Brief description of the injection molding process settings

Before molding, the plastic must be fully dried. When the material containing moisture enters the mold cavity, it will cause silvery blemishes on the surfaces of the parts, and even water decomposition will occur 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 a suitable moisture content.

Mold temperature setting

(a) The mold temperature affects the molding cycle and the forming quality. In actual operation, it is set by the lowest mold temperature of the used material, and then it is appropriately adjusted according to the quality condition.

(2) In the correct sense, the mold temperature refers to the temperature of the mold cavity surface when the molding is performed. In the mold design and the condition setting of the molding process, it is important not only to maintain the proper temperature but also to make it uniform. Distribution.
(C) The uneven mold temperature distribution will lead to non-uniform shrinkage and internal stress, so that the forming port is easily deformed and warped.

(D) increase the mold temperature to obtain the following effects;

1. Add the crystallinity and more uniform structure of the molded product.

2. The molding shrinkage is more sufficient and the post-shrinkage is reduced.

3. Improve the strength and heat resistance of molded products.

4. Reduce internal stress residuals, molecular alignment and deformation.

5. Reduce the flow resistance during filling and reduce the pressure loss.

6. Make the appearance of the molded product more glossy and good.

7. Increase the chance of flashing on molded products.

8. Increase the chance of near-gate location and reduce recesses in the far runner area.

9. Reduce the apparent degree of bonding

10. Increase the cooling time.

Metering and plasticization

(a) In the molding process, the control of the injection volume (measurement) and the uniform melting of the plastic (plasticization) are performed by the plasticating unit of the injection molding machine (Plasticating unit).

Barrel Temperature

Although about 60 to 85% of the melting of the plastic is due to the heat generated by the rotation of the screw, the molten state of the plastic is still greatly affected by the temperature of the heating cylinder, especially near the temperature in the front area of ​​the nozzle - the temperature in the front zone. When it is high, drooling and the phenomenon of pulling the wire when taking out the parts are likely to occur.

Screw speed

(1) The melting of plastics is generally due to the heat generated by the rotation of the screw. Therefore, if the speed of the screw is too fast, the following effects will occur:

1) Thermal decomposition of plastics.

2) Shorten the glass fiber (fiber-reinforced plastic).

3) The screw or heater cartridge wears faster.

(2) The setting of the rotation speed can be measured by the size of its circumferential-to-universe screw speed:

Peripheral speed = n (rotation speed) * d (diameter) * π (pi)

In general, plastics with good low temperature viscosity and good heat stability, the peripheral speed of screw rod rotation can be set up to about 1m/s, but the plastic with poor thermal stability should be as low as about 0.1.

(3) In actual applications, we can reduce the screw speed as much as possible, so that the rotary feed can be completed before the mold is opened.

Back pressure (BACK PRESSURE)

(1) When the screw is rotated and fed, the pressure accumulated by the glue advanced to the front end of the screw is called back pressure. When the injection molding is performed, it can be adjusted by adjusting the oil discharge pressure of the injection oil cylinder. The back pressure can have the following: The effect: 1) more uniform melting of the melt. 2) More uniform dispersion of toner and filler. 3) Allow the gas to exit from the letdown port. 4) Accurate measurement of feed.

(2) The level of back pressure is determined by the viscosity and thermal stability of the plastic. Too high back pressure delays the feeding time, and due to the increase of rotational shear force, the plastic can easily overheat. Generally 5 ~ 15kg/cm2 is appropriate.

Song Back (SUCK BACK, DECOMPRESSION)

(1) When the rod is rotated and feeding is completed, the screw is properly retracted, and the pressure of the melt at the front end of the screw can be reduced. This is called loosening and the effect can prevent drooling of the nozzle portion.

(2) Insufficient, easy to make the main road (SPRUE) stick mold; and too much loose, you can inhale air, make the formation of air marks.

Stabilization Molding Number Setting (1) Pre-Acknowledgement and Preliminary Setting Confirm that the material is dry, the mold temperature, and the heating barrel temperature are set correctly and can be processed. Check the mold opening and closing movement and distance setting. The injection pressure (P1) is set at 60% of the maximum value. The holding pressure (PH) is set at 30% of the maximum value. The injection speed (V1) is set at 40% of the maximum value. The screw rotation speed (VS) is set at about 60 RPM. The back pressure (PB) is set at about 10 kg/cm2. The loose exit setting is set at 3mm. The pressure switching position is set at 30% of the screw diameter. For example, for a φ100 mm screw, set 30 mm. The metering stroke is set slightly shorter than the calculated value. The total shot time is slightly shorter and the cooling time is set longer.

(2) Manually operate the parameter to modify the locking mold (recognize the rise of high pressure) and advance the injection base. Take it out manually until the screw stops completely and pay attention to the stop position. Screw feed back. After cooling, the mold is opened to remove the molded product. Repeat steps (1) to (4). The screw will eventually stop at 10% to 20% of the screw diameter, and the molded product will have no short shots, burrs, whitening, or cracking.

(c) Correction of the semi-automatic operation parameters Correction of the metering stroke [Measurement end point] Increase the injection pressure to 99%, temporarily adjust the holding pressure to 0, and adjust the measurement end point S0 forward to a short shot, and then adjust it backwards. Flash occurs, with the middle point as the selected position. The correction of the exit speed returns the PH to the original level, adjusts the injection speed up and down, finds the individual speeds for short shots and flash, and takes the middle point as the appropriate speed [this stage can also enter the parameter setting for multi-section speed corresponding to the appearance problem. set]. Keep the pressure correction up and down to maintain pressure, find out the surface depression and burrs of the individual pressure, with the middle point as the choice of pressure. The retention time of the dwell time [or injection time] is gradually extended to hold time until the weight of the molded article is obviously stable and is suitable for Ming Shi choice. The cooling time correction gradually reduces the cooling time and confirms that the following conditions are met:

(1) The molded product is ejected, clipped, trimmed, and the package is not whitened, chipped, or deformed.

(2) Mold temperature is stable and stable. Simple algorithm for cooling time of products with meat thickness over 4mm:

1) Theoretical cooling time = S(1+2S)....... Mold temperature is 60 degrees or less.

2) Theoretical cooling time = 1.3S (1 + 2S).. Mold 60 degrees or more [S indicates the maximum thickness of the molded product].

Modification of plasticizing 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) To confirm that the measurement time is stable, try to adjust the gradient of the heating ring temperature.

(4) Check if there is any drip in the nozzle, if there is a pig tail or sticking die in the main channel, or if there is an air mark on the finished product, adjust the nozzle part temperature or loosen the distance.

Sectional pressure maintenance and multi-segment firing rate utilization (1) In general, the injection should be based on the principle of high speed without affecting the appearance, but it should be performed at a relatively low speed before switching between gates and holding pressure;

(2) The packing pressure should be gradually reduced so as to avoid the residual stress of the molded product being too high and the molded product to be easily deformed.

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