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.


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).

Excessive Back Pressure, Screw RPM, or Injection Speed

Explanation: Increasing back pressure, screw speed, and/or injection fill rate beyond recommendations for specific materials will result in thermal degradation of the plastic through increased shear heat. The overheated resin forms a weak molecular bond resulting in brittleness.

Solution: Follow the material supplier's recommendations concerning these parameters and do not go beyond or above their suggested values.

Excessive Nozzle Temperature

Explanation: A nozzle that is too hot will overheat the material passing through. This may cause a separation of molecules due to localized thermal degradation and will result in a lack of proper molecular bonding, which will cause brittleness.

Solution: 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

Explanation: Low injection pressure may result in the formation of weld line areas in the molded part. This is caused by a non-filling condition because the plastic solidifies before it can fully pack the cavity. The result is a weak area that tends to appear brittle and may even exhibit signs of cracking.

Solution: 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

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.

Condensation or Leaks

Explanation: Condensation being formed on the surface of a cold mold in humid conditions will cause moisture to form and be carried into the cavity by the molten plastic. This moisture forms a barrier to localized molecular bonding and this will result in brittleness. Also, if there is a water leak in the cavity due to cracks (no matter how small) moisture will be picked up by the incoming plastic and cause brittleness. 

Solution: 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

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.

Excessive Moisture

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.

Degraded Resin

Explanation: It is certainly possible to degrade a material by overheating it. This can be done in a variety of ways, including too high a barrel temperature, too long a residence time in the barrel, too much restriction in the runner system or gate, and too fast of an injection fill rate. Molecules of the plastic will break their bonds when overheated and the material may even char or burn locally. With the broken bonds, the material is weak and brittle.

Solution: 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

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.

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