深圳大学：《塑料成型工艺与模具》课程教学资源（教材讲义）Chapter 5 Mold Testing.pdf_大学文库

System Whether entrance/exit of the hot and cold medium channel Eyeballing are marked Whether connector of tubes has been assembled Eyeballing er block and been assembled Whether all screws are secure Sampling System ctIon Whether the thimble requiring anti-reverse is provided IPQC with an anti-reverse unit. Whether all problem points listed in the MPI have been IPQC Whether all parts have been assembled. Eyeballing Miscellaneous Whether the interposing space of the moving and fixedEyeballing molds over the parting line has been aired for 0.5mm. Whether the oil stain and smudge on the mold seat, core Eyeballing ave been Whether the wedge block is assembled Eyeballing 5.3 TIR After PSI starts the testing, after which the testing workers shall fill in the tiR and meanwhile the problems shall be brought into and analyzed within the range of the TiR standard problem database. wherein the analysis include the problem points, description of problem points, origin of problem as well as modification countermeasures etc. If it is beyond the problem database, consideration need be made on whether the current problem database shall be expanded. Common testing problems and countermeasures are as follows 1. Cannot Be Sufficiently Filled The plastic melt condenses and solidifies before it can smoothly fill in the cavity part far away from e gate or that with thin section, the cavity therefore is insufficiently filled and the shape and structure of the molded products are incomplete, which is intrinsically due to the low fluidity and poor filling capacity of the plastic melt. The extrinsic cause, however, is due to the large resistance against filling and the insufficient filling power. Specific causes thereof are as follow The plastic melt is provided with high flow viscosity, resulting in poor flow and filling capacity; 2 The plastic melt has poor plasticity, excessively low temperature for plastication and unbalanced melt temperature, which altogether result in the poor melt fluidity;

Whether entrance/exit of the hot and cold medium channel are marked. System Eyeballing Whether connector of tubes has been assembled. Eyeballing Whether the positioning block and positioning ring have been assembled. Eyeballing Whether all screws are secure. Sampling Inspection Positioning System Whether the thimble requiring anti-reverse is provided with an anti-reverse unit. IPQC Whether all problem points listed in the MPI have been settled. IPQC Whether all parts have been assembled. Eyeballing Whether the interposing space of the moving and fixed molds over the parting line has been aired for 0.5mm. Eyeballing Whether the oil stain and smudge on the mold seat, core and cavity have been cleaned. Eyeballing Miscellaneous Whether the wedge block is assembled Eyeballing 5.3 TIR After PSI starts the testing, after which the testing workers shall fill in the TIR and meanwhile the problems shall be brought into and analyzed within the range of the TIR standard problem database, wherein the analysis include the problem points, description of problem points, origin of problem as well as modification countermeasures etc. If it is beyond the problem database, consideration need be made on whether the current problem database shall be expanded. Common testing problems and countermeasures are as follows: 1. Cannot Be Sufficiently Filled The plastic melt condenses and solidifies before it can smoothly fill in the cavity part far away from the gate or that with thin section, the cavity therefore is insufficiently filled and the shape and structure of the molded products are incomplete, which is intrinsically due to the low fluidity and poor filling capacity of the plastic melt. The extrinsic cause, however, is due to the large resistance against filling and the insufficient filling power. Specific causes thereof are as follows: ① The plastic melt is provided with high flow viscosity, resulting in poor flow and filling capacity; ② The plastic melt has poor plasticity, excessively low temperature for plastication and unbalanced melt temperature, which altogether result in the poor melt fluidity;

3 Mold temperature is overlow or unbalanced, temperature of some parts is too low and temperature of melt injected into the cavity drops too fast; a The flow of melt in the mold is too long or too complicated, the resistance against filling is and the filling is not smooth, even resulting in stagnation 6 Temperature of the nozzle is too low and the cold-slug well of feed system is improperly set resulting in jam and stagnation when the front cold material of melt enters the cavity 6 During the filling, molds are not sufficiently aired and the gas has not been discharged in time thereby resulting in the expansion of filling resistance O The actual quantity of injection for the injection machine is too small and the melt required filling is insufficient, thereby directly resulting in material shortage in the cavity; 8 The injection stroke of screw is ended and the plastic stocked in the plastication room is scarce, resulting in poor transmission of injection pressure and insufficient post-stage power of filling: 9 Injection rate of screw is excessively low, the injection pressure is overlow and the filling power is insufficient (0 Wall thickness of products is uneven and the filling resistance is (10 Wall of products is too thin and the filling resistance is large; above all, temperature of melt drops too fast when it flows through and the hertz increases Solutions O Use plastics with low viscosity and sound fluidity or adjust prescription of plastics and reduce iscosity of melt 2 Improve barrel temperature, increase injection pressure, raise melt temperature to reduce erection degree, and improve plastication quality 3 Improve mold temperature and uniformity thereof, raise nozzle tem nd properly cold-slug well Increase dimension of the runner and gate and properly set gate position to avoid overlong flow during filling and overlarge loss of