Analysis of the causes and countermeasures of fusion joint marks during TPE/TPR molding

  • Time of issue:2021-11-02
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Analysis of the causes and countermeasures of fusion joint marks during TPE/TPR molding

1.What is fusion mark

  When the TPE melt fills the cavity, if two or more strands of melt have cooled at the front part when they meet, so that they cannot be completely fused, a linear groove is produced at the confluence, forming a fusion mark.

  2. Analysis of the causes of fusion marks and countermeasures

  2.1. Material temperature is too low.

  Low-temperature melt material has poor divergence and convergence performance, easy to form fusion marks. If the TPE material inside and outside the surface in the same part of the fusion fine lines, often due to the material temperature is too low caused by poor fusion. In this regard, the barrel and nozzle temperature can be properly increased, or extend the injection cycle to promote the material temperature rise. At the same time, the amount of coolant passed through the mold should be restrained to properly increase the temperature of the mold.       In general, the strength of the TPE material fusion marks is poor, if the corresponding part of the mold fusion marks for local heating, improve the local temperature of the fusion parts of the molding parts, often improve the strength of the TPE|TPR plastic fusion parts. If, due to special needs, low-temperature molding process must be used, the injection speed and injection pressure can be appropriately increased, so as to improve the melt convergence properties. Can also be appropriate in the raw material formula with a small amount of additional lubricants to improve the fluidity of the melt.

  2.2. Mold defects.

  Try to use the gate form with less diversion and choose the gate position reasonably. Choose the gate position to avoid inconsistent filling rate and interruption of the mold flow. If possible, a little bit of glue should be used. In order to prevent the low temperature melt from being injected into the mold cavity to produce fusion marks, the mold temperature can be increased at the same time, set up cooling cavities in the mold.

  2.3. Poor mold exhaust.

  First of all, you should check whether the mold exhaust hole is blocked by the melt curing or other objects, and whether there are foreign objects at the gate. If there is still charring point after the blockage is cleared, the exhaust hole should be increased at the sink point of the mold, or by repositioning the gate, or by appropriately reducing the closing force and increasing the exhaust gap to speed up the sink flow. In terms of process operation, we can also take auxiliary measures such as reducing the material temperature and mold temperature, shortening the high-pressure injection time and reducing the injection pressure.

  2.4. Improper use of mold release agent.

  In the injection molding, generally only in the threads and other parts that are not easy to release the mold evenly coated with a small amount of release agent, in principle, the amount of release agent should be reduced as much as possible.

  2.5 TPE material structure design is not reasonable.

  If the wall thickness of TPE plastic parts designed too thin or thin thickness disparity and too many inserts, will cause poor fusion. When designing the structure of the plastic part, it should be ensured that the thinnest part of the plastic part must be greater than the minimum wall thickness allowed during molding. In addition, the use of inserts should be minimized and the wall thickness as much as possible to the same.

  2.6. The fusion angle is too small.

  Different rubber and plastic materials have their own extreme fusion angle. Two streams of material converge if the convergence angle is less than the ultimate fusion angle, there will be fusion marks, if greater than the ultimate fusion angle, fusion marks will disappear. The ultimate fusion angle value is generally around 135 degrees.

  2.7. Other reasons.

  When the use of raw materials, moisture or volatile content is too high, the mold in the oil stains are not clean, the mold cavity with cold material or melt within the fiber filler distribution is poor, mold cooling system design is unreasonable, the melt curing too fast, the temperature of the insert is too low, the nozzle hole is too small, the injector plasticizing ability is not enough, the plunger or injector barrel pressure loss, can lead to different degrees of bad fusion.

  In this regard, in the operation process, should be taken for different situations, respectively, pre-drying of raw materials, regular cleaning of the mold, change the mold cooling water channel design, control the flow of cooling water, improve the temperature of the insert, change to a larger aperture nozzle, change to a larger size injection machine and other measures to be resolved.

  The melt marks, in addition to causing the appearance of the parts, but also may lead to insufficient strength of the parts, easy to fracture, etc..

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