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

(1) Mold temperature affects molding cycle and molding quality. In actual operation, it is set from the lowest appropriate mold temperature of material used, and then adjusted appropriately according to quality status.
(2) Correctly speaking, mold temperature refers to temperature of cavity surface when molding is performed. In mold design and molding engineering condition setting, it is important not only to maintain a proper temperature, but also to make it evenly distributed.
(3) Uneven mold temperature distribution will lead to uneven shrinkage and internal stress, which makes molding mouth prone to deformation and warping.
(4) Increasing mold temperature can obtain following effects;
1. Add crystallinity of molded product and a more uniform structure.
2. Make molding shrinkage more fully and reduce post-shrinkage.
3. Improve strength and heat resistance of molded products.
4. Reduce internal stress residue, molecular alignment and deformation.
5. Reduce flow resistance during filling and reduce pressure loss.
6. Make appearance of molded product more shiny and good.
7. Increase chance of burrs on molded products.
8. Increase location near gate and reduce chance of recession at far gate.
9. Reduce obvious degree of bonding line
10. Increase cooling time.

 

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

Although about 60~85% of melting of plastic is due to heat generated by rotation of screw, melting state of plastic is still greatly affected by temperature of heating cylinder, especially temperature near front of nozzle-when temperature in front zone is too high, phenomenon of drooling and wire drawing when taking out parts is prone to occur.

(1) Melting of plastic is mainly caused by heat generated by rotation of screw. Therefore, if screw speed is too fast, following effects will be caused:
1) Thermal decomposition of plastics.
2) Glass fiber (fiber-added plastic) is shortened.
3) Screw or heating cylinder wears faster.
(2) Speed setting can be measured by size of its circumferen-tial screw speed:
Peripheral speed = n (rotation speed) * d (diameter) * π (circumference ratio). Generally, for low-viscosity plastics with good thermal stability, circumferential speed of 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 screw speed as much as possible so that rotating feed can be completed before mold is opened.

(1) When screw rotates and feeds, pressure accumulated by melt advancing to front end of screw is called back pressure. During injection molding, it can be adjusted by adjusting return pressure of injection hydraulic cylinder. Back pressure can be as follows Effect:
1) Melt glue more evenly.
2) Toner and filler are more evenly dispersed.
3) Make gas exit from blanking port.
4) Metering of feed is accurate.
(2) Level of back pressure is determined by viscosity of plastic and its thermal stability. Too high back pressure will delay feeding time, and increase in rotational shear force will easily cause plastic to overheat. Generally 5~15kg/cm2 is appropriate.

(1) After rod rotation is completed, screw is properly retracted to reduce melt pressure at the front of screw. This is called loosening, and its effect can prevent dripping of nozzle.
(2) Insufficient, it is easy to cause sprue (SPRUE) to stick to mold; too much loosening can suck in air and cause air marks in molded product.

1. Confirm whether material is dry, mold temperature and heating cylinder temperature are correctly set, reach a processable state.
2. Check action and distance setting of opening and closing mold and ejection.
3. Injection pressure (P1) is set at 60% of maximum value.
4. Keep pressure (PH) set at 30% of maximum value.
5. Injection speed (V1) is set at 40% of maximum value.
6. Screw speed (VS) is set at about 60RPM.
7. Back pressure (PB) is set at about 10kg/cm2.
8. Set loose and retract at 3mm
9. Position of holding pressure switch is set at 30% of screw diameter. For example, for a screw with φ100mm, set 30mm.
10. Metering stroke is set slightly shorter than calculated value.
11. Total injection time is slightly shorter, and cooling time is slightly longer.

 

1. Lock mold (confirm rise of high pressure), and injection seat moves forward.
2. Manually shoot until screw completely stops, and pay attention to stop position.
3. Screw revolves back to feed material.
4. After cooling, open mold and take out molded product.
5. Repeat steps ⑴~⑷, screw will finally stop at 10%~20% of screw diameter, and molded product has no short shots, burrs, whitening, or cracking.

1. Correction of metering stroke [metering end point]. Increase injection pressure to 99%, temporarily adjust holding pressure to 0, adjust metering end point S0 forward to occurrence of short shots, then back to occurrence of burrs, with middle point as selected position.
2. Correction of output speed to restore PH to original level, adjust injection speed up and down to find out individual speeds where short shots and burrs occur, use middle point as appropriate speed. [At this stage, you can also enter parameter setting that corresponds to appearance problems at multiple speeds.].
3. Correction of holding pressure. Adjust holding pressure up and down to find out individual pressures that cause surface depressions and burrs, choose middle point to hold pressure.
4. Correction of holding time [or injection time] gradually extends holding time until weight of molded product is obviously stable, which is a clear choice.
5. Modification of cooling time gradually reduce cooling time, confirm that following conditions can be met:
(1) Molded product is ejected, clipped, trimmed, and packaging will not be whitened, cracked or deformed.
(2) Mold temperature can be balanced and stable. A simple algorithm for cooling time of products with a meat thickness of more than 4mm:
1) Theoretical cooling time = S (1+2S)....... Mold temperature is below 60 degrees.
2) Theoretical cooling time = 1.3S (1+2S)...... Mold is 60 degrees or more [S represents maximum thickness of molded product].

