Polycarbonate (PC) is an engineering plastic with excellent performance. It not only has high transparency and excellent impact toughness, but also is creep-resistant, non-toxic, has a wide operating temperature range, good dimensional stability, excellent electrical insulation and good weather resistance, so it is widely used in instrumentation, lighting appliances, electronic and electrical equipment, household appliances, packaging and other industries. In addition, with the rapid advancement of polymer material technology, new modified varieties of polymer materials continue to emerge, which also greatly expands its application fields. At the same time, its processing technology has attracted more and more attention from all aspects.
PC is a linear polymer containing benzene rings, isopropyl groups, and acetate bonds in main chain structure of molecule. This structure makes it both rigid and flexible, and has good high-temperature resistance. However, it also has shortcomings such as high melt viscosity and sensitivity to moisture, which makes injection molding difficult. Its processing technology characteristics are that it has no obvious melting point, and melt viscosity is high in normal processing temperature range of 230-320℃. Viscosity is less sensitive to shear rate but more sensitive to temperature, which is similar to Newtonian fluid behavior; it is sensitive to moisture, and resin is easily hydrolyzed at high temperatures; products are prone to internal stress, etc. It can be seen that PC is a plastic that is difficult to process. Therefore, in actual production process, we encountered many problems. Now we will analyze and discuss some of the more common product defects.

 

Relatively speaking, PC has better heat resistance. Usually when processing ordinary PC materials, its melting temperature can be set at 240-300℃. Even if it stays for a long time, it generally will not decompose. But why do discolorations often occur when producing some electrical products? This is because current market competition is fierce. In order to reduce production costs, most manufacturers use PC modified materials or recycled materials when producing medium and low-end electrical products. Some manufacturers even use materials with their own added flame retardants, fillers, etc. Because these materials are miscellaneous and plasticization requirements are high, process control is more difficult, resulting in various problems.
In response to above phenomenon, it is necessary to consider and find solutions from following aspects:
(1) In terms of process conditions, melting temperature is mainly considered. Generally, barrel temperature must be reduced step by step, especially temperature of the first two stages, and different temperatures should be used for different materials. If polyethylene (PE) modified PC is used to produce large electrical products, barrel temperature should generally be controlled at around 230℃; For another example, when using ABS or PS modified PC to produce small electrical devices such as switches and sockets, barrel temperature should generally be controlled at around 250℃; while when using PBT modified PC to produce lighting products, barrel temperature should generally be controlled at around 280℃. Of course, final selection of molding temperature must also comprehensively consider product shape, size, mold structure, product performance requirements, etc.
Second step is to fully dry raw materials to reduce possibility of catalytic cracking of hot melt by trace moisture. In addition, if screw speed is too fast, back pressure is too high, injection rate is too fast, nozzle hole diameter, runner, gate size, etc. are too small, melt will generate high shear heat, causing PC to melt rupture, and it is easy for gas in mold cavity to not be discharged in time, causing local burns and blackening of product. .

(2) In terms of equipment, due to high viscosity of PC melt, poor fluidity, high injection pressure required, strong bonding with metallurgy, and decomposition products are highly corrosive to metals, when selecting processing equipment, it is required to use small or specially designed, chrome-plated screws, and plasticizing system does not allow dead corners, dead materials, gaps, cracks, etc.

 

Generally speaking, if there is no problem with process conditions, but molten material is discolored during air injection, it means that there is a problem with plasticizing system. Plasticizing system needs to be checked one by one, starting from nozzle, to nozzle flange, three small parts, screw, and barrel. Sometimes product will periodically change color in two or three molds at regular intervals. This is mostly related to presence of dead materials in plasticizing system, because when PC decomposition exceeds a certain amount, it has its own catalytic effect, causing a large melt to decompose, especially plastics with flame retardants added. This requires identifying dead spots such as screw sticking, stocking, barrel sticking, etc., which need to be solved through cleaning, repairing, and polishing.
(3) In terms of materials and operating methods, if you find black spots as soon as you turn on machine, it is probably related to material stored in barrel.
Therefore, attention must be paid to operation method. When material stored in barrel is PC before starting machine, barrel must be cleaned 3 to 4 times with new material at molding temperature (injection into air). If stored material is other materials, especially materials with poor thermal stability such as PVC, POM, etc., this requires that temperature cannot be raised when starting up, PC materials cannot be used to clean barrel, and can only be cleaned with materials with good thermal stability such as PS, PE, etc. at a lower temperature. After cleaning, raise barrel temperature to normal PC processing temperature, then clean it with PC material before processing.
During processing, if production needs to be temporarily suspended, barrel temperature needs to be lowered to below 160℃ (because glass transition temperature of PC is 160℃) to prevent material from decomposing and discoloring for too long. When completing production task, use PS, PE and other materials with good thermal stability to clean barrel, then stop machine after emptying it. If there is discoloration during production process, you need to first check whether there are any problems with materials, such as whether they are mixed with other materials and foreign matter, whether there are quality problems with new materials, whether gate materials are qualified, whether mixing method is correct, etc. After eliminating them one by one, check for other reasons.
Another factor is serious environmental pollution, such as a lot of dust floating in the air, mold being contaminated, self-drying hopper filter not working and sucking in more dust particles, etc. This requires processing workshop to be kept clean and tidy at all times. It is best to cover air inlet and outlet of hopper with fine gauze. This is very necessary when processing transparent products.

