Why do injection molded products burn?

Factor 1:

When plastic is injected into a large cavity under high pressure, melt fracture is very likely to occur. At this time, transverse fractures appear on the surface of the melt, and the fracture area is roughly mixed in the surface of the plastic part to form a burnt spot. Especially when a small amount of melt is directly injected into a cavity that is prone to being too large, melt fracture is more serious, and the burnt spot is larger.

The essence of melt fracture is caused by the elastic behavior of polymer melt. When the melt flows in the barrel, the melt near the barrel is subjected to friction with the barrel wall, resulting in greater stress and a lower melt flow rate. Once the melt is injected from the nozzle, the stress of the tube wall disappears, while the melt flow rate in the middle of the barrel is extremely high. The melt at the barrel wall is accelerated by the melt at the center. Since the flow of the melt is relatively continuous, the flow rates of the inner and outer melts will be rearranged and tend to the average speed.

Factor 2:

During this process, the melt in the electronic product shell processing factory will undergo a sharp stress change and produce strain. Due to the extremely fast injection speed, the stress is particularly large, which is far greater than the strain capacity of the melt, resulting in melt fracture.

If the melt encounters a sudden shape change in the flow channel, such as diameter shrinkage, expansion, and dead corners, the melt stays and circulates at the dead corners. It is different from the normal melt in terms of force, and the shear deformation is large. When it is mixed into the normal flow material and injected, the deformation recovery of the two is inconsistent and cannot be bridged. If the difference is large, fracture and rupture will occur, and its manifestation is also melt fracture.

From the above, it can be seen that to overcome the difficulty of melt fracture and avoid the occurrence of paste spots:
Pay attention to eliminating the dead corners in the flow channel and make the flow channel as streamlined as possible;
Appropriately increase the material temperature and reduce the relaxation time of the melt to make its deformation easy to recover and bridge;
Add low molecular weight substances to the raw materials, because the lower the molecular weight of the melt, the wider the distribution, and the more conducive to reducing the elastic effect;
Appropriately control the injection speed and screw speed;
It is very important to reasonably set the gate position and choose the correct gate form. Practice shows that the use of expanded point gates and latent gates (tunnel gates) is more ideal. The best location for the gate is when the melt is injected into the transition cavity before entering the larger cavity. Do not allow the flow to enter the larger cavity directly.

Factor 3: Improper control of molding conditions

This is also an important reason for the scorching and burning spots on the surface of plastic parts, especially the injection speed has a great influence on it. When the flow is slowly injected into the cavity, the flow state of the melt is laminar; when the injection speed rises to a certain value, the flow state gradually changes to turbulent.

Under normal circumstances, the surface of the plastic part formed by laminar flow is relatively bright and smooth. The plastic part formed under turbulent conditions is not only prone to burn spots on the surface, but also prone to pores inside the plastic part. Therefore, the injection speed should not be too high, and the flow should be controlled to fill the mold in a laminar state.

If the temperature of the melt is too high, it is easy to cause the melt to decompose and coke, resulting in burn spots on the surface of the plastic part. Generally, the screw speed of the injection molding machine should be less than 90r/min, and the back pressure should be less than 2mpa, so as to avoid excessive friction heat generated by the barrel.

If excessive friction heat is generated during the molding process due to the long rotation time of the screw when it retracts, it can be overcome by appropriately increasing the screw speed, extending the molding cycle, reducing the screw back pressure, increasing the temperature of the barrel feeding section, and using raw materials with poor lubricity.

During the injection process, too much reflux of the melt along the screw groove and resin retention at the check ring will cause the melt to decompose. For this, a resin with a higher viscosity should be selected, the injection pressure should be appropriately reduced, and an injection molding machine with a larger length-to-diameter ratio should be used. The check rings commonly used in injection molding machines are more likely to cause retention, causing it to decompose and discolor. When the decomposed and discolored melt is injected into the cavity, a brown or black focus is formed. For this, the screw system centered on the nozzle should be cleaned regularly.

Factor 4: Mold failure

If the mold vent of the electronic product shell processing factory is blocked by the release agent and the solidified material precipitated from the raw material, the mold vent is not set enough or the position is incorrect, and the filling speed is too fast, the air in the mold that has not been discharged in time is adiabatically compressed to produce high-temperature gas, which will decompose and coke the resin. In this regard, the obstruction should be removed, the clamping force should be reduced, and the poor exhaust of the mold should be improved.

The determination of the mold gate form and position is also very important. The flow state of the molten material and the exhaust performance of the mold should be fully considered during the design.

In addition, the amount of mold release agent should not be too much, and the cavity surface should maintain a high degree of finish.

Factor 5:

The raw materials do not meet the requirements
If the moisture and volatile content in the raw materials are too high, the melt index is too large, and the lubricant is used excessively, it will cause burning and burning spots.

In this regard, the raw materials should be treated with a hopper dryer or other pre-drying methods, and resins with a smaller melt index should be used instead, and the amount of lubricant should be reduced.