Reasons and solutions for injection molding defects melt marks

Weld Lines, among the many defects of injection molded products, are the most common, except for a few injections molded parts with very simple geometry.

The melt marks not only affect the appearance of the part, but also the mechanical properties of the part, such as impact strength, tensile strength, elongation at break, etc., to varying degrees. In addition, melt marks also bring serious impact to the product design and life of the plastic parts, therefore, should be avoided or improved as much as possible.

The main reasons for melt marks are: molten plastic in the cavity encounter inserts, holes, flow rate incoherent areas or mold filling material flow interrupted areas, the convergence of multiple melts; occurring gate injection filling mold, the material can not be completely integrated.

 

Causes and solutions for melt marks.

1. Temperature is too low
The low-temperature melt has poor diverging and converging properties, which makes it easy to form fusion marks. If the inner and outer surface of the plastic parts 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 appropriately increased or extend the injection cycle to promote the rise in material temperature. At the same time, the amount of cooling water through the mold should be restrained to properly increase the temperature of the mold.

In general, the strength of the plastic fusion marks is poor, if the corresponding part of the mold to produces fusion marks for local heating, improve the local temperature of the fusion parts of the molding parts, often can improve the strength of the plastic fusion parts.

If due to special needs, must use low-temperature molding process, can be appropriate to improve the injection speed pole increase injection pressure, so as to improve the melt convergence properties. Can also be appropriate in the raw material formula with a small number of additional lubricants to improve the fluidity of the melt.

2. Mould defects
The structural parameters of the mold casting system have a great influence on the fusion condition of the flowing material because the poor fusion is mainly produced by the shunting and convergence of the melt. Therefore, we should try to use the gate form with less diversion and choose the location of the gate reasonably to avoid inconsistent filling rate and interruption of the mold flow. If possible, a one-point gate should be used because it does not produce multiple streams and the melt does not converge from two directions, which makes it easy to avoid fusion marks.

If the gate is too much or too small in the casting system, if the positioning of multiple gates is not correct or the spacing from the gate to the fusion of the flowing material is too large, if the inlet port of the mainstream and the cross-section of the manifold of the casting system are too small, the resistance of the material flow is too large, which will cause bad fusion and make the surface of the plastic parts have more obvious fusion marks. In this regard, the number of gates should be reduced as much as possible, the location of the gates should be set reasonably, the cross-section of the gates should be increased, the auxiliary runners should be set, and the diameter of the main flow channel and manifold should be expanded.

In order to prevent the low-temperature melt from injecting into the mold cavity, cold cavities should be set in the mold while increasing the mold temperature.

In addition, the fusion marks of the plastic parts are often due to high pressure filling the mold and producing flying edge, and after the production of such flying edge fusion marks will not produce shrinkage, so such flying edge is often not as a fault elimination, but in the mold on the part of the flying edge to open a very shallow small groove, the fusion marks on the plastic parts will be transferred to the additional flying edge winglet, after the plastic parts are formed and then remove the winglet, which is also a common method to eliminate the fusion marks fault.

3. Poor mold exhaust
When the fusion line of the melt and the mold line or inlay overlap, the air in the mold cavity with multiple streams of material to catch the pressure can be discharged from the mold gap or inlay.

However, when the fusion line and the mold line or inlay do not overlap, and the exhaust hole is not set when the mold cavity by the flow of material to drive the residual air can not be discharged, the air bubbles under high pressure is strongly squeezed, the body gradually become smaller, and finally compressed into a point, due to the molecular kinetic energy of the compressed air under high pressure into heat, resulting in the melt at the point of convergence temperature rise, when its temperature is equal to or slightly higher than the decomposition temperature of raw materials, the fusion point will appear yellow dots if its temperature is much higher than the decomposition temperature of raw materials, the fusion point will appear black dots.

In general, the surface of the plastic parts near the fusion marks of such spots always appear repeatedly in the same location, and the appearance of the site is always regular at the point of convergence, in the process of operation, should not be mistaken for such spots as impurity spots. The main reason for this kind of spots is due to the bad exhaust of the mold, which is the carbonization point formed after the decomposition of the melt at high temperatures.

After this kind of failure, first of all, you should check whether the mold exhaust hole is blocked by the solidification of molten material or other objects, and whether there are foreign objects at the gate. If the blockage is removed and the carbonization point still appears, the venting hole should be increased at the molten material rendezvous point. You can also speed up the merging of the sink material by repositioning the sprue or by appropriately reducing the combined force and increasing the exhaust gap. In the 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.

4. Improper use of mold release agent
The amount of release agent is too much or incorrectly selected varieties can cause fusion marks on the surface of plastic parts. In 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.

The selection of various release agents must be determined according to the molding conditions, the shape of the plastic parts and raw material varieties and other conditions. For example, pure zinc stearate can be used for all kinds of plastics except polyamide and transparent plastics, but mixed with oil can be used for polyamide and transparent plastics. Another example is toluene solution of silicone oil can be used for various plastics, and it can be used for a long time after painting once, but it needs to be heated and dried after painting, so the usage is complicated.

5. Plastic structure design is not reasonable
If the wall thickness of the plastic parts designed too thin can be too thick and thin disparity and too many inserts, can cause poor fusion. Thin-walled parts molding, due to the melt curing too fast, easy to produce defects, and melt in the process of filling the mold always converge at the thin wall to form fusion marks, once the thin wall fusion marks, it will lead to a reduction in the strength of the plastic parts, affecting the use of performance.

Therefore, when designing the structure of the plastic part, it should be ensured that the thinnest part of the plastic part must be larger than the minimum wall thickness allowed during molding. In addition, the use of inserts should be minimized and the wall thickness should be as uniform as possible.

6. Other reasons
When the moisture or volatile content of the raw materials used is too high, the oil in the mold is not cleaned, there is cold material in the mold cavity or poor distribution of fiber filler in the melt, the mold cooling system is not designed properly, the melt cures too quickly, the temperature of the insert is too low, the nozzle hole is too small, the plasticizing capacity of the injection molding machine is not enough, the pressure loss in the barrel of the injection molding machine is too large, all of which can lead to different degrees of poor 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 settings, control the flow of cooling water, improve the temperature of the insert, change to a larger aperture nozzle, change to a larger size injection molding machine and other measures to solve