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Process sharing - TPU injection molding process

Process sharing - TPU injection molding process
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  • Categories:Industry news
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  • Time of issue:2021-01-04 10:18
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(Summary description)

Process sharing - TPU injection molding process
Top

(Summary description)

  • Categories:Industry news
  • Author:
  • Origin:
  • Time of issue:2021-01-04 10:18
  • Views:
Information

The hardness of TPU is usually expressed by the internationally recognized Shaw hardness. Generally, the hardness below 100 degrees is expressed by A, such as 80A, 90A, 95A, 98A, etc., while the hardness above 100 degrees is expressed by D, such as 50D, 60D, 64D, 71D, 74D, etc. TPU molding process has a variety of methods: including injection molding, blow molding, compression molding, extrusion molding, etc., among which injection molding is the most commonly used. The function of injection molding is to process TPU into the required parts, which is divided into three discontinuous stages of pre-molding, injection and machine. There are two types of injectors: plunger type and screw type. The screw injector is recommended because it provides uniform velocity, plasticization, and melting.

1, injection machine design injection machine barrel lining with copper and aluminum alloy, screw chrome plated to prevent wear. The length diameter ratio of screw L/D=16~20 is good, at least 15; Compression ratio 2.5/1~3.0/1. The feeding section length is 0.5l, the compression section is 0.3l, and the metering section is 0.2L. The check ring should be mounted near the top of the screw to prevent reflux and maintain maximum pressure. It is appropriate to use a self-flow nozzle for TPU processing. The outlet is inverted cone, the nozzle diameter is more than 4mm, less than 0.68mm for the inlet of the main channel bushing ring. The nozzle should be equipped with controllable heating belt to prevent material solidification. From the economic point of view, the injection amount should be 40%~80% of the amount. Screw rotation speed 20~50r/min.

2. Mold Design Mold design should pay attention to the following points:

(1) The shrinkage rate of molded TPU parts is affected by the hardness of raw materials, thickness of the parts, shape, molding temperature and mold temperature and other molding conditions. The shrinkage usually ranges from 0.005 to 0.020cm/cm. For example, a 100×10×2mm rectangular specimen shrinks at the sprue in the length direction and flow direction, and the hardness of 75A is 2 ~ 3 times greater than that of 60D. The influence of TPU hardness and manufacturing thickness on shrinkage rate is shown in Figure 1. It can be seen that when the hardness of TPU is between 78A and 90A, the shrinkage rate of parts decreases with the increase of thickness. When the hardness was 95A ~ 74D, the shrinkage rate increased slightly with the increase of thickness.

(2) Flow passage and cold hole main passage are a section of passage in the mold connecting the injection machine nozzle to the shunt passage or cavity. The diameter should be expanded inward at an Angle of more than 2o, so as to facilitate the flow passage to unmold. The shunt duct is the channel connecting the main duct and each cavity in the multi-groove mold. The arrangement on the mold should be symmetrical and equidistant. The runner can be round, semicircular or rectangular with a diameter of 6~9mm. The runner surface must be polished like the die cavity to reduce flow resistance and provide faster die filling speed. The cold feed hole is a hole at the end of the main stream to trap the cold feed produced between two injections at the nozzle end, so as to prevent the shunt or gate from blocking. Cold material mixed with the cavity, the product is easy to produce internal stress. The diameter of the cold feed hole is 8~10mm and the depth is about 6mm.

