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1、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 INJE
2、CTION 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
3、 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
4、 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 IN
5、JECTION 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
6、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
7、 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 temper
8、ature 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-thu
9、mb 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 tempera
10、ture 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 fas
11、t 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
12、bubbles because they were not able to reach the vented areas of the mold. Solution: Start with the suppliers 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 PRESSU
13、RE 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 finish
14、ed 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 suppl
15、ier is the best source of information regarding proper back pressure settings. MOLDIMPROPER VENTINGExplanation: 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 necessaril
16、y 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 bl
