The precision injection molding refers to the back of the formation product accuracy(including size precision, shape precision and mutual position precision) and the surface quality (for example surface roughness, stress condition and superficial air bubble, silver grain conditions and so on),it requests the very high handicraft technology. The precision injection formation must use precision injection mould. The design and the manufacture of precision injection mould play an important role in the precision product size precision, therefore, injection mould formation and spare part’s shape, the size, the precision, the surface quality, the intensity, and the rigidity all have special requests. Its characteristic mainly concentrates the following several aspects.
一、Mould Precision
(1)Must guarantee the precision injection product accuracy, first must guarantee the mould precision, including the mould cavity size precision, the profile precision and so on. The concrete measures include: the size of against pole is smaller than the convention mould size, the cavity number is not too excessive, thickness of the ledger wall, the backing strip, the backing and cavity wall should big somewhat and so on.
(2)Mould Processing Nature In the mould design processing, must consider mould processing nature, for example: when design the shape of complex precision injection mould,the best design make the cavity piece together structure, not only it is advantageous to the abrasive machining and exhaust and heat treatment, but must guarantee piecing together precision., in order to avoid the product appearing piecing together seam grain. In addition, but also must consider the installment position about the temperature measurement and the cooling system.
(3) Product Separates From the Film the sharp of precision injection molding product is generally complex and processing injection pressure is high, therefore, the product separate from the membrane is difficult. In order to prevent product separating from the membrane distort may affect the precision, when design the mould, besides must consider the separated membrane ascent, but also must enhance the cavity and the flow channel smooth finish, reduce the cavity number, apply less and short flow channels, and the push pedal seprating film as soon as possible.
(4) Mould Material Quality Since the precision forming mould must withstand the high injection pressure and the high locking force, and keep high accuracy for a long time, therefore, manufacture mould material have to choose high quality alloy steel with high hardness degree、good friction nature、good corrosion resistance and the high mechanical strength.
二. Precision Injection Plastic Forming Machine
High quality product of precision injection formation need to carry on in the special-purpose precision injection machine. The precision injection formation craft raises some requests for injection machine.
(1) Injection Power Is Great Precision injection plastic forming machine has bigger power than an ordinary injection machine to satisfy high-pressured and the high speed injection condition, thus guarantee the product size deviation scope is small, the size stability is high.
(2) Control Precision Is High The higher controlling precision of injection plastic forming machine mainly manifests the following several aspects.
2.1 Craft Parameter Redundant precision Is Very High Precision injection plastic forming machine implements the multistage reaction control, for the injection quantity, the injection pressure, guarantees pressure, multifeedback pressure and screw rod rotational speed ,use PID for the material tube temperature and the spray nozzle temperature (proportion integral differential) to control, cause to control temperature precision in ±0.5℃, thus guaranteed these craft parameter stability and the reproducibility, avoided affecting the product precision because of the craft parameter changed. In addition, injection plastic forming machine can warning and diagnosing the disadvantages causing the entire craft process at the steady state.
2.2 Temperature Precision Regulation Injection process involves the temperature very much, have the plastic temperature, the material tube temperature, the spray nozzle temperature, the hydraulic pressure return route temperature, the mold cavity and the mold body various spare parts temperature. The first three major effect material pours into in front of the cavity changing characteristic, latter three is mainly influence formation condition, they are connected closely with the handicraft quality,therefore, if the temperature controls is inaccurate, it will be unable to guarantee the craft redundant precision.
The mold temperature control, injected the shaper to strengthen precisely to the product in the mold the cooling period finalize the design control, as well as the product drawing of patterns took out when to ambient temperature control. Must use the mold warm machine control, guarantees the mold a temperature truehearted nature. The oil temperature change can cause the hydraulic fluid viscosity and the current capacity changes, causes the injection craft parameter the undulation, thus influence product precision. The precise injection machine adopts the heating and the cooling closed-loop control to the hydraulic fluid, causes the oil temperature to stabilize in 50-55℃.
2.3 Has the very strong plastic ability, can guarantee the material obtains the even consistent plastic effect injection the shaper all to use the big actuation torque generally precisely, the screw rod has the stepless speed regulation function, as well as has the high grade mix effect screw rod.
