Analysis and solution of common defects in PC injection molding
Polycarbonate (PC) is an engineering plastic with excellent properties. It has not only high transparency, excellent impact toughness, but also creep resistance, non-toxicity, wide use temperature range, good dimensional stability, excellent electrical insulation, and weather resistance. Therefore, it is widely used in instrumentation, lighting appliances, electrical and electronic equipment, household appliances, packaging and other industries.
In addition, with the rapid advancement of polymer material technology, its modified new varieties continue to appear, which has also greatly expanded its application fields. At the same time, its processing technology is receiving more and more attention from all sides.
PC is a linear polymer containing a benzene ring, isopropyl group, and vinegar bond in the molecular main chain structure. This structure makes it both rigid and flexible, and has good high temperature resistance. High melt viscosity and insufficient sensitivity to moisture bring certain difficulties to the injection molding process.
Its processing characteristics are no obvious melting point, melt viscosity is high in the normal processing temperature range of 230-320 ° C, viscosity is less sensitive to shear rate and greater sensitivity to temperature, similar to Newtonian fluid behavior; for moisture Sensitive, the resin is easy to hydrolyze at high temperature; the product is prone to internal stress. It can be seen that PC is a more difficult plastic to process.
Therefore, in the actual production process, we have encountered many problems. Several kinds of more common product defects are analyzed and discussed.
Discoloration of a product, blackening, yellow streaks and black spots
Relatively speaking, the heat resistance of PC is better. Generally, when processing ordinary PC materials, its melting temperature can be set between 240 and 300 ° C. Even if it stays for a long time, it generally does not decompose. But why do we often see discoloration in the production of some electrical products? This is because the market is now highly competitive. In order to reduce production costs, most manufacturers use PC modified materials or recycled materials to produce low- and medium-grade electrical products. Some manufacturers use their own blended materials such as flame retardants and fillers. Because these materials are mixed, and the plasticization requirements are high, it is difficult to control the process, which causes problems.
In view of the above phenomena, we need to consider and find solutions from the following aspects:
(1) Mainly consider the melting temperature in terms of process conditions
Generally, the barrel temperature should be lowered step by step, especially the temperature of the first two sections, and different temperatures should be used for different materials, such as polyethylene (PE) modified PC to produce large electrical products, the barrel temperature should generally be controlled at About 230 ℃; when using ABS or PS modified PC to produce small electrical devices such as switches and sockets, the barrel temperature should generally be controlled at about 250 ℃; and when PBT modified PC is used to produce lighting products, the barrel temperature should generally be controlled at Around 280 ° C. Of course, the final selection of the molding temperature must also consider the product shape, size, mold structure, product performance requirements and other aspects.
The second is to fully dry the raw materials to reduce the possibility of trace moisture to the hot melt catalytic cracking. In addition, if the screw speed is too fast, the back pressure is too high, the injection rate is too fast, and the nozzle hole size, runner, and gate size are too small, etc., the melt will generate high shear heat, resulting in melt fracture of PC. Moreover, it is easy for the gas in the mold cavity to be exhausted in time, resulting in partial burns and darkening of the product.
(2) Equipment
Due to the high melt viscosity of PC, the poor fluidity, the high injection pressure required, the strong bonding with metallographic materials, and the strong corrosion of the decomposition products, the selection of pc machining equipment requires the use of small or specially designed, chrome Screw, and the plasticizing system does not allow dead angles, dull material, notches, cracks and so on.
Generally speaking, if the process conditions are okay and the melt is discolored during air injection, this indicates that there is a problem with the plasticizing system. You need to check the plasticizing system one by one, starting from the nozzle, to the nozzle flange, three small pieces, Screw and barrel. Sometimes the product will periodically appear two or three mold large pieces of discoloration, which is mostly related to the existence of dullness in the plasticizing system. Because the PC decomposition product has a self-catalytic effect when it exceeds a certain amount, a large amount of melt is decomposed. , Especially plastics with flame retardants.
