On-Line Solutions To Injection Molding Problems
Barrel Temperature Too High
Explanation: If the barrel temperature is too high, the resin absorbs an excessive amount of heat and this increases the size of the voided area between the plastic molecules. Upon cooling, the skin of the material solidifies first and the remaining resin closes up the excessively large molecules and voids as it cools, pulling the solidified skin with it. The larger the molecules and voids, the greater the amount of shrinkage.
Solution: Decreasing the barrel temperature allows the molecules and voids to expand and contract normally and provide consistent shrinkage values. Shrinkage is impossible to predict accurately, but keeping the material and mold temperatures at the right settings (as recommended by the material suppliers) will minimize the effective shrinkage.
Insufficient Injection Pressure Or Time
Explanation: Injection pressure must be high enough to push molten material into the mold, through the runners and gates, and into the cavity image area. It should be used to force material into every part of the mold until it is packed solidly. The proper amount of pressure held for the proper amount of time ensures that all the resin molecules are held closely together while they cool and solidify. This minimizes the amount of shrinkage that will take place after the part is removed from the mold. But, if inadequate pressure is used or if it is applied for too short a period of time, the molecules will not be constrained during the solidification phase and the entire part will shrink excessively after removal from the mold.
Solution: Increase the amount of pressure or the time applied. Upon initial startup, the mold should be filled incrementally starting with intentional short shots (if the mold design allows) and progressively increasing pressure shot-by-shot until the mold is filled and packed properly. Then, parts should be inspected for critical dimensions. If the mold is new, and dimensions are incorrect, the mold should be returned to the moldmaker for adjustments. If the mold has already been in production and the dimensions are wrong, process parameters can be adjusted to make the part shrink less or more, whichever is required.
Mold Temperature Too High
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. Upon ejection from the mold the material will be allowed to contract more than normal and excessive shrinkage will occur.
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.
Small Gates And/Or Runners
Explanation: Gates and/or runners that are too small will cause excessive restriction to the flow of the molten plastic. Many plastics will then begin to solidify before they fill the cavity. The result is a material that is not fully contained within the metal mold surfaces and is allowed to shrink beyond normal values.
Solution: Examine the gates and runners to optimize their size and shape. Do not overlook the sprue bushing as a long sprue may solidify too soon. Use a heated bushing or extended nozzle to minimize sprue length. Ask the material supplier for data concerning gate and runner dimensioning for a specific material and flow rate.
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 a less dense material filling the cavity image. The lower this density, the higher the amount of shrinkage that will occur after ejection.
Solution: Utilize a material that has the stiffest flow possible without causing non-fill. 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 shrinkage may occur.
Solution: If possible, operate 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.
Excessive shrinkage can be defined as an extreme decrease in the dimensions of a molded part after it has cooled to room temperature.
Some common causes and solutions are listed below.
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