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