(3) The sprue and the vent sprue are the passages connecting the main stream or the shunt to the cavity. Its cross-sectional area is usually less than the runner, is the smallest part of the runner system, the length should be short. The shape of the gate is rectangular or round, the size increases with the thickness of the product, the thickness of the product is less than 4mm, the diameter is 1mm; Thickness 4~8mm, diameter 1.4mm; The thickness is over 8mm and the diameter is 2.0mm ~2.7mm. The gate location is generally selected in the product the thickest and does not affect the appearance and use of the place, and the mold wall at right angles to prevent shrinkage cavity, to avoid rotation. The exhaust product is a kind of groove air outlet opened in the mold, in order to prevent the molten material into the mold into the gas, the gas cavity out of the mold. Otherwise it will make the products with air holes, welding bad, filling mold dissatisfaction, and even because the air is compressed to produce high temperature will be burned products, pieces of internal stress. The exhaust port can be set at the end of the molten material flow in the mold cavity or on the parting surface of the mold. It is 0.15mm deep and 6mm wide. Care must be taken to control the mold temperature as evenly as possible to avoid warping and torsion of the parts.

 

3. Molding conditions

The most important molding conditions for TPU are temperature, pressure, and time affecting plasticization flow and cooling. These parameters will affect the appearance and performance of TPU parts. Good processing conditions should be able to obtain uniform white to beige parts.

(1) Temperature The temperature that needs to be controlled in the TPU process of molding includes the temperature of the feed barrel, the temperature of the nozzle and the temperature of the mold. The first two temperatures mainly affect the plasticization and flow of TPU, while the latter one affects the flow and cooling of TPU. A. Cartridge temperature The selection of cartridge temperature is related to the hardness of TPU. TPU with high hardness has high melting temperature and the maximum temperature at the end of the barrel is also high. The temperature range of the barrel used for processing TPU is 177~232℃. Generally, the temperature distribution of the barrel is gradually increased from one side of the hopper (the back end) to the nozzle (the front end), so that the TPU temperature can rise steadily to achieve the purpose of uniform plasticization. B. Nozzle temperature The nozzle temperature is usually slightly lower than the maximum temperature of the barrel to prevent the drooling that may occur in the straight-through nozzle. If a self-locking nozzle is used to eliminate salivation, the nozzle temperature can also be controlled within the maximum temperature range of the barrel. C. Mould temperature mould temperature has great influence on the intrinsic performance and apparent quality of TPU products. Its height depends on many factors such as TPU crystallinity and product size. Mold temperature is usually controlled by a constant temperature cooling medium such as water. TPU has high hardness, high crystallinity and mold temperature. For example, Texin, hardness 480A, mold temperature 20 ~ 30℃; Hardness 591A, mold temperature 30 ~ 50℃; Hardness 355D, mold temperature 40 ~ 65℃. The mold temperature of TPU products is generally 10 ~ 60℃. The mold temperature is low, the melt freezes prematurely and produces streamline, and is not conducive to the growth of spherulites, making the product crystallinity is low, there will be a late crystallization process, resulting in the product shrinkage and performance changes. B. Pressure injection process is pressure including plasticizing pressure (back pressure) and injection pressure. When the screw recedes, the pressure on the top of the melt is back pressure, which is regulated by the relief valve. Increasing the back pressure will increase the melt temperature, reduce the plasticizing speed, make the melt temperature uniform, color mixing uniform, and let out the melt gas, but will prolong the forming cycle. TPU usually has a back pressure of 0. 3 ~ 4 mpa. Injection pressure is the pressure exerted by the top of the screw on TPU. Its function is to overcome the flow resistance of TPU from the barrel to the cavity, to feed the mold filling rate of the molten material, and to compacted the molten material. TPU flow resistance and filling rate are closely related to melt viscosity, which in turn is directly related to TPU hardness and melt temperature. In other words, melt viscosity not only depends on temperature and pressure, but also depends on TPU hardness and deformation rate. The higher the shear rate is, the lower the viscosity is. The higher the TPU hardness, the greater the viscosity. Under the condition of constant shear rate, the viscosity decreases with the increase of temperature, but under high shear rate, the viscosity is not affected by temperature as much as that under low shear rate. The injection pressure of TPU is generally 20~110MPa. The holding pressure should be about half of the injection pressure, and the back pressure should be 1. Below 4MPa to make TPU plasticized evenly. C. The time required to complete an injection process is called the molding cycle. The forming cycle includes filling time, pressure holding time, cooling time and other time (mold opening, mold stripping, mold closing, etc.), which directly affects labor productivity and equipment utilization rate. The forming cycle of TPU is usually determined by hardness, workpiece thickness and configuration. TPU has a high hardness and a short cycle, while plastic parts have a long thick cycle and complex plastic parts have a long cycle. The forming cycle is also related to mold temperature. TPU molding cycle is generally between 20~60s. D. Injection speed Injection speed is mainly determined by the configuration of TPU products. The product with thick end face needs lower injection speed, while the product with thin end face needs faster injection speed. E. Low shear rate is usually required for processing TPU products by screw rotation, so a low screw rotation rate is advisable. The screw speed of TPU is generally 20~80r/min, and 20~40r/min is preferred. (4) Shutdown treatment Because degradation may occur after prolonged TPU at high temperature, after shutdown, PS, PE, acrylic plastic or ABS should be used for cleaning; Heat should be turned off if shutdown lasts more than 1 hour.

