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Flow lines can be defined as linear grooving, or circular ripples, on the surface of a molded part that indicate the direction of material flow within the cavity of the mold.

Possible Causes & Remedies:

Machine Cause


Remedies: Increasing the injection pressure will force the molten plastic against the mold cavity steel before the plastic solidifies, removing the flow lines and duplicating the cavity finish.


Remedies: Optimize the residence time by making sure the mold is sized to the proper machine. Also, optimize the cycle time to ensure the material residence time is adequate to properly melt the plastic.


Remedies: Increase the barrel temperature to that recommended by the material supplier. Adjust as needed to eliminate the flow lines. And, remember to keep the profile set so the material is heated from the rear towards the front of the barrel.


Remedies: Increase the nozzle temperature. As a rule-of-thumb, the nozzle temperature should be set at 10 degrees F higher than the setting for the front zone of the barrel. This helps compensate for heat loss due to metal-to-metal contact between the nozzle and the sprue bushing, and keeps the material hot enough to pack the mold, eliminating flow lines.


Remedies: Increase the cycle time. The easiest change to make is to add time to the cooling portion of the cycle. That is when the plastic is absorbing the most heat in the barrel. Increase barrel temperatures 10 degrees F at a time, allowing 10 cycles between changes to re-stabilize the process.

Mould Cause


Remedies: Increase the mold temperature to the point that the material has proper flow and packs out the mold. Start  with the material suppliers recommendations and adjust accordingly. Allow 10 cycles for every 10-degree change for the process to re-stabilize.


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. At least 30% of the parting line perimeter should be vented, but additional vents can be selectively placed for any area where flow lines appear.


Remedies: Examine the gates and runners to determine if any burrs or other obstructions exist. If possible, perform a computer analysis to determine the proper sizing and location of gates and runners. Ask the material supplier for data concerning gate and runner dimensioning for a specific material and flow rate.

Material Cause


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.


Remedies: If it is determined that 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


Remedies: 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.

(Source: plastictroubleshooter.com)

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