Steps and measures to solve the problem of bubbles of trapped gases

1. Material dryness

Confirm whether the material is water-absorbent or not. ， It is recommended to use the dehumidification drying system as much as possible, which not only controls the water content of the plastic stably in order to minimize the viscosity variation during the injection molding process, but also reduces the risk of cracking and volatile gases caused by the setting of the drying temperature and drying time.
 

2. Screw plasticizing stage

Move back the injection seat of the injection machine. The melt is injected into the air and observed. If bubbles are observed in the melt currently, the source of the bubbles is the plasticizing operation prior to the cavity filling process.
Plasticizing speed and cycle time should be reduced as much as possible to avoid the risk of generating gases due to the shear overheating or the plastic staying in the tube for too long. In addition, try to use lower tail zone temperatures to maintain gas venting paths between pellets in the feed section, or set higher back pressures to allow air to escape through the inlet.
 

3. Melt filling stage

1. (1) Turn off the holding pressure and check if the bubbles are still present during the melt filling phase of the mold cavity. If so, the next check is to understand the melt filling flow pattern to determine if the gas is trapped by the melt during the filling of the part. The short injection test is performed by turning off the holding pressure and gradually reducing the injection volume from 99% to 10% of the volumetric filling volume in 10% decrements. (Note: The velocity control of the filling process is required for this test. )
2. (2) Examine the filling pattern of each part to see if the melt flows freely before the flow wave, or if there is stagnation before the flow wave as the part is filled. Do bubbles always appear in the same area? If so, this means that there is a fixed place where the bubbles are generated. Watch for track effects or melt injection, which causes air to be trapped in the melt.
3. (3) Examine the ribs or bump features. If short injection occurs in these locations, this means that there is air trapped in these areas and when the ribs are filled, the air is pushed out to form bubbles. Sometimes you will see a bunch of bubbles coming from near this feature. Try slowing down the fill rate before the melt flows through the feature or change the geometry of the rib or bump to allow the air to escape as the melt fills the feature.

Note: Do the bubbles appear only after the part is 85% filled? If so, this may be a mold venting problem, check the design of the venting ports for proper venting.

Phenomenon and main cause of bubble formation as vacuum bubbles
After the tests in the three steps mentioned above, if bubbles are not formed during the filling stage, the chances of them being vacuum bubbles are greatly increased. Vacuum bubbles tend to form in the thick flesh of the part. The center of the thick section of the part cools down slowly, and as the surface layer cools down and solidifies rapidly, the melt is pulled to the surface layer by the shrinkage phenomenon during the cooling and contraction of the plastic, resulting in a cavity in the thick part due to the uneven volume contraction. If a higher mold temperature is set, the thickness of the cured surface layer is not sufficient, and the bubbles disappear, resulting in surface dents or shadow marks and forming vacuum bubbles.

Insufficient effective melt filling volume during the pressure holding phase of the plastic is the main cause of pits or voids, so the first step is to make sure that the amount of screw residue is present, and the second step is to make sure that the runners or pouring gates don't freeze prematurely. The settings of higher holding pressures and longer holding time are used to get more fill into the mold cavity, but very often the pouring gate freezes before the part gets enough shrinkage to fill the part due to the wrong size of the pouring gate. Changing the pouring gate location to fill a thicker area of the mold may allow more plastic into the part before the pouring gate freezes. In addition to increasing the melt filling volume, ways to dampen the amount of melt shrinkage on cooling, such as using non-shrinking plastic fillers, using nucleating agents for lower flow plastics and semi-crystalline plastics, or lowering the melt temperature, are also very effective countermeasures in practice.

Note: Do not set the mold temperature too low, although this strategy is a "temporary" solution to the problem of cooling shrinkage, it will increase the risk of more dimensional instability and mechanical deterioration.

The most effective solution to eliminate vacuum bubbles or dents
The most effective way to eliminate vacuum bubbles or dents is to make the wall of the part thinner and to minimize the difference in the thickness of the part. The reasonable thickness of plastic parts is 2~4 mm. Using reinforcing ribs, box or wave contour design instead of increasing the flesh thickness to reinforce the structural strength of the parts, and the design of reducing the thickness of the thick parts where there is a difference in the thicknesses of the parts, these countermeasures will save more plastic volume and cycle time, and enhance the economic efficiency of injection molding.

Finally, in order to eliminate the source of Venturi effect, we can apply dye near the hot runner nozzle and on the manifold surface for the match between the nozzle tip and the runner bushing, and be careful not to allow any dye to enter the runner. If the dye shows up at the start of the molding process, we can find the source of the problem.

There are many reasons for bubble defects in plastic parts, and adjusting the injection molding conditions should not be the only recourse when bubbles are found. Based on many years of experience, adjusting the injection molding conditions is often accompanied by a risk of failure from other potential defects. A systematic investigation of the causes of bubble defects should be made, and the potential failure risk of each countermeasure should be recognized in order to choose the correct elimination approach.

FCS launches diagnostic service by injection molding experts
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FCS has launched the "Expert Diagnostic Service" for customers in the field of plastic injection molding, guiding them to identify the real causes of production problems through scientific methods and data, to break through the bottlenecks in production, and to further build up their own elite production teams.


Consultation window for "Expert Diagnostic Service":
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Email:  vincentlin@fcs.com.tw

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Causes and Countermeasures of Injection Molding Defects - Bubbles(1)