Plastic Troubleshooter
On-Line Solutions To Injection Molding Problems


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.

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.


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.


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.


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.

Defect - 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.


Some common causes and solutions are listed below.


NOTE: For more detailed information on the causes and solutions of injection molding defects you can find it in our BOOK, or ONLINE SEMINAR.


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