This is to find out the blank points such as screw sticky material, storage material, barrel sticky material, etc., which need to be resolved by cleaning, repairing and polishing.
(3) If there are black spots found in the materials and operation methods as soon as the machine is turned on, this is mostly related to the barrel stock.
Therefore, it is necessary to pay attention to the operation method. When the barrel is stocked with PC before starting, the barrel must be cleaned with new materials at the molding temperature 3 to 4 times (injection into the air). If the stored materials are other materials, especially Materials with poor thermal stability, such as PVC, POM, etc., require that the temperature cannot be increased when the machine is turned on, and that the barrel cannot be cleaned with PC materials. Only materials with good thermal stability such as PS, PE can be used at lower temperatures. After the material is cleared and cleaned, the temperature of the barrel is raised to the normal machining temperature of the PC, and then washed with the PC material before machining.
During the process, if the production needs to be temporarily suspended, the temperature of the barrel must be reduced to below 160 ° C (the glass transition temperature of the PC is 160 “C) to prevent the material from discoloring for a long time. When the production task is completed, Can use PS, PE and other materials with good thermal stability to clean the barrel, and shut down after emptying.
If there is always discoloration during the production process, you need to first check whether the materials are defective, such as whether other materials and foreign materials have been mixed, whether there are quality problems with the new materials, whether the gate materials are qualified, and whether the compounding method is correct. After eliminating one by one, check other reasons. Another factor is that the environmental pollution is relatively serious, such as a lot of dust in the air, molds are polluted, self-drying hopper filter does not work and sucks more dust particles. This requires the machining workshop to be kept clean and tidy at all times. It is best to cover the inlet and outlet of the hopper with a fine gauze cloth, which is very necessary when machining transparent products.
The product appears silver streaks, bubbles, and vacuum bubbles
The appearance of silver streaks, bubbles, and vacuum bubbles in products is one of the common product defects of PC materials. The causes of these defects are many and complicated, so it is difficult to judge and eliminate them.
Silver streaks (or air streaks) refer to defects that appear in the direction of the melt on the surface of the product due to gas interference during the filling process. The composition of gas mainly includes water vapor, air, decomposition gas and solvent gas. Among them, water vapor, decomposition gas and air are more common.
When these gases exceed a certain limit, the pressure in the mold cavity is lost after injection molding, and the gas close to the surface of the product will emerge, and will be etched along the direction of the flow into a series of large and small bubble points that flicker under the light. Called silver or air lines. In fact, the presence of gas during the injection process is inevitable, and a considerable part of it remains in the plastic.
When the pressure in the mold is large enough and the gas content does not exceed a certain limit, the gas is dissolved in the plastic in a dispersed state; but when the pressure in the mold is not large enough and the gas content exceeds a certain limit, these gases Released from the molten plastic one after another, reaching the surface of the product to form silver streaks, trapped inside the thick wall and becoming air bubbles.
Whether it is the silver streaks on the surface of the product or the bubbles in the wall of the product, it may be the result of the action of one of the four gases or the combination of several gases. It interacts with raw materials, molds, and plasticizing systems. , The adjustment of process parameters, and even changes in weather (especially changes in humidity) and other factors have a great relationship. So the problem is more complicated. However, the focus of the problem and the countermeasures should be focused on the gas, that is, how to control the gas content.
(1) Water vapor Generally speaking, if air bubbles are irregularly dispersed on the surface of the product, it is mostly caused by water vapor.
The hot melt of PC is very sensitive to moisture, and the moisture content is required to be below 0.02%. Therefore, to control the moisture content, it is necessary to fully dry the material. Generally, the drying temperature of PC material is about 120 ° C, and the drying time is about 4h. The time should not be too long. If it exceeds 10h, the material is easy to deteriorate, especially the material with flame retardant should not be dried for too long; and the drying method is based on The effect of the dehumidifier dryer is the best, and it has no effect on the material. Check whether the drying effect is good. You can use the air injection method to see if the shot is continuous, smooth and not white.
