There are two kinds of quality of the products, namely: the quality of the inner material and the quality of the appearance. The quality of the inner material is mainly the mechanical strength, and the magnitude of the internal stress directly affects the mechanical strength of the product. The main reason for the internal stress is the crystallinity of the product and the
plastic molding. The orientation of the molecules is determined. The appearance quality of the product is the surface quality of the product, but the internal stress causes the warpage deformation of the product to affect the appearance quality. The appearance quality of the products is: insufficient products, product dents, weld marks, flash, bubbles, silver, black spots, deformation, cracks, delamination and discoloration, etc., all with molding temperature, pressure, flow, time and position. related.
1. Molding temperature: barrel temperature, plasticizing temperature, nozzle temperature, mold temperature and plastic drying temperature.
Barrel temperature: It is the melting temperature of the plastic. The temperature of the barrel is set too high. The viscosity of the plastic is low after melting. Under the same injection pressure and flow rate, the injection speed is fast, the molded product is easy to fly, silver wire, discoloration and brittleness.
The temperature of the cylinder is too low, the plastic plasticization is poor, the viscosity is high, the injection speed is slow under the same injection pressure and flow rate, the molded product is easy to be insufficient, the weld mark is obvious, the size is unstable, and there is a cold block in the product.
Nozzle temperature: The nozzle temperature is set high, and the nozzle is easy to flow, resulting in a cold wire in the product. Low nozzle temperatures cause blockage in the mold casting system. The injection pressure should be increased to inject the plastic, but there is a cold block in the molded product.
Mold temperature: The mold temperature is high, the injection pressure and flow rate can be set lower, but under the same pressure flow rate, the product is easy to fly, warping deformation, product sticking mold difficulty. The mold temperature is low, under the same injection pressure and flow rate, the product is insufficiently formed, bubbles, weld lines and the like.
Plastic drying temperature: Various plastics have different drying temperatures. ABS plastic is generally set to a drying temperature of 80 ~ 90 ° C, otherwise it is not easy to dry and volatilize moisture and residual solvent, the product is easy to have silver wire and bubbles, the strength of the product will also decline.
2. Molding process pressure: Pre-plastic back pressure: injection pressure, mold clamping pressure, pressure holding, mold and neutron pressure, shooting table pressure, etc.
Pre-plastic back pressure: high back pressure and high storage density, and a large amount of storage in an equal storage volume. Low back pressure, low storage density and low storage. After setting the storage position, and then making a large adjustment of the back pressure, you must pay attention to reset the storage position, otherwise it will easily cause the product to fluff or insufficient.
Injection pressure: different varieties of plastics have different melt viscosities, and amorphous plastics have a large viscosity change with plasticizing temperature. Injection pressure is set based on the weld viscosity of the plastic and the plastic flow ratio. If the injection pressure is set too low, the product is undershot, dents, weld lines and dimensional instability. The injection pressure is too high, and the product is flashed, and the discoloration and sticking are difficult to eject.
Clamping pressure: depending on the projected area of ​​the mold cavity and the injection pressure. The mold clamping pressure is insufficient, and the product is easy to fly and the weight is increased. If the clamping force is too large, it is difficult to open the mold. Generally, the clamping pressure is set to not exceed 120 par/cm2.
Holding pressure: When the injection is over, continue to give the screw a pressure to hold the pressure. At this time, the product in the mold cavity has not been frozen. The pressure can continue to refill the mold cavity to ensure that the product is full. The pressure holding and pressure setting are too high, which brings great resistance to the mold and the ferrule. The product is easy to be white and the top is warped. In addition, the mold runner gate is easily filled with plastic, and the gate is broken in the flow channel. Under-pressure products have dents and dimensional instability.
The mold and neutron pressure, set the mold and neutron pressure principle is to set the pressure according to the size of the mold cavity area and the core projection area of ​​the ferrule, and the geometric complexity of the molded product. size. Generally, the pressure of the mold and the neutron cylinder should be set to push the product.
3. Flow rate: related to the speed of the injection molding machine, such as screw speed, injection speed, mold and neutron speed.
