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Super complete TPU molding process and defect analysis! Quick look at

Super complete TPU molding process and defect analysis! Quick look at

  • Categories:Industry news
  • Author:
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  • Time of issue:2021-01-11 09:04
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(Summary description)

Super complete TPU molding process and defect analysis! Quick look at

(Summary description)

  • Categories:Industry news
  • Author:
  • Origin:
  • Time of issue:2021-01-11 09:04
  • Views:



TPU injection molding process, the most important molding conditions are affecting the plasticizing flow and cooling temperature, pressure, time. In view of the characteristics of TPU, especially its processing temperature fluctuation range is particularly narrow, the injection molding machine must be equipped with accurate temperature regulation and control system, otherwise it will cause defects.




TPU processing equipment and processing technology are as follows:

1. Injection molding machine design

The material barrel of injection molding machine is lined with copper and aluminum alloy, and the screw is chrome plated to prevent wear. L/D of screw is 16~20. Compression ratio 2.5/1~3.0/1; Feeding section length 0.5L, compression section 0.3L, metering section 0.2L; The stop ring should be installed near the top of the screw to prevent backflow and maintain maximum pressure. The processing TPU should use the gravity nozzle, the outlet is inverted cone, the diameter of the nozzle is more than 4mm, less than the main channel ring inlet 0.68mm, the nozzle should be equipped with a controllable heating belt to prevent material solidification. From the economic point of view, the injection volume should be 40%~80% of the quota. Screw speed 20~50r/min. 1) Shrinkage of molded TPU parts is affected by hardness of raw materials, thickness of parts, shape, forming temperature and mold temperature, etc. The shrinkage rate is usually 0.005~0.020cm/cm.

2) flow passage and cold material hole

The main channel is the channel from the mold connecting the nozzle to the split channel or cavity. The diameter should be expanded inward with an Angle of more than 20, so as to facilitate the demoulding of the tautness in the flow channel



Split channel is the channel connecting the main channel and each cavity in the multi-groove die, which should be symmetrical and equidistant. The flow channel can be circular, semicircular or rectangular, with a diameter of 6~9mm, a diameter of 8~10mm and a depth of about 6mm. The surface of the runner must be polished to reduce flow resistance and provide a faster filling speed.

3) Sprue and exhaust

The gate is connected to the main channel or the shunt channel and the cavity channel. Its cross-sectional area is usually smaller than the runner, and it is the smallest part of the runner system, and its length should be short. The shape of the gate is rectangular or circular, 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 above 8mm, and the diameter is 2.0~2.7mm. The gate position is generally selected in the thickest products and does not affect the appearance and use of the place, and the mold wall at right angles, to prevent shrinkage cavity, to avoid spinning lines.

3. Moulding process conditions

1) The temperature should be controlled by the temperature of the cylinder, nozzle and mold. The first two temperatures mainly affect the plasticization and flow of TPU, while the latter temperature affects the flow and cooling of TPU. A. Drum temperature The selection of cylinder temperature is related to TPU hardness. The melting temperature of TPU with high hardness is high, so is the maximum temperature at the end of the barrel. Processing T cylinder temperature range of 177~232℃. The temperature distribution of the cylinder generally increases from one side of the hopper (back end) to the nozzle (front end), so as to make the TPU temperature rise steadily to achieve the purpose of uniform plasticizing. B. Nozzle temperature The nozzle temperature is usually slightly lower than the maximum cylinder temperature to prevent the molten material from salivating in the straight-through nozzle. If self-locking nozzle is used to prevent drooling, the nozzle temperature can also be controlled within the maximum temperature range of the cylinder. C. Mold temperature Mold temperature has a great influence on the internal properties and apparent quality of TPU products. It depends on factors such as TPU crystallinity and product size. Mold temperature is usually controlled by constant temperature cooling medium such as water. TPU hardness is high, crystallinity is high, and mold temperature is also high. The mold temperature of TPU products is generally 10 ~ 60℃.

