On-Line Solutions To Injection Molding Problems
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.
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.
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.
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.
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.
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.
Some common causes and solutions are listed below.
NOTE: For more detailed information on the causes and solutions of this defect, you can find it in our BOOK, or ONLINE SEMINAR.
Copyright by IPLAS and Douglas M. Bryce
Worldwide Rights Reserved