Four points of attention when burning injection products

Four points of attention when burning injection products

Factor 1: Melt fracture

When the melt is injected into a cavity with a large volume under high-speed and high-pressure conditions, the melt fracture phenomenon easily occurs. At this time, the surface of the melt breaks laterally, and the fracture area is rough and is mixed with the surface of the plastic part to form a stain. In particular, when a small amount of molten material is directly injected into an easily enlarged cavity, the melt fracture is more severe and the larger the spotted appearance is.

The nature of the melt fracture is due to the elastic behavior of the polymer melt. When the melt flows in the barrel, the melt near the barrel is subjected to friction by the barrel wall, the stress is greater, and the melt flow rate is higher. Small, once the molten material is injected from the nozzle, the stress acting on the tube wall disappears, and the flow velocity of the melt in the middle of the barrel is extremely high. The melt at the barrel wall is accelerated by the melt at the center, because the flow of the melt is Relatively continuous, the internal and external melt flow rate will rearrange and tend to average speed.

In this process, the molten material will undergo sudden stress changes and will produce strain. Because the injection speed is extremely fast, the stress received is extremely large, far greater than the strain capacity of the melt, resulting in melt fracture.

If the melt encounters abrupt shape changes in the flow path, such as diameter shrinkage, expansion and dead angle, etc., the melt stays in the dead corner and circulates. It is different from the normal force of the frit, and the shear deformation is larger. Mixed into the normal flow of material, when the two due to the deformation recovery is inconsistent, can not be bridged, if the disparity is very large, then the rupture fracture occurs, its form of expression is also the melt fracture.

From the above we can see that to overcome the trapped melt fracture, to avoid the generation of stains:

Pay attention to eliminate the dead angle in the flow channel and streamline the flow channel as much as possible;

Appropriately increase the material temperature, reduce the relaxation time of the melt, and make it easy to recover and bridge the deformation;

Adding low-molecular-weight materials to the raw materials, because the lower the molecular weight of the melt, the wider the distribution, which is more beneficial to the reduction of elastic effects;

Proper control of injection speed and screw speed

It is very important to set the gate position and choose the correct gate form. Practice has shown that with the enlarged point gate, the latent gate (tunnel gate) is ideal. The position of the gate is preferably selected in the molten material first into the transition chamber and then into a larger chamber, do not make the stream directly into the larger chamber.

Factor 2: Improper control of molding conditions

This is also an important reason that causes burnt and stain on the surface of plastic parts, especially the size of the injection speed has a great influence on it. When the flow material is injected into the cavity at a slow speed, the flow state of the molten material is laminar; when the injection speed When it rises to a certain value, the flow state gradually becomes turbulent.

Under normal circumstances, the surface of plastic parts formed by laminar flow is brighter and smoother. The plastic parts formed under turbulent flow conditions are not only susceptible to surface smearing, but also tend to produce air holes inside the plastic parts. Therefore, the injection speed should not be too high and the flow material should be controlled under laminar flow.

If the temperature of the frit is too high, the frit may decompose and coke, resulting in the appearance of plastic parts on the surface. General injection molding machine screw rotation should be less than 90r/min, back pressure is less than 2mpa, so as to avoid excessive friction heat barrel.

If excessive friction heat is generated during the molding process due to the excessive rotation time of the screw, the screw rotation speed may be appropriately increased, the molding cycle may be lengthened, the back pressure of the screw may be reduced, the temperature of the feed section of the cylinder may be increased, and raw materials having poor lubricity may be used. Ways to overcome it.

During the injection process, too much molten material flows back along the screw groove and the resin stays at the check ring, which leads to depolymerization of the melt. In this regard, the resin with higher viscosity should be used, and the injection pressure should be appropriately reduced to replace the injection molding machine with a large diameter. Injection molding machine commonly used anti-reverse ring are more likely to cause retention, disintegration and discoloration, when the decomposition of discolored melt injected into the cavity, the formation of a brown or black focus. In this regard, the nozzle-centric screw system should be regularly cleaned.

Factor 3: Die failure

If the mold vent is blocked by the release agent and solidified material precipitates, the mold exhaust is not set or the position is incorrect, and the mold filling speed is too fast, the mold will not be able to exhaust the air in an adiabatic compression and the high temperature gas will cause the resin to decompose and coke. . In this regard, the obstruction should be removed to reduce the mold clamping force and improve the mold exhaust.

The determination of the form and position of the mold gate 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 can not be too much, the cavity surface to maintain a high degree of finish.

Factor 4: Raw materials do not meet the requirements

If the content of water and volatile matter in the raw material is too high and the melt index is too large, excessive use of the lubricant can cause scorching and spot defects.

In this regard, hopper dryers or other pre-drying methods should be used to treat the raw materials, replacing the resin with a lower melt index and reducing the amount of lubricant.