2) Pressure pressure includes plasticizing pressure (back pressure) and injection pressure. When the screw recedes, the pressure on the top of the molten material is the back pressure, which is adjusted by the relief valve. Increasing the back pressure will increase the melt temperature, reduce the plasticizing speed, make the melt temperature uniform, color mixture uniform, discharge of melt gas, but will prolong the molding cycle. The injection pressure is the pressure applied by the top of the screw on TPU, which is used to overcome the flow resistance of TPU from the cylinder to the cavity, to fill the mold rate of the molten material, and to compact the molten material. TPU flow resistance and filling rate are closely related to melt viscosity, and melt viscosity is directly related to TPU hardness and melt temperature, that is, melt viscosity is not only determined by temperature and pressure, but also determined by TPU hardness and deformation rate. The higher the shear rate, the lower the viscosity. The higher the hardness of TPU, the higher the viscosity. The injection pressure of TPU is generally 20~110MPa. The holding pressure is about half of the injection pressure, and the back pressure should be below 1.4MPa to make the TPU plasticized evenly.

3) Time The time required to complete an injection process is called the molding cycle. The molding cycle includes mold filling, pressure holding, cooling and other times, which directly affects productivity and equipment utilization. TPU forming cycle is usually determined by hardness, thickness and structure of parts, and the TPU forming cycle is generally between 20~60s.

4) Injection speed The injection speed is mainly determined by the structure of TPU products. Products with thick end face need lower injection speed, while products with thin end face need faster injection speed.

5) Screw speed processing of TPU products requires low shear rate, and lower screw speed is appropriate. TPU screw speed is generally 20~80r/min, preferably 20~40r/min.

4, stop processing

Because TPU may degrade after prolonged time at high temperature, it should be cleaned with PS, PE, acrylic plastic or ABS after shutdown. The heating should be turned off if the machine is shut down for more than 1 hour.

5. Post-processing of products

Due to uneven plasticizing in the cylinder or different cooling rates in the mold cavity, TPU often produces uneven crystallization, orientation and shrinkage, resulting in the presence of internal stress in the products, which is more prominent in thick-walled products or products with metal inserts. The products need to be annealed. The annealing temperature depends on the hardness of TPU products. The annealing temperature of products with high hardness is also higher, and the temperature of products with low hardness is also lower. Too high temperature may cause warping or deformation of products, too low can not achieve the purpose of eliminating internal stress. TPU annealing should be carried out at low temperature for a long time and the products with low hardness should be placed at room temperature for several weeks to achieve the best performance. Hardness in Shore A85 below annealing 80℃×20h, A85 above 100℃×20h can be.

6, Mosaic injection molding

In order to meet the requirements of assembly and use strength, TPU parts have embedded metal inserts. The metal inserts are placed in predetermined places in the mold and then injected into a monolithic product. TPU products with embedded parts are not firmly bonded to TPU due to the great difference in thermal properties and shrinkage between metal inserts and TPU. The solution is to preheat the metal insert, because after preheating the insert reduces the melt temperature difference, so in the injection process can make the melt around the insert cooling slower, more uniform contraction, a certain amount of hot material feeding effect. It should be noted that the TPU used does not contain lubricants.

7, recycling material utilization

In the process of TPU processing, unqualified products and other waste can be recycled. 100% recycled material is not mixed with new material, and the mechanical properties decline is not too serious, and it is completely available. However, in order to maintain the physical and mechanical properties and injection conditions at the best level, it is recommended that the proportion of recycled material should be 25%~30%. Common defects of TPU products are shown as follows: 01 Surface sag The surface of products is sag, which will reduce the quality and strength of finished products and affect the appearance of products. Causes of depression and the use of raw materials, forming technology and die design are related.


























02 products have air bubbles


During injection molding, bubbles sometimes appear in the products, which will affect the strength and mechanical properties, and greatly reduce the appearance of the products.






















Warpage and deformation of products


The reasons for warping and deformation of injection molding products are too short cooling time, too high mold temperature, unevenness and asymmetry of flow system.



















04 products have flow marks


Flow mark is the flow trace of molten material, with the gate as the center and showing a striped appearance. Flow marks are caused by the material cooling too fast in the forming space at the beginning and the boundary between the material and the subsequent flow.





















05 products have rough edges for a variety of reasons, such as raw material problems, or injection molding machine problems, or improper adjustment, and even the mold itself may also be. When determining the cause of burrs, proceed from easy to difficult. (1) Check whether the raw material is thoroughly roasted, whether mixed with sundries, whether mixed with different kinds of raw materials, raw material viscosity effect; (2) the correct adjustment of the pressure control system of the injection molding machine and the adjustment of the injection speed must match the clamping force used; (3) whether there is wear in some parts of the mold, whether the vent is blocked, whether the flow channel design is reasonable; (4) Whether there is deviation between the parallelism of the injection molding machine template, whether the force distribution of the template tie rod is uniform, whether the screw reverse ring and the melt cylinder are worn.





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