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Solve the problem of craze and bubble of injection molded plastic shell

Release time:2022-03-24

When the plastic is disturbed by gas during the filling process, crazes or tiny bubbles often appear on the surface of the plastic shell, or bubbles form in the thick wall of the plastic shell. The main source of these gases is that the raw materials contain water or volatile substances or excessive lubricants, or the material temperature is too high, and the plastic is degraded by heating for a long time to produce degradation gases.

1、 Equipment

The nozzle hole is too small, the material drips water or wiredrawing at the nozzle, there are obstacles or burrs in the barrel or nozzle, and friction thermal decomposition materials are generated when high-speed materials flow.

2、 Mold aspect

(1) Due to design defects, such as poor gate position, too small gate, asymmetric gate layout of multi gate plastic shell, too small runner, unreasonable mold cooling system, too large mold temperature difference, etc., molten material flows discontinuously in the mold cavity, blocking the air channel.

(2) The parting surface of the mold lacks necessary exhaust holes, or the exhaust holes are insufficient, blocked, poorly positioned, and there are no exhaust holes in the machining gaps of inserts, thimbles, etc., so that the air in the mold cavity cannot be discharged when the plastic enters.

(3) The surface roughness of the mold is poor, and the friction resistance is large, causing local hot spots and decomposing the plastic.


3、 Technical aspects

(1) If the material temperature is too high, it will cause decomposition. If the barrel temperature is too high or heating fails, gradually reduce the barrel temperature. If the temperature of the feeding section is too high, part of the plastic will melt prematurely and fill the spiral groove, and the air cannot be discharged from the feeding port.

(2) The injection pressure is small and the holding time is short, so that the melt will not cling to the surface of the cavity.

(3) If the injection speed is too fast, the molten plastic will decompose under the action of large shear force and produce decomposition gas; The injection speed is too slow to fill the cavity in time, resulting in insufficient surface density of the plastic shell and crazing.

(4) Insufficient material, too large feeding buffer, too low material temperature or too low mold temperature will affect the flow and forming pressure of molten material and produce bubbles.

(5) Multi stage injection reduces crazing: medium speed injection fills the runner → slow filling the gate → fast injection → low pressure slow filling the mold, so that the gas in the mold can be eliminated in time at each stage.

(6) When the screw is pre molded, the back pressure is too low and the rotation speed is too high, which makes the screw return too fast, and the air is easy to be pushed to the front end of the barrel with the material.

4、 Raw materials

(1) Raw materials mixed with heterogeneous plastics or granular materials mixed with a large amount of powder are easy to entrain air when melting, and sometimes crazes will appear. When raw materials are contaminated or contain harmful chips, raw materials are easy to be decomposed by heating.

(2) The structure of recycled material is loose, and the amount of air stored in micropores is large; The regeneration times of recycled materials are too many or the proportion with new materials is too high (generally less than 20%).

(3) Volatile solvents contained in raw materials or liquid additives in raw materials, such as white oil as dyeing additives, silicone oil as lubricants, dibutyl plasticizers, stabilizers, antistatic agents, etc., are used too much or mixed unevenly, and enter the mold cavity in a stacked state to form crazing.

(4) Plastics do not dry or absorb moisture from the atmosphere. Raw materials should be fully dried and dry hoppers should be used.

(5) Some brands of plastics cannot withstand high temperature or long-term heating. Especially when there is a small amount of water, catalytic cracking reaction may occur. For this kind of plastic, stearic acid and its salts (50g per 10kg of material) and other external lubricants should be considered to reduce the processing temperature as much as possible.

5、 Design of plastic shell

If the wall thickness is too thick, the external and internal cooling rates are different. During mold manufacturing, the size of main runner, shunt runner and gate should be appropriately increased.

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