BLISTERS

 

 Blisters can be defined as raised defects on the surface of a molded
part caused by trapped gases in the part that could not escape before
the surface began to ``skin'' during the molding process.

MACHINE

Cycle Time Too Short

Explanation: In the process of minimizing overall cycle times to reduce manufacturing costs, many molders reduce the cooling portion of the cycle. This results in the surface skin of the molded part not being fully solidified when the part is ejected from the mold. Because of this, some of the gases that are formed during molding are allowed to expand against this soft skin. Blisters are formed because the gases are not constrained.

Solution: Although it may impact the manufacturing cost, the way to minimize blisters caused by short cooling time is to increase the ``mold closed'' portion of the cycle. Reducing mold temperatures may also help, but this practice may cause undue stress because of the increased injection pressure requirements. And, an increase of back-pressure may help remove the gases before they enter the mold.

Screw Rotation Too Fast

Explanation: A screw rotation speed that is too high will tend to ``whip'' air into the molten plastic. This excessive air may not be drawn out of the material during the molding process and pockets of the air may be forced to the surface of the molded part, forming blisters. Excessive rotation speed also causes a shearing action that can cause plasticizing gases to develop. These gases can also cause blisters.

Solution: Slowing down the screw rotation will minimize the amount of air that is drawn into the material and will also minimize screw shear. Start at 100 RPM and adjust up or down as needed in 10-RPM increments.

Low BaCk Pressure

Explanation: The back pressure setting controls the density of the melt. A low setting results in a melt that is not dense enough to push out excessive gases. The gases may cause blisters.

Solution: Increasing the back pressure setting will make the melt denser and help remove gases and minimize trapped air volume.

Injection Speed Too High

Explanation: While it is usually best to inject at high speeds, too high a speed will cause a turbulence that traps air pockets. The air may not have a chance to escape through normal venting practices and may show up as surface blisters.

Solution: Reducing the injection speed will reduce the tendency for turbulence and trapped air pockets will not form.

MOLD

Low Mold Temperature

Explanation: As molten material enters the mold, the material starts to cool down immediately and a ``skin'' begins to form on the surface of the part being molded. If this skin forms too quickly, any air or gas that is in the material will not be allowed time enough to escape through proper venting methods. A mold that is too cold will cause this skin to form too early resulting in the air and gases being trapped and forming blisters.

Solution: Increasing the mold temperature will help to allow gases to escape by delaying the hardening of the surface skin.

Mold Temperature Too High

Explanation: Trapped air in a molten plastic will stay trapped inside the molded part if the skin forms properly before the part is ejected from the mold. A mold that is too hot does not allow that skin to form in time and the ejected part has a soft skin. Trapped air and gases can force their way through this skin and form blisters on the ejected parts surface.

Solution: Decrease the mold temperature to allow the skin to form in the right amount of time. This varies with various plastics. An increase in cooling time can also help form the required skin hardness but should only be used as a temporary fix as it will increase the cost of the molded part.

Improper Gate Location

Explanation: As material travels through the gate and enters the cavity it seeks the path of least resistance. An improper gate location can cause the material to take an improper path and not properly push the trapped air out in front of it.

Solution: Consideration of material flow paths and vent locations at the mold design stage will minimize trapped air blister problems on new molds. Existing molds may require relocating the gate. Gates should normally be located in the thickest section of the part.

Insufficient Venting

Explanation: The correct size, location, and shape of vents should be considered and analyzed in the mold design stages. Inadequate venting will not allow trapped and gases to escape from a mold and this may result in blisters, burns, or other defects.

Solution: Venting is a very important part of the hole molding process. The parting line perimeter of the cavity should contain vents equal to 30% of that perimeter. Another rule-of-thumb is to place a vent at every inch along the parting line perimeter. And, the runner should be vented.

MATERIAL

Use Of Regrind That Is Too Coarse

Explanation: Using excessively coarse regrind increases the amount (volume) of air that gets trapped in the melt because the coarse, uneven particles of regrind create ``pockets'' of air between them and the smaller, consistently sized particles of virgin material. These pockets of air get pushed into the molded part and may result in surface blisters.

Solution: One remedy is to use a finer gauge screen in the granulator. This will produce smaller particle sizes. Another remedy is to limit the amount of regrind that is used to less than 5%. Also, an increase in back pressure may help blend out the trapped air. And, the final solution is to use only virgin material.

Use of Highly Volatile Resins

Explanation: Some molding materials (such as liquid crystal polymer) release a large amount of volatile gases during the plasticizing portion of the molding process. These gases need a chance to escape from the injection barrel before being injected into the mold.

Solution:  Back pressure control can be used to accommodate this condition, and some success has been achieved through utilization of vented barrels on the machine itself.

Excessive Moisture

Explanation: Improperly dried or stored molding material will contain excessive moisture because all plastics either have a tendency to absorb moisture from the atmosphere or hold moisture that has accumulated through condensation or spillage. When processed, this moisture turns to steam in the melt flow and will form pockets of trapped gas that may show up as blisters in the molded part.

Solution: Properly dry the material before using and store it correctly to minimize future absorption or accumulation of moisture. Most materials need to be dried to a dew point reading of between -20 and -40 degrees F. This equates to a level of less than 0.10% by weight. And remember that the material must be used within two hours of drying or moisture can accumulate again. Even regrind must be dried before using if it is allowed to stand for more than two hours.

OPERATOR

Early Gate Opening

Explanation: It is possible that the machine operator can cause blisters by opening the safety gate too soon. Depending on the age and type of molding machine, this can cause the mold to open before a hard skin has had time to form on the molded part. Trapped gases will be allowed to expand and form blisters on the surface of the part.

Solution: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an  emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles.

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