The plastic must be sufficiently dried before molding. When the moisture-containing material enters the cavity, it will cause silver-like ecchymosis on the surface of the workpiece, and even water decomposition will occur at high temperatures, resulting in deterioration of the material. Therefore, the material must be pretreated before forming, so that the material can maintain proper moisture.
First, the mold temperature setting
(1) The mold temperature affects the molding cycle and the forming quality. In actual operation, the setting is started by using the lowest appropriate mold temperature of the material, and then appropriately adjusted according to the quality condition.
(2) Properly speaking, the mold temperature refers to the temperature of the cavity surface when the forming is carried out. In the mold design and the setting conditions of the forming engineering, it is important to maintain not only the proper temperature but also the uniformity. Distribution.
(3) The uneven mold temperature distribution causes uneven shrinkage and internal stress, which makes the forming port susceptible to deformation and warpage. (4) Increasing the mold temperature to obtain the following effects;
1. Add crystallinity and uniform structure of the molded product. 2. The molding shrinkage is sufficient, and the rear shrinkage is reduced.
3. Improve the strength and heat resistance of the molded product.
4. Reduce residual internal stress, molecular alignment and deformation.
5. Reduce the flow negative pressure during filling and reduce the pressure loss.
6. Make the molded product look shiny and good.
7. Increase the chance of burrs on the molded product.
8. Increase the chance of a near gate location and a reduction in the depression of the gate.
9. Reduce the apparent extent of the bond line
10. Increase the cooling time.
Metering and plasticizing
(1) In the molding process, the control (metering) of the injection amount and the uniform melting (plasticization) of the plastic are carried out by the plasticizing unit of the injection machine.
1. Heating cylinder temperature (Barrel Temperature)
Although the melting of plastics, about 60-85% is due to the thermal energy 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 the temperature near the front of the nozzle - the temperature in the front zone When it is high, it is easy to drop the material and take the wire when the part is taken out.
2. Screw speed (screw speed)
(1) 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 has the following effects:
1) Thermal decomposition of plastics.
2) Glass fiber (plus fiber plastic) is shortened.
3) The screw or heating cylinder wears up quickly.
(2) The setting of the speed can be measured by the size of the circumferential speed (circumferen-tial screw speed):
Circumferential speed = n (speed) * d (diameter) * π (pi) In general, a plastic with a low viscosity and good thermal stability can be set to a circumference of about 1 m/s with a screw rod rotating, but the thermal stability is poor. Plastics should be as low as 0.1.
(3) In practical applications, we can reduce the screw speed as much as possible so that the rotary feed can be completed before the mold is opened.
3. Back pressure (BaCK PRESSURE)
(1) When the screw is rotated and fed, the pressure accumulated by the melt propelled to the front end of the screw is called back pressure. During injection molding, it can be adjusted by adjusting the oil pressure of the injection hydraulic cylinder. The back pressure can be as follows: Effect:
1) Melt glue is more uniformly melted.
2) The toner and the filler are more uniformly dispersed.
3) Let the gas exit from the drop port.
4) The measurement of the feed is accurate.
(2) The level of back pressure is determined by the viscosity of the plastic and its thermal stability. The too high back pressure delays the feeding time and also causes the plastic to overheat due to the increase of the rotating shear force. Generally, it is suitable for 5~15kg/cm2.
4. Retreat (SUCK BaCK, DECOMPRESSION)
(1) After the rod is rotated and the feed is finished, the screw can be appropriately withdrawn, and the melt pressure at the front end of the screw can be lowered. This is called loosening, and the effect is to prevent dripping of the nozzle portion.
(2) Insufficient, it is easy to make the main channel (SPRUE) stick mold; and too much retreat, it can absorb air, causing gas marks on the molded product.
Set number of stability molding
(1) Confirmation beforehand and preliminary setting
1. Confirm that the material is dry, the mold temperature and the temperature of the heating cylinder are correctly set and reach the processable state.
2. Check the opening and closing mode and the action and distance setting of the 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 approximately 60 RPM.
7. The back pressure (PB) is set at approximately 10 kg/cm2.
8. Loose release is set at 3mm
9. The position of the pressure holding switch is set at 30% of the screw diameter. For example, a screw of φ100 mm is set to 30 mm.
10. The metering stroke is set slightly shorter than the calculated value.
11. The total shot time is slightly shorter and the cooling time is set longer.
(2) Manual operation parameter correction
1. Lock the mold (confirm the rise of the high pressure) and advance the injection seat.
2. Shoot manually until the screw stops completely and pay attention to the stop position.
3. The screw retracts the feed.
4. After cooling, open the mold and take out the molded product.
5. Repeat steps (1) to (4). The screw is finally stopped at 10% to 20% of the screw diameter, and the molded product has no short shots, burrs and whitening, or cracking.
(3) Correction of semi-automatic operation parameters
1. Correction of measuring stroke [measuring end point] Increase the injection pressure to 99%, and temporarily adjust the holding pressure to 0, adjust the measuring end point S0 to the short shot, and then adjust the back to the occurrence of burrs. Click to select a location.
2. The speed correction corrects the PH to the original level, and adjusts the injection speed up and down to find out the individual speeds of short shots and burrs, and the middle point is the appropriate speed. [This stage can also enter the multi-segment speed corresponding to the appearance problem. Parameter setting].
3. Maintain the pressure correction up and down to maintain the pressure, find out the individual pressure of the surface depression and burrs, and choose the pressure at the middle point.
4. The correction of the holding time [or injection time] gradually lengthens the holding time until the weight of the molded product is obviously stable.
5. Correction of the cooling time gradually reduce the cooling time and confirm that the following conditions can be met:
(1) The molded product is ejected, pinched, trimmed, and packaged without whitening, cracking or deformation.
(2) The mold temperature can be balanced and stable. Simple algorithm for cooling time of products with a 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 60 degrees or more [S indicates the maximum meat thickness of the molded product].
6. Correction 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) Confirm that the measurement time is stable. Try to adjust the gradient of the heating coil temperature.
(4) Confirm whether the nozzle has dripping material, whether the pig tail or the mold is generated in the main flow, whether there is any air mark in the finished product, etc., and adjust the nozzle part temperature or the retreat distance appropriately.
7. Segment pressure and multi-stage rate of fire
(1) In general, the injection should be based on high speed without affecting the appearance, but should be carried out at a lower speed before switching between gates and holding pressure;
(2) The pressure should be gradually reduced to avoid excessive stress in the molded product, which makes the molded article easy to deform.
