plastic mould, injection mould

Procedures for control of injection pressure

1.4.3.2 procedures for control of injection pressure
    The control of the injection pressure is usually divided into an injection pressure, the second injection pressure (holding pressure) or more than three times the injection pressure control. Switching time pressure if it is appropriate to prevent mold pressure is too high, to prevent overflow or lack of material are all very important. Mold packing stage the specific volume depends on the gate closed when the melt pressure and temperature. If the holding pressure time to switch to products from the cooling phase of the same pressure and temperature, then the specific volume of products will not change. Molding at a constant temperature to determine product size is the most important parameters of the packing pressure, affecting the product dimensional tolerances of the most important variable is the holding pressure and temperature. For example: after the end of the filling, packing pressure decreased immediately, when the surface to form a certain thickness, packing pressure to rise again, thus forming a low clamping force of large thick-walled products, elimination of collapse pits and flash.
Holding pressure and the speed is usually highest when the plastic cavity filling pressure and speed of 50% to 65%, which is holding pressure lower than the injection pressure of about 0.6 ~ 0.8MPa. As the packing pressure lower than the injection pressure, holding pressure in the considerable time, the pump load is low, solid pump life is extended, while oil pump motor power consumption is also reduced.
    Three pressure injection filling both the smooth parts, and weld line does not appear, depression, flash and warpage. For thin-walled parts, small pieces of long, long process of molding large parts or even the cavity mold configuration is not balanced and not too close parts of the molding as a whole.
    1.4.3.3 the amount injected into the mold cavity filling process control of plastic
    Regulate the use of certain measures in advance so near the end of the injection stroke, the screw end is still a small amount of residual melt (buffer capacity), according to the mold filling was further applied injection pressure (injection pressure two or three times) , add little melt. This can prevent depression or adjustment products product shrinkage.
    1.4.3.4 screw back pressure and speed of the process control
    High back pressure can be intense shear melt at low speeds, will also get a long plastic tube in the machine plasticizing time. Therefore the current use of more back pressure and speed of parallel programming control. For example: measurement of the whole trip in the screw first high speed, low back pressure, and then switch to a lower speed, high back pressure, and then switch to high back pressure, low speed, and finally in the low back-pressure, low speed for plastics , so that the front screw melt most of the pressure be released, reducing the rotational inertia of the screw, thereby improving the accuracy of measurement of the screw. Often result in excessive back pressure increases the degree of stain color; pre-plastic bodies in increased mechanical wear screw and barrel; pre-plastic periodic extension of decline in production efficiency; nozzle prone to drooling, regeneration feeding amount increased; even with self-locking nozzle, if the back pressure higher than the design of the spring closure pressure may also cause fatigue failure. So, back pressure must be properly adjusted.
    As technology advances, the small computer into the injection molding making machine control system, using a computer to control the injection molding process has become possible. Japan Steel Works N-PACS (micro-computer control system) can do four feedback control (holding pressure adjustment, molded adjustment, automatic measurement adjustment, resin temperature adjustment) and four process control (injection speed process control, pressure inspection, screw speed process control, process control back pressure).
    1.4.4 Preparation before injection molding
    Preparation before forming may include many things. Such as: materials processing performance test (determination of plastic flow, moisture content, etc.); raw materials, dyeing and processing of pre-selected grain; aggregates warm and dry; insert the cleaning and preheating; test mode and barrel cleaning and so on.
    1.4.4.1 Pretreatment of raw materials
    According to the characteristics of plastic and feeding conditions, usually in response to raw material before forming the appearance and performance testing process. If the plastic used for the powder, such as: polyvinyl chloride, and dry ingredients should be mixed; if the products have color requirements, you can add the right amount of coloring agent or masterbatch; supply of aggregates often contain different levels of water , flux and other volatile low molecular weight compounds, especially those with moisture, water is always a tendency to exceed the processing of plastic allowed limits. Therefore, must be dried before processing and measurement of water content. Sensitive at high temperature polycarbonate water requirements of the moisture content of 0.2% or less, or even 0.03% to 0.05%, so commonly used vacuum oven drying. Has to be properly dry and sealed plastic to prevent plastic from the air and then the loss of moisture and drying effects of the hopper can be used for this purpose drying room for the injection molding machine to provide continuous hot, dry material, to streamline operations, keep clean, improve quality, increase in injection rate are favorable. The added material drying hoppers generally the injection molding machine with a feeding amount of 2.5 times per hour.
1.4.4.2 Insert the preheating
    Injection molding products for the assembly and strength requirements, the need of embedded metal insert products. Injection molding, placed in the cavity of the cold and hot metal insert with plastic melt cools, the metal and plastic shrink significantly different rates, often around the inserts have a great internal stress (especially as polystyrene rigid-chain polymers of ethylene and other more significant). The existence of such internal stress cracks around the insert, resulting in significantly reduced performance products. This can be a large selection of thermal expansion coefficient of metal (aluminum, steel, etc.) for the inserts and the inserts (especially a large metal insert) to warm up. Also, when designing products around the insert arranged in a large thick-walled and other measures.
    1.4.4.3 barrel cleaning
    The beginning of a new injection molding machines purchased before using, or need to change products in production, the replacement of raw materials, change the color or find a break in the phenomenon of plastic, injection molding machines are required to clean or take apart and wash barrel.
    Washing machines generally use the heating cylinder tube cleaning method. Cleaning materials in the plastic materials (or plastic recycled material.) For heat-sensitive plastics, such as the existence of PVC material, can be low density polyethylene, polystyrene and other materials for cleaning the transition, then the processing of new materials replacement of cleaning materials transition.
    1.4.4.4 Selection of release agent
    Release agent is easy to make plastics material ejection. Zinc stearate applies in general outside the plastic polyamide; liquid paraffin used for polyamide plastic is better; silicone oil is expensive, cumbersome to use, less use.
    Should be controlled using the amount of release agent, try to use less or no. Excessive spraying will affect the product appearance, color decoration on the products also have adverse effects.


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