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注塑机加工时背压的形成
一、背压的形成
在塑料熔融、塑化过程中，熔料不断移向料筒前端（计量室内），且越来越多，逐渐形成一个压力，推动螺杆向后退。为了阻止螺杆后退过快，确保熔料均匀压实，需要给螺杆提供一个反方向的压力，这个反方向阻止螺杆后退的压力称为背压。
背压亦称塑化压力，它的控制是通过调节注射油缸之回油节流阀实现的。预塑化螺杆注塑机注射油缸后部都设有背压阀，调节螺杆旋转后退时注射油缸泄油的速度，使油缸保持一定的压力；全电动机的螺杆后移速度（阻力）是由AC伺服阀控制的。
二、适当调校背压的好处
1、能将炮筒内的熔料压实，增加密度，提高射胶量、制品重量和尺寸的稳定性。
2、可将熔料内的气体“挤出”，减少制品表面的气花、内部气泡、提高光泽均匀性。减慢螺杆后退速度，使炮筒内的熔料充分塑化，增加色粉、色母与熔料的混合均匀度，避免制品出现混色现象。
3、减慢螺杆后退速度，使炮筒内的熔料充分塑化，增加色粉、色母与熔料的混合均匀度，避免制品出现混色现象。
4、适当提升背压，可改善制品表面的缩水和产品周边的走胶情况。
5、能提升熔料的温度，使熔料塑化质量提高，改善熔料充模时的流动性，制品表面无冷胶纹。
三、背压太低时，易出现下列问题
1、背压太低时，螺杆后退过快，流入炮筒前端的熔料密度小（较松散），夹入空气多。
2、会导致塑化质量差、射胶量不稳定，产品重量、制品尺寸变化大。
3、制品表面会出现缩水、气花、冷料纹、光泽不匀等不良现象。
4、产品内部易出现气泡，产品周边及骨位易走不满胶。
四、过高的背压,易出现下列问题
1、炮筒前端的熔料压力太高、料温高、粘度下降,熔料在螺杆槽中的逆流和料筒与螺杆间隙的漏流量增大,会降低塑化效率(单位时间内塑化的料量).
2、对于热稳定性差的塑料(如:PVC、POM等)或着色剂，因熔料的温度升高且在料筒中受热时间增长而造成热分解，或着色剂变色程度增大，制品表面颜色/光泽变差。
3、背压过高，螺杆后退慢，预塑回料时间长，会增加周期时间，导致生产效率下降。
4、背压高，熔料压力高，射胶后喷嘴容易发生熔胶流涎现象，下次射胶时，水口流道内的冷料会堵塞水口或制品中出现冷料斑。
5、在啤塑过程中，常会因背压过大，喷嘴出现漏胶现象，浪费原料并导致射嘴附近的发热圈烧坏。
6、预塑机构和螺杆筒机械磨损增大。
五、背压的调校
注塑背压的调校应视原料的性能、干燥情况、产品结构及质量状况而定，背压一般调校在3-15kg/cm3。当产品表面有少许气花、混色、缩水及产品尺寸、重量变化大时，可适当增加背压。当射嘴出现漏胶、流涎、熔料过热分解、产品变色及回料太慢时可考虑适当减低背压。
背压是注塑成型工艺中控制熔料质量及产品质量的重要参数之一，合适的背压对于提高产品质量有着重要的作用，不可忽视！
部分型号部分物性表
| Physical Properties | Metric | English | Comments |
|---|---|---|---|
| Specific Gravity | 0.940 g/cc | 0.940 g/cc | 23°C; ASTM D792 |
| Mechanical Properties | Metric | English | Comments |
| Hardness, Shore A | 96 | 96 | 2 mm, 15 sec; ISO 868 |
| 97 | 97 | Air at 302°F, 168 hr; ASTM D573 | |
| Tensile Strength at Break | 13.0 MPa | 1890 psi | Across Flow; ASTM D412 |
| 13.2 MPa | 1910 psi | Air at 302°F, 168 hr; ASTM D573 | |
| Tensile Stress |
7.10 MPa @Strain 100 % |
1030 psi @Strain 100 % |
Across Flow; ASTM D412 |
| Elongation at Break | 527 % | 527 % | Air at 302°F, 168 hr; ASTM D573 |
| 620 % | 620 % | Across Flow; ASTM D412 | |
| Tear Strength | 48.0 kN/m | 274 pli | 73°C, Die C; ASTM D624 |
| Compression Set | 52 % | 52 % | 158°F, 22 hr; ASTM D395 |
| Thermal Properties | Metric | English | Comments |
| Brittleness Temperature | -53.9 °C | -65.0 °F | ASTM D746 |
| UL RTI, Electrical | 100 °C | 212 °F | UL 746 |
| UL RTI, Mechanical with Impact |
90.0 °C @Thickness 1.10 mm |
194 °F @Thickness 0.0433 in |
UL 746 |
|
95.0 °C @Thickness 1.60 mm |
203 °F @Thickness 0.0630 in |
UL 746 | |
|
95.0 °C @Thickness 3.00 mm |
203 °F @Thickness 0.118 in |
UL 746 | |
| Flammability, UL94 |
HB @Thickness 1.10 mm |
HB @Thickness 0.0433 in |
|
|
HB @Thickness 1.60 mm |
HB @Thickness 0.0630 in |
||
|
HB @Thickness 3.00 mm |
HB @Thickness 0.118 in |
||
PP (polypropylene # folding glue | soft) /8706J/ MITSUBISHI of Japan
Uses: other
Remark: high gloss
Important beter: melt flow rate: 15 g/10min
Manufacturer: MITSUBISHI Chemical Industrial Co., Ltd.
