1. The appearance and classification of the appearance of injection molded parts

The bulge of injection molded parts refers to the phenomenon that the local volume of injection molded parts increases and expands and bulges on the surface. Through the observation of the section of the bulge, it can be divided into two types: hollow bulge (Fig. 1) and solid bulge (Fig. 2).

In terms of the timing of the appearance of the bulge, some appear after molding and demoulding (Figure 3), and some appear after the plastic part is processed (after secondary heating).

(Figure 1 Hollow bulge)

(Figure 2 Solid bulge)

2. Bulge mechanism of injection molded parts

Through a large number of physical analysis and research, it is believed that the bulge of the plastic part is because the locality of the plastic part is not completely cooled and hardened after the mold is formed, and the gas is released under the action of internal pressure; or after the second heating during the post-processing of the plastic parts, the gas in the inner layer of the plastic parts expands, which causes the local expansion and bulging of the plastic parts. 

The gas inside the bulge mainly comes from the raw material containing moisture or low molecular weight volatile substances or excessive lubricant additives; it may also be the melt degradation gas (the material temperature is too high and the melt is degraded by heating for a long time), mold trapped gas, etc,.

3. Analysis of the causes of bulging of injection molded parts and discussion on improvement methods

1. Hollow bulge: The inner cavity of the bulge contains gas, but it is not polluted by external abnormal substances, which can be improved from 4 aspects:

Analysis and method of forming process aspect

① The filling speed is too fast, and the gas in the cavity is not discharged smoothly, causing trapped air;

② The back pressure of the melt is low and the rotation speed is too high, so that the air in the screw chamber is pushed to the front end of the barrel together with the melt;

③ Insufficient molding cooling, appropriately reduce the mold temperature and extend the mold opening time;

④ The feed-stock temperature is set too high, the barrel temperature is too high or the heating is out of balance, resulting in decomposition gas;

⑤ The nozzle has obstacles or burrs, and when the high-speed material flow passes through, it produces strong shear friction and thermal decomposition (disassembly and cleaning).

Analysis aspect of the mould

① Mold trapped gas. An exhaust slot is added at the end of the feed flow to ensure that the exhaust port is large enough so that the gas has enough time and space to be discharged;

② Poor gate location and unreasonable gate arrangement for multi-gate parts cause the melt to be trapped in the mold cavity and block the air discharge channel;

③ The design of the water way of the mold is unreasonable, and the local temperature is too high.

Analysis aspect of the plastic part structure

① The difference of wall thickness and thickness is large, and the deviation of cooling rate is large, try to ensure the uniform wall thickness;

② Avoid the sharp corner structure of the workpiece and avoid local air pockets (air trapped).

Raw material aspect

① The particles are not sufficiently dried, and the allowable moisture content of the molding process should be met;

② There are too many powdered scraps in the particles, and the powdered scraps are easily decomposed by heat;

③ The use or proportion of recycled materials is too high (not recommended);

④ Excessive amount of low molecular weight volatile lubricant agents in the raw materials;

⑤ Improper or wrong use of injection molding with the addition of color master batch.

2. Solid bulge: it contains cold blocks of injection molding material;

① The drooling at the nozzle produces cold material, forming a "Cold bulge";

② The equipment is abnormal, and the heating ring of the extended nozzle section is not heated to produce cold material;

③ The gate valve needle is not tightly sealed, and there is cold material overflowing; or the slag is scraped from the inclined top, and it is wrapped into the normal melt to form a "Cold bulge" during filling.

Solid bulge: it contains external foreign body contamination.

① The cleaning is not thorough. When switching among different materials, the nozzle of the material pipe of the machine needs to be disassembled and cleaned (the nozzle is the easiest to remain the material)

② Incompatible material particles are mixed into the raw material to cause a reaction; or different melting temperatures are different, the impurities cannot be melted.