Silver streaks, bubbles, and vacuum bubbles in products are one of common product defects of PC materials. Causes of these defects are many and complex, so they are difficult to judge and eliminate.
Silver streaks (or gas streaks) refer to defects in direction of molten material on the surface of product caused by gas interference during mold filling process. Components of gas mainly include water vapor, air, decomposition gas and solvent gas, among which water vapor, decomposition gas and air are the most common. When these gases exceed a certain limit, mold cavity will lose pressure after injection molding, and gases close to the surface of product will emerge, etching along direction of material flow into a series of large and small bubble points that sparkle under light, called silver streaks or gas streaks.
In fact, presence of gas is inevitable during injection molding process, and a considerable part of it remains inside plastic. When pressure inside mold is large enough and gas content does not exceed a certain limit, gas will dissolve into plastic in a dispersed state; but when pressure inside mold is not high enough and gas content exceeds a certain limit, these gases will be released from molten plastic one after another, reach surface of product and form silver streaks, and become trapped in thick wall to become bubbles.
Whether it is silver streaks on the surface of product or bubbles in the wall of product, it may be result of main action of one of four gases or result of combined action of several gases. It interacts with factors such as raw materials, molds, plasticizing systems, adjustment of process parameters, and even changes in weather (especially changes in humidity), so this problem is relatively complex. But no matter what, focus of problem and the solutions should be focused on gas, that is, how to control gas content.
(1) Water vapor. Generally speaking, if bubbles are scattered irregularly on the surface of product, it is mostly caused by water vapor.
PC hot melt materials are very sensitive to moisture and require a moisture content of less than 0.02%. Therefore, in order to control moisture content, material must be fully dried. Generally, drying temperature of PC materials is around 120℃, and drying time is about 4 hours. Time should not be too long. If it exceeds 10 hours, material will easily deteriorate. In particular, materials with added flame retardants should not be dried for too long; as for drying method, dehumidification dryers have the best effect and have no impact on materials. To check whether drying effect is good, you can use air injection method to see whether injected material is continuous, smooth and does not emit white gas.
(2) If air bubble particles are extremely fine and dense, they are mainly distributed around gate of product, forming ray-like or fan-shaped patterns, which are mostly caused by air.
Source of air is air entrained in material. When there is a lot of gate material and particle sizes vary greatly, air is easily entrained. Therefore, when using sprue material, it is best to screen out powder. If back pressure during melting is too low and screw speed is high, screw will retreat too quickly and air will easily be pushed to the front of barrel along with material. Therefore, it is generally recommended to extend melting time as much as possible during cooling time, which is very helpful to improve plasticizing quality.
If temperature of unloading section is not well controlled, if temperature is too high, part of material will melt prematurely and block path for air to exit unloading port; if temperature is too low, preheating will be insufficient, causing part of pellets to enter homogenization section and become entrapped in air. In addition, if amount of loosening is too large, air will be sucked in. In above situation, generally adjusting screw speed, back pressure and loosening amount can roughly solve problem of exhaust during mold filling process.
In order to ensure smooth filling of PC materials with higher melt viscosity, it is generally necessary to increase melt temperature and increase injection pressure. Under high temperature and pressure, if injection rate is fast, melt will suddenly enter through narrow runner and gate into mold cavity with a large free space. In this way, gas released from melt entrains air in runner and mold cavity to form a high-speed injection state. Traces of dispersed airflow, namely air streaks, will appear on the surface of condensed plastic.
In addition, if there are many corners in mold cavity, difference in thickness is too large, or there are many inserts, and gate position is inappropriate, melt will flow into mold cavity, stirring up air in mold to form eddy currents, air marks will be formed in certain parts. For example, in switch and socket panels of molded electrical products, this situation often occurs because sockets, interfaces and switches are concentrated in one location.
Solution to this flaw is to:
On the one hand, mold is modified, mold exhaust is strengthened, and gate position is optimized; On the other hand, reduce mold filling rate, especially injection rate in areas with air marks.
(3) Decomposition gas. Since PC materials need to be molded at high temperatures, some decomposition is inevitable. However, how to avoid large-scale decomposition and how to eliminate gas is worth exploring.
As with above-mentioned discoloration, main cause of decomposition is excessive melt temperature. For example, if barrel temperature is set too high, or heating coil of barrel is out of control, you should start with nozzle and check heating coil section by section to reduce barrel temperature; melt stays in barrel for too long (for example, large equipment is used to produce small products, and the amount of cushioning pad is too large), molding cycle is too long, or dead material in barrel and material in dead corner are decomposed due to long-term heating; Or melt is subject to strong shearing in barrel, such as screw compression ratio is too large, screw speed is too high, back pressure is too high, etc., it will also decompose. In addition, nozzle aperture is too small, mold gate and runner are too small, and cavity resistance is large, which can cause passing melt to decompose due to local overheating due to friction. Therefore, when processing PC materials, nozzle hole diameter, gate, and runner size should be larger, exhaust groove should be deep, and it is not suitable to make thin-walled products.
Another important reason is that PC itself is of poor quality and easy to decompose. This is often ignored by users, problem is pushed to mold and processing equipment, so that correct solution to problem cannot be found.
(4) Solvent gas. Solvent gas is mainly related to quality of operations in production, such as unclean cleaning of barrel, too much additives, etc. Most of solvent gas can be removed by sufficient drying, and it does not have a great impact on gas marks.
Sometimes it is difficult to distinguish whether bubble points exist inside transparent products or are vacuum bubbles. It is generally believed that if a bubble point is found at the moment of mold opening and volume does not change after a period of storage, it is a bubble, which is caused by gas interference; if it appears and becomes larger during demolding and cooling process, it is a vacuum bubble. Formation of vacuum bubbles is due to insufficient material or low pressure during mold filling. Under rapid cooling of mold, surface of molten material in contact with mold wall first solidifies, then molten material in the central part cools and shrinks, causing volume to shrink and form a hollow point, that is, a bubble point.
Solution is:
Increase injection pressure, injection time and material volume;
Adjust material temperature: When vacuum bubble is far away from gate, increase material temperature to make melt flow smoothly and pressure can be transmitted to part far away from gate;
When vacuum bubble is near gate, material temperature can be lowered to reduce shrinkage;
Appropriately increase mold temperature, especially local mold temperature where vacuum bubble is formed;
Set gate in thick-walled part of product to improve flow conditions of nozzle, runne, gate and mold exhaust condition;
Shorten cooling time of product in mold, and if necessary, put product into hot water to cool slowly;
Products molded with point gates can be molded at slow speed and low temperature to solve vacuum bubble problem. When there are vacuum bubbles on runner, runner size can be increased.
In addition, during production process, it was also found that bubbling appeared in thick-walled parts of PC products soon after demoulding. This was caused by insufficient cooling that caused gas inside PC to expand. Generally, it can be solved by extending cooling time, strengthening cooling effect, increasing holding pressure and time, and delaying decomposition of PC.

