: for crystalline plastic injection products, internal stress is also generated by different crystal structures and crystallinity of various parts inside product. In addition, there are also internal forces in the configuration and internal stress of demoulding, but proportion of internal stress is very small.
plastic injection products 

Factors affecting internal stress of plastics

(1) Rigidity of molecular chain

The greater rigidity of molecular chain, the higher melt viscosity, the lower mobility of polymer molecular chain, and the poor resilience of reversible high elastic deformation, which tends to produce residual internal stress. For example, some polymers containing a benzene ring in molecular chain, such as PC, PPO, PPS, etc., internal stress of corresponding products is too large.

(2) Polarity of molecular chain

The greater polarity of a molecular chain, the greater attraction between molecules, so that mutual movement between molecules is difficult, and degree of recovery of reversible elastic deformation is reduced, resulting in large residual internal stress. For example, some plastics having a polar group such as a carbonyl group, an ester group or an eye group in a molecular chain have a large internal stress of corresponding product.

(3) Steric hindrance effect of substituted groups

The larger volume of macromolecular pendant group, the greater residual internal stress caused by free movement of macromolecular chain. For example, phenyl group of polystyrene substituent group is bulky, and thus internal stress of polystyrene article is large. Order of internal stress of 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 a resin with a large molecular weight and a narrow molecular weight distribution
The larger molecular weight of polymer, the greater force and entanglement of macromolecular chain, and the stronger resistance to stress cracking of product; the wider molecular weight distribution of polymer, the larger low molecular weight component, it is easy to form microscopic tear first, causing stress concentration and cracking of product.
2) Select a resin with low impurity content
Impurities in polymer are concentration of stress, which will reduce original strength of plastic, and impurity content should be minimized.
3) Blending modification
Resin which is prone to stress cracking is blended with a suitable other resin to reduce existence of internal stress. For example, when PC is mixed with an appropriate amount of PS, PS is dispersed in PC continuous phase in a nearly bead shape, which can relieve internal stress along spherical surface and prevent crack from expanding, thereby achieving purpose of reducing internal stress. For example, if a proper amount of PE is mixed in PC, outer edge of PE pellet can form a closed cavitation zone, and internal stress can be appropriately reduced.
4) Enhanced modification
Reinforcement modification with reinforcing fibers can reduce internal stress of product because fibers are entangled with many macromolecular chains, thereby improving stress cracking ability. For example, stress-tolerant cracking ability of 30% GF+PC is 6 times higher than that of pure PC.
5) Nucleation modification
By adding a suitable nucleating agent to crystalline plastic, many small spherulites can be formed in article to reduce internal stress and disperse.

(2) Control of molding processing conditions

In molding process of plastic injection products, any molding factor that can reduce orientation of polymer molecules in product can reduce orientation stress; any process conditions that can uniformly cool polymer in product can reduce internal stress of cooling; Any processing method that contributes to demolding plastic product is beneficial to reduce internal stress of demoulding. Processing conditions that have a large influence on internal stress are mainly as follows.
1) barrel temperature
Higher barrel temperatures favor a reduction in orientation stress because at higher barrel temperatures, melt plasticization is uniform, viscosity decreases, fluidity increases, and molecular orientation is small during melt-filled cavities. Therefore, orientation stress is small. At lower barrel temperatures, melt viscosity is higher, molecular orientation during filling process is more, residual internal stress is larger after cooling and setting. However, if temperature of cylinder is too high, it is not good. If it is too high, cooling is insufficient, and deformation is easily caused during demolding. Although orientation stress is reduced, cooling stress and demolding stress are increased.
2) mold temperature
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, cooling will be uneven, which will cause a large difference in shrinkage, thereby increasing internal stress of cooling; on the other hand, if mold temperature is too low, melt temperature increases rapidly after melt enters mold, melt viscosity increases rapidly, causing mold to be filled at a high viscosity, and degree of orientation stress is significantly increased. Mold temperature has a great influence on crystallization of plastic. The higher mold temperature, the more favorable grain stacking is, defects inside crystal are reduced or eliminated, thereby reducing internal stress. In addition, mold temperature requirements are different for plastic products of different thicknesses. For thick-walled products, mold temperature should be higher. Taking PC as an example, relationship between internal stress and mold temperature is shown in Table 5-5.
3) injection pressure
Injection pressure is high, shearing force is large during melt filling process, and chance of generating orientation stress is also large. Therefore, in order to reduce orientation stress and eliminate release stress, injection pressure should be appropriately lowered. Taking PC as an example, relationship between internal stress and injection pressure is shown in Table 5-6. .
4) holding pressure
Effect of holding pressure on internal stress of plastic products is greater than influence of injection pressure. In pressure holding stage, as melt temperature decreases, melt viscosity increases rapidly. At this time, if a high pressure is applied, forced orientation of molecular chain is inevitably caused, thereby forming a larger orientation stress.
5) injection speed
The faster injection speed, the more likely degree of orientation of molecular chains is increased, resulting in greater orientation stress. However, injection speed is too low, and after plastic melt enters cavity, it may be layered to form a melt mark, resulting in a stress concentration line, which is prone to stress cracking. Therefore, injection speed is moderate. It is preferable to use a variable speed injection to end filling at a gradually decreasing speed.
6) holding time
The longer holding time, the greater shearing action of plastic melt, resulting in greater elastic deformation and freezing more orientation stress. Therefore, orientation stress increases remarkably with increase of dwell time and increase of feed amount.
7) mold residual pressure
Injection pressure and holding time should be properly adjusted so that residual pressure in mold is close to atmospheric pressure when open mold, thereby avoiding a larger internal stress of mold release.

