Description: 

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.

Possible Causes & Remedies:

Machine Cause

◇ CYCLE TIME TOO SHORT

Remedies: 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

Remedies: 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

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

◇ INJECTION SPEED TOO HIGH

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

Mould Cause

◇ LOW MOLD TEMPERATURE

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

◇ MOLD TEMPERATURE TOO HIGH

Remedies: 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

Remedies: 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

Remedies: 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 Cause

◇ USE OF REGRIND THAT IS TOO COARSE

Remedies: 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

Remedies: 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

Remedies: 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 Cause

◇ EARLY GATE OPENING

Remedies: 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.