FLASH

 

 Flash can be defined as excess plastic material forced out of the
cavity. This occurs at any point where two mold surfaces meet.

Machine

Excessive Residence Time In Barrel

Explanation: Ideally, a shot size should equal 50% of the capacity of the barrel. That results in processing one cycle while preparing for the next cycle. However, this is a general statement because, depending on the material, the ratio can be as small as 20% for non-heat-sensitive materials such as polypropylene and up to 80% for heat-sensitive material such as PVC. As the ratio drops, the time of residence by the material in the barrel increases, and the material gets hotter. It will flow much easier at this point and enter areas where it could not at its normal viscosity. This results in a flashing condition in those areas.

Solution: Strive for a 50% shot-to-barrel ratio. This is ideal but can go as low as 20% if the material is not too heat sensitive (like polypropylene) and up to 80% if the material is extremely heat sensitive (like PVC). It is not a good idea to empty the barrel every shot because more time will be required to bring the next mass of material up to proper heat and degradation may occur.

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.

High Barrel Temperatures

Explanation: High barrel temperatures have the same effect as long residence time. The plastic material becomes more fluid than it should and can enter small openings and crevices that it could not normally enter at the right viscosity.

Solution: Reduce the barrel temperature to that recommended by the material supplier. And, remember to keep the profile set so the material is heated from the rear towards the front of the barrel.

Excessive Cycle Times

Explanation: If the total cycle time is too long, there is a good possibility that the material is overheating in the barrel. This will cause the material to become too fluid and creates the potential for flashing in areas where it would not normally do so.

Solution: Reduce the cycle time. Normally, this can come from the cooling portion of the cycle, but make sure the other functions are not excessive. For instance, injection hold time only needs to be long enough for the gate to freeze. After that, the hold pressure has no effect on the material in the cavity. So, the hold time is an area that should be considered for time reduction. Other functions should also be analyzed.

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.

MOLD

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.

Improper Venting

Explanation: If vents are machined too deeply for a specific material the molten plastic can leak into the vents and become flash on the molded part. 

Solution: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. The viscosity of the plastic being molded determines the depth of the vent. Stiff materials can utilize deeper vents but fluid materials require thinner vents. In either case, the concept is to remove air from the mold as fast as possible with as deep a gate as the material viscosity will allow.

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%.

Sprue Bushing Too Long

Explanation: In a standard mold design, the sprue bushing extends through the A half of the mold until it touches the parting line at the ``B'' half. A runner is then machined across the face of the bushing to allow molten plastic to flow into the mold. If the sprue bushing is too long, it will keep the ``B'' half from closing tightly against the A half and a gap will form at the parting line. Molten material will leak into this gap causing flash.

Solution: Reduce the length of the sprue bushing. This is easily done by grinding the face back enough to form a small pad of material to ensure the bushing does not touch against the ``B'' half. The thickness of the pad should be limited to approximately 1/32'' so it will not affect the overall cooling time of the cycle. This pad will also act as a cold well.

Improper Stack-Up Dimensions

Explanation: Unless a mold is cut-in-the-solid, there are many plates, blocks, and other components used in its construction. Each half of the mold must be constructed so that these items ``stack-up'' to specific dimensions. If this is not the case, these items will have gaps that the molten material can flow through and cause flash.

Solution: A new mold should have the dimensions checked and adjusted even before the mold is placed in a press. As molds age, the components are exposed to compression and fatigue and may relax. They need to be adjusted periodically to ensure that the stack-up dimensions are still proper. Proper stack-up results in a preload of approximately 0.003'' on the cavity block faces.

MATERIAL

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 has a stiffer consistency. It is better to use the stiffest flow possible because it improves physical properties of the molded part. However, the stiff material will require higher injection pressures, which may blow the mold open and cause flash at the parting line. If an easy flow material is used, the physical properties will not be as great. In addition, the material will flow into very thin areas and could create flash where the stiffer materials would not.

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.

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.

OPERATOR

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. 

HOME  DEFECTS  RESOURCES  BOOKSTORE  SHOWCASE  SEMINARS  CONTACT US
Copyright Texas Plastic Technologies
Worldwide rights reserved