During the injection process, the phenomenon that the nozzle material (runner aggregate) sticks in the mold runner after the mold is opened and cannot be peeled off is called water injection.Mouth material sticks to the mold. The main reason why the nozzle material sticks to the mold is that the apertures of the injection machine nozzle and the sprue bushing (main flow bushing) do not match.Burrs (undercuts) are generated and cannot be taken out of the mold smoothly. The causes and solutions for this phenomenon are shown in the table below.
Reasons and solutions for the nozzle material to stick to the mold
| Cause Analysis | Solution Method |
|---|---|
| ① Excessive glue pressure or low pressure | ① Reduce glue pressure or low pressure |
| ② Coating temperature too high | ② Lower coating temperature |
| ③ Main runner inlet and sprue bushing not fitting well | ③ Improve fitting condition between main runner inlet and sprue bushing |
| ④ Main runner surface not smooth or has sharp corners | ④ Polish main runner or improve sharp corners |
| ⑤ Main runner inlet diameter smaller than nozzle diameter | ⑤ Enlarge main runner inlet diameter |
| ⑥ Main runner inlet rounded R smaller than nozzle head R | ⑥ Enlarge main runner inlet rounded R |
| ⑦ Main runner center hole not aligned with nozzle center | ⑦ Adjust nozzle hole center to align on the same line |
| ⑧ Runner outer side damaged or sprue head not smooth | ⑧ Repair mold, repair damaged areas, clean sprue head (prevent producing burrs) |
| ⑨ Main runner without pull hook | ⑨ Add pull hook at the end of the sprue or Z-shaped pull hook |
| ⑩ Main runner dimensions too large or cooling time insufficient | ⑩ Reduce main runner dimensions or extend cooling time |
| ⑪ Main runner demolding angle too small | ⑪ Increase main runner demolding angle |
Mold opening difficulties and solutions
During the injection molding production process, if there is excessive clamping force, mold core misalignment, guide pillar wear, or the mold is in a high-pressure clamping state for a long time, causing mold deformation and "biting force", the mold will not be able to be opened. This phenomenon is collectively referred to as mold opening difficulty. When larger plastic parts, molds with deeper cavities, or injection molding machines adopt a toggle clamping mechanism, the above-mentioned undesirable phenomena are more likely to occur. The causes and solutions for this phenomenon are shown in the table below.
Reasons for difficulties in mold making and their solutions.
| Cause Analysis | Solution Method |
|---|---|
| ① Mold cavity force causes deformation of the mold, resulting in "biting" | ① Reinforce the mold, reduce clamping force |
| ② Guide posts/guides too loose, clamping force too high | ② Clean/lubricate guide posts/guides, replace bushings |
| ③ Cooling time too long, leading to high pressure locked state | ③ Shorten cooling time (increase pressure) |
| ④ Single parting surface loose, mold thickness inconsistent | ④ Reinforce loose mold, adjust mold thickness screw |
| ⑤ Ejection mechanism friction insufficient | ⑤ Increase ejection force or replace with a larger ejection mechanism |
| ⑥ Mold exhaust system blocked, resulting in "negative pressure" | ⑥ Clean exhaust slots / vents inside the mold cavity or parting surface (or through exhaust channels) |
| ⑦ Uneven pulling force from three plates (insufficient strength) | ⑦ Increase the strength of the pulling hooks |
Note: For injection molding machines with a standard toggle-type clamping mechanism, the mold opening force can only reach approximately 80% of the rated clamping force.
Air entrapment caused by hot runner systems and its solutions
When using a hot runner system with a needle valve gate for injection molding, if a semi-hot, semi-cold runner system is used, as shown in the figure, the air in the cold runner must be able to be smoothly and completely expelled as the melt flows from the needle valve nozzle into the cold runner. Otherwise, the gas will be trapped by the melt flowing out of the needle valve gate, leading to the formation of bubbles in the molded product.
In hot runner molds used for long production runs, due to high temperatures or the accumulation of volatile substances, the venting of the cold runner connected to the hot runner can be affected. If the cold runner itself lacks proper venting or has poor venting efficiency, it can lead to gas entering the mold cavity and causing bubbles in the molded product.