pressure when the melt enters cavity; 5 Properly design mold exhausting scheme and change number of position of gate as well as position of parting line when necessary to achieve sound exhausting effect; 6 Select proper injection machine and correctly adjust screw injection stroke to ensure sufficient actual quantity of injection; meanwhile, it should be ensured that the melt stored in the plastication room at the front of the screw can perfectly transmit pressure. Check the plastic raw material in the hopper of injection machine in due course to see whether it is sufficient Increase injection pressure, injection rate and the filling power of melt; meanwhile, check whether the non-return valve on the screw is damaged to avoid pressure caused by the counterflow of melt as well as loss of melt during injection; 8 Improve structure of plastic parts and design of wall thickness, reduce complication of structure, improve uniformity of wall thickness and properly design wall thickness to reduce structural resistance during the melt filling stroke 2. Welding mark The thread mark formed at the front confluence of two or more melt flows in the cavity is called welding mark or welding line. Usually, apart from low mechanical intensity, such defect as depression and aberration may also occur at places where welding mark appears For plastic parts with holes and inserts or those of complicated structure and with uneven wall

③ Mold temperature is overlow or unbalanced, temperature of some parts is too low and temperature of melt injected into the cavity drops too fast; ④ The flow of melt in the mold is too long or too complicated, the resistance against filling is large and the filling is not smooth, even resulting in stagnation; ⑤ Temperature of the nozzle is too low and the cold-slug well of feed system is improperly set, resulting in jam and stagnation when the front cold material of melt enters the cavity; ⑥ During the filling, molds are not sufficiently aired and the gas has not been discharged in time, thereby resulting in the expansion of filling resistance; ⑦ The actual quantity of injection for the injection machine is too small and the melt required for filling is insufficient, thereby directly resulting in material shortage in the cavity; ⑧ The injection stroke of screw is ended and the plastic stocked in the plastication room is scarce, resulting in poor transmission of injection pressure and insufficient post-stage power of filling; ⑨ Injection rate of screw is excessively low, the injection pressure is overlow and the filling power is insufficient. ⑩ Wall thickness of products is uneven and the filling resistance is large; ⑩ Wall of products is too thin and the filling resistance is large; above all, temperature of melt drops too fast when it flows through and the hertz increases. Solutions: ① Use plastics with low viscosity and sound fluidity or adjust prescription of plastics and reduce viscosity of melt; ② Improve barrel temperature, increase injection pressure, raise melt temperature to reduce erection degree, and improve plastication quality; ③ Improve mold temperature and uniformity thereof, raise nozzle temperature and properly set cold-slug well ; ④ Increase dimension of the runner and gate and properly set gate position to avoid overlong flow during filling and overlarge loss of pressure when the melt enters cavity; ⑤ Properly design mold exhausting scheme and change number of position of gate as well as position of parting line when necessary to achieve sound exhausting effect; ⑥ Select proper injection machine and correctly adjust screw injection stroke to ensure sufficient actual quantity of injection; meanwhile, it should be ensured that the melt stored in the plastication room at the front of the screw can perfectly transmit pressure. Check the plastic raw material in the hopper of injection machine in due course to see whether it is sufficient; ⑦ Increase injection pressure, injection rate and the filling power of melt; meanwhile, check whether the non-return valve on the screw is damaged to avoid pressure caused by the counterflow of melt as well as loss of melt during injection; ⑧ Improve structure of plastic parts and design of wall thickness, reduce complication of structure, improve uniformity of wall thickness and properly design wall thickness to reduce structural resistance during the melt filling stroke. 2. Welding Mark The thread mark formed at the front confluence of two or more melt flows in the cavity is called welding mark or welding line. Usually, apart from low mechanical intensity, such defect as depression and aberration may also occur at places where welding mark appears. For plastic parts with holes and inserts or those of complicated structure and with uneven wall

hickness or those with overlarge structural dimension and requiring multi-gate feeding, plastic melt will form two or more flow filling in the cavity and the welding structure shall be unavoidably formed at the front confluence. The key index for measuring welding quality is the welding intensity at the welding mark, whereas the position of welding mark of which certain appearance quality is required follows in the second place The most influential factor for welding intensity is the temperature during th confluence or me actual engineering practice, commonly-used methods for treating welding mark include First, improve welding temperature to a degree as high as possible. Properly improve the melt plastication temperature, nozzle temperature, mold temperature as well as pre-warming temperature for inserts; reduce flow stroke or flow time of melt prior to confluence(sometimes the number of gates can be increased at no spare); improve screw rate to achieve rapid filling, which can not only reduce heat loss during the filling but also can increase the shearing friction heat during melt flow to remedy the heat loss of melt; optimize the design of feed system accompanied with proper injection pressure, increase shearing friction heating during the flow, improve