Among engineering plastics and plastic molding for china, resins that have cavity surface temperatures above 100°C are gradually increasing. If the surface temperature of the cavity is above 90°C, it is normally difficult for us to increase the temperature if we perform temperature control using hot water.
(1) Oil temperature control
In oil temperature control, oil comes from a circulation pump by a hose through a passage designed inside the mold plate and the casting cavity, thereby maintaining a constant temperature. Once this temperature rises to the set temperature, we can maintain a relatively constant temperature.
However, the disadvantage of this method is that it takes time for the temperature to rise. Moreover, it can also cause burns hazard to people while handling the oil, and there is also the problem of waste oil disposal later.
In the process of temperature control using an electric heater (cartridge heater), the temperature comes constant using this heater in combination with a temperature sensor (temperature pair...). Due to its high heat capacity, it has the advantage of rapid temperature rise. We use this technique in plastic molding for china.
However, the temperature near the heater will be high and the temperature far away from the heater will be low and thus it is difficult to maintain a uniform temperature distribution.
In addition, because the heater has a limited lifespan, it is also necessary to replace it periodically.
Mounting holes and clearances will be important while mounting the heater. If the clearance is too large, the heater will be in idle operation and therefore its life will be shortened.
In order to realize precise temperature control, we need to put an insulation plate between the plate of the injection machine and the plate of the mold mounting plate. Or it is also possible to design insulation panels around the mold.
In the case of plastic molding for china, since this resin needs to maintain the mold cavity temperature at more than 100°C, temperature control using oil or temperature control using a heater will be necessary. The temperature control method commonly common by commercially available heaters is the ON-OFF control method. We control the temperature by turning on or off the power to the heater.
Since the ON-OFF control method is easy to do with a simple switching device. Although the cost of this type of controller is relatively low. The fluctuations in the cavity surface temperature will become large. And therefore its disadvantage is that it is difficult to stabilize the temperature.
If the cavity surface temperature is not stable, it will cause fluctuations in material shrinkage, dimensions and surface gloss in precision molded products.
PID plastic molding for china control is a control method that can shorten the time we require for the temperature. It is to become stable by controlling using the following types:
The heater can have a maximum load of 10kW with 200V three-phase AC. Therefore, as a usage example, we can take two 200V 1kW three-phase heaters and integrate them in the dynamic side mold plate and the stationary side dies plate, and performs a stable control of the surface temperature. casting cavity surface using PID control method.
Insulation panels are important to stabilize the temperature of plastic molding for china, or to save energy while maintaining the temperature. They are common as a mandatory part in the injection molding of engineering plastics or super engineering plastics.
During the thermoplastic molding process, we can obtain the molded product of the desired size by using shrinkage when molding. Casting shrinkage is a phenomenon in which the volume of molten plastic filling the inside of the mold shrinks when it is cooled and solidified.
The extension of this plastic molding for china shrinkage is known as the “casting shrinkage”. And it is precisely known from experience and experiment. By designing the mold with the size of the casting cavity larger by an amount equal to the shrinkage, we can shape the molded product so that it has the desired size.
The value of the shrinkage factor is usually a number between 2/1000 and 20/1000 (range from 0.2% to 2%).
If the shrinkage factor is represented by the symbol α (alpha), then it is easy to define by Equation 1.
α = (L0 − L) L0
If further considered, the coefficient of shrinkage affects by the following factors:
The range of the basic shrinkage factor comes by the type of plastic material common. However, it also varies according to the plastic molding for china and the quality class of the material.
The molding shrinkage coefficient varies according to the cavity surface temperature during the plastic injection process. In general, the coefficient of shrinkage tends to be large if the temperature is high.
The shrinkage coefficient varies according to the magnitude of the holding pressure after the resin injects and the time that the pressure maintains. In general, the shrinkage coefficient tends to be smaller when the holding pressure is high and the holding time is long.
The shrinkage factor also varies according to the wall thickness of the molded product. It tends to be larger if the wall thickness is larger.
The aspect ratio varies according to the shape and size of the plastic molding for china casting gate. In general, it tends to decrease as the cross-sectional area of work becomes large. In addition, the aspect ratio also tends to be smaller in the side gate type than in the pinpoint gate or the submarine gate.
Usually, there will be a large difference in the coefficient of shrinkage between natural materials and materials with glass fibers. The coefficient of shrinkage tends to be smaller in the case of fiberglass materials. In practice, the shrinkage factor for mold design is determined by exhaustively studying the above conditions.