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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
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  • Categories:Industry news
  • Author:
  • Origin:
  • 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
Top

(Summary description)

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

 

 

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.

 

产生原因

处理方法

模具进料不足

增加进料量

熔料温度高

降低熔料温度

注射时间短

增加注射时间

注射压力低

提高注射压力

合模压力不足

适当调高合模压力

模温不当

调整至适当温度

浇口不对称

调整模具入口大小或位置

凹陷部位排气不良

在凹陷部位设排气孔

模具冷却时间不够

延长冷却时间

螺杆止逆环磨损

更换

制品厚薄不均

增加射压

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.

 

产生原因

处理方法

原料潮湿

彻底焙干

注料不足

检查温度、注射压力及注射时间

注射速度太快

降低注射速度

原料温度过高

降低熔料温度

背压低

提高背压至适当

成品断面,肋或柱过厚

变更成品设计或溢口位置

浇道溢口太小

加大浇道及入口

模具温度不平均

调整模具温度至均匀

螺杆后退速度过快

降低螺杆后退速度

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.

 

产生原因

处理方法

制品脱模时尚未冷却

延长冷却时间

制品形状及厚薄不对称

变更成型设计或增加加强肋条

填料过多

减少注射压力、速度、时间和原料剂量

浇口进料不均

更改浇口或增加浇口数量

顶出系统不平衡

调整顶出装置位置

模具温度不均

调整模具温度至均衡

原料缓冲过度

降低原料缓冲

 

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.

 

产生原因

处理方法

原料熔融不佳

提高熔料温度、背压,加快螺杆转速

原料不洁或掺有他料,干燥不足

检查原料,彻底焙干

模具温度太低

提高模具温度

浇口附近温度太低

提高温度

浇口太小或位置不当

加大浇口或更改位置

保压时间短

延长保压时间

注射压力或速度不当

调整至适当

成品断面厚薄相差太多

变更成品设计

 

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.

 

■来源丨网络整理

 

 

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