Description:

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

Possible Causes & Remedies:

Machine Cause

◇ EXCESSIVE RESIDENCE TIME IN BARREL

Remedies: 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

Remedies: 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

Remedies: 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

Remedies: 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

Remedies: 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.

Mould Cause

◇ IMPROPER PARTING LINE SEAL

Remedies: 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

Remedies: 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

Remedies: 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

Remedies: 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

Remedies: 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 Cause

◇ IMPROPER FLOW RATE

Remedies: 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

Remedies: 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 Cause

◇ INCONSISTENT PROCESS CYCLE

Remedies: 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. 

(Source: plastictroubleshooter.com)