 

(5) Product post-treatment TPU, due to uneven plasticization in the barrel or different cooling rates in the mold cavity, will often produce uneven crystallization, orientation and contraction, thus resulting in the internal stress of the product, which is more prominent in thick wall products or products with metal inserts. There is internal stress in the storage and use of products often occur mechanical properties decline, silver surface and even deformation and cracking. The solution to these problems in production is to anneal the products. Annealing temperature depends on the hardness of TPU products. The annealing temperature of products with high hardness is also higher, and the hardness of products with low temperature is also lower. Too high temperature may cause warping or deformation of the product, too low to eliminate the purpose of internal stress. TPU should be annealed at low temperature for a long time. The products with low hardness can achieve the best performance after being placed at room temperature for several weeks. The hardness was annealed at 80℃×20h under Shao Er A85, and 100℃×20h above A85 was sufficient. Annealing can be carried out in a hot air oven, pay attention to the location of local overheating and make the product deformation. Annealing can not only eliminate internal stress, but also improve mechanical properties. Since TPU is a two-phase form, phase mixing occurs during TPU hot processing. During rapid cooling, due to the high TPU viscosity and slow phase separation, sufficient time must be given to separate the phases and form microzones, so as to obtain the best performance.

(6) Inlay and injection molding in order to meet the needs of assembly and use strength, TPU parts are embedded with metal inserts. The metal inserts are first placed in a predetermined position within the mold and then injected into an integral product. TPU products with embedded metal inserts are not firmly bonded with TPU due to the great difference in thermal performance and shrinkage rate between metal inserts and TPU. The solution is to preheat the metal inserts, because the preheat inserts reduce the melt temperature difference, so in the injection process can make the melt around the insert cooling slower, more uniform shrinkage, a certain amount of hot material supplement shrinkage, to prevent excessive internal stress around the insert. TPU inlay molding is relatively easy, inlay shape is not limited, as long as the inlay after degreasing, it is heated at 200 ~ 230℃ 1. The peel strength can reach 6 ~ 9kg/25mm for 5 ~ 2min. To obtain a stronger bond, the insert can be coated with adhesive, heated at 120℃, and then injected. In addition, it should be noted that the TPU used should not contain lubricants.

(7) Reuse of recycled Materials During TPU processing, the waste materials such as main stream channel, shunt channel and unqualified products can be recovered and reused. According to the experimental results, the 100% recovered material is not mixed with new material, and the mechanical property degradation is not too serious, so it can be fully utilized. However, in order to keep the physical and mechanical properties and injection conditions at the optimal level, it is recommended that the proportion of recovered material should be 25%~30%. It should be noted that the recycled materials and new materials of the varieties and specifications had better be the same, contaminated or annealed recycling materials to avoid use, recycling materials do not store too long, it is best to immediately granulated, dry use. The melt viscosity of the recovered material generally to decline, molding conditions to be adjusted.

 

 

 

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