Due to high viscosity and poor fluidity of PC melt, products are prone to "fingerprints" and turbulent flow marks. These two phenomena are more common when processing switch sockets and general electrical device panels, and because of their similar shapes, these two phenomena are sometimes difficult to distinguish. In fact, causes of these two phenomena are different, and their solutions are also different.
(1) "Fingerprints" are named because their shape resembles human fingerprints. Sometimes they are also called ripples, vibration patterns or shock patterns, which means that their patterns are like those formed by stones dropped on calm water.
Main reason is that PC melt viscosity is too high. When injection pressure and injection rate are small, melt fills mold in a stagnant form. As soon as front-end melt contacts cold mold surface, it quickly condenses and shrinks, while hot molten material at the back expands shrunk cold material under pressure and continues to move forward. Alternating process of this process forms vertical corrugated lines on direction of material flow.
Solution is:
To increase temperature, it is mainly to increase temperature of nozzle, temperature of front end of barrel and temperature of mold, especially temperature where ripples are generated. This is to reduce melt viscosity of PC and improve melt fluidity. And if product is relatively precise and has strict appearance requirements, it is necessary to add a mold temperature controller to accurately control mold temperature at around 120℃.
Increasing injection rate and injection pressure is mainly to increase melt flow rate at "fingerprint" and prevent melt from flowing in a stagnant form. If "fingerprints" occur in the center of product or away from gate, multi-stage injection must be used to adjust injection rate step by step.
Modifying mold is mainly to reduce resistance encountered by melt during mold filling process, such as increasing size of runner and gate; paying attention to polishing nozzle hole and runner; enlarging exhaust ditch and groove; setting up inserts and ejector air guide devices; improve mold exhaust condition; set up a large enough cold material trap to reduce flow blocking effect of front end cold material.
(2) Turbulent flow marks refer to irregular flow lines centered on gate on PC products. Unlike "fingerprint" lines, turbulent flow marks appear along direction of material flow rather than perpendicular to direction of material flow. Reason may be that molten material injected into mold cavity is subject to a large impact, causing it to sometimes stick and sometimes slip on cold mold.
Solution is:
Increase melt temperature to reduce premature cooling of melt;
Increase temperature of mold, especially temperature of parts where turbulent flow marks appear, to prevent molten material from sliding due to premature cooling in mold cavity;
Use multi-stage injection to appropriately reduce injection rate and injection pressure in areas where turbulent flow marks appear;
Change gate position to change flow pattern of molten material;
Enlarge cold material well to prevent cold material from sliding in mold; use materials with good fluidity instead to make molten material fill mold smoothly.

