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Under normal circumstances, the gloss on the surface of injection molded parts is mainly determined by the type of plastic, colorants, and surface finish of the mold. However, it is common for other reasons to cause surface color and gloss defects, as well as surface darkening and other defects in products. The reasons and solutions for this are analyzed as follows:

(1) Poor mold finish, rust stains on the surface of the mold cavity, and poor mold exhaust.

(2) There are defects in the pouring system of the mold, and it is necessary to increase the cold well, flow channel, polishing main channel, diversion channel, and gate.

(3) If the material temperature and mold temperature are too low, local heating at the gate can be used if necessary.

(4) The processing pressure is too low, the speed is too slow, the injection time is insufficient, and the back pressure is insufficient, resulting in poor compactness and dark surface color.

(5) Plastic should be fully plasticized, but to prevent material degradation, it should be stable when heated, and cooled sufficiently, especially for thick walled materials.

(6) Prevent cold materials from entering the parts, and if necessary, use self-locking springs or lower the nozzle temperature.

(7) Excessive use of recycled materials, poor quality of plastics or colorants, mixed with water vapor or other impurities, and poor quality of lubricants used.

(8) The locking force should be sufficient.

Before forming and processing, the plastic must be thoroughly dried. After the material containing moisture enters the mold cavity, it will cause silver streaks to appear on the surface of the workpiece, and even cause water decomposition at high temperatures, leading to material degradation. Therefore, before forming and processing, it is necessary to pre-treat the material to maintain appropriate moisture content.

1、 Mold temperature setting

（1） The mold temperature affects the forming cycle and quality. In practical operation, it is set from the lowest appropriate mold temperature of the material used, and then adjusted appropriately according to the quality status.

（2） To put it correctly, mold temperature refers to the temperature on the surface of the mold cavity during forming. In mold design and forming engineering conditions, it is important not only to maintain an appropriate temperature, but also to ensure its uniform distribution.

（3） Uneven mold temperature distribution can lead to uneven shrinkage and internal stress, making the forming mouth prone to deformation and warping.

（4） Increasing the mold temperature can achieve the following effects;

1. Add the crystallinity and relatively uniform structure of the formed product.

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

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

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

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

6. Make the formed product have a shiny and good appearance.

7. Increase the opportunity for burrs to occur in formed products.

8. Increase the chances of near gate areas and reduce the chances of dents in far gate areas.

9. Reduce the degree of obvious bonding lines

10. Increase cooling time.

Measurement and Plasticization

（1） In the molding process, the control (measurement) of injection volume and the uniform melting (plasticization) of plastic are handled by the plasticizing unit of the injection machine.

1. Barrel Temperature

Although about 60-85% of the melting of plastic is due to the thermal energy generated by the rotation of the screw, the melting state of plastic is still greatly affected by the temperature of the heating cylinder, especially when the temperature near the nozzle front zone is too high, which is prone to material dripping and wire pulling when taking out the workpiece.

2. Screw speed

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

1) Thermal decomposition of plastics.

2) Glass fiber (reinforced plastic) is shortened.

3) Accelerated wear of screws or heating cylinders.

(2) The setting of rotational speed can be measured by the size of its circumferential screw speed:

Circumferential speed=n (rotational speed) * d (diameter) * π (circumference)

Usually, plastics with low viscosity and good thermal stability can have a screw rod rotation speed of about 1m/s, but plastics with poor thermal stability should have a speed as low as around 0.1.

(3) In practical applications, we can try to lower the screw speed as much as possible, so that the rotating feed is completed before mold opening.

3. Back pressure

(1) When the screw rotates to feed, the pressure accumulated by the molten adhesive pushed to the front end of the screw is called back pressure. During injection molding, it can be adjusted by adjusting the oil return pressure of the injection hydraulic cylinder. Back pressure can have the following effects:

1) Melting glue melts more evenly.

2) Colorants and fillers are more evenly dispersed.

3) Allow the gas to exit from the discharge port.

4) The measurement of incoming materials is accurate.

(2) The level of back pressure is determined by the viscosity and thermal stability of the plastic. If the back pressure is too high, it can delay the feeding time and increase the rotational shear force, which can easily cause the plastic to overheat. It is generally recommended to use 5-15kg/cm2.

4. BACK, DECOMPRESSION

(1) After the rod rotation feeding is completed, the screw can be appropriately withdrawn, which can reduce the melt pressure at the front end of the screw. This is called loosening, and its effect can prevent material dripping at the nozzle.

(2) Insufficient, easy to cause the sprue to stick to the mold; And too much looseness can suck in air, causing air marks on the formed product.

Number setting for stable molding

（1） Pre confirmation and preparation settings

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

2. Check the action and distance settings of opening and closing the mold and ejecting.

3. Set the injection pressure (P1) to 60% of the maximum value.

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

5. Set the ejection velocity (V1) to 40% of the maximum value.

6. The screw speed (VS) is set at approximately 60RPM.

7. Set the back pressure (PB) at approximately 10kg/cm2.

8. Set the loosening to approximately 3mm.

9. The position for pressure maintaining switching is set at 30% of the screw diameter. for example φ 100mm screw, set 30mm.

10. Set the measuring stroke to be slightly shorter than the calculated value.

11. Set the total shooting time to be slightly shorter and the cooling time to be slightly longer.

（2） Manual operation parameter correction

1. Lock the mold (confirm the rise of high pressure) and advance the injection seat.

2. Manually shoot until the screw stops completely and pay attention to the stop position.

3. Screw rotation back feeding.

4. After cooling, open the mold and take out the formed product.

5. Repeat steps (1) to (4) until the screw finally stops at a position between 10% and 20% of the screw diameter, and the formed product has no short shot, burrs, whitening, or cracking.

（3） Correction of semi-automatic operation parameters

1. The correction of metering stroke [metering end point] increases the injection pressure to 99%, and temporarily sets the pressure holding to 0. Adjust the metering end point S0 forward to the occurrence of short shot, and then backward to the occurrence of rough edge, with its middle point as the selected position.

2. Correction of discharge speed restores the PH to the original level, adjusts the discharge speed up and down, and identifies individual velocities where short shots and burrs occur. The appropriate speed is the midpoint. This stage can also enter parameter settings that correspond to appearance issues with multiple speeds.

3. Adjust the holding pressure up and down to identify the individual pressures that cause surface dents and burrs, and select the middle point as the holding pressure.

4. The correction of the holding time [or injection time] gradually extends the holding time until the weight of the formed product is significantly stable, which is a wise choice.

5. Gradually adjust the cooling time and confirm that the following conditions can be met:

(1) The molded product will not whiten, crack or deform when pushed out, clamped out, trimmed, or 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)... mold temperature below 60 degrees Celsius.

2) Theoretical cooling time=1.3S (1+2S)... The mold is above 60 degrees [S represents the maximum flesh thickness of the formed product].

6. Correction of plasticizing parameters

(1) Confirm whether the back pressure needs to be adjusted;

(2) Adjust the screw speed to make the metering time slightly shorter than the cooling time;

(3) To confirm whether the measurement time is stable, try adjusting the gradient of the heating coil temperature.

(4) Confirm whether there is any material dripping from the nozzle, whether there is pig tail or mold sticking in the main channel, and whether there are air marks in the finished product. Adjust the temperature or loosening distance of the nozzle appropriately.

7. Flexible use of pressure maintaining and multi stage firing rate

(1) Generally speaking, injection should follow the principle of high speed without affecting the appearance, but should be carried out at a lower speed before passing through the gate and pressure switching;

(2) The pressure should be gradually reduced to avoid high residual stress in the formed product, which can easily deform the formed product.