flow state of melt and strengthen the capacity of confluence Next, change position of welding mark and locate it on a part insensitive to the mechanical property and appearance quality of products; change wall thickness on the premises of not influencing requirement for use of the plastic parts During mold design, preferably reduce the number of gates; properly design the cold-slug well and the exhausting measure for molds at places where welding mark occurs; use release agent as less as possible; sufficiently dry the raw materials 3. Shrink Mark, Depression and Shrinkage Porosity Both the shrink mark and shrinkage porosity are defects of injection products due to the molding shrinkage at thick walls where no sufficient follow-up compensation is made. During the injection molding process, the surface layer of products often condenses first, followed by the center layer, which will cause sinking on the surface of parts with thick walls, thereby producing obvious inner concave on surface of products, wherein the small concave is called shrink mark and the large concave is called depression; conversely, when parts with thick walls have condensed and when the surface intensity and rigidity are large enough to resist the shrinkage stress resulted from the subsequent shrink of the center part, a large shrinkage porosity or a series of minute shrinkage holes will form inside the parts with thick walls. Usually when such shrinkage porosities or holes appear, depression will more or less occur at the surface of products Although shrink mark has no impact on the structural intensity of products, it will cause rather obvious visual defect on the surface whereof, depression influences both surface quality and structural property of products; shrinkage porosity can bring bad effect on the structural property of products. The hree defects usually occur at parts with relatively thick walls or at hot spot of products such as the back of lug boss, stiffening rib as well as straight gate of sprue During actual production, optimization of structure and material of product, the injection technics as well as mold structure are usually adopted to avoid shrinkage porosity and meanwhile to control the surface shrink mark of products at the minimum degree that can be accepted by users. The specific solutions are as follows O Optimize the structural design of injection products. The injection products should not be to thick and when necessary the stiffening rib can be added to reduce wall thickness; the wall thickness

thickness or those with overlarge structural dimension and requiring multi-gate feeding, plastic melt will form two or more flow filling in the cavity and the welding structure shall be unavoidably formed at the front confluence. The key index for measuring welding quality is the welding intensity at the welding mark, whereas the position of welding mark of which certain appearance quality is required follows in the second place. The most influential factor for welding intensity is the temperature during the confluence of melt. In actual engineering practice, commonly-used methods for treating welding mark include: First, improve welding temperature to a degree as high as possible. Properly improve the melt plastication temperature, nozzle temperature, mold temperature as well as pre-warming temperature for inserts; reduce flow stroke or flow time of melt prior to confluence (sometimes the number of gates can be increased at no spare); improve screw rate to achieve rapid filling, which can not only reduce heat loss during the filling but also can increase the shearing friction heat during melt flow to remedy the heat loss of melt; optimize the design of feed system accompanied with proper injection pressure, increase shearing friction heating during the flow, improve flow state of melt and strengthen the capacity of confluence. Next, change position of welding mark and locate it on a part insensitive to the mechanical property and appearance quality of products; change wall thickness on the premises of not influencing requirement for use of the plastic parts. During mold design, preferably reduce the number of gates; properly design the cold-slug well and the exhausting measure for molds at places where welding mark occurs; use release agent as less as possible; sufficiently dry the raw materials. 3. Shrink Mark, Depression and Shrinkage Porosity Both the shrink mark and shrinkage porosity are defects of injection products due to the molding shrinkage at thick walls where no sufficient follow-up compensation is made. During the injection molding process, the surface layer of products often condenses first, followed by the center layer, which will cause sinking on the surface of parts with thick walls, thereby producing obvious inner concave on surface of products, wherein the small concave is called shrink mark and the large concave is called depression; conversely, when parts with thick walls have condensed and when the surface intensity and rigidity are large enough to resist the shrinkage stress resulted from the subsequent shrink of the center part, a large shrinkage porosity or a series of minute shrinkage holes will form inside the parts with thick walls. Usually when such shrinkage porosities or holes appear, depression will more or less occur at the surface of products. Although shrink mark has no impact on the structural intensity of products, it will cause rather obvious visual defect on the surface whereof; depression influences both surface quality and structural property of products; shrinkage porosity can bring bad effect on the structural property of products. The three defects usually occur at parts with relatively thick walls or at hot spot of products such as the back of lug boss, stiffening rib as well as straight gate of sprue. During actual production, optimization of structure and material of product, the injection technics as well as mold structure are usually adopted to avoid shrinkage porosity and meanwhile to control the surface shrink mark of products at the minimum degree that can be accepted by users. The specific solutions are as follows: ① Optimize the structural design of injection products. The injection products should not be too thick and when necessary the stiffening rib can be added to reduce wall thickness; the wall thickness