Screw speed: In addition to adjusting the pre-plastic flow rate, it is mainly affected by the pre-plastic back pressure. If the pre-plastic flow rate is adjusted to a large value, the pre-plastic back pressure is high. As the screw rotates, the plastic has a large shear force in the barrel, and the plastic molecular structure is easily cut. The product has black spots and black streaks, which affect the appearance quality and strength of the product. And the barrel heating temperature is not easy to control. The pre-plastic flow setting is too low, and the pre-plastic storage time is prolonged, which will affect the molding cycle.
Injection speed: It is reasonable to set the injection speed, otherwise it will affect the product quality, the injection speed is too fast, and the product has bubbles, charring, discoloration and so on. If the injection speed is too slow, the product is insufficiently formed, and there is a weld mark or the like.
Mode and neutron flow: should not be set too large, otherwise the ejection and core pulling action is too fast, resulting in unstable ejection and ferrule, the product is easy to white.
3. Time setting: There are drying time, injection and holding time, cooling time, etc. in the injection molding process.
Drying time: It is the drying time of plastic raw materials, and all kinds of plastics have the best drying temperature and time. The drying temperature of ABS plastic is 80-90 ° C for 2 hours. ABS plastic is generally 24 hours, the water absorption is 0.2 to 0.4%, and the water content that can be injection molded is 0.1 to 0.2%.
Injection and pressure holding time: The control methods of the computer injection machine are equipped with multi-stage injection, gradable pressure regulation, speed regulation and adjustment of injection plastic volume. The speed of the plastic injected into the mold cavity reaches a constant speed, and the appearance and quality of the molded product are improved.
Therefore, the injection process is commonly used for position control, without time control, and the pressure is controlled by time. For example, the pressure holding time is long, the product density is large, the weight is heavy, the internal stress is large, the demoulding is difficult, the whitening is easy, and the molding cycle is prolonged. The holding time is too short, the product is prone to dents, and the size is unstable.
Cooling time: It is to ensure the stable setting of the product. It is enough to cool the molding time after the plastic injected into the mold cavity is molded into a product. Otherwise, the product is easy to warp and deform when the mold is opened, and the ejection is easy to deform and white. The cooling time is too long, and the molding cycle is extended, which is uneconomical.
4. Position control: Some injection molding machines have mold shift position, die position, stock storage and injection position control.
The position of the mold is from the mold opening to the closed mold to lock the entire moving distance called the mold shift position. The best position for mold shifting is to remove the product smoothly. The mold opening distance is too large and the molding cycle is long.
The position of the die can be easily removed by simply controlling the position of the die from the mold.
Storage position: Firstly, the amount of plastic injected in one time of the molded product should be ensured, and secondly, the amount of storage in the barrel should be controlled. The storage position control is more than one injection, and the product is easy to fly, and the product is insufficiently formed.
The amount of material stored in the cylinder is too much, the plastic stays in the barrel for a long time, the product is easy to fade, and the strength of the molded product is affected. On the contrary, it affects the quality of plastic plasticizing, and it is not added to the mold during the pressure keeping, and the product is not formed with dents.
to sum up:
The quality of injection molded products involves product design, plastic materials, mold design and processing quality, selection of injection molding machines and process adjustment.
For normal production molds, it is necessary to ensure and improve the quality of the product, only the adjustment of the injection process; that is, the temperature, pressure flow rate, time and position adjustment.
Process adjustments must be fully integrated. If the size of the product changes greatly, it can be considered from the plastic raw material, whether it is crystalline plastic or amorphous plastic, the molecular weight of the plastic, and so on. It is a crystalline plastic and a macromolecular plastic. The crystallinity and molecular orientation of the product should be considered. The shape of the product changes with the flow during molding, which causes the change and size of the stress in the product, resulting in shrinkage of the product.
Solve the small size change of the product, improve the material temperature, reduce the mold temperature, control the injection volume, reduce the injection pressure, increase the injection speed, instantly fill the mold cavity, reduce the pressure holding time, and extend the cooling time. To ensure the product density when the product is shot. It is achieved to reduce the internal stress of the product, thereby reducing the shrinkage of the product.
In short, the injection process adjustment can not be started from a certain point, must start from the injection process principle. Comprehensive and balanced consideration of issues, can be adjusted from multiple aspects one by one or multiple issues at a time. However, the adjustment method and adjustment principle should be determined by the quality and process conditions of the products produced at that time.
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