Nozzle