must be kept as uniform as possible and the shape and structure should be simple and symmetric; the parts with insufficient shrinkage compensation can be approached from structure of products, whereby the section of runner can be enlarged by adding stiffening rib to make it easier for shrinkage compensation; on important surface of products, structural intersection should be reduced as much as possible so that hot spot thereon can be reduced accordingly; at positions such as hot spot where depression may occur, thickness of cavity section can be properly incre compensation to counteract the shrinkage depression value; decorative patterns can also be designed at positions with shrink mark to cover up the visual defect caused by surface depression. 2 Optimize prescription of plastics and preferably use resins with low shrinkage; reduce dosage of recycled materials; dry the materials as much as possible; add proper amount of lubrication additive to improve fluidity of melt and to reinforce the effect of shrinkage compensation; select proper reinforcing filling material to reduce shrinkage and to improve the materials capacity for resisting shrinkage stress so that surface depression can be avoided 3 Optimize injection technics. Properly improve injection and dwell pressure as well as the injection rate to increase melts compression tightness; prolong time of injection and dwell(the freezing of gate should not occur too early) to realize sufficient shrinkage compensation; properly reduce melt temperature and properly raise mold temperature(whereas cooling should be reinforced at parts with hick walls)on the premises that the ejection quality and dimension precision of products after ejection are ensured; properly increase melt plastication quantity to ensure sufficient material supply and effective transmission of pressure; when depression occurs around the insert, the pre-warming temperature of insert should be increased to a certain degree. In addition, depression and shrinkage porosity which are unavoidable due to the structural property of products can be removed by adopting gas-assisted injection molding 4 Optimize mold design. Properly increase section dimension of gate and runner; the position of gate is set at thick walls of the plastic parts and should be as symmetric as possible; improve mold exhausting condition; set cold-slug well with sufficient capacity to avoid cold materials entering into the cavity and influencing filling and shrinkage compensation; properly dispose cooling water channel and intensify cooling at depressed parts with thick walls to achieve effective shrinkage compensation on the basis that uniform cooling for all parts of the cavity can be ensured; carefully analyze whether a "bottleneck"exists at the runner of melt within the cavity, otherwise, replace gate position or increase number of gate and when necessary, enlarge section of runner where the bottleneck is located; for products with thick walls, fan gate or flush joint gate can be adopted to shift the possible depression and shrinkage porosity to the gate 4. Flow mark Flow marks refer to the obvious flow cracks of melt that can be found near the gate. Sometimes it also called flow lines. Flow marks not only influence the surface quality of plastic parts but also can influence their mechanical property. According to the causes of flow marks and their different appearance features, they fall into such types as ejection flow mark, wheel wave flow mark centered around the gate turbulence flow mark and nebulous flow mark ① Ejection Flow Mark It is a snake-shaped squirt flow formed upon the melts entering the cavity with thick walls and large section with excessively high injection rate. Such snake-shaped squirt flow can stay at the surface of plastic parts, whereby influencing the appearance quality, and meanwhile, minute welding mark may

must be kept as uniform as possible and the shape and structure should be simple and symmetric; the parts with insufficient shrinkage compensation can be approached from structure of products, whereby the section of runner can be enlarged by adding stiffening rib to make it easier for shrinkage compensation; on important surface of products, structural intersection should be reduced as much as possible so that hot spot thereon can be reduced accordingly; at positions such as hot spot where depression may occur, thickness of cavity section can be properly increased through reverse compensation to counteract the shrinkage depression value; decorative patterns can also be designed at positions with shrink mark to cover up the visual defect caused by surface depression. ② Optimize prescription of plastics and preferably use resins with low shrinkage; reduce dosage of recycled materials; dry the materials as much as possible; add proper amount of lubrication additive to improve fluidity of melt and to reinforce the effect of shrinkage compensation; select proper reinforcing filling material to reduce shrinkage and to improve the material’s capacity for resisting shrinkage stress so that surface depression can be avoided. ③ Optimize injection technics. Properly improve injection and dwell pressure as well as the injection rate to increase melt’s compression tightness; prolong time of injection and dwell (the freezing of gate should not occur too early) to realize sufficient shrinkage compensation; properly reduce melt temperature and properly raise mold temperature (whereas cooling should be reinforced at parts with thick walls) on the premises that the ejection quality and dimension precision of products after ejection are ensured; properly increase melt plastication quantity to ensure sufficient material supply and effective