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Injection molding process setting

Publisher: Administrator    Date:2021-08-02

Before molding, the plastic must be fully dried. After the material containing moisture enters the mold cavity, it will cause silver tape-like flaws on the surface of the part, and even hydrolysis at high temperatures, resulting in deterioration of the material. Therefore, the material must be pretreated before the molding process, so that the material can maintain the proper moisture.

1. Mold temperature setting (1) Mold temperature affects the molding cycle and molding quality. In actual operation, it starts with the lowest appropriate mold temperature of the material used, and then adjusts it appropriately according to the quality condition. (2) To put it right, the mold temperature refers to the temperature of the cavity surface when the molding is performed. In the design of the injection mold and the condition setting of the molding process, it is important not only to maintain an appropriate temperature, but also to allow Its even distribution. (3) Uneven mold temperature distribution will lead to uneven shrinkage and internal stress, which makes the molding mouth prone to deformation and warping.

(4) Increasing the mold temperature can obtain the following effects: 1. Add the crystallinity of the molded product and a more uniform structure. 2. Make the molding shrinkage more fully and reduce the post-shrinkage. 3. Improve the strength and heat resistance of molded products. 4. Reduce internal stress residue, molecular alignment and deformation. 5. Reduce the flow resistance during filling and reduce pressure loss. 6. Make the appearance of the molded product more shiny and good. 7. Increase the chance of burrs on molded products. 8. Increase the location near the gate and reduce the chance of recession at the far gate. 9. Reduce the obvious degree of the joint line. 10. Increase the cooling time.

Metering and plasticization. (1) In the molding process, the control (metering) of the injection volume and the uniform melting (plasticization) of the plastic are performed by the plasticating unit of the injection machine. 1. Barrel Temperature (Barrel Temperature) Although plastic About 60~85% of the melting is due to the heat generated by the rotation of the screw, but the melting state of the plastic is still greatly affected by the temperature of the heating cylinder, especially the temperature near the nozzle front area-when the front area temperature is too high The phenomenon of drooling and wire drawing when taking out the parts is easy to occur.

2. Screw speed (1) The melting of plastic is mainly caused by the heat generated by the rotation of the screw. Therefore, if the screw speed is too fast, the following effects will be caused: 1) Thermal decomposition of the plastic. 2) The glass fiber (fiber-added plastic) is shortened. 3) The screw or heating cylinder wears faster. (2) The setting of the speed can be measured by the size of its circumferential speed (circumferen-tial screw speed): Circumferential speed = n (rotation speed) * d (diameter) * π (circumference ratio) Generally, low viscosity has good thermal stability For plastics, the circumferential speed of the screw rod rotation can be set to about 1m/s, but for plastics with poor thermal stability, it should be as low as about 0.1. (3) In practical applications, we can reduce the screw speed as much as possible so that the rotating feed can be completed before the mold is opened.

3. BACK PRESSURE (1) When the screw rotates and feeds, the pressure accumulated by the melt advancing to the front end of the screw is called back pressure. During injection molding, you can adjust the ejection pressure of the injection hydraulic cylinder. To adjust, the back pressure can have the following effects: 1) Melt the glue more uniformly. 2) Toner and filler are more evenly dispersed. 3) Make the gas exit from the blanking port. 4) The metering of the feed is accurate. (2) The level of back pressure is determined by the viscosity of the plastic and its thermal stability. Too high back pressure will delay the feeding time, and the increase in the rotational shear force will easily cause the plastic to overheat. Generally 5~15kg/cm2 is appropriate. 4. SUCK BACK (DECOMPRESSION) (1) After the rod rotates and feeds, the screw is properly retracted to reduce the melt pressure at the front of the screw. This is called loosening, and its effect can prevent dripping of the nozzle. (2) Insufficient, it is easy to make the sprue (SPRUE) stick to the mold; and too much loosening can suck in air and cause air marks in the molded product.

Stable molding number setting (1) Pre-confirmation and preliminary setting 1. Confirm that the material drying, mold temperature and heating cylinder temperature are correctly set and reach a processable state. 2. Check the action and distance setting of opening and closing the mold and ejection. 3. The injection pressure (P1) is set at 60% of the maximum value. 4. Keep the pressure (PH) set at 30% of the maximum value. 5. The injection speed (V1) is set at 40% of the maximum value. 6. The screw speed (VS) is set at about 60RPM. 7. The back pressure (PB) is set at about 10kg/cm2. 8. The loose retraction is set at about 3mm. 9. The holding pressure switch position is set at 30% of the screw diameter. For example, for a screw with φ100mm, set 30mm. 10. The metering stroke is set slightly shorter than the calculated value. 11. The total injection time is slightly shorter, and the cooling time is slightly longer. .(2) Manual operation parameter correction 1. Lock the injection mold (confirm the rise of high pressure), and the injection seat moves forward. 2. Manually shoot until the screw completely stops, and pay attention to the stop position. 3. The screw revolves back to feed the material. 4. After cooling, open the mold and take out the molded product. 5. Repeat the steps ⑴~⑷, the screw will finally stop at 10%~20% of the screw diameter, and the molded product has no short shots, burrs, whitening, or cracking.

(3) Modification of semi-automatic operation parameters 1. Modification of metering stroke [metering end point] Increase the injection pressure to 99%, temporarily adjust the holding pressure to 0, and adjust the metering end point S0 forward until a short shot occurs, and then backward Adjust to the occurrence of burrs, and use the middle point as the selected position. 2. Correction of the output speed to restore the PH to the original level, adjust the injection speed up and down to find out the individual speeds where short shots and burrs occur, and use the middle point as the appropriate speed. [This stage can also enter the multi-stage speed corresponding to the appearance problem Parameter setting]. 3. Correction of the holding pressure. Adjust the holding pressure up and down to find out the individual pressures that cause surface depressions and burrs, and choose the middle point to hold the pressure. 4. The correction of the holding time [or injection time] gradually extends the holding time until the weight of the molded product is obviously stable, which is a clear choice. 5. Modification of the cooling time gradually reduce the cooling time, and confirm that the following conditions can be met: (1) The molded product is ejected, clipped, trimmed, and the packaging will not be whitened, cracked or deformed. (2) The mold temperature can be balanced and stable. A simple algorithm for the cooling time of products with a meat thickness of 4mm or more: 1) Theoretical cooling time=S(1+2S)……. The mold temperature is below 60 degrees. 2) Theoretical cooling time=1.3S(1+2S)……. 60 degrees above the injection mold [S represents the maximum thickness of the molded product]. 6. Modification of plasticization parameters (1) Confirm whether the back pressure needs to be adjusted; (2) Adjust the screw speed so that the metering time is slightly shorter than the cooling time; (3) Confirm whether the metering time is stable, try to adjust the temperature gradient of the heating ring . (4) Check whether there is any dripping in the nozzle, whether pig tails or sticky molds occur in the main runner, and whether the finished product has air marks, etc., and adjust the nozzle temperature or loosening distance appropriately. 7. Utilization of stage holding pressure and multi-stage shooting speed (1) Generally speaking, in the case of not affecting the appearance, the injection should be based on the principle of high speed, but it should be carried out at a lower speed before passing through the gate and switching the holding pressure (2) The holding pressure should be gradually lowered to avoid too high residual stress in the molded product, which makes the molded product easily deformed.
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