Drying before plastic injection molding

The plastic must be fully dried before molding. When materials containing moisture enter the mold cavity, silver ribbon-like flaws will appear on the surface of the part, and may even decompose with water at high temperatures, resulting in material deterioration. Therefore, the material must be pretreated before molding so that the material can maintain appropriate moisture.

Mold temperature setting

   ⑴Mold temperature affects the molding cycle and molding quality. In actual operation, it is set starting from the lowest appropriate mold temperature of the material used, and then adjusted appropriately according to the quality condition.

   ⑵Correctly speaking, mold temperature refers to the temperature of the surface of the mold cavity when molding is in progress. In terms of mold design and molding engineering conditions setting, it is important not only to maintain an appropriate temperature, but also to distribute it evenly. .

   ⑶ Uneven mold temperature distribution will lead to uneven shrinkage and internal stress, thus making the molding port prone to deformation and warping.

   ⑷ Raising the mold temperature can achieve the following effects;

   ①Added molded products have a higher crystallinity and more uniform structure.

   ② Make the molding shrinkage more sufficient and reduce the post-shrinkage.

   ③Improve the strength and heat resistance of molded products.

   ④ Reduce internal stress residual, molecular alignment and deformation.

   ⑤ Reduce flow resistance during filling and reduce pressure loss.

   ⑥ Make the appearance of the molded products glossier and better.

   ⑦ Increase the chance of burrs in molded products.

   ⑧Increase the chance of dents near the gate and reduce the chance of dents far from the gate.

   ⑨ Reduce the obvious degree of bonding lines

   ⑩Increase the cooling time.

Dosing and plasticization

   ⑴In the molding process, the plasticizing unit of the injection machine is responsible for the control (metering) of the injection volume and the uniform melting (plasticization) of the plastic.

   ①Heating barrel temperature (Barrel Temperature)

   Although about 60 to 85% of the melting of plastic is due to the heat energy generated by the rotation of the screw, the melting state of the plastic is still greatly affected by the temperature of the heating barrel, especially the temperature near the front area of the nozzle - the temperature in the front area is too high When it is high, it is easy to cause material dripping and string pulling when taking out the parts. The following table shows the appropriate material temperature, mold temperature and molding shrinkage of several plastics.

②Screw speed

   A. The melting of plastic is generally due to the heat generated by the rotation of the screw. Therefore, if the screw rotates too fast, it will have the following effects:

   a. Thermal decomposition of plastics.

   b. Glass fiber (fiber-added plastic) is shortened.

   c. The screw or heating barrel wears out faster.

   B. The setting of rotation speed can be measured by the size of its circumferential speed (circumferen-tial screw speed):

   Circumferential speed = n (rotation speed) * d (diameter) * π (circumferential ratio)

   Generally, for plastics with low viscosity and good thermal stability, the peripheral speed of screw rotation can be set to about 1m/s, but for plastics with poor thermal stability, it should be as low as about 0.1.

   C. In practical applications, we can lower the screw speed as much as possible so that the rotating feed can be completed before mold opening.

   ③Back pressure

   A. When the screw rotates and feeds material, the pressure accumulated by the melt pushed to the front end of the screw is called back pressure. During injection molding, it can be adjusted by adjusting the oil withdrawal pressure of the injection hydraulic cylinder. The back pressure can be as follows Effect:

   a. The melt melts more uniformly.

   b. Colorants and fillers are more evenly dispersed.

   c. Let the gas exit from the blanking port.

      d. Accurate measurement of feed materials.

   B. The level of back pressure is determined by the viscosity and thermal stability of the plastic. Too high back pressure will prolong the feeding time and easily cause the plastic to overheat due to the increase in rotational shear force. Generally, 5~15kg/cm2 is appropriate.

   ④SUCK BACK, DECOMPRESSION

   A. After the rod rotation and feeding are completed, the screw is properly withdrawn to reduce the melt pressure at the front end of the screw. This is called loosening, and its effect can prevent dripping at the nozzle.

   B. If it is insufficient, it is easy for the sprue (SPRUE) to stick to the mold; if it is too loose, air can be sucked in, causing air marks on the molded product.

Parameter settings for stable molding

   1. Prior confirmation and preliminary settings

   ⑴Confirm whether the material dryness, mold temperature and heating cylinder temperature are correctly set and reach a processable state.

   ⑵ Check the mold opening and closing and ejection movements and distance settings.

   ⑶Injection pressure (P1) is set at 60% of the maximum value.

   ⑷ Keep the pressure (PH) set at 30% of the maximum value.

   ⑸The injection speed (V1) is set at 40% of the maximum value.

   ⑹Screw speed (VS) is set at about 60RPM.

   ⑺The back pressure (PB) is set at about 10kg/cm2.

   ⑻ Loose withdrawal is set at 3mm.

   ⑼The position of pressure maintaining switching is set at 30% of the screw diameter. For example, for a φ100mm screw, set it to 30mm.

   ⑽Set the metering stroke slightly shorter than the calculated value.

   ⑾The total injection time is slightly shorter and the cooling time is slightly longer.

   2. Manual operation parameter correction

   ⑴Lock the mold (confirm the rise of high pressure) and move the injection base forward.

   ⑵ Inject manually until the screw stops completely, and pay attention to the stop position.

   ⑶The screw rotates back to feed.

   ⑷After cooling, open the mold and take out the molded product.

   ⑸ Repeat steps 1 to 4 until the screw finally stops at 10% to 20% of the screw diameter, and the molded product has no short shots, burrs, whitening, or cracking.

   3. Correction of semi-automatic operating parameters

   ⑴Correction of the metering stroke [Measuring end point] Increase the injection pressure to 99%, temporarily adjust the holding pressure to 0, adjust the metering end point S0 forward to the occurrence of short shot, and then adjust it backward to the occurrence of burrs, and then adjust it to the middle point To select a location.

   ⑵Correction of the injection speed returns the PH to the original level, adjusts the injection speed up and down, finds out the individual speeds where short shots and burrs occur, and uses the midpoint as the appropriate speed. [At this stage, you can also enter the parameters of multiple speeds corresponding to appearance problems. set up].

   ⑶ Correction of holding pressure Adjust the holding pressure up or down to find out the individual pressures that cause surface dents and burrs, and use the midpoint as the selected holding pressure.

   ⑷Correction of the holding time [or injection time] and gradually extending the holding time until the weight of the molded product is obviously stable is an appropriate choice.

   ⑸ Correction of the cooling time. Gradually reduce the cooling time and confirm that the following conditions can be met: 1. The molded product will not be whitened, cracked or deformed when it is ejected, clamped, trimmed and packaged. 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)...... The mold is above 60 degrees [S represents the maximum thickness of the molded product].

   ⑹Correction of plasticizing parameters

   ① Confirm whether the back pressure needs to be adjusted;

   ②Adjust the screw speed so that the metering time is slightly shorter than the cooling time;

   ③ Confirm whether the measurement time is stable, and try adjusting the gradient of the heating coil temperature.

   ④Confirm whether there is any dripping material from the nozzle, whether there is pig tailing or mold sticking in the main flow channel, whether there are air marks on the finished product, etc., and adjust the nozzle temperature or loosening distance appropriately.

   ⑺Use of stage pressure holding and multi-stage firing rate

   ① Generally speaking, without affecting the appearance, the injection should be at high speed, but it should be done at a lower speed before passing through the gate room and switching between pressure holding;

   ② The holding pressure should be gradually reduced to prevent the residual stress in the molded product from being too high and making the molded product easily deformed.