(3) Heat treatment of plastic products

Heat treatment of plastic article refers to a method of removing internal stress by leaving molded article at a certain temperature for a certain period of time. Heat treatment is the best way to eliminate orientation stress in plastics. For injection molded parts with high rigidity and high glass transition temperature of polymer molecular chains; parts with large wall thickness and metal inserts; parts with high temperature range and high dimensional accuracy. Heat treatment must be carried out on parts with large internal stress, are not easy to eliminate and mechanically processed.
Heat treatment of article can cause polymer molecules to change from an unbalanced conformation to a balanced conformation, so that forced freezing of unstable high elastic deformation energy obtains thermal relaxation, thereby reducing or substantially eliminating internal stress. Heat treatment temperature is usually higher than use temperature of workpiece by 10~20℃ or lower than heat distortion temperature of 5~10℃. Heat treatment time depends on type of plastic, thickness of part, heat treatment temperature and injection molding conditions. Normal thickness of part can be heat treated for 1-2 hours. As thickness of part increases, heat treatment time should be extended. Increasing heat treatment temperature and prolonging heat treatment time have similar effects, but effect of temperature is more pronounced.
Heat treatment method is to put workpiece into a liquid medium such as water, glycerin, mineral oil, ethylene glycol and liquid paraffin, or put it in an air circulating oven to a specified temperature, stay for a certain time, then slowly cool to room temperature. Experiments show that parts after demolding are immediately subjected to heat treatment, which is more effective in reducing internal stress and improving performance of parts. In addition, increase mold temperature, prolong cooling time of workpiece in mold, heat treatment after demolding has a similar heat treatment effect.
Although heat treatment is one of effective ways to reduce internal stress of part, heat treatment usually only reduces internal stress to extent permitted by use conditions of part, and it is difficult to completely eliminate internal stress. When PC part is heat treated for a long time, PC molecular chain may undergo order rearrangement or even crystallization, thereby reducing impact toughness and notched impact strength. Therefore, heat treatment should not be used as sole measure to reduce internal stress of part.
plastic injection products 

(4) Design of plastic products

1) shape and size of plastic products
In specific design of plastic products, in order to effectively disperse internal stress, principle should be followed: shape of product should be as continuous as possible, avoiding acute angles, right angles, gaps and sudden enlargement or contraction.
For edge of plastic product, it should be designed as rounded corners, wherein radius of fillet should be greater than 70% of thickness of thinner adjacent walls; radius of fillet is determined according to shape of product.
For parts with large wall thickness differences, cooling internal stress and orientation internal stress are likely to occur due to different cooling rates. Therefore, it should be designed as a part with same wall thickness as possible. If wall thickness is not uniform, a gradual transition of wall thickness difference is required.
2) Reasonable design of metal inserts
Thermal expansion coefficient of plastic and metal differs by 5 to 10 times. Therefore, when plastic product with metal inserts is cooled, degree of shrinkage formed is different. Because shrinkage of plastic is relatively large, metal insert is tightly held, inner layer of plastic around insert is subjected to compressive stress, and outer layer is subjected to tensile stress, resulting in stress concentration.
When specifying juice insert, following points should be noted to help reduce or eliminate internal stress.
a. Select plastic parts as inserts as much as possible.
b. Select as much as possible a metal material with a small difference in thermal expansion coefficient of plastic as an insert material, such as aluminum, aluminum alloy and copper.
c. Apply 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 difference in shrinkage.
d. Surface degreasing treatment of metal insert can prevent grease from accelerating stress cracking of product.
e. Metal insert is subjected to a suitable pre-heat treatment.
f. Thickness of plastic around metal insert should be sufficient. For example, outer diameter of insert is D, thickness of plastic around insert is h, then thickness of aluminum insert plastic h ≥ 0.8D; for copper insert, thickness of plastic h ≥ 0.9 D.
g. Metal inserts should be designed in a rounded shape, preferably with a fine knurling.
3) design of upper hole of plastic products
All shape, number and position of holes in plastic product have a great influence on degree of internal stress concentration.
In order to avoid stress cracking, it is forbidden to open prismatic, rectangular, square or polygonal holes in plastic products. A circular hole should be opened as much as possible, wherein elliptical hole has the best effect, and long axis of elliptical hole should be parallel to direction of external force. If a circular hole is opened, circular hole of same diameter can be opened, and center connecting line of adjacent two circular holes can be parallel to direction of external force, so that effect similar to elliptical hole can be obtained; there is also a method of opening a symmetrical hole around a circular hole to disperse internal stress.