Cold spot is one of common defects at gate of PC products. Phenomenon is that product has foggy or bright spots near gate, or curved scars from gate that stick to the surface of product like earthworms.
Main reason for its formation is that molten material entering mold cavity is pushed forward by cold material or cold material is later squeezed into mold cavity due to excessive pressure holding. Front material transfers heat due to contact between nozzle and cold template or cooling effect of runner. When it enters mold cavity, it is pushed by hot melt, thus forming a cold material spot. Cold material spots will spread out and become smoky or mushy turbid spots on thinner products, while on free-flowing thick-walled products, they will leave a scar that curves like an earthworm. As for cold material spots caused by excessive pressure holding, it is caused by holding time being too long and cold material on runner and gate continuing to be squeezed into product when holding pressure is too high. This cold material spot often forms a circular bright spot in a small area close to gate.
There is also a situation where melt is quickly squeezed into a small gate and causes melt rupture around gate, or smoke cloud-like or ray-like bright spots appear at gate due to interference of gas in mold. Cold spot not only damages apparent quality of product, but also affects effect of subsequent processes such as spraying or electroplating, and also reduces mechanical strength of product to varying degrees.
Possible solutions to this defect are:
Increase temperature of barrel and nozzle, and increase temperature of mold to reduce impact of cold material;
Slow down injection rate and increase injection pressure to avoid melt rupture or interference from gas in mold;
Adjust injection time and pressure holding time to avoid overfilling;
Reasonable mold gate design can reduce or avoid formation of cold spots in advance. Traditional and effective method is to set up a cold slug trap at the end of runner so that forward material sinks into it without entering mold cavity. In addition to cold slug trap, some molds also need to consider rationality of form, size and location of gate;
Strengthen mold exhaust; remove pollutants in materials, enhance drying effect of materials, reduce or replace lubricants, and use as little release agent as possible.

