Injection Mold And Product Quality

If debris is formed around the casting pin or feed channel during the mold filling phase, it may be an indication that the spray rate is too high, the closing force on the machine is too low, or the mold is too weak. In this case, we tried to solve the problem by selecting acetals with better fluidity, but it was not completely successful. Another option is to increase the wall thickness of the part or change the grid pattern of round holes. However, this is not necessary. Instead, the so-called flow controller is selected, in which the wall thickness is increased at the cell pattern.

Cast pin with incorrect size

A cast pin with the wrong size usually means that the cast pin is too thin relative to the wall thickness of the part, or the nozzle diameter of the machine is too small. Especially for partially crystallized plastics, it is important that the dimensions of these parts be correct. Pictures of products with high impact load and failed the mechanical strength test, because when the mold was installed on the machine, it forgot to replace the machine nozzle. There may also be situations where the nozzle is too large and causes other problems, such as leakage between the nozzle and the mold

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Feed channel deformation

The most common mistake of feed channels is that they are designed too thin relative to the part wall thickness. As a result, the crystalline plastic freezes in the casting tube before the block is properly sealed. Another common mistake is the imbalance of the casting channel, so the filling and sealing methods of the shell are different. The image shows an example of a poorly designed cast duct system.

Wrong cold swap bag

In the previous section, it was reported that the cold cavity has two different functions in the mold. It captures the raw material cooled in the nozzle and must be shaped to help pull out the cast pin. When using partially crystallized plastics, cold plug bags are very important because the risk of the cold plug is greatest when using these raw materials. If the cold plug bag cannot be used for aesthetic reasons, a special method can be used, which reduces the risk of material cooling in the nozzle. Here, the nozzle is removed from the mold after the quantitative feeding. In addition, the raw material must be wetted (the raw material is sucked back into the cylinder) to allow any cold plug to melt again. Another option is to use a hot runner system.

Feed point error

The most common problem with entry points is that they are too small. In this case, the partially crystallized raw material solidifies at the feed point before compacting the product, and sufficient shrinkage compensation has been made. This can lead to internal inclusions, suction and abnormal product size. If large volume objects are to be filled, the cutting of raw materials will result in the decomposition of raw materials at high spray rates. This can also happen if the edge of the feed point is too sharp.

The casting channel may also deform. The image shows the tapered right casting channel. This is a very typical shape of the casting channel and works well with amorphous plastics, but not for partially crystalline plastics. The reason is that raw materials solidify too early. When using partially crystallized plastic, the casting channel on the left shows the correct design. A good way to check whether the partially crystallized raw materials are fully filled in the mold is to observe the injection mold parts through many points to see whether there are inclusions in the parts.

Insufficient degassing

If there are no degassing channels in the mold, they will be blocked by dust (decomposed polymer), or they are too small, the air in the mold space will not escape when filling the mold. In this case, the air is compressed and heated to more than 1000 degrees, resulting in the thermal cracking of the plastic material. This phenomenon is called the diesel effect. In general, problems can lead to discoloration of raw materials, in which case, in most cases, the workpiece will not be fully filled. For black raw materials, it may be difficult to detect the fire trace caused by the destruction of raw materials. However, the flame trail is usually lighter than that of black plastic.

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Incorrect tempering of die

In the previous section, the tempering system of the die was discussed. It was found that the correct temperature of the die shell is very important to obtain high-quality green body, because the surface quality, mechanical strength, size and twist depend on the die. A common mistake of the spray type is that the tempering ability is insufficient even if the tempering channel is properly designed. This may be due to rusty tempered pipes. Sometimes it is negligent to temper a fixed and movable half die with only one tempering device, although two separate devices are usually recommended. The order in which the different tempering channels are connected to each other may also be important.

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