transmission of pressure; when depression occurs around the insert, the pre-warming temperature of insert should be increased to a certain degree. In addition, depression and shrinkage porosity which are unavoidable due to the structural property of products can be removed by adopting gas-assisted injection molding. ④ Optimize mold design. Properly increase section dimension of gate and runner; the position of gate is set at thick walls of the plastic parts and should be as symmetric as possible; improve mold exhausting condition; set cold-slug well with sufficient capacity to avoid cold material’s entering into the cavity and influencing filling and shrinkage compensation; properly dispose cooling water channel and intensify cooling at depressed parts with thick walls to achieve effective shrinkage compensation, on the basis that uniform cooling for all parts of the cavity can be ensured; carefully analyze whether a “bottleneck” exists at the runner of melt within the cavity, otherwise, replace gate position or increase number of gate and when necessary, enlarge section of runner where the bottleneck is located; for products with thick walls, fan gate or flush joint gate can be adopted to shift the possible depression and shrinkage porosity to the gate. 4. Flow Mark Flow marks refer to the obvious flow cracks of melt that can be found near the gate. Sometimes it is also called flow lines. Flow marks not only influence the surface quality of plastic parts but also can influence their mechanical property. According to the causes of flow marks and their different appearance features, they fall into such types as ejection flow mark, wheel wave flow mark centered around the gate, turbulence flow mark and nebulous flow mark. ① Ejection Flow Mark It is a snake-shaped squirt flow formed upon the melt’s entering the cavity with thick walls and large section with excessively high injection rate. Such snake-shaped squirt flow can stay at the surface of plastic parts, whereby influencing the appearance quality, and meanwhile, minute welding mark may

form at the snake-shaped folding; moreover, it can also result in poor exhausting and reduce the quality of plastic parts. The ejection is mainly due to overhigh screw rate as well as improper position and type The treatment measures include optimizing the position, form and dimension of gate to enlarge contact between melt at the gate and the surface wall of cavity and to reduce melt shearing rate and shearing stress; reducing injection rate of screw to avoid overhigh rate when the melt is injected into mold cavity. Lapping gate, tab gate and fan gate can be adopted to achieve such purpose ② Wheel Wave Flow Mark When melt with unsteady flow is injected into the mold cavity through the runner and gate in semi-solidified state, refluence and stagnation can form during its flow in the cavity due to the extrusion and pushing by follow-up melt that is continuously injected therein, and thereby wheel wave flow mark centering around the gate occurs on the surface of plastic parts The treatment measures include raising mold temperature and nozzle temperature, improving injection rate, increasing injection and dwell pressure, properly designing size and position of cold-slug well and optimizing design of feed system so that the unsteady flow of melt can be improved through the shearing heat during the flow and filling of melt ③ Turbulence Flow Mark When the melt flows from a runner with narrow section into a cavity with large section or when the runner is very narrow and the surface roughness is rather poor, turbulence flow will form within the runner(including the cavity), thereby resulting in the turbulence flow mark on the surface of plastic The treatment measures include properly reducing screw injection rate or controlling it through hierarchical injection; opening a gate on the thick walls or side walls of plastic parts, wherein the type of gate should be propitious for the melt's steady entering the cavity, such as tab gate, fan gate and slice gate; sometimes the section dimension of runner and gate can be properly enlarged to reduce flow resistance; optimizing structure of plastic parts when necessary to avoid uneven wall thickness wherein melt in the cavity can flow from narrow section to large and thick section; in addition, the fluidity of melt can be improved through increasing temperature of mold, barrel as well as nozzle ④ Nebulous flow Mark The volatile gas produced from the resin in the melt and the lubricating agent under processing temperature is stored in between the melt flow and surface wall of cavity and interferes the flow of melt. whereby the nebulous flow mark is formed. The treatment measures withal are: improving the exhausting condition of molds, properly increasing dimension of gate and reducing melt filling rate properly reducing mold temperature and melt plastication temperature, replacing additives such as lubricating agent which can produc olatile gas 5. Crazing poge crazing is silvery white filamentous speckle occurring on the surface or near-surface of injection ducts along the direction of melt flow. It is resulted from the moving of gas in the resin which is then stored in between the melt and surface wall of cavity and subsequently crushed. Crazing sometimes is also called mica mark, silver line or silver speckle The most essential reason for the occurrence of crazing is that volatile gas is contained in the melt withal the following measures can be taken O As for raw materials. In allusion to degradation gas crazing, resins with uniform granularity are

