Plastic Troubleshooter
On-Line Solutions To Injection Molding Problems

Clamp Opens Too Quickly

Explanation: To produce more parts in a given amount of time, molders sometimes cause the clamp to open before the part has cooled enough in the mold. The part may not have shrunken enough on to the ``B'' half of the mold and wants to still stay on the ``A'' half. This results in a temporary tug-of-war until the part snaps back onto the ``B'' half as the mold continues to open. The snapping action can permanently distort the part.

Solution: Make sure the mold is closed for the proper amount of time, and allow it to open slowly for the first 1/2'' or so. That should be enough to allow the part to shrink to the ``B'' half. Then, the mold can continue opening more quickly.

Ejector System Not Level

Explanation: Due to wear, lack of lubrication, age, improper settings, or a variety of other reasons, the machines ejector system may go out of alignment. The system may come forward at a jaunted angle and force the molds ejector to come forward at the same angle. The molded part may be ejected in a cocked manner and may take a permanent set in that distorted condition.

Solution: Analyze the entire ejection system of the machine. Readjust and align anything that is out of adjustment and make sure locks are used to prevent future movement. Lubricate the system regularly and make sure you are using guided ejector systems in the molds if possible.


Temperature Too Low

Explanation: Some materials (such as polyesters) require mold temperatures that are above the boiling point of water (212 degrees F). All materials require mold temperatures high enough to sustain proper flow and packing in the mold. Mold temperatures that are too low result in inadequate filling and uneven packing. Uneven packing results in uneven shrinkage and this will cause bowing and distortion.

Solution: Raise the mold temperature to that recommended by the material supplier for the specific material being molded. This may require the use of an oil heater, or cartridge heaters placed in the mold, if the requirement is higher than 200 degrees F.


Melt Index Too Low

Explanation: Every material is available in a range of Melt Index values. This number (average is 14) indicates the flow-ability of a material. The higher the number, the easier the material flows. If the material is purchased at the low end of the range it will be stiffer than at the high end of the range. Stiffer materials are more difficult to push and will require higher injection pressures to fill the mold. Unfortunately, these high pressures tend to over-pack the material and less shrinkage occurs. When the mold opens the parts may distort.

Solution: Use a Melt Index (MI) value that is mid-range to start with. You can always request a higher or lower value later. But, be practical. The value must have a tolerance applied. For example, if you determine that a 14 MI is the right value, understand the tolerance factor will allow an MI of at least 13 to 15.


Parts Mishandled After Ejection

Explanation: Even if parts are molded properly and have no uneven shrinkage, they are still hot enough to be distorted by the operator if they are mishandled after they are ejected. Shoving the parts together into a box, or laying them so they are not flat, may cause the parts to distort and hold the distorted shape as they cool further.

Solution: Instruct the operator on the proper method of handling the parts. Parts should be allowed to air cool for a minimum of six cycles before being packaged, and then they should be packaged loosely.



Bowing can be defined as a condition of being unintentionally bent into theshape of a bow. This shape can be either convex or concave and is usuallycaused by a differential in shrinkage rates from one face of a part to an opposite face, but can also be caused by mechanical distortion.


Some common causes and solutions are listed below.


Defect - Bowing

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