Factor 1: Melt rupture

When the melt is injected into a cavity with a large volume under high-speed and high-pressure conditions, it is easy to cause melt fracture. At this time, the melt surface is laterally fractured, and the fracture area is roughly mixed on the surface of the plastic part to form a paste. Especially when a small amount of melt is directly injected into a cavity that is easy to be too large, the melt fracture is more serious, and the larger the smear appears.

Melt fracture due to the nature of the elastic behavior of the polymer melt produced when the melt in the cartridge flows, the melt near the cylinder bore wall by friction, stress is large, the melt flow rate Smaller, once the molten material is injected from the nozzle, the stress acting on the tube wall disappears, and the flow rate of the molten material in the middle of the barrel is extremely high. The molten material at the wall of the barrel is carried by the molten material at the center and is accelerated, due to the flow of the molten material It is relatively continuous, the flow speed of the internal and external melts will be rearranged, tending to the average speed.

In this process, the melt will undergo a sudden stress change and will produce strain. Due to the extremely fast injection speed, the stress received is particularly large, which is much greater than the strain capacity of the melt, resulting in melt fracture.

If the melt encounters sudden shape changes in the flow channel, such as diameter shrinkage, expansion, and dead corners, the melt stays and circulates at the dead corner. It is different from the normal melt. When mixed into the normal flow and injected, the deformation recovery of the two is inconsistent and cannot be bridged. If the disparity is very large, fracture and fracture will occur, and its manifestation is also melt fracture.

It can be seen from the above that it is necessary to overcome the rupture of the melted melt and avoid the occurrence of smears.

One is to pay attention to eliminate dead angles in the flow channel, making the flow channel as streamlined as possible;

The second is to appropriately increase the material temperature, reduce the slack time of the melt, and make its deformation easy to recover and bridge;

The third is to add low molecular weight materials to the raw materials, because the lower the molecular weight of the melt, the wider the distribution, the more beneficial to reduce the elastic effect;

The fourth is to properly control the injection speed and screw speed;

Fifth, it is very important to set the gate position and select the correct gate form. Practice has shown that with an enlarged point gate, the latent gate (tunnel gate) is ideal. It is best to choose the position of the gate after the molten material is injected into the transition cavity before entering the larger cavity, and do not let the flow material directly enter the larger cavity.

Factor 2: Improper control of molding conditions

This is also an important cause of scorching and smearing on the surface of plastic parts, especially the size of the injection speed has a great influence on it. When the flow material is slowly injected into the cavity, the flow state of the melt is laminar; when the injection speed When it reaches a certain value, the flow state gradually becomes turbulent.

In general, the surface of plastic parts formed by laminar flow is brighter and smoother. The plastic parts formed under turbulent flow are not only prone to smears on the surface, but also tend to generate air holes inside the plastic parts. Therefore, the injection speed cannot be too high, and the flow material should be controlled to fill the mold under laminar flow.

If the temperature of the melt is too high, it is easy to cause decomposition and coking of the melt, resulting in paste spots on the surface of the plastic part. The screw rotation speed of the general injection molding machine should be less than 90r / min, and the back pressure is less than 2mpa, so as to avoid excessive frictional heat generated by the barrel.

If the rotation time when the screw returns is too long during the molding process and excessive friction heat is generated, the screw rotation speed can be appropriately increased to extend the molding cycle, reduce the back pressure of the screw, increase the temperature of the feeding section of the barrel, and use poorly lubricating materials Ways to overcome.

During the injection process, too much melt backflow along the screw groove and resin retention at the check ring will cause the melt to depolymerize and decompose. In this regard, the resin with higher viscosity should be selected, the injection pressure should be appropriately reduced, and the injection machine with a larger diameter should be used. The anti-return rings commonly used in injection molding machines are more likely to cause stagnation and make them decompose and change color. When the melted material that decomposes and changes color is injected into the cavity, a brown or black focus is formed. In this regard, the screw system centered on the nozzle should be cleaned regularly.

Factor three: mold failure

If the mold vent hole is blocked by the release agent and the solidified material precipitated from the raw material, the mold exhaust setting is not sufficient or the position is incorrect, and the filling speed is too fast, the air inside the mold is too late to be adiabatically compressed to generate high-temperature gas will cause the resin to decompose and coking . In this regard, the obstruction should be removed, the mold clamping force should be reduced, and the exhaust of the mold should be improved.

The determination of the mold gate form and position is also very important. In the design, the flow state of the melt and the exhaust performance of the mold should be fully considered.

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

Factor four: raw materials do not meet the requirements

If the content of moisture and volatile matter in the raw material is too high, the melt index is too large, and excessive use of lubricant will cause scorch and smear failure.

In this regard, the hopper dryer or other pre-drying methods should be used to process the raw materials, replace with a resin with a smaller melt index, and reduce the amount of lubricant.