Brittleness can be defined as the tendency of a molded plastic part to break or crack under conditions in which it would not normally do so. At times the part may also shatter.
Possible Causes & Remedies:
◇ IMPROPER SCREW DESIGN
Remedies: Use an injection screw with the proper compression ratio. The material supplier is the best source for this information. While the general purpose screw that comes with the machine is adequate for many situations, specific screw designs are available for almost any specific material.
◇ SHORT CYCLE TIME
Remedies: Increasing the cycle time will allow longer residence time for the material, and it will heat and melt better. However, this will add to the cost of molding, so alternative actions should be taken first. For example, increasing the barrel temperature, increasing screw RPM and increasing back pressure will have the same effect. Be cautious, though. Too great an increase in these areas will cause degradation of the material (see the next paragraph).
◇ EXCESSIVE BACK PRESSURE, SCREW RPM, OR INJECTION SPEED
Remedies: Follow the material supplier's recommendations concerning these parameters and do not go beyond or above their suggested values.
◇ EXCESSIVE NOZZLE TEMPERATURE
Remedies: A reduction of nozzle temperature to that of the front zone is a good starting point. Gradually adjust the nozzle temperature, if necessary, to optimum conditions. Normally that optimum temperature would be 10 degrees F above the front zone, but it may vary with specific materials.
◇ LOW INJECTION PRESSURE
Remedies: Increasing the injection pressure helps to force the weld line areas together, minimizing the tendency to crack or appear brittle.
◇ GATE AND/OR RUNNER RESTRICTIONS
Remedies: Examine the gates and runners and, if possible, perform a computer simulation to determine the optimum size and shape of runner and gate for the specific parts(s) being molded. Remember that too large a gate and runner is just as detrimental as too small a gate and runner. Follow the material supplier's recommendations.
◇ CONDENSATION OR LEAKS
Remedies: Raising the mold temperature will eliminate "sweating” (condensation) on the mold surfaces. Check for crack conditions in the mold cavities. Sometimes, leaking “O” ring seals will be the source of water leakage.
◇ RESIN TOO COLD
Remedies: Increasing the barrel temperature, and/or back pressure, will help soften and homogenize the plastic and result in stronger molecular bonds.
◇ EXCESSIVE MOISTURE
Remedies: 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 supplier's 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.
◇ DEGRADED RESIN
Remedies: Reduce the temperature of the plastic. Shortening the overall cycle time, reducing residence time, reducing barrel temperatures, or moving the mold to another machine with a smaller barrel can accomplish this. Runners and gates also should be analyzed to improve any shearing situation.
◇ INCONSISTENT PROCESS CYCLE
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.
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