(1) Confirm whether back pressure needs to be adjusted;
(2) Adjust screw speed to make metering time slightly shorter than cooling time;
(3) To confirm whether measurement time is stable, try to adjust temperature gradient of heating ring.
(4) Confirm whether there is dripping in nozzle, whether pig tails or sticky molds occur in main runner, whether finished product has air marks, etc., and adjust nozzle temperature or loosening distance appropriately.

(1) Generally speaking, in the case of not affecting appearance, injection should be carried out at a high speed, but it should be carried out at a lower speed before passing through gate and switching between holding pressure;
(2) Holding pressure should be gradually reduced to avoid too high residual stress in molded product, which makes molded product easily deformed.
Mold temperature affects molding cycle and molding quality. In actual operation, it is set from the lowest appropriate mold temperature of material used, and then adjusted to a higher level according to quality condition.

 

To put it right, mold temperature refers to temperature of cavity surface when molding is performed. In mold design and molding engineering condition setting, it is important not only to maintain an appropriate temperature, but also to make it evenly distributed.
Uneven mold temperature distribution will cause uneven shrinkage and internal stress, which makes molding mouth prone to deformation and warping.
Following effects can be obtained by increasing mold temperature:
Add crystallinity of molded product and a more uniform structure.
Molding shrinkage is sufficient, and post shrinkage is reduced.
Improve strength and heat resistance of molded products.
Reduce internal stress residue, molecular alignment and deformation.
Reduce flow resistance during filling and reduce pressure loss.
Make appearance of molded product more shiny and good.
Increase chance of burrs on molded products.
Increase location near gate and reduce chance of recession at far gate.
Reduce obvious degree of joint line.
Increase cooling time.
Pressure adjustment in injection molding process. Whether it is hydraulic or electric injection molding machine, all movements in injection molding process will generate pressure. Only by properly controlling required pressure can a finished product of reasonable quality be produced.
Pressure control and metering system is on hydraulic injection molding machine, all movements are executed by oil circuit responsible for following operations:
Screw in plasticizing stage rotates.
Slide forehearth (nozzle is close to nozzle bushing)
Axial movement of injection screw during injection and holding pressure
Close base material on ejector rod until toggle rod is fully extended or piston mold clamping stroke has been completed.
Start assembly of ejector rod to eject parts
On a full-voltage machine, all movements are performed by a brushless synchronous motor equipped with permanent magnets. Through ball bearing screw that has been used in machine tool industry, rotary motion is converted into linear motion. Efficiency of entire process partly depends on plasticization process, among which screw plays a very critical role.
Screw must ensure that material is melted and homogenized. This process can be adjusted with help of back pressure to avoid overheating. Mixing element must not generate excessively high flow rates, otherwise it will cause polymer degradation.
Each polymer has a different maximum flow rate. If this limit is exceeded, molecules will stretch and polymer backbone will break. However, focus is still to control forward axial movement of screw during injection and pressure holding. Subsequent cooling process, including internal stress, tolerance and warpage, is very important to ensure product quality. All this is determined by quality of mold, especially when optimizing cooling channel and ensuring effective closed-loop temperature regulation. System is completely independent and will not interfere with mechanical adjustments.
Mold movement such as mold closing and ejection must be precise and efficient. Speed distribution curve is usually used to ensure that moving parts are accurately approached. Contact maintenance force can be adjusted. Therefore, it can be concluded that, without considering energy consumption and mechanical reliability, with same additional conditions (such as mold quality), product quality is mainly determined by system that controls forward movement of screw. On a hydraulic injection molding machine, this adjustment is achieved by detecting oil pressure.
Specifically, oil pressure activates a set of valves through control panel, fluid acts through manipulator, is adjusted and released.
Injection speed control includes options such as open loop control, semi-closed loop control and closed loop control. Open loop system relies on a shared proportional valve. Proportional tension is applied to fluid of the required ratio, so that fluid generates pressure in injection barrel, and injection screw moves at a certain forward speed.
Semi-closed loop system uses a closed loop proportional valve. Loop is closed at position where closed port is located, and closed port controls flow rate of oil by moving in valve. Closed loop system is closed at screw translation speed. A speed sensor (usually a potentiometer type) is used in closed loop system to detect tension drop regularly. Oil flowing out of proportional valve can be adjusted to compensate for speed deviation.
Closed-loop control relies on dedicated electronic components integrated with machine. Closed-loop pressure control can ensure uniform pressure during injection and pressure holding stages, as well as uniform back pressure in each cycle. Proportional valve is adjusted by detected pressure value, and deviation compensation is performed according to set pressure value.
Generally speaking, hydraulic pressure can be monitored, but detecting melt pressure in nozzle or cavity is another effective method. A more reliable solution is to manage proportional valve by reading nozzle or cavity pressure readings. Adding temperature detection on basis of pressure detection is particularly conducive to process management.
Knowing actual pressure that material can withstand can also help predict actual weight and size of molded part based on set pressure and temperature conditions. In fact, by changing holding pressure value, more materials can be introduced into mold cavity to reduce component shrinkage and meet design tolerances (including preset injection shrinkage). When approaching melting conditions, semi-crystalline polymers show a great change in specific volume. In this regard, overfilling will not hinder ejection of part.