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


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.

Cooling Time Too Short

Explanation: Plastic material must be held (constrained) in the mold for a long enough period of time to allow a solid skin to form on the part. If cooling time is too short, the material will continue to move as it cools outside the press and, with nothing to constrain it, will distort until it stabilizes.

Solution: Increase the cooling portion time of the cycle to ensure proper skin solidification of the plastic material.

Inadequate Packing Of Molecules

Explanation: The amount of material being injected into the cavity determines whether or not adequate pressures can build up against that material and pack the molecules together to hold them in place during the cooling portion of the cycle. Also, the amount of time used for the injection ``hold'' phase of the cycle determines how much packing is achieved. Too little hold time will result in the material partially being sucked back out of the cavity, which relaxes the total pressure being held against the material remaining in the cavity. Too much hold time may result in the material solidifying without being able to properly shrink slightly away from the cavity walls. This is called ``over-packing'' and the parts will be distorted.

Solution: Adjust the material feed so that a ``cushion'' of material is present at the end of the injection stroke. The standard requirement is 1/8'' but it can go to 1/4'' for most materials. Adjust the injection hold time to stay engaged until the gate freezes. Then, the screw can be retracted without affecting material that is in the cavity.


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.

Inconsistent Mold Temperature

Explanation: A molded part must be cooled in a consistent manner. Uneven cooling will result in uneven shrinkage and that can cause bowing. There should be no hot spots or cold areas in the mold around the molded plastic. In fact, there should be no more than a 10 degree F difference between any two points on the cavity surfaces when measured with a surface pyrometer. A difference of more than 10 degrees will result in uneven cooling. Bowing can also occur if one half of the mold is hotter than the other. The part will want to stay to the hotter half and this can cause bowing as the mold opens.

Solution: First, make sure that each mold half has its own temperature control system. Then, check for hot or cold spots in the mold using a surface pyrometer. The only areas of concern are where the plastic will be molded (including the runner). The rest of the mold does not matter. If you find more than a 10 degree F difference, adjustments must be made to the cooling system or pattern by adding cooling lines, bubblers, or other methods.

Improper Gate Location

Explanation: A gate should be designed and located such that the material flow will be consistent in all directions. The material should flow equally through the cavity both in speed and volume. And, in multiple cavity molds each cavity should finish filling at the exact same instant. This results in even fill, even packing, and even shrinkage and is known as a balanced runner system. If the system is unbalanced, or improperly located, uneven pressures will cause uneven shrinkage and bowing will occur.

Solution: Always locate the gate in the thickest section of the part and allow material to flow from thick to thin. Rectangular parts should have the material flow across the width. Circular parts should be centrally gated. This will result in fast filling and even shrinkage.


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.

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