On-Line Solutions To Injection Molding Problems
Excessive Injection Pressure
Explanation: If too much injection pressure is used, the clamp unit may not be able to hold the mold closed, especially on a machine with a hydraulic clamp. The mold will blow open and plastic material will leak out around the perimeter of the parting line. This leakage is called flash. It can also occur between any mating steel surfaces of the mold such as around ejector pins and slides. Major mold damage may occur as a result of flashing.
Solution: Reducing the injection pressure reduces the tendency for the material to flash. In addition, make sure there is a properly raised shutoff land around the perimeter of the cavity. This will focus the clamp force and allow less clamp tonnage to be used. Without the shutoff land on the mold, the machine may not be able to generate enough clamp force to keep the mold closed under normal injection pressure.
Inadequate Clamp Force Setting
Explanation: In both hydraulic and mechanical clamp machines, the clamp unit position must be set at the beginning of a molding run and readjusted as the run progresses, due to thermal expansion of the mold and machine. If this readjustment requirement is ignored, the clamp unit position may shift to the point of not fully closing the mold against the incoming injection pressure, and flashing will occur at the parting line.
Solution: Size the mold to run in the proper machine. This is done by calculating the molding surface area (area of the part to be molded) and multiplying it by a factor of from 2 to 6. The higher number is used for stiff material (like polycarbonate) and the lower number for easy-flowing materials. That will give the number of tons needed to keep the mold closed, assuming there is a proper shutoff land on the mold. You must calculate the total area so include all cavities and the runner system.
Improper Parting Line Seal
Explanation: The parting line(s) of a mold must be machined to very close tolerances and parallelism to seal properly when the mold is clamped shut. If the parting line is not parallel, or is otherwise improperly machined or designed, molten plastic material will be forced out of the areas that are not closed tightly, and flash will form.
Solution: Check for proper parting line seal. Make sure there is a shutoff land around the perimeter of the part. There should also be pads around the leader pins at the same height as the shutoff land to ensure parallelism when the mold is clamped. Use a dial indicator to check the flatness (or parallelism) of the parting line surfaces. They should be within 0.002'' (or less) over the entire parting line surface.
Inadequate Mold Supports
Explanation: Components called support pillars are used in the construction of a mold to provide extra compression support behind the cavity retainer plates on the ejector half of the mold. These pillars are used to fill in the vacant areas present in the U-shaped ejector housing and, when properly positioned, keep the mold from collapsing into the ejector housing during the injection phase of the molding cycle. If there are too few pillars or they are not positioned properly, the mold plates will deflect when injection pressure is applied and molten material will flow into the distorted areas, causing flash.
Solution: Ensure that adequate support exists. An example of the importance of support pillars can be seen by the following: If a 12'' x 15'' mold base is used without any pillars, the maximum amount of projected part area that the mold could produce without plates deflecting would be 14 square inches. If four 1-1/4'' diameter support pillars are properly placed in the same mold, the allowed projected area would increase to 56 square inches, an improvement of 400%.
Excessive Mold Lubricant
Explanation: If a material is stiff, a lubricant can be added to improve the flow. If this is an external lubricant such as a mold release agent, it is difficult to control the amount of lubricant being used and the material may become more fluid than required. The result could be flashing where the material would not do so without lubricant.
Solution: If it a lubricant must be used, have the material manufacturer (or a compounder) add it directly to the pellets. That will result in more uniform blending and all the material will have the same flow rate.
Inconsistent Process Cycle
Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in erratic heating of the material in the injection barrel. If such a condition exists, the material will not be of consistent flow rate and the easier flowing portions may cause flashing.
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.
Flash can be defined as excess plastic material forced out of the cavity or runner system. This occurs at any point where two mold surfaces meet.
Some common causes and solutions are listed below.
Copyright by IPLAS and Douglas M. Bryce
Worldwide Rights Reserved
NOTE: For more detailed information on the causes and solutions of this defect, you can find it in our BOOK, or ONLINE SEMINAR.