Warpage can be defined as a dimensional distortion in a molded product after it
is ejected from the mold at the end of the injection molding process. Warpage is
sometimes called ``potato-chipping'' because the part tends to appear wavy.
Inadequate Injection Pressure or Time
Explanation: If too little injection pressure is used the plastic material will tend to cool and solidify before the mold is packed out. If no packing is achieved the individual molecules are not held tightly together and have space to move while the part is cooling. Also, if the injection hold time is not long enough, the packing process is minimized and the molecules can relax before full solidification occurs. In either case, as the part cools it is uncontrolled and the plastic is allowed to move because it is not being constrained. Each area of the part cools at a different rate and warpage will occur due to the differences.
Solution: Increase the injection pressure or time applied. This will ensure the total part is cooling while constrained and the tendency for warpage will be minimized.
Inadequate Residence Time
Explanation: Residence time is the amount of time a material must spend being exposed to heat in the barrel. The time is determined by the ability of the specific resin to absorb heat enough to be properly processed. Inadequate residence time will result in under-heated material, which causes the material to be stiff. It will cool off before the mold is packed and individual molecules will be unconstrained while they solidify. Molecules that are not constrained during cooling will shrink at differing rates throughout the part and warpage will occur.
Solution: Increase the residence time by adding time to the cooling portion of the cycle. While increased cycle time may add cost to the final product, each material requires a specific minimum amount of time to absorb heat in the barrel, and if the time is not long enough warped parts will occur.
Barrel Temperature Too Low
Explanation: When barrel temperatures are too low, the material will not have a chance to heat to the proper flow temperature. The cold material gets pushed into the mold but solidifies before the molecules are packed and constrained. This results in warpage as the molecules shrink at varying rates.
Solution: Increase the barrel temperature. This will allow the material to come to proper heat and it will fill the mold before solidification takes place. The molecules will be packed and constrained as they cool, thus shrinking at uniform rates, minimizing the chance of warpage.
Nozzle Temperature Too Low
Explanation: The nozzle, being the final transfer point between the heated barrel and the mold, is a critical area and must be scrutinized whenever splay patterns occur. If the nozzle is too cold, the plastic material may slow down as it travels through the area and the molecules will not get packed under constraint. They will shrink at varying rates and cause warpage.
Solution: Increase the nozzle temperature 10 degrees F at a time until the warpage disappears. If splay still appears, reduce the temperature of the nozzle, and make sure the nozzle being used is of the proper design for the material being molded. There are many different nozzle designs and some may interfere with proper flow if they are not designed for the material in use.
Excessive Stress Buildup
Explanation: The injection molding process tends to build up physical stress in a molded part due to the stretching and squeezing action that takes place on the individual plastic molecules as they are heated, expanded, cooled and contracted. They must be allowed to relax and recover in a constrained position before they solidify or the stress will be locked in. It will then be released as the part cools after being ejected from the mold and warpage will occur.
Solution: Increase the barrel temperature and decrease injection pressure until the stress is minimized. It can never be eliminated but lower pressure will result in lower stress. And, higher barrel temperatures allow the use of lower injection pressures.
Explanation: Gates that are too small will cause excessive restriction to the flow of the molten plastic as it passes through. This restriction may cause additional physical stress to the plastic molecules as they are stretched and squeezed again going through the gate area. The stress gets released after ejection and the parts will warp.
Solution: Optimize gate size and shape. The material supplier can provide data on proper sizing and shape, or use a computer finite element analysis program to help make the determination.
Mold Temperature Too Low
Explanation: Generally, a hot mold will allow a material to stay molten longer than a cold mold and cause the molecules to stay fluid longer before they cool and solidify. If the mold is too cold the molecules will solidify before they are packed and will shrink at differing, uncontrolled rates. This is a prime cause of warpage.
Solution: Increase the mold temperature to the point at which the material has the proper flow and packs out the mold with maximum fill. Start with the material suppliers recommendations and adjust accordingly. Allow 10 cycles for every 10-degree change for the process to re-stabilize.
Uneven Mold Temperatures
Explanation: The plastic molecules must cool and shrink evenly to resist warpage. If the mold is not cooling the plastic in a uniform manner the molecules will have varying cooling and shrinking characteristics and this will cause warpage.
Solution: Check the surfaces of the mold that are in contact with the molten plastic. Use a fast-acting pyrometer to determine if there is more than a 10 degree F difference between any two points, even between the two mold halves. A difference greater than 10 degrees F will cause too great a difference in shrink rates and warpage will occur.
Explanation: It is possible that either the ejection system of the mold or the press will not be operating properly. If the part is warm enough and the ejection force is not even and exactly perpendicular to the part, stresses will be set up in the part as it tries to resist the ejection activity. These stresses will cause warpage of the part as it cools after being ejected.
Solution: Inspect and adjust the ejection system(s) as required. Make sure all adjusting devices are locked down to eliminate slipping, and that all components are properly lubricated. It may be necessary to use a guided ejection system that utilizes leader pins and bushings to keep the system in line and even.
Improper Flow Rate
Explanation: Resin manufacturers supply specific formulations in a range of standard flow rates. Thin-walled products may require an easy flow material while thick-walled products can use a material that is stiffer. It is better to use as stiff a flow as possible because that improves physical properties of the molded part. But the stiff material will be more difficult to push and this may result in the material solidifying before full packing takes place. The molecules will be left to shrink at different rates and warpage will occur.
Solution: Utilize a material that has the stiffest flow possible without causing warpage. Contact the material supplier for help in deciding which flow rate should be used for a specific application.
Inconsistent Process Cycle
Explanation: The machine operator may be opening the gate too soon, thereby effectively shortening the overall cycle time. This would cause the part to be ejected before the skin has formed properly and excessive, uncontrolled shrinkage may occur. The varying shrinkage rates will cause warpage.
Solution: If possible, run the machine on the automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is necessary. In addition, instruct all employees on the importance of maintaining consistent cycles.
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