Short Shots and/or Non-fill can be defined as an incomplete molded part caused by insufficient material injected into the mold. Non-fill can be an extension of a flow line or knit line condition.
Possible Causes & Remedies:
◇ INSUFFICIENT MATERIAL IN HOPPER
Remedies: It is best to use an automated system that replenishes material in the hopper as it is used. That way the machine will never run out of plastic. If manual systems are utilized instead, the employee in charge must understand the importance of keeping the hopper filled. Alarm units can be used to emit an audible signal when the material in the hopper reaches a preset level.
◇ LOW BARREL TEMPERATURE
Remedies: Increasing the barrel temperatures will allow the flow fronts to stay hotter longer and complete the filling of the mold. Make sure the proper profile is being used and that the material heats progressively as it travels through the barrel from rear to front.
◇ INADEQUATE BACK PRESSURE
Remedies: Increase the back pressure to increase the melt temperature and improve the ability for the fronts to flow. This is best accomplished by starting at the minimum of 50 psi and increasing in 10-psi increments until the proper flow is attained. Do not exceed 300 psi. The higher the back pressure the hotter the plastic, and excessive back pressure will thermally degrade the plastic.
◇ INJECTION PRESSURE OR SPEED TOO LOW
Remedies: Increase the injection pressure or speed. While these two parameters are related, it is not proper to adjust them both at the same time. Adjust them each independently and monitor the results closely to determine whether or not the other needs adjustment. As a rule-of-thumb, it is best to make adjustments in increments of no more than 10% of the original setting.
◇ EXCESSIVE NON-RETURN VALVE CLEARANCE
Remedies: Inspect the non-return valve mechanism and replace worn or damaged components. This wear is normal but is accelerated by molding materials that have reinforcements (such as glass) in them. The valve should be inspected at least every three months.
◇ BRIDGING IN FEED THROAT
Remedies: Decrease the feed throat temperature. The material supplier can provide the proper temperature value for a specific material. Make sure there is no obstruction in the water line used for cooling the feed throat.
◇ INSUFFICIENT BARREL CAPACITY
Remedies: Place the mold in a machine that utilizes the 20% to 80% rule. This states that, ideally, a barrel should be sized such that 50% of the capacity is used every shot, but based on heat sensitivity of the material being used, that ratio can be between 20% for most materials and 80% for heat sensitive materials. This formula allows enough time for the material to absorb heat properly before being molded.
◇ LOW MOLD TEMPERATURE
Remedies: 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
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.
◇ IMPROPER GATE LOCATION
Remedies: Relocate, or redesign, the gate so that the molten plastic is directed against an obstruction such as a core pin. This will cause the material to disperse and continue to flow instead of slowing down.
◇ INSUFFICIENT 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.
◇ 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.
◇ INCONSISTENT PROCESS CYCLE
Remedies: If possible, run the machine on the automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is necessary. In addition, instruct all employees on the importance of maintaining consistent cycles.
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