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针对加工工艺特点正确选择模温控制系统
不同原料性能不同,它们有着不同的加工工艺特点,模具温度对工件表质量流动性、收缩率、注塑周期的影响,是塑料加工制造中必须控制的因素。研究显示,24%的废品是由模具温度不当所致。为此,在从注塑、吹塑到挤塑的各种塑料加工领域中广泛采用了模温控制系统,以将模具加热到工作温度,并使模具温度保持在一定的精度范围。
然而,如果模具控制系统设计、选用不当,不但无法提高产品的成型效率,稳定产品品质和降低不良率,也致使其他设备自动化的改良失去意义。
正确选择适当的模温控制系统可以从以下几方面考虑。
确定塑料加工工艺要求的模具温度
各种塑料原材料要求的加工成型温度、模具温度不同。"各种塑料加工模具温度表"给出了一些主要塑料加工所需的模具温度。
确定模温机的传热介质类型
模温控制系统的工作原理为,泵浦驱动传热介质(通常为水或油)从装有内置加热器和冷却器的水箱中到达模具,再从模具回到水箱。控制器根据温度传感器测量的热流体温度,或/和模具内部温度,调节热流体的温度从而调节模具的温度。
根据使用的传热介质的类型,模温机为水式模温机、加压水模温机或油式模温机。
一般水式模温机控制温度范围较低,油式模温机控制温度范围较高。水式模温机的工作温度为90℃,加压水式模温机的工作温度为160℃~180℃,油式模温机用于工作温度≥150℃的场合,最高工作温度为350℃。
就传热介质本身而言,以水作传热介质经济环保,即使发生泄漏,也可直接排放,不会污染环境,但水容易使水箱、流道腐蚀和结垢,所以除需做防腐蚀剂预处理外,在使用中还要定期除锈。以导热油作传热介质,导热系数只有水的1/3,成本相对较高,容易结焦。
特殊应用环境也是选择模温机需要考虑的因素。如油温机在光学镜片的加工中受到限制,因为油受热蒸发,会影响光学镜片的品质。
温度控制精度
要使注塑加工中模具的温度保持基本恒定,其关键是要控制模具内的热传导平衡。模具内部由热塑性塑料带来的热量,通过热传导传递给模具,传导至模具的热量一部分通过热辐射直接在空气中散失,另一部分通过与模温机的热交换,传递给传热介质(水或油)。
模温控制的实现,有基于热流体温度的模温控制、基于模具温度的模温控制,以及联合模温控制几种方式。用户应根据实际情况,考察模温机操作的实现,选择能够满足自己温度控制精度要求的产品。
基于热流体温度模温控制,控制面板上显示的温度和模具温度并不一致,由于注射周期、注射速度、熔化温度及环境温度等因素没有得补偿,模具的温度波动相对较大。基于热流体温度模温控制可以满足大多数情况要求,也是最常见方法。
基于模具温度的模温控制,由温度传感器安装于模具内部,控制面板上设置、显示温度模具温度一致,适用于温度控制精度要求较高的情况。
联合控制综合了以上方法,适用于高精度塑料加工制造。在联合控制中,安放温度传感器在模具中的位置必须考虑模具形状、结构及冷却通道的位置,将其放置在对加工件质量起决定性作用的地方。
目前,绝大多数模温机采用了PID控制技术,对影响模具温度的各种因素进行补偿。
在模温机选配过程中,加热功率和媒体循环的压力、流量也都是必须考虑的因素。
其中:
P模具所需加热能量(kW)
W模具重量(kg)
C模具比热(kcal/kg℃)
T1所需模温(℃)
T2开始生产时模温(℃)
S安全系数,取值1.5~2
t由开始生产达到所需模温的时间(h)
媒体循环所需压力
模具越细,所需要的压力越大,且需加上流道的阻抗。
P=0.1×H×α
P压力(kg/cm2)
H水头高度(m)
α传热媒体的比热。水=1;油=0.7~0.9
媒体循环所需流量
流量越大,传热效果越好。
L媒体循环所需流量(L/min)
Q模具需移出或移入的热量(kcal/h)
c传热媒体的比热。水=1;油=0.7~0.9
ΔT循环媒体进出模具的温差(℃)
α媒体循环的比重
通常,模温机专业厂商会提供其产品泵浦特性以及模温机型号功能比较,这些都是选择模温机的参考。以晏邦产品进行说明。
·确定需要达到的温度范围:"各种塑料加工模具温度表"提了一些主要塑料加工所需的模具温度,用户需根据加工工艺要求,确定需要达到的温度范围。
·传热介质的选择:一般水温机控制温度范围较低,油温机控制温度范围较高。就模温机而言,如需采用水为传热介质,应选择YBM-D系列;如采用油或水为传热介质,则应选择YBM-I型。
·根据"型号功能表",在允许的最长加热时间内选择最适合的型号。
例如:模重120kg,模温需要在25分钟内达到120℃,YBM-1IH(高温型)为最佳选择;
模重120kg,模温需要在10分钟内达到120℃,YBM-2IH(高温型)为最佳选择;
模重120kg,模温需要在25分钟内达到80℃,YBM-1D或YBM-1I(一般型)为最佳选择。
部分型号部分物性表
PA6 (polycaprolactam # nylon 6) /EMI 260.7A/ RTP
Use: other
Important beters: density: 1.21 g/cm3 molding shrinkage: 1% notch impact strength: 53 tensile strength: 52 MPa bending strength: 83 MPa flexural modulus: 2410 MPa
Manufacturer: RTP company, USA
PA6 (polycaprolactam # nylon 6) /EMI 262A/ RTP
Use: other
Important beters: density: 1.24 g/cm3 water absorption: 1.4% molding shrinkage: 0.9% notched impact strength: 53 tensile strength: 83 MPa bending strength: 124 MPa flexural modulus: 3100 MPa, hot debation temperature: 82 degrees.
