BUBBLES (Voids)

 

 Bubbles can be defined as a voided area trapped within a molded plastic part.
It differs from a blister in that there is no surface protrusion with a bubble.
Bubbles are usually caused by trapped gases or air pockets,
but can also be caused by differential shrinking.
 

Machine

Low Injection Pressure or Hold Time

Explanation: If injection pressure or hold time are too low, the molten material is not forced into the mold cavity and trapped gases and air will form voids because the gases will not be forced out of the mold through vent paths.

Solution: Increase the injection pressure and/or the hold time to help force the gases out as the plastic is pushed into the cavity.

Insufficient Material

Explanation: Too little material feed will have the same effect as low injection pressure. The material will not be forced into the cavity and gases will be trapped, forming voids due to a lack of molecular packing.

Solution: It is important to establish a feed setting that allows a 1/8'' to 1/4'' cushion of material at the end of the injection stroke. Without this cushion, there is no material against which holding pressure can be applied to force material into the cavity.

Improper Injection Temperature Profile

Explanation: The injection temperature profile addresses four heating zones of the injection barrel. These are commonly known as rear, center, front, and nozzle. The rear is also know as the feed zone, the center is known as the transition zone, and the front is known as the metering zone. The purpose of the feed zone (rear) is to start the material through the heating process. The heat is kept lower at this point but high enough to begin softening the plastic. The transition zone (center) heats the plastic higher and begins to compress it, squeezing out the trapped gases. In the metering zone (front) the material is brought up to the final, ideal temperature and is further compressed and sheared, which also introduces more heat. In the nozzle zone the material is simply kept at the upper temperature as it is injected into the mold. Any imbalance in the temperature values of these zones may result in plastic particles that are not properly melted at the right time. This will not allow gases to escape and voided areas (bubbles) will appear.

Solution: Maintain a proper temperature profile. This is readily obtained from the resin supplier, but a rule-of-thumb sets the temperature controls at increments of 50 to 100 degrees F from rear to front, and the nozzle at the same temperature as the front zone. An air shot from the nozzle should produce a bubble-free stream of plastic that has the approximate consistency of warm honey. Remember that the temperature control settings are not the same as the actual temperature of the plastic. They are usually 50 to 100 degrees higher than the actual plastic temperature to accommodate the rapid travel of material through the barrel.

Excessive Injection Speed

Explanation: The injection speed determines how fast the material is injected into the mold. If it is too slow, the material tends to cool off and solidify before the mold is fills, which results in a short shot. If it is too fast, the material tends to tumble and become turbulent, which traps air and gases in the resin. These gases then show up as bubbles because they were not able to reach the vented areas of the mold. 

Solution: Start with the supplier's recommendations for injection fill speed. Adjust up or down according to the results. If bubbles appear, slow down the rate. If short shots appear, speed up the rate.

Insufficient Back Pressure

Explanation: Back pressure is used to help mix the material and homogenize it. It also helps remove trapped air and densifies the melt. If back pressure is insufficient the gases and trapped air are not allowed to escape and remain in the plastic melt as bubbles that can be molded into the finished part.

Solution: Increase the back pressure. Most materials will benefit from a back pressure that is approximately 50 psi. But, some materials require higher settings: in some cases up to 300 psi. However, be cautious, because too high a back pressure will degrade any material. The material supplier is the best source of information regarding proper back pressure settings.

MOLD

Improper Venting

Explanation: Most molds do not have adequate venting. Usually the moldmaker elects to ``wait and see'' where the venting needs to be located and then assigns an arbitrary size. While size is not necessarily as important as location, there is a tendency to use a minimum number of oversized vents rather than an adequate number of properly sized vents. If improper venting is used (or no venting), any trapped air or generated gases cannot escape. This will result in voids, bubbles, shorts, and burns.

Solution: Vent the mold even before the first shot is taken by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up approximately 30% of the perimeter of the molded part. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part. Another rule-of-thumb is to place a vent at every 1-inch dimension around the perimeter of the cavity. You cannot have too many vents.

Section Thickness Too Great

Explanation: Most plastic parts are not of one continuous wall thickness. There is usually a need to change the wall thickness for such reasons as additional strength. Unfortunately, when that happens, there is a pressure loss in the thicker section as the molten material shrinks more there as it solidifies. The material pulls away from the cavity wall leaving a voided area. If the void is captured below the part surface, the void will appear as a bubble.

Solution: A good rule-of-thumb is that any wall thickness should not exceed any other wall thickness by more than 25%. There will be little tendency for bubbles at that ratio. Metal inserts can be used to core out sections that do not meet that ratio, or ``overflow'' wells might be used to move the voided area off the primary part surface. However, the overflow would then need to be removed from the molded part.

Improper Runners or Gates

Explanation: Runners or gates that are too small will restrict the molten material in the flow pattern and may cause non-packed parts. If gates are placed to flow material from a thin section into a thicker section, the restricted flow in the thin section will keep the thicker section from packing. Both of these conditions can result in a loss of filling pressure and cause sinks to evolve in the molded part. These sinks can take the shape of bubbles and voids if they are trapped within the part rather than on the surface.

Solution: Gates should be of a depth that is equal to at least 50% of the wall they are placed at and should always be located to flow material from the thickest section to the thinnest. Runner diameters should be adequate to avoid a pressure drop as the material fills. Thus, the farther the travel, the larger the initial runner diameter should be. Gates and runners should be machined in the mold to be ``steel safe'' so they can be increased by removing metal. It is a good practice to place gates and runners in individual inserts so they can be easily replaced and/or reworked.

Low Mold Temperature

Explanation: A mold that is too cold for a specific resin or product design will not allow the molten material to fill and pack all of the mold properly before the resin starts to solidify. Any air or gases present in the plastic at the time will be trapped under the surface as bubbles.

Solution: Raise the mold temperature in increments of 10 degrees F until the bubbles disappear. Allow 10 cycles for each 10-degree adjustment (up or down) for the mold temperature to stabilize.

MATERIAL

Excessive Moisture

Explanation: Excessive moisture is one of the most frequent causes of bubbles. Moisture causes bubbles because the water droplets actually turn to pockets of steam when heated in the injection unit, causing voided areas between molecules. If the voided areas are trapped beneath the surface of the part they appear as bubbles.

Solution: Although it is commonly understood that non-hygroscopic materials do not require drying, do not take chances. Dry all materials. It may be that fillers used in the material are hygroscopic and they will absorb moisture. Every plastic material requires specific drying conditions. And each material should be dried according to the material suppliers recommendations. The desired moisture content is between 1/10th of 1 percent and 1/20th of 1 percent by weight. This means the dry air being used to take moisture from the material should have a dew point of -20 to -40 degrees F.

OPERATOR

Inconsistent Process Cycle

Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in excessive residence time of the material in the injection barrel. If such a condition exists, materials may flow more easily and be injected too quickly, resulting in trapped air and gases being held in the resin and not being vented as required. The gases will form bubbles if held under the molded part surface.

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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