(2) If the air bubbles are extremely fine and dense, they are mainly distributed around the gate of the product, forming radiating lines or fan-shaped lines, which are mostly caused by air.
The air source is: Entrained air in the material. When the gate material is more, and the particle size is very different, it is easy to entrain air. Therefore, when using gate materials, it is best to screen out the powder. If the back pressure is too low and the screw speed is too high during melting, the screw will retreat too fast, and the air will easily push to the front of the barrel with the material. Therefore, it is generally recommended to extend the melting time as much as possible during the cooling time. Very helpful.
If the temperature of the lower section is not well controlled, too high temperature will cause a part of the material to melt prematurely and block the passage of air exiting the lower opening; while if the temperature is too low, the preheating will not be enough, so that some of the pellets will enter the homogenization section and be wrapped. air. In addition, too much looseness will also suck air. In the above situation, generally adjusting the screw speed, back pressure and loosening amount can generally be solved. Vent during mold filling.
In order to make the PC material with higher melt viscosity fill the mold smoothly, it is generally to increase the melt temperature and increase the injection pressure. At high temperature and pressure, if the injection rate is fast, the melt will suddenly pass through the narrow flow channel and gate into the mold cavity with a large free space, so that the gas released from the melt will entrain the flow channel and the mold. The air in the cavity forms a high-speed spray state, and traces of dispersed airflow, called air ripples, appear on the surface of the condensed plastic.
In addition, if there are many corners in the mold cavity, the thickness is too large, or there are too many inserts, and the gate is not positioned properly, the melt will surge into the mold cavity, the air in the mold will be vortexed, and gas will be formed in a certain location. Patterns, such as the switch socket panel of molded electrical products, often occur because their sockets, interfaces, and switches are concentrated in one place. The solution to this defect is to modify the mold on the one hand, strengthen the mold venting, and optimize the gate position; on the other hand, reduce the filling rate, especially the injection rate of the air-stripe parts.
(3) Decomposed gas As the PC material needs to be molded at high temperature, some decomposition is inevitable, but how to avoid a large amount of decomposition and how to exclude the gas is worth exploring.
As with the discoloration described above, the main reason for the generation of decomposition gas is the excessively high melt temperature. For example, if the barrel temperature is set too high, or the heating coil of the barrel is out of control, the nozzle should be started, and the heating coil should be checked step by step to reduce the barrel temperature; the melt stays in the barrel for too long (such as production The product uses large equipment, the cushion is too large), the molding cycle is too long, or the dead material in the barrel and the stock in the dead corner are decomposed due to prolonged heating; or the melt is subjected to strong shear in the barrel, If the compression ratio of the screw is too large, the screw speed is too high, and the back pressure is too large, it will also decompose.
In addition, the nozzle aperture is too small, the mold gate, the runner is too small, and the cavity resistance is large, etc., which can cause the passing melt to decompose due to local overheating due to friction. Therefore, when machining PC materials, the nozzle aperture and gate taken The size of the flow channel is large, the exhaust groove is deep, and it is not suitable to make thin-walled products.
Another important reason is that the PC itself is of poor quality and easy to decompose. This is often ignored by users, and the problem is pushed to the mold and machining equipment, so that the correct way to solve the problem cannot be found.
(4) Solvent gas Solvent gas is mainly related to the operation quality in production, such as the machine barrel is not clean, the additive is excessive. The solvent gas can be removed by sufficient drying to remove most of it, and it has little effect on air marks.
It is sometimes difficult to distinguish the bubble point inside a transparent product from a bubble or a vacuum bubble. It is generally believed that if a bubble point has been found at the moment of mold opening, and there is no change in volume after storage for a period of time, it is a bubble, which is caused by gas interference; if it appears and becomes large during the mold cooling process, it is a vacuum bubble.