Manufacturer: RTP company, USA
PA6 (polycaprolactam # nylon 6) /0348/ TPC
Use: other
Important beters: density: 3.47 g/cm3 water absorption: 0.1% molding shrinkage: 0.2% notched impact strength: 37.4 tensile strength: 44.8 MPa, elongation at break: 1% bending strength: 48.3 MPa flexural modulus: 6890 MPa thermal debation temperature: 167 C.
Manufacturer: TP Composites, USA
PA6 (polycaprolactam # nylon 6) /BT33 ST/ perrite
Use: other
Feature remark: improved impact resistance, granular material
Important beters: notch impact strength: 17 tensile strength: 89 MPa flexural modulus: 4420 MPa
Manufacturer: perrite, UK
PA6 (polycaprolactam # nylon 6) /BT34 ST/ perrite
Use: other
Feature remark: improved impact resistance, granular material
Important beters: notch impact strength: 13 tensile strength: 75 MPa flexural modulus: 3400 MPa
Manufacturer: perrite, UK
PA6 (polycaprolactam # nylon 6) /BT36 ST/ perrite
Use: other
Feature remark: improved impact resistance, granular material
Important beters: notch impact strength: 17 tensile strength: 130 MPa flexural modulus: 8300 MPa
Manufacturer: perrite, UK
PA6 (polycaprolactam # nylon 6) /PA 66-01/ Lucent
Use: other
Characteristic notes: general purpose
Important beters: molding shrinkage: 1.4% notch impact strength: 42.7 bending strength: 110 MPa flexural modulus: 2830 MPa thermal debation temperature: 85 centigrade
Manufacturer: Lucent Polymer Co, USA
PA6 (polycaprolactam # nylon 6) /ADDINYL A2 W/ Addiplast of France
Use: other
Characteristic notes: production stage, fast
Important beters: density: 1.14 g/cm3 flexural modulus: 3050 MPa thermal debation temperature: 90 C
Manufacturer: Addiplast company, France
PA6 (polycaprolactam # nylon 6) /ADDINYL B2 I/ Addiplast of France
Use: other
Characteristic notes: Halogen-free, flame retardancy
Important beters: density: 1.2 g/cm3 flexural modulus: 3300 MPa hardness: 82 thermal debation temperature: 70 C
Manufacturer: Addiplast company, France
PA6 (polycaprolactam # nylon 6) /C2 V40/ Addiplast of France
Use: household appliance components
Characteristic notes: high rigidity
Important beters: density: 1.46 g/cm3 flexural modulus: 11500 MPa hardness: 85 thermal debation temperature: 235 C
Manufacturer: Addiplast company, France
PA6 (polycaprolactam # nylon 6) /E M/SL4 MB/ TPC
Use: other
Important beters:
Manufacturer: TP Composites, USA
PA6 (polycaprolactam # nylon 6) /B11^ F30/ polykemi
Use: other
Important beters: density: 1.36 g/cm3, molding shrinkage: 0.9% notched impact strength: 10 bending strength: 230 MPa bending modulus: 8800 MPa, VEKA softening point: 512 DEG heat debing temperature: 200 c.