PP (polypropylene # folding glue | soft) /3300F/ MITSUBISHI of Japan
Application: film grade
Important beter: melt flow rate: 6 g/10min
Manufacturer: MITSUBISHI Chemical Industrial Co., Ltd.
PP (polypropylene # folding glue | soft) /1500F/ MITSUBISHI of Japan
Application: film grade
Important beter: melt flow rate: 10 g/10min
Manufacturer: MITSUBISHI Chemical Industrial Co., Ltd.
PP (polypropylene # folding glue | soft /XIB/ MITSUBISHI petrochemical)
Uses: other
Important beter: melt flow rate: 1.2 g/10min
MITSUBISHI Petrochemical Co., Ltd.
PP (polypropylene # folding glue | soft) /8836J/ MITSUBISHI of Japan
Uses: other
Remark: high gloss
Important beter: melt flow rate: 30 g/10min
Manufacturer: MITSUBISHI Chemical Industrial Co., Ltd.
PP (polypropylene # folding glue | soft) /8826J/ MITSUBISHI of Japan
Uses: other
Characteristic note: high rigidity
Important beter: melt flow rate: 30 g/10min
Manufacturer: MITSUBISHI Chemical Industrial Co., Ltd.
PP (polypropylene # folding glue | soft /XIA/ MITSUBISHI petrochemical)
Uses: other
Important beter: melt flow rate: 0.9 g/10min
MITSUBISHI Petrochemical Co., Ltd.
PP (polypropylene # folding glue | soft /1035H/ America viima)
Uses: other
Important beters: melt flow rate: 35 g/10min notched impact strength: 32 tensile strength: 32 MPa, elongation at break: 200% bending modulus: 1150 MPa, hot debation temperature: 98 DEG C
Manufacturer: America viima
PP (polypropylene # folding glue | soft /361/ DuPont Dow)
Uses: other
Important beters: melt flow rate: 2 g/10min density: 0.9 g/cm3 tensile strength: 32 MPa VEKA softening point: 129 degrees of hot debation temperature: 80 degrees centigrade
Manufacturer: Dow DuPont Co
PP (polypropylene # folding glue | soft /DFDA-1980/ DuPont Dow)
Uses: other
Important beter: density: 1.6 g/cm3
Manufacturer: Dow DuPont Co
PP (polypropylene # folding glue | soft) /J700/ Daqing / North China Petroleum Refinery
Uses: other
Important beter:
Manufacturer: PetroChina North China
PP (polypropylene # folding glue | soft) /S900/ Lanzhou / Huabei Petrochemical
Uses: other
Important beter:
Manufacturer: PetroChina North China
PP (polypropylene # folding glue | soft) /500P/ Daqing / Southern China oil refinery
Uses: other
Important beter:
Manufacturer: PetroChina Southern China
PP (polypropylene # folding glue | soft) /H9018/ Lanzhou Petrochemical / oil Southern China
Uses: other
Important beter:
Manufacturer: PetroChina Southern China
PP (polypropylene # folding glue | soft) /H9008/ Lanzhou Petrochemical / oil Southern China
Uses: other
Important beter:
Manufacturer: PetroChina Southern China
PP (polypropylene # folding glue | soft) /HPPSS/ Fujian United
Uses: other
Important beter:
Manufacturer: Fujian United Petrochemical Co., Ltd.
PP (polypropylene # folding glue | soft) /C703/ Fushun / Huabei Petrochemical
Uses: other
Important beter:
Manufacturer: PetroChina North China
PP (polypropylene # folding glue | soft) /H9018/ Lanzhou Petrochemical / oil in Northwest China
Uses: other
Important beter:
Manufacturer: Northwest of CNPC
PP (polypropylene # folding glue | soft) /H9018/ Lanzhou / Huabei Petrochemical
Uses: other
Important beter:
Manufacturer: PetroChina North China
PP (polypropylene # folding glue | soft) /PP-07/ Yuyao.
Uses: automotive parts
Characteristic note: impact resistance, aging resistance, 30% minerals,
Important beters: melt flow rate: 4.5 g/10min density: 1.13 g/cm3 molding shrinkage: 0.9% notched impact strength: 9 tensile strength: 27 MPa bending strength: 42 MPa hardness: 67, hot debation temperature: 125 DEG C
Manufacturer: Yuyao Zhong FA Engineering Plastics Co., Ltd.