(5) Design of plastic mold

When designing plastic mold, pouring system and cooling system have a great influence on internal stress of plastic products. Following points should be noted in specific design.
1 gate size
An oversized gate will require a longer hold-up time. Feed flow during cooling process will freeze more orientation stress, especially when filling cold material, which will cause a large internal stress near gate.
Properly reducing size of gate can shorten pressure feeding time and reduce pressure inside mold when gate is sealed, thereby reducing the orientation stress. However, too small a gate will result in prolonged filling time, resulting in a shortage of products.
2 gate location
Location of gate is determined by flow of plastic melt, flow distance and flow direction of flow within mold cavity. When gate is set at the largest thickness part of product, injection pressure, holding pressure and dwell time can be appropriately reduced, which is advantageous for reducing orientation stress. When gate is provided at thin-walled portion, wall thickness at gate should be appropriately increased to reduce orientation stress near gate.
The longer flow distance of melt in cavity, the greater probability of producing an orientation stress. For this reason, for plastic parts with a large wall thickness, long flow and large area, a plurality of gates should be appropriately distributed, which can effectively reduce orientation stress and prevent warpage.
In addition, since vicinity of gate is inner stress-prone zone, a ear-protection gate can be designed in vicinity of gate, so that internal stress is generated in ear-protection area, and ear protector with larger internal stress is removed after demolding, thereby reducing internal stress in plastic product.
3 runner design
Designing short and thick flow paths reduces pressure loss and temperature drop of melt, and reduces injection pressure and cooling rate, thereby reducing orientation stress and cooling pressure.
4 cooling system design
Distribution of cooling water channel should be reasonable, so that vicinity of gate, away from gate area, thick wall, thin wall are uniformly and slowly cooled, thereby reducing internal stress.
5 ejection system design
It is necessary to design a proper demoulding taper, a higher core finish and a larger area of ejector portion to prevent forced release from demoulding. Method of inspecting stress of plastic part is mainly a solvent impregnation method. Dip with glacial acetic acid for 30 s, dry, whitish area is stress concentration. When stress is large, plastic will crack. The more cracks, the greater stress. It can also be immersed in 2rain, crack is deeper and more obvious. It can be immersed in 1:1 mixture of methyl ethyl ketone and acetone for 15s instead of glacial acetic acid impregnation.
plastic injection products 

Method for relieving stress

Method for relieving stress is a heating method, that is baking at 65 to 70℃ for 4 hours. Small pieces can be soaked in a 25% acetone solution for 30rain to eliminate stress. When stress is too large, both methods are invalid and parts cannot be plated.
Plastic plating is widely used in electronics industry, national defense research, household appliances and daily necessities. It saves metal materials, simplifies processing, reduces equipment weight, improves part appearance, electrothermal performance and mechanical strength of materials. Quality of plastic plating is not only closely related to electroplating process and operation, but also five aspects of material selection, structural design, plastic mold, plastic forming process and post-treatment process.

1.1 Internal stress test

Test piece was immersed in glacial acetic acid at 25℃ for 3 min, and internal stress was judged according to degree of "whitening" on the surface of test piece. The larger internal stress, the more serious "whitening" phenomenon. This method can roughly explain condition of internal stress.

1.2 Determination of coating peel strength

Peeling strength was measured by a peeling method: a strip of 10 mm width was cut on test piece, tip was picked up by 30 to 40 mm, and peeled off by a tensile machine in a direction perpendicular to surface of plating layer (90 ° ± 5 °).