The formation of vacuum bubbles is due to insufficient material or low pressure when filling the mold. Under the rapid cooling of the mold, the surface of the melt in contact with the mold wall first solidifies, and then the melt of the central part cools and shrinks, causing the volume to shrink and form a hollow. Bubble point.
The solution is:
Increase injection pressure, injection time and material volume;
- Adjust the material temperature: when the vacuum bubble is far from the gate position, increase the material temperature to make the melt flow smoothly, and the pressure can be transmitted to the part far from the gate;
- When the vacuum bubble is near the gate, the material temperature can be reduced to reduce the shrinkage;
- Properly increase the temperature of the mold, especially the temperature of the mold where the vacuum bubble is formed;
- Set the gate in the thick wall of the product to improve the flow condition of the nozzle, runner and gate and the mold exhaust condition;
- Shorten the cooling time of the product in the mold, if necessary, slowly cool the product into hot water;
- Products molded with point gates can be processed at low speed and low temperature to solve the problem of vacuum bubbles. When there are vacuum bubbles on the flow channel, the size of the flow channel can be increased.
- In addition, during the production process, it was also found that the bubbling phenomenon occurred in the thick-walled parts soon after the PC product was demoulded, which was caused by insufficient expansion of the gas inside the PC.
- Generally, it can be solved by extending the cooling time, enhancing the cooling effect, increasing the holding pressure and time, and delaying the decomposition of the PC.
“Fingerprint” and turbulent marks appear on the product
Because PC melt has high viscosity and poor fluidity, the products are more prone to “fingerprints” and turbulent flow marks. These two phenomena are more common when cnc machining switch socket parts and general electrical device panels, and these two phenomena are sometimes difficult to distinguish due to their similar shapes. In fact, the causes of these two phenomena are different, and their solutions are also different.
(1) “Fingerprint” is named because it is shaped like a human hand’s fingerprint, and is sometimes called a ripple, a vibration pattern, or a vibration pattern, that is, its pattern is like a stone formed on a calm water surface. The main reason for this is that the viscosity of the PC melt is too large. When the injection pressure and injection rate are small, the melt is filled in a stagnation mode. As soon as the front-end melt contacts the cold mold surface, it will quickly condense and shrink. The hot molten material later expands and shrinks the cold material under the impetus of the pressure to continue to advance. This process is carried out alternately, and a vertical corrugation line is formed on the feeding side of the material.
The solution is:
Increasing the temperature is mainly to increase the temperature of the nozzle, the temperature of the front end of the barrel and the temperature of the mold, especially the temperature at the place where the ripples are generated.
This is to reduce the melt viscosity of PC and improve the melt fluidity. And if the product is more precise and strict on the appearance, it is necessary to add a mold temperature machine to accurately control the mold temperature at about 120 ° C.
The increase of injection rate and injection pressure is mainly to increase the melt flow rate at the “fingerprint” and prevent the melt from flowing in the form of stagnation. If the “fingerprint” is generated in the center of the product or away from the gate, multi-stage injection is used to adjust the injection rate step by step.
The modification of the mold is mainly to reduce the resistance to the melt during the filling process, such as increasing the size of the runner and the gate; pay attention to the polishing of the nozzle hole and the runner; increase the exhaust groove and groove; Ejector device for ejector; improve the exhaust condition of the mold; set a large enough cold material trap to reduce the blocking effect of the cold material at the front end.
Turbulent flow marks are irregular flow lines around the gate on the PC product. Unlike the “fingerprint” line, the turbulence marks appear along the direction of the flow rather than the direction of the vertical flow. The reason may be that the molten material injected into the mold cavity is subject to a large impact, which causes it to stick and slip when it is on the cold mold.
