Plastics are disturbed by gas during the filling process,
and often silver stripes or tiny bubbles appear on the surface of the product
or bubbles are formed in the thick wall of the product. The sources of these
gases are mainly moisture or volatile substances or excessive lubricants in the
raw materials. It may also be that the material is too hot and the plastics are
degraded for a long time when heated to produce degradation gases.
1. Equipment
The nozzle
hole is too small, the material is drooling or drawing at the nozzle, there are
obstacles or burrs on the barrel or nozzle, and frictional heat is generated
when the high-speed material flows to decompose the material.
2. Mold
(1) Due to design defects, such as: poor gate position,
too small gate, asymmetric gate arrangement of multi-gate parts, small runners,
unreasonable mold cooling system, so that the mold temperature difference is
too large, etc. The melt flow is discontinuous in the mold cavity, which blocks
the air passage.
(2) The mold parting surface lacks the necessary vents or
the vents are insufficient, blocked, poorly positioned, and there are no
processing gaps such as inserts and thimbles to vent, causing the air in the
cavity to be unable to enter the plastic when the plastic enters. Leave at the
same time.
(3) The
surface roughness of the mold is poor, and the frictional resistance is large,
causing local hot spots and decomposing the passing plastic.
3. Processing
(1) The material temperature is too high, causing
decomposition. The temperature of the barrel is too high or the heating is out
of adjustment. The temperature of the barrel should be reduced step by step.
The temperature of the feeding section is too high, so that part of the plastic
melts and fills the screw groove prematurely, and the air cannot be discharged
from the feeding port .
(2) The injection pressure is small and the pressure holding time is short, so that the melt does not adhere to the surface of the cavity.
(3) The injection speed is too fast, which will cause the
molten plastic to be decomposed by large shearing action, resulting in
decomposition gas; the injection speed is too slow to fill the cavity in time,
resulting in insufficient surface density of the product to produce silver
streaks.
(4) Insufficient amount of material, too large feeding cushion, too low material temperature or too low mold temperature will affect the flow and molding pressure of the melt, and generate bubbles.
(5) Use multi-stage injection to reduce silver streaks: medium-speed injection fills the runner → slow-speed fills the gate → fast injection → low-pressure and slow-speed injection to fill the mold, so that the gas in the mold can be removed in time in each section.
(6) When the screw is pre-molded, the back pressure is too low and the speed is too high, which makes the screw return too fast, and the air is easily pushed to the front of the barrel along with the material.
Four, raw materials
(1) When raw materials are mixed with dissimilar plastics
or granules are mixed with a large amount of powder, it is easy to entrain air
when melting, and sometimes silver streaks appear. When the raw materials are
contaminated or contain harmful scraps, the raw materials are easily decomposed
by heat.
(2) The structure of the recycled material is loose, and the amount of air stored in the micropores is large; the number of regenerations of the recycled material is too much or the ratio with the new material is too high (generally less than 20%)
(3) The raw materials contain volatile solvents or the liquid additives in the raw materials such as white oil, lubricant silicone oil, plasticizer dibutyl ester, stabilizer, antistatic agent, etc., too much or unevenly mixed. The accumulated state enters the cavity, forming a silver streak.
(4) The plastic has not been dried or absorbed moisture from the atmosphere. The raw materials should be fully dried and a drying hopper should be used.
(5) Some brands of plastics cannot withstand higher temperatures or longer heating times. Especially when they contain trace amounts of moisture, catalytic cracking reactions may occur. For this type of plastics, consider adding external lubricants such as stearic acid. And its salts (50g per 10kg material can be added) in order to reduce the processing temperature as much as possible.
Five, product design
The wall thickness is too thick, and the cooling rate is different between the surface and the inside. The size of the main runner, runner and gate should be appropriately increased during mold manufacturing
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