form at the snake-shaped folding; moreover, it can also result in poor exhausting and reduce the quality of plastic parts. The ejection is mainly due to overhigh screw rate as well as improper position and type of gate. The treatment measures include optimizing the position, form and dimension of gate to enlarge contact between melt at the gate and the surface wall of cavity and to reduce melt shearing rate and shearing stress; reducing injection rate of screw to avoid overhigh rate when the melt is injected into mold cavity. Lapping gate, tab gate and fan gate can be adopted to achieve such purpose. ② Wheel Wave Flow Mark When melt with unsteady flow is injected into the mold cavity through the runner and gate in semi-solidified state, refluence and stagnation can form during its flow in the cavity due to the extrusion and pushing by follow-up melt that is continuously injected therein, and thereby wheel wave flow mark centering around the gate occurs on the surface of plastic parts. The treatment measures include raising mold temperature and nozzle temperature, improving injection rate, increasing injection and dwell pressure, properly designing size and position of cold-slug well and optimizing design of feed system so that the unsteady flow of melt can be improved through the shearing heat during the flow and filling of melt. ③ Turbulence Flow Mark When the melt flows from a runner with narrow section into a cavity with large section or when the runner is very narrow and the surface roughness is rather poor, turbulence flow will form within the runner (including the cavity), thereby resulting in the turbulence flow mark on the surface of plastic parts. The treatment measures include properly reducing screw injection rate or controlling it through hierarchical injection; opening a gate on the thick walls or side walls of plastic parts, wherein the type of gate should be propitious for the melt’s steady entering the cavity, such as tab gate, fan gate and slice gate; sometimes the section dimension of runner and gate can be properly enlarged to reduce flow resistance; optimizing structure of plastic parts when necessary to avoid uneven wall thickness wherein melt in the cavity can flow from narrow section to large and thick section; in addition, the fluidity of melt can be improved through increasing temperature of mold, barrel as well as nozzle. ④ Nebulous Flow Mark The volatile gas produced from the resin in the melt and the lubricating agent under processing temperature is stored in between the melt flow and surface wall of cavity and interferes the flow of melt, whereby the nebulous flow mark is formed. The treatment measures withal are: improving the exhausting condition of molds; properly increasing dimension of gate and reducing melt filling rate; properly reducing mold temperature and melt plastication temperature; replacing additives such as lubricating agent which can produce volatile gas. 5. Crazing Crazing is silvery white filamentous speckle occurring on the surface or near-surface of injection products along the direction of melt flow. It is resulted from the moving of gas in the resin which is then stored in between the melt and surface wall of cavity and subsequently crushed. Crazing sometimes is also called mica mark, silver line or silver speckle. The most essential reason for the occurrence of crazing is that volatile gas is contained in the melt, withal the following measures can be taken: ① As for raw materials. In allusion to degradation gas crazing, resins with uniform granularity are

required to be used, dosage of recycled materials should be reduced, promptly clear the remnant foreign materials in the barrel, and when necessary replace the additives such as lubricating agent which can decompose easily; in allusion to aqueous vapor crazing, the plastic raw materials should be strictly dried and the plastication back pressure of screw should be improved (2 As for molding technics In allusion to degradation crazing, the temperature of barrel and nozzle should be reduced, the retention time of melt in the barrel should be shortened to avoid partial temperature rising, properly reduce plastication and injection pressure to avoid high temperature of materials under high pressure; in allusion to aqueous vapor crazing, properly raise plastication pressure increase compression ratio of screw, reduce rotation speed of screw and use barrel and screw with exhausting function 3 As for mold design and injection operation. In allusion to degradation crazing, properly increase section dimension of sprue, runner as well as gate, reduce shearing rate of melt, expand space of cold-slug well and improve exhausting condition of molds; in allusion to aqueous vapor crazing, add mold exhausting channel, check whether the cooling water channel is leaking and whether the surface wall of cavity is affixed with moisture 6. Air Cavitation, Air Bubble and Burn Mark During the process of injection molding, due to the poor exhausting condition of molds, the water vapor in the melt, the air in cavity, the volatile gas from volatile substance in raw materials as well as the material decomposition gas either remain inside the products to form air bubble or are stored in between the melt and surface wall of cavity to form air cavitation which causes inner concave on the surface of products. Sometimes, at parts with thick walls, gas may also exist in the shrinkage porosity caused by insufficient shrinkage compensation. The occurrence of air cavitation and air bubble may further result in inadequate filling in the cavity and insufficient dwell, and may even lead to the rising of gas temperature due to the compression of melt, thereby occurring such phenomenon as degradation, discoloration or scorching of plastics around the air cavitation or air bubble ( As for injection raw material. If the raw material contains too much water, additive of decomposition gas and recycled materials may occur therein and thereby forming gas sources. Hence the raw material of plastics should be strictly dried and barrels with exhausting function should be applied to the injection machine, preferably reduce or replace the additives easy to decompose, and reduce the use of remnant materials or recycled materials 2 As for molding technics. Air bubble or air cavitation may occur in the plastic parts due to improperly-controlled material temperature and mold temperature, overfast injection speed, overlow injection