When producing PC transparent products such as sunglasses, windshields, eye masks and other products, deformation, astigmatism, poor transparency and cracking of products are often found. This is mainly due to internal stress within products. In fact, there is also internal stress inside opaque products, but it is not obvious. Internal stress refers to stress inside plastic caused by improper molding, temperature changes, etc. in the absence of external force. Its essence is caused by high elastic deformation of plastic molecules being frozen in product.
Internal stress of plastic products can affect mechanical properties and performance of products, such as warping, deformation and even small cracks; optical properties of products become worse, and products become turbid. Internal stress will also cause injection molded products to show higher mechanical properties in flow direction, while strength perpendicular to flow direction is lower, making product performance uneven and affecting use of product. Especially when product is heated or comes into contact with organic solvents, cracking of product will be accelerated.
Internal stress of PC products is mainly caused by orientation stress and temperature stress, and is sometimes related to improper demolding. Orientation stress: When internal macromolecules of injection molded products are oriented, internal stress is easily generated, causing stress concentration.
During injection molding, melt cools rapidly, melt viscosity is higher at lower temperatures, and oriented molecules cannot fully relax. Internal stress thus generated has an impact on mechanical properties and dimensional stability of product. Therefore, melt temperature has the greatest impact on orientation stress. When melt temperature is increased, melt viscosity decreases, so shear stress and orientation decrease.
In addition, degree of relaxation of orientation stress is greater at high melt temperatures, but when viscosity decreases, pressure transmitted from screw of injection molding machine to mold cavity increases, which may increase shear rate, resulting in an increase in orientation stress. If holding time is too long, orientation stress will increase; increasing injection pressure will also cause orientation stress to increase due to increase in shear stress and shear rate. Thickness of product also affects internal stress. Orientation stress decreases as thickness of product increases. Because thick-walled products cool slowly, melt cools and relaxes in mold cavity for a long time, oriented molecules have sufficient time to return to random state. If mold temperature is high and melt cools slowly, orientation stress can be reduced.
(2) Temperature stress. Temperature difference between melt temperature and mold temperature is very large during injection molding of plastics, which causes melt close to mold wall to cool quickly, resulting in stress that is unevenly distributed within volume of product.
Due to large specific heat capacity and small thermal conductivity of PC, surface layer of product cools much faster than inner layer. Solidified shell formed on the surface of product will hinder free shrinkage of interior as it continues to cool, resulting in tensile stress inside product and compressive stress in outer layer. The greater stress generated by shrinkage of thermoplastic plastics, the lower stress generated by compaction of material in mold, that is, holding time is short and holding pressure is low, which can greatly reduce internal stress. Product shape and size also have a great impact on internal stress. The greater ratio of surface area to volume of product, the faster surface cooling, the greater orientation stress and temperature stress. Orientation stress mainly occurs in thin surface layer of product. Therefore, it can be considered that orientation stress should increase as ratio of product surface to its volume increases. If thickness of product is uneven or the product has metal inserts, it is easy to produce directional stress, so inserts and gates should be located at thick wall of product.

 

From above analysis, it can be seen that due to structural characteristics of plastics and limitations of injection molding process conditions, it is impossible to completely avoid internal stress. The only way is to minimize internal stress or try to make internal stress evenly distributed within product.
Method is:
Injection temperature has a great influence on internal stress of product. Therefore, barrel temperature should be appropriately increased to ensure good plasticization of material, make components uniform to reduce shrinkage and reduce internal stress; increase mold temperature to slow down cooling of product to relax orientation molecules and reduce internal stress.
Excessive injection molding pressure can increase orientation of plastic molecules and produce large shear forces, causing plastic molecules to be arranged in an orderly manner and increasing orientation stress of product. Therefore, a lower injection molding pressure should be used as much as possible; if holding time is too long, pressure in mold will increase due to pressure compensation effect, molten material will have a higher extrusion effect, and degree of molecular orientation will increase, which will increase internal stress of product. Therefore, holding time should not be too long.
Impact of injection rate on internal stress of injection molded parts is much smaller than factors such as temperature and pressure. However, it is best to use variable speed injection, that is, rapid mold filling. When mold cavity is full, switch to low speed. On the one hand, variable speed injection speeds up mold filling process and reduces weld marks. On the other hand, low speed pressure holding can reduce molecular orientation.
Reasonably design gate location, and it is best to use slot-shaped or fan-shaped gates for flat products;
Ejection device should be designed to eject a large area;
Draft must be large. Try to use better materials (less impurities, larger molecular weight) without gate materials. When product has a metal insert, insert material needs to be preheated (generally required to reach about 200℃) to prevent internal stress caused by inconsistent linear expansion coefficients of metal material and plastic material. Transition point needs to be an arc transition.
After mold is released, heat treatment can be used to eliminate internal stress. Heat treatment temperature is about 120℃ and time is about 2 hours. Its essence is to make chain segments and links in plastic molecules have certain mobility, frozen elastic deformation is relaxed, and 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 product to be opaque, streaky or cracked.