On-Line Solutions To Injection Molding Problems
Improper Screw Design
Explanation: An injection screw with too low of a compression ratio will not properly mix and melt the material. This results in weak bonding of individual plastic resin molecules. The weak molecular bonds cause the molded part to be brittle.
Solution: 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
Explanation: The overall machine cycle time may be so short that the material does not have enough residence time in the injection barrel to melt to the proper consistency. This results in material that does not get thoroughly mixed and melted. A weak bond occurs between molecules and the part is brittle due to that weakness.
Solution: 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).
Gate And/Or Runner Restrictions
Explanation: Gates and runners that are too small will cause restrictions to the flow of molten plastic. These restrictions cause the material to heat up due to shearing friction and the material will thermally degrade. This results in weak molecular bonding and causes the molded parts to be brittle.
Solution: 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.
Resin Too Cold
Explanation: A cold resin will result in a poorly mixed and blended melt. There may even be ``clumps'' of unmelted resin that are not bonded to other ``clumps''. This results in a weak area lacking proper molecular bonding with localized brittleness or cracking.
Solution: Increasing the barrel temperature, and/or back pressure, will help soften and homogenize the plastic and result in stronger molecular bonds.
Explanation: Excessive moisture is one of the most frequent causes of brittleness. Moisture causes brittleness because the water droplets actually turn to steam when heated in the injection unit, and this steam explodes throughout the plastic, interfering with molecular bonding, causing voided areas between molecules. This causes those areas to be extremely weak and brittle. The voided areas easily break apart when exposed to any mechanical forces.
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 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.
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, heat sensitive materials will degrade due to erratic heating in the barrel, resulting in brittleness.
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.
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.
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