pressure, too large or too small quantity of injection, insufficient dwell, unbalanced or insufficient cooling etc. Therefore, the injection rate of melt should be properly reduced to ensure sufficient time for gas in the cavity to be discharged; properly improve injection pressure and mold temperature, keep appropriate molding pressure in the cavity and properly prolong dwell time, dosage of injection should be kept in a proper amount; reinforce the effect of uniform cooling; improve temperature at pre-warming section of barrel; increase length of pre-warming section, reinforce dehumidifying and exhausting effect of plastics in the barrel; properly reduce temperature of plastic melt and the time of storing at high-temperature section to avoid degradation which will produce decomposition gas 3 As for product structure and mold design. Optimize mold exhausting scheme, and exhausting channel should be particularly set at confluence of front materials of plastic melt in the cavity; optimize

required to be used, dosage of recycled materials should be reduced, promptly clear the remnant foreign materials in the barrel, and when necessary replace the additives such as lubricating agent which can decompose easily; in allusion to aqueous vapor crazing, the plastic raw materials should be strictly dried and the plastication back pressure of screw should be improved. ② As for molding technics. In allusion to degradation crazing, the temperature of barrel and nozzle should be reduced, the retention time of melt in the barrel should be shortened to avoid partial temperature rising, properly reduce plastication and injection pressure to avoid high temperature of materials under high pressure; in allusion to aqueous vapor crazing, properly raise plastication pressure, increase compression ratio of screw, reduce rotation speed of screw and use barrel and screw with exhausting function. ③ As for mold design and injection operation. In allusion to degradation crazing, properly increase section dimension of sprue, runner as well as gate, reduce shearing rate of melt, expand space of cold-slug well and improve exhausting condition of molds; in allusion to aqueous vapor crazing, add mold exhausting channel, check whether the cooling water channel is leaking and whether the surface wall of cavity is affixed with moisture. 6. Air Cavitation, Air Bubble and Burn Mark During the process of injection molding, due to the poor exhausting condition of molds, the water vapor in the melt, the air in cavity, the volatile gas from volatile substance in raw materials as well as the material decomposition gas either remain inside the products to form air bubble or are stored in between the melt and surface wall of cavity to form air cavitation which causes inner concave on the surface of products. Sometimes, at parts with thick walls, gas may also exist in the shrinkage porosity caused by insufficient shrinkage compensation. The occurrence of air cavitation and air bubble may further result in inadequate filling in the cavity and insufficient dwell, and may even lead to the rising of gas temperature due to the compression of melt, thereby occurring such phenomenon as degradation, discoloration or scorching of plastics around the air cavitation or air bubble. ① As for injection raw material. If the raw material contains too much water, additive of decomposition gas and recycled materials may occur therein and thereby forming gas sources. Hence, the raw material of plastics should be strictly dried and barrels with exhausting function should be applied to the injection machine; preferably reduce or replace the additives easy to decompose, and reduce the use of remnant materials or recycled materials. ② As for molding technics. Air bubble or air cavitation may occur in the plastic parts due to improperly-controlled material temperature and mold temperature, overfast injection speed, overlow injection pressure, too large or too small quantity of injection, insufficient dwell, unbalanced or insufficient cooling etc. Therefore, the injection rate of melt should be properly reduced to ensure sufficient time for gas in the cavity to be discharged; properly improve injection pressure and mold temperature, keep appropriate molding pressure in the cavity and properly prolong dwell time; dosage of injection should be kept in a proper amount; reinforce the effect of uniform cooling; improve temperature at pre-warming section of barrel; increase length of pre-warming section, reinforce dehumidifying and exhausting effect of plastics in the barrel; properly reduce temperature of plastic melt and the time of storing at high-temperature section to avoid degradation which will produce decomposition gas. ③ As for product structure and mold design. Optimize mold exhausting scheme, and exhausting channel should be particularly set at confluence of front materials of plastic melt in the cavity; optimize

design of product structure and mold feed system(especially gate position and section dimension of gate) to avoid severely uneven wall thickness, wherein the gate should be set at the thick wall of plastic parts, and the section shape and dimension of gate should be suitable for the weight of plastic parts to avoid such occurrences as unbalanced flow, stagnation and track effect etc and to achieve rational flow of melt in the mold, thereby enabling the gas under extrusion of melt filling to be discharged smoothly througl sound exhausting method during the extrusion, especially at the final confluence of melt. 7. Overflow and Flash During the injection and filling process, the plastic melt enters into the clearance of parting line clearance of sliding surface of the slide, gap between inserts and holes of ejector pin which are related with mold cavity structure, and overflow thereby occurs. These surplus materials overflowing from cavity shall attach to the injection products after ejection, thereby forming flash. The occurrence of flash not only increases the workload of operators but also reduces productivity and influences ejection and the quality of products. The specific causes are as follows O Insufficient clamping force 2 Unbalanced mold clamping and fluctuated clamping force B Overlow or overhigh viscosity of plastic raw materials. Overlow viscosity shall increase the possibility of occurring flash despite the high precision of fit clearance; if the viscosity is too high, the flow and filling resistance of melt will increase, and consequently the molding pressure within the cavity shall rise and flash will form due to the insufficient clamping force. 