Excessively long molding cycle and solutions.
In the injection molding process, the reasons for abnormally prolonged molding cycles and corresponding measures are as follows:
① High plastic temperature, resulting in excessively long cooling time for the product. Therefore, the barrel temperature should be lowered, the screw speed or back pressure should be reduced, and the temperature of each section of the barrel should be properly adjusted.
② High mold temperature, resulting in a long melt solidification time. However, high mold temperature is often beneficial for molding and achieving a good product appearance; therefore, the cooling of the water channels should be specifically enhanced.
③ Unstable molding time. Automatic or semi-automatic molding mode should be used.
④ Insufficient heating of the barrel, resulting in excessively long plasticization time. A machine with a larger plasticizing capacity should be used, or the preheating of the plastic raw materials should be enhanced.
⑤ Nozzle drooling. The machine's injection is unstable during the injection molding process. The temperature of the barrel and nozzle should be controlled, or a self-locking nozzle should be used.
⑥ The wall thickness of the plastic part is too thick, resulting in an excessively long solidification time. The mold structure should be improved to minimize the wall thickness of the plastic part.
⑦ Reducing the proportion of mineral fillers in the material, low thermal conductivity, and low crystallization rate of the material can also lead to excessively long molding cycles.
⑧ Other effective measures to shorten the molding cycle: using a hot runner mold to shorten the molding cycle. As shown in the figure, for a large plastic part, CAE mold flow analysis was performed. The original mold used a conventional runner, requiring 51.28s; after changing to a hot runner mold, the molding cycle only required 25.63s.
Other abnormal phenomena in injection molding and their solutions.
During the injection molding process, due to the influence of various factors such as plastic raw materials, molds, injection molding machines, molding process, operating methods, workshop environment, and production management, many abnormal phenomena can occur. In addition to the aforementioned defects, other abnormal phenomena such as broken pillars and excessive plastic may also occur. The causes and solutions for these abnormal phenomena are shown in the table below.
Other abnormal phenomena and their solutions
| Defect Phenomenon | Cause Analysis | Solution Method |
|---|---|---|
| Flash | ① Injection pressure or holding pressure too high ② Insufficient mold clamping force or uneven light on the parting surface, insufficient cooling time ③ Rubber material too soft | ① Reduce injection pressure or holding pressure ② Increase mold clamping force, polish (smooth) mold ③ Lower melt temperature, reduce small material proportion |
| Multiple lines | Mold (low-temperature area) components with scratches, rough processing / poor exhaust Components damaged / worn parts have very small gaps | Inspect mold and replace/repair internal components with related wear conditions |
| Burn marks | Mold (low-temperature area) has convex points, flowers, scratches, pits, grooves, and mold damage, etc. | Repair mold, improve surfaces with scratches/pits on the mold, prevent wear and pressure marks |
| Top needle convex defect | Internal过长 or protruding too much | Shorten top needle length or adjust amount |
| Top needle dent | Top needle has internal objects, top needle body length insufficient or top needle head too thin | Increase top needle internal objects, increase top needle length or thicken head |
| Top needle perforation | Top needle rear card top needle hole, resulting in "broken needle" | Inspect/replace top needle, clean left side in the middle of top needle seat, add top needle seat (prevent wear) |
| Top needle dent | Top needle hole wall, melt enters the interior | Expand top needle hole, increase production run, reduce top needle temperature, reduce ejection speed, reduce injection pressure / reduce top needle seat pressure |
| Ejector pin marks | Ejector pin flat, ejector pin too many, ejector pin diameter too large, ejection speed too fast, ejection delay too long | More ejector pins, production in ejector pin mechanism, reduce ejector pin diameter, reduce ejection speed, reduce ejection delay / retention pressure |
| Mold sticking on fixed side | Low pressure force too high, too short cooling (shrinkage), material good, pressure too high | More/less material, use release agent / raise mold temperature, reduce injection pressure and holding pressure, increase pressure |
| Letter block (alphabet block) surface | Letter / alphabet block (alphabet block) direction opposite during letter block sliding | Correct letter block installation or add positioning pin |