The solution is to increase the melt temperature to reduce the premature cooling of the melt; increase the temperature of the mold, especially the temperature of the turbulent flow marks, to prevent the melt from sliding prematurely in the mold cavity; using multi-stage injection, The injection rate and injection pressure of the turbulent flow marks will be appropriately reduced; change the position of the gate to change the flow of the molten material; increase the cold material well to prevent the cold material from sliding in the mold; use a material with good fluidity, Make the melt filling smooth.
Cold spots on four products
Cold spot is one of the common defects in the gate area of PC products. The phenomenon is that the product has a fog or bright color paste near the gate, or a curved scar like an earthworm from the gate that sticks to the surface of the product. The reason for its formation is mainly caused by the advance of the cold front of the molten material entering the mold cavity or the cold material that is subsequently squeezed into the mold cavity due to excessive pressure holding. The front material is caused by the contact of the nozzle with the cold template or the cooling of the flow channel. The action dissipates the heat, and when it enters the mold cavity, there is the subsequent pushing of the hot melt, so a cold spot is formed.
Cold material spots will be spread out on thinner products to become smoke-like or paste-like turbidity marks, while free-flowing thick-walled products will leave a scar that bends into the shape of an earthworm. As for the cold spot caused by excessive holding pressure, it is because the holding time is too long. When the holding pressure is too large, the cold material on the runner and the gate will continue to be squeezed into the product. Spots often make a small area near the gate form round bright spots.
Another type is that the molten material is quickly squeezed into a small gate and the melt is broken around the gate, or a smoke cloud or a bright spot appears at the gate due to the interference of the gas in the mold. Cold spots not only damage the apparent quality of the product, but also affect the effect of subsequent processes such as spraying or plating, and also reduce the mechanical strength of the product to varying degrees.
Possible solutions to this deficiency are:
- Increase the barrel and nozzle temperature and increase the mold temperature to reduce the impact of cold materials;
- Slow down the injection rate and increase the injection pressure to avoid the melt rupture or the interference of the gas in the mold;
- Adjust injection time and dwell time to avoid overfilling;
- Reasonable mold gate design can reduce or avoid the formation of cold material spots in advance. The traditional and effective method is to open a cold material well at the end of the runner to make the forward material sink into the well without entering the mold cavity. Some molds In addition to the cold material trap, the rationality of the form, size and location of the gate also needs to be considered;
- Strengthen mold exhaust; remove pollutants in the material, strengthen the drying effect of the material, reduce or exchange lubricants, and minimize the use of mold release agents.
Internal stress of transparent products
In the production of PC transparent products such as sunglasses, windshields, eye masks and other parts, deformation, astigmatism, poor transparency and cracking are often found. This is mainly due to the internal stress inside the product.
In fact, there are internal stresses inside the opaque products, but the performance is not obvious. Internal stress refers to the stress generated in the plastic due to improper molding, temperature changes, etc. in the absence of external forces. The essence of plastic stress is caused by the high elastic deformation of plastic molecules that are frozen in the product.
The internal stress of plastic products can affect the mechanical properties and use properties of the products, such as warping, deformation and even small cracks; the optical properties of the products become worse, and the products become turbid. The internal stress will also make the injection molded product show higher mechanical properties in the flow direction, while the strength perpendicular to the flow direction is lower, making the product performance uneven, thereby affecting the use of the product. Cracking is accelerated especially when the article is heated or in contact with organic solvents.
The internal stress of PC products is mainly caused by orientation stress and temperature stress, and sometimes related to improper mold release. Orientation stress Injection molding products tend to generate internal stress after orientation of macromolecules, resulting in stress concentration. The melt is rapidly cooled during injection, the melt viscosity is higher at a lower temperature, and the oriented molecules cannot relax sufficiently. The internal stress thus generated has an impact on the mechanical properties and dimensional stability of the product.