Manufacturer: polykemi company
PA6 (polycaprolactam # nylon 6) /B11^ F45/ polykemi
Use: other
Important beters: density: 1.51 g/cm3, molding shrinkage: 0.8% notched impact strength: 15 bending strength: 280 MPa bending modulus: 12800 MPa, VEKA softening point: 200 DEG heat debing temperature: 210 C.
Manufacturer: polykemi company
PA6 (polycaprolactam # nylon 6) /B11^ F50/ polykemi
Use: other
Important beters: density: 1.56 g/cm3, molding shrinkage: 0.8% notched impact strength: 13 bending strength: 290 MPa bending modulus: 14000 MPa, VEKA softening point: 200 DEG heat debing temperature: 215 C.
Manufacturer: polykemi company
PA6 (polycaprolactam # nylon 6) /B12^ F15/ polykemi
Use: other
Important beters: density: 1.23 g/cm3, molding shrinkage: 0.3% bending strength: 190 MPa, bending modulus: 5000 MPa hardness: 79 VEKA softening point: 200 DEG heat debing temperature: 200 c.
Manufacturer: polykemi company
PA6 (polycaprolactam # nylon 6) /B12^ F20/ polykemi
Use: other
Important beters: density: 1.27 g/cm3, molding shrinkage: 0.3% notched impact strength: 8 bending strength: 200 MPa bending modulus: 6000 MPa, VEKA softening point: 200 DEG heat debing temperature: 200 c.
Manufacturer: polykemi company
PA6 (polycaprolactam # nylon 6) /B12^ F25/ polykemi
Use: other
Important beters: density: 1.32 g/cm3, molding shrinkage: 0.3% notched impact strength: 9 bending strength: 220 MPa bending modulus: 7700 MPa, VEKA softening point: 200 DEG heat debing temperature: 200 c.
Manufacturer: polykemi company
PA6 (polycaprolactam # nylon 6) /B12^ F30/ polykemi
Use: other
Important beters: density: 1.36 g/cm3, molding shrinkage: 0.3% bending strength: 240 MPa, bending modulus: 8800 MPa hardness: 80 VEKA softening point: 200 DEG heat debing temperature: 205 C.
Manufacturer: polykemi company
PA6 (polycaprolactam # nylon 6) /B12^ F40/ polykemi
Use: other
Important beters: density: 1.46 g/cm3, molding shrinkage: 0.3% notched impact strength: 11 bending strength: 206 MPa bending modulus: 10200 MPa, VEKA softening point: 220 DEG heat debing temperature: 205 C.
Manufacturer: polykemi company
PA6 (polycaprolactam # nylon 6) /B12^ F45/ polykemi
Use: other
Important beters: density: 1.51 g/cm3, molding shrinkage: 0.2% notched impact strength: 11 bending strength: 200 MPa bending modulus: 8000 MPa, VEKA softening point: 200 DEG heat debing temperature: 210 C.
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