TL;DR
Short shots on the Roboshot are usually a worn check ring losing shot size, or insufficient injection speed or pack pressure for the part geometry. Confirm the check ring seals before changing process parameters.
What you might see
- parts missing material in thin sections
- gate area fully filled but far end short
- shot size reading below target on controller
- cushion consistently too large
Likely causes
Worn check ring (non-return valve) allowing backflow and reducing effective shot size
Insufficient injection speed or pack pressure for the wall thickness and flow length
Inadequate material feed, bridging in hopper or low dryer throughput
Blocked or insufficient mold venting preventing gas escape
Required tools
- Part weight scale (0.01 g resolution minimum)
- Brass vent cleaning brush and solvent
- Milacron Roboshot controller access (process parameters)
- Screw removal tools (for check ring inspection)
- Replacement check ring (Milacron-specified)
Safety first
- Hot melt at injection temperatures can cause severe burns. Wear heat-resistant gloves and face protection when purging or inspecting the barrel.
- The clamp unit operates at high force. Never reach into the mold area while the machine is in automatic or semi-automatic mode.
- Lock out the main disconnect and bleed hydraulic pressure before removing the injection unit or screw.
Procedure
- 1
Record the current shot size, cushion, and fill time from the Roboshot controller before making any change.[1]
- 2
Run three consecutive air shots and observe whether the screw forward position is consistent. Inconsistent forward positions confirm check ring leakage.
- 3
Inspect the hopper and feed throat for bridging or low material level. Confirm the dryer dew point and throughput rate are within spec for the resin.
- 4
Increase injection speed in 5% increments, up to the Roboshot barrel specification maximum, and observe whether the short improves.[1]
- 5
If speed increase helps but fill is still short, increase pack pressure in 5% increments while monitoring part weight.
- 6
Inspect the mold vents. Clogged vents trap gas and create a back-pressure wall that stops fill. Clean vents with a brass brush and solvent.
- 7
If shot inconsistency persists after vent cleaning and parameter adjustment, remove the screw and inspect the check ring for wear, scoring, or cracking.
- 8
Replace the check ring if the seat or ring face shows visible wear. Use Milacron-specified components only.
Sources
Milacron Roboshot All-Electric Injection Molding Machine Operator Manual, Milacron
Milacron Roboshot Injection Molding Machine Operation and Maintenance Manual, injection unit and process parameter procedures (general)
View source
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