Therefore, the melt temperature has the greatest influence on the orientation stress. When the melt temperature is increased, the melt viscosity decreases, so the shear stress and orientation are reduced. In addition, the degree of orientation stress relaxation is high at high melt temperatures, but when the viscosity is reduced, the pressure transmitted by the injection molding machine screw to the mold cavity increases, which may increase the shear rate and cause the orientation stress to increase. If the dwell time is too long, the orientation stress will increase. Increasing the injection pressure will also increase the orientation stress due to the increase of shear stress and shear rate.
The thickness of the product also has an effect on the internal stress. The orientation stress decreases as the thickness of the product increases, because the thick-walled product cools slowly, the melt cools and relaxes in the mold cavity for a long time, and the orientation molecules have sufficient time to return to the random state. If the mold temperature is high, the melt is cooled slowly, which can reduce the orientation stress.
(2) Temperature stress The temperature difference between the melt temperature and the mold temperature of the plastic during injection is very large, which makes the melt close to the mold wall cool more quickly, so that the stress is unevenly distributed in the product volume. Because the specific heat capacity of PC is large and the thermal conductivity is small, the surface layer of the product cools much faster than the inner layer. The solidified shell layer formed on the surface of the product will hinder the free shrinkage when the interior continues to cool. The layer then generates compressive stress.
The greater the stress caused by shrinkage of thermoplastics, the lower the stress generated by the material in the mold due to compaction, that is, the short holding time and low holding pressure can greatly reduce the internal stress. The shape and size of the product also have a great influence on the internal stress. The larger the ratio of the surface area to the volume of the product, the faster the surface is cooled, and the greater the orientation stress and temperature stress. Orientation stress is mainly generated in the thin surface layer of the product. Therefore, it can be considered that the orientation stress should increase as the ratio of the surface of the product to its volume increases. If the thickness of the product is uneven or the product has metal inserts, it is easy to produce orientation stress, so the inserts and gates should be located at the thick wall of the product.
From the above analysis, it can be known that due to the structural characteristics of plastics and the limitations of the injection molding process conditions, it is impossible to completely avoid the internal stress. It can only reduce the internal stress as much as possible or evenly distribute the internal stress in the product.
The method is: the injection temperature has a great influence on the internal stress of the product, so the cylinder temperature should be appropriately increased to ensure that the material is plasticized well, the components are uniform to reduce the shrinkage and reduce the internal stress; increase the mold temperature to make the product Slow cooling to relax the orientation molecules and reduce internal stress.
Excessively high injection pressure can increase the orientation of plastic molecules and generate large shear forces, orderly arrange the plastic molecules, and increase the orientation stress of the product. Therefore, try to use a lower injection pressure; Too long, the pressure in the mold is increased due to the pressure compensation effect, the melt has a higher compression effect, the molecular orientation is increased, and the internal stress of the product is increased, so the holding time should not be too long.
The impact of injection rate on the internal stress of the injection molded part is much smaller than the factors such as temperature and pressure, but it is best to use variable speed injection, that is, fast filling. When the cavity is full, switch to low speed. Reduced weld marks, on the other hand, low-speed holding pressure can reduce molecular orientation.
Reasonably design the gate position. It is best to use slot-shaped or fan-shaped gates for flat products. The ejection device should be designed to eject in a large area. The demolding slope should be large. Use better materials (less impurities and high molecular weight) as much as possible, without gate materials.
When the product has a metal insert, the insert material needs to be heated in advance (generally, it is required to be about 200 ° C) to prevent the internal stress of the metal material from inconsistent with the coefficient of linear expansion of the plastic material. The transition needs to use an arc transition.
After ejection, heat treatment can be used to eliminate internal stress. The temperature of the heat treatment is about 120 ° C, and the time is about 2h. The essence is to make the segments and links in the plastic molecules have a certain ability to move, the frozen elastic deformation is relaxed, and the oriented molecules return to a random state. It is best not to use a release agent, otherwise it will easily cause internal stress and cause the product to be opaque, streaked or cracked.
