Defects And Solutions For Injection Molding

- Jul 13, 2018-

7. Burns

The solutions adopted are different depending on the burn caused by mechanical, mold or molding conditions.

(1) Mechanical reasons, for example, due to overheating of the barrel due to abnormal conditions, pyrolysis of the resin, injection into the product after burns, or stagnant resin due to nozzles and screw threads, check valves, etc. in the material. After disintegrating and discoloring, it is brought into the product, and there is a dark brown burn mark in the product. At this time, the nozzle, screw and barrel should be cleaned.

(2) The reason for the mold is mainly due to poor exhaust. This type of burn usually occurs in a fixed place and is easily distinguished from the first case. At this time, attention should be paid to measures such as adding a venting groove to the exhaust rod.

(3) In terms of molding conditions, when the back pressure is above 300 MPa, the barrel is partially overheated and burns are caused. When the screw speed is too high, overheating will also occur, which is generally in the range of 40 to 90 r/min. When there is no venting groove or the venting groove is too small, the injection speed is too high, which may cause burnt of the overheated gas.

8. Silver wire

The silver wire is mainly caused by the hygroscopicity of the material. Therefore, it should generally be dried under conditions of 10 to 15 C lower than the heat distortion temperature of the resin. For the more demanding PMMA tree wax series, it needs to be dried for 4~6h under the condition of about 75t). Especially when using the automatic drying hopper, it is necessary to select a reasonable capacity according to the molding cycle (forming amount) and the drying time, and it is also necessary to start the baking before a few hours before the start of the injection.

In addition, too long a material stagnation time in the material will produce silver wire. When mixing different kinds of materials, such as polystyrene and ABS resin, AS resin, polypropylene and polystyrene, etc., it is not suitable to mix.