4 Overhigh injection pressure, overfast injection rate, excessively small section dimension of gate overhigh temperature of barrel, nozzle and mold, which altogether result in the overlow flow viscosity 5 Deflected position of mold cavity, which result in unilateral tension of the mold during injection and thereby forming flash 6 Overlarge fit clearance of relevant parts of mold cavity due to deformation, abrasion and foreign matters existing on the parting line, or due to the overlow design precision for the clearance of moving or fixed part of mold as well as the inadequate processing and assembly O Overlarge or overdeep exhausting clearance The solutions include O Use injection machines with large clamping force, wherein the rated clamping force must be higher than the tension of injection molds during filling, otherwise, mold expansion and flash will occur 2 Check the clamping system of injection machine, especially whether the toggle unit is straight under clamping state, dispel such factors as unbalanced clamping and fluctuated clamping force of melt during the process of injection and filling 3 Optimize gate position and achieve balanced mold-filling; avoid unbalanced clamping and partial over-dwell for melt in parts where filling has been completed 4 Properly reduce injection pressure and dwell pressure and switch pressure in time 6 Optimize prescription of plastic raw materials, properly reduce dosage of lubricating agent, keep e viscosity of plastics within a proper range; appropriately reduce temperature of barrel, nozzle and molds and injection rate, properly increase section dimension of gate and thereby optimizing the flow and filling state of melt 6 Check the attaching degree of parting line for deformation or foreign matters(if any ) and polish the part of parting line with flash; check the places that have gliding or insert fit with the molding

design of product structure and mold feed system (especially gate position and section dimension of gate) to avoid severely uneven wall thickness, wherein the gate should be set at the thick wall of plastic parts, and the section shape and dimension of gate should be suitable for the weight of plastic parts to avoid such occurrences as unbalanced flow, stagnation and track effect etc and to achieve rational flow of melt in the mold, thereby enabling the gas under extrusion of melt filling to be discharged smoothly through sound exhausting method during the extrusion, especially at the final confluence of melt. 7. Overflow and Flash During the injection and filling process, the plastic melt enters into the clearance of parting line, clearance of sliding surface of the slide, gap between inserts and holes of ejector pin which are related with mold cavity structure, and overflow thereby occurs. These surplus materials overflowing from cavity shall attach to the injection products after ejection, thereby forming flash. The occurrence of flash not only increases the workload of operators but also reduces productivity and influences ejection and the quality of products. The specific causes are as follows: ① Insufficient clamping force. ② Unbalanced mold clamping and fluctuated clamping force. ③ Overlow or overhigh viscosity of plastic raw materials. Overlow viscosity shall increase the possibility of occurring flash despite the high precision of fit clearance; if the viscosity is too high, the flow and filling resistance of melt will increase, and consequently the molding pressure within the cavity shall rise and flash will form due to the insufficient clamping force. ④ Overhigh injection pressure, overfast injection rate, excessively small section dimension of gate, overhigh temperature of barrel, nozzle and mold, which altogether result in the overlow flow viscosity of melt. ⑤ Deflected position of mold cavity, which result in unilateral tension of the mold during injection and thereby forming flash. ⑥ Overlarge fit clearance of relevant parts of mold cavity due to deformation, abrasion and foreign matters existing on the parting line, or due to the overlow design precision for the clearance of moving or fixed part of mold as well as the inadequate processing and assembly. ⑦ Overlarge or overdeep exhausting clearance. The solutions include: ① Use injection machines with large clamping force, wherein the rated clamping force must be higher than the tension of injection molds during filling, otherwise, mold expansion and flash will occur. ② Check the clamping system of injection machine, especially whether the toggle unit is straight under clamping state; dispel such factors as unbalanced clamping and fluctuated clamping force of melt during the process of injection and filling. ③ Optimize gate position and achieve balanced mold-filling; avoid unbalanced clamping and partial over-dwell for melt in parts where filling has been completed. ④ Properly reduce injection pressure and dwell pressure and switch pressure in time. ⑤ Optimize prescription of plastic raw materials, properly reduce dosage of lubricating agent, keep the viscosity of plastics within a proper range; appropriately reduce temperature of barrel, nozzle and molds and injection rate, properly increase section dimension of gate and thereby optimizing the flow and filling state of melt. ⑥ Check the attaching degree of parting line for deformation or foreign matters (if any), and polish the part of parting line with flash; check the places that have gliding or insert fit with the molding