Injection Molding Typical Problems And Solutions

Burns

Depending on the burns caused by different reasons such as mechanical, mold or molding conditions, different solutions are adopted.

(1) Mechanical reasons, for example, due to abnormal conditions caused by the barrel overheating, so that the resin decomposition at high temperatures, burns injected into the product, or due to the nozzle and screw threads within the material Jane, check valve and other parts of the resin stagnation, decomposition and discoloration of the products brought into the product, in the product with a black-brown burn marks. At this time, the nozzle, screw and barrel should be cleaned;

(2) The cause of the mold, mainly due to poor exhaust. This burn generally occurs in a fixed place, easy to distinguish from the first case. At this time, attention should be paid to take measures such as adding exhaust groove anti-exhaust rod;

(3) In terms of molding conditions, when the back pressure is above 300MPa, it will make the barrel partly overheat and cause burns. Screw speed is too high, will also produce overheating, generally in the range of 40 ~ 90r/min is good. When there is no exhaust tank or the exhaust tank is small, the injection speed is too high will cause overheating gas burns.

silver line

Silver line is mainly caused by the hygroscopicity of the material. Therefore, it should generally be dried under conditions 10 to 15 ℃ lower than the heat distortion temperature of the resin. For the higher requirements of the PMMA resin series, need to dry under the conditions of about 75 ℃ 4 ~ 6 h. Especially in the use of automatic drying hopper, need to be based on the molding cycle (molding volume) and the drying time to choose a reasonable capacity, but also in the injection before the start of a few hours before the start of the first boot drying material.

In addition, the material stagnation time in the material brief is too long will also produce silver line. When mixing different types of materials, for example, polystyrene and ABS resin, AS resin, polypropylene and polystyrene should not be mixed.

Air bubbles

According to the causes of air bubbles, the countermeasures to solve the problem are as follows:

(1) When the wall thickness of a product is large, its outer surface cools faster than that of the center, so as cooling proceeds, the resin in the center expands toward the surface while contracting, resulting in underfilling in the center. This situation is called vacuum bubble. The solutions are mainly:

(a) According to the wall thickness, determine a reasonable gate, sprue size. The general height of the gate should be 50% to 60% of the wall thickness of the product.

b) Until the gate is closed, leave a certain amount of supplementary injection material.

(c) The injection time should be slightly longer than the gate sealing time.

d) Reduce the injection speed and increase the injection pressure.

e) Use materials with high melt viscosity grade.

(2) Bubbles caused by the generation of volatile gases, the solution is mainly:

(a) Sufficient pre-drying.

(b) Reduce the resin temperature to avoid the production of decomposition gases.

(3) Bubbles caused by poor fluidity can be solved by increasing the temperature of the resin and mold, and increasing the injection speed.