1.3 High and low temperature impact test to verify coating adhesion

Method is proposed by West German Plastic Electroplating Workers Association. Method is simple and easy to reproduce. Specific operation process is: heat preservation in a hot water bath at 80℃ ± 5℃ for 1 h, and after taking it out, put it in a low temperature water bath of 5℃ ± 5℃ for 30s, and then transfer it to a hot water bath. After 3 cycles, if coating is free of blistering, peeling, wrinkling, etc., it is considered qualified.
Material selection of Electroplated Plastics

2.1 Material selection of Electroplated Plastics

There are many plastics that can be used for electroplating, but there are great differences in processing properties, mechanical properties, material costs, plating costs, ease of plating, and dimensional accuracy of various materials. ABS plastic has excellent comprehensive properties, is widely used, and is easy to form. Surface is easily etched to obtain high plating adhesion, so it is currently used most in electroplating.
In addition, it has been found by infrared spectroscopy that there are reactive groups such as -COOH, -CHO, -OH, -SO3H on the surface of chemically roughened plastic, and these polar groups can chemically bond with metal plating player, thereby increasing bonding strength of plating layer. The higher butadiene content of ABS plastic, the greater bonding strength of coating. Content of butadiene in electroplated ABS plastic is 22% to 24%. Tests have shown that plating adhesion of electroplated ABS resin 301M is one time higher than that of electroless ABS resin PA-757.

2.2 Influence of plastic parts structure on electroplating

Original structure of test piece (knob) has a right angle and a sharp edge. During high and low temperature impact test, it is found that foaming parts were mainly concentrated near right angle, sharp edge and gate. These parts were found to have internal stresses during test, which had an adverse effect on adhesion of coating. Right angle and sharp edge are changed into a circular arc for electroplating test, plating layer is well combined with substrate. On the other hand, right-angled and sharp-edged edges tend to cause excessive tip current density during electroplating, resulting in loose coating and poor bonding, even scorching or breakdown of chemical pre-plating.

2.3 Effect of plastic mold on plating of plastic parts

During test, it was found that there was a flow mark on the surface of original knob, which could not be covered after plating, which affected apparent quality. At the same time, due to poor roughness of mold cavity of plastic mold, surface of knob is not bright enough, and finally brightness of coating is affected. Plating piece (test piece) used for measuring peeling strength has a good appearance quality after injection molding, and plating layer is bright. On the other hand, when designing plastic molds (such as casting systems and demolding mechanisms), care should be taken to minimize internal stress of parts to be plated.

2.4 Effect of plastic forming process on plastic plating

(1) Screw injection machine should be used to ensure uniform distribution of B component in ABS plastic. In addition, it should be noted that selected injection machine will cause internal stress on the part and affect bonding strength of coating.
(2) Drying of raw materials. ABS plastic particles are easy to absorb moisture. If they are not dried, they will cause defects such as bubbles, silver wires and lack of luster on the surface of part, which will affect appearance and bonding force of coating.
(3) Selection of injection process parameters. Injection process parameters should be selected so that internal stress of part is as small as possible, appearance defects such as flow marks and corrugations are overcome. Such as, appropriately increasing processing temperature and mold temperature, reducing injection pressure, shortening dwell time, and appropriately reducing injection speed all reduce internal stress of part to varying degrees.
(4) It is not allowed to use oil as a release agent, otherwise roughening will be uneven and bonding strength of plating metal cannot be guaranteed. When necessary, talc or soapy water can be used as a release agent.

2.5 Effect of post-treatment on plating of plastic parts

Due to injection molding conditions, choice of injection machine, shape of product, improper mold design, etc., plastic parts may have internal stress in different parts, which may cause local roughening insufficient, makes activation and metallization difficult, and eventually metallized layer will not be resistant to collision and bonding force will decrease. Tests have shown that both heat treatment and surface treatment can reduce, eliminate internal stress, increase adhesion of coating by 20% to 60%. ABS plastic parts are heat-treated, internal molecules are rearranged to make molecules evenly arranged, especially butadiene particles have a spherical structure, which significantly reduces internal stress. Prolonging heat treatment time appropriately can reduce internal stress to a minimum. Use of a full-surface agent to treat plastic parts not only eliminates internal stress but also degreases, thereby increasing bonding strength of coating. In high and low temperature impact test, parts that have not been post-treated have blistering, and post-treated parts have no obvious change, indicating that post-treatment can greatly reduce internal stress of part.

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