2.4 Locking Force Control Precision Is Very High Locking force is too small, the product can cause overflows under the high pressure and affect the precision; The locking force is too big,the product can affect the precision because of the mold distortion. Therefore, precision injection plastic forming machine enhances the control precision to the locking force.
(3) Hydraulic System Reaction Speed Is Very Quick
Injection plastic formation usually applys the high speed punch film craft, because between the high and low pressure and the high and low speed change quickly, therefore it requests hydraulic system with the very quick reaction rate to satisfy the precision injection formation craft. Therefore, in the hydraulic system, using the high sensitivity and the quick reaction hydraulic pressure part and applying the inserted and installed proportion technology, in the oil duct,it reduces from the controlled part to the implement processing . In addition, using the accumulator, both enhanced the hydraulic system reaction rate and can attract inspire, the uniform pressure and the energy conservation.
(4)Opening, Locking System Technical Level Are Very High
Precision injection forming machine’s opening and locking system are keen and accurate and own reliable rigidity. Precision injection forming machine develops to the direction of high pressure and the ultrahigh pressure to reduce the product shrinkage and enhance the product density; the injection speed develops to the high speed direction to satisfy the complex shape product injection formation compressor, therefore, Precision injection forming machine’s opening and locking system must have enough rigidity to avoid distorting during the formation processing, affecting the product precision. during the locking system ,to the moved mould、stationary mould、the pulling pole and the clamping mechanism structural element must do careful design and selection from enhancing the rigid angle.
(5) Formation Craft
Precision injection forming technic is similar to the ordinary injection molding formation process, it also includes three aspects: before formation preparation, injection formation process and production post-processing. Its characteristic mainly manifests in choice and control to the formation technic condition , it may summarize: the injection molding pressure is high, the injection molding speed is quick, the temperature control is accurate.
5.1 Injection Pressure Is High Ordinary injection formation needs the injection pressure is generally 40-180MPa, but precision injection plastic formation needs to enhance to 180-250 Mpa and sometimes it is even higher may up to 400 Mpa.
The Precision Injection Formation Uses the High-Pressured Injection Goal As Follows.
(1)Enhance the Product Precision And the Quality For example: when the injection pressure enhances to 400 Mpa, the product formation shrinkage and the shrinkage undulation change extremely low, but it’s not affect the product precision.
(2)Improve Product Formation Performance Enhance the injection pressure may cause the formation melt flowing compared to increase, thus it can product formation performance and produce the very thin product. This is because the huge pressure function may produce more friction heat, it causes the material viscosity drop greatly, thus the melt material can fill each position through the ultra thin cavity in the short time.
(3)It’s Advantageous In the Full Display Injection Machine Function Melt actual injection speed cannot achieve the injection machine design value as a result of the interval channel resistance restriction, to enhance the injection pressure is advantageous in the overcome the flow channel resistance and guarantee the injection speed funtion exertion ,it enables the material melt to fill each spot of complex cavity in the extremely short time.
5.2 Injection Speed Is Quick The higher injection pressure enables the injection speed to achieve the quite quick degree. The injection speed is more quicker, material melt packing ability is more stronger, it may form all kinds of complex products. The injection speed is quick, the filling mold time is short, the material rheology is very small, can by the even and continuous formation, it is advantageous to reduce the shrinkage,increase the handicraft size precision,the shape precision and improve the product state of stress.
5.3 Temperature Control Precision Formation temperature is the most important factor to affect the plastic handicraft quality, because the most basic principle of forming is depend on the heat flowing denaturalization of the plastic. The formation temperature is a big concept, including the material tube temperature, the spray nozzle temperature, the mould temperature, the oil temperature and the environment temperature, also speaking of the material tube temperature, in the different spot has the different request , therefore, the so multitudinous invariable factors bring the difficulties for the temperature strict accuracy control.
The temperature parameter determination is decided by the handicraft structure shape,size, the formation equipment performance, the formation mould structure and the cavity number, the handicraft qualitative index, the forming material variety and the trademark and so on, but it is finally determined must regard the special details and the experimental mould.
In precision injection formation process, if the temperature control is not enough, then the plastic melt fluidity, the product formability and the shrinkage are unstable, therefore,it is also unable to guarantee the product precision.
In summary, injection formation craft needs to the best melt pressure, the melt speed, the melt temperature. The precision injection machine and the injection mould provide the hardware support for it, but software support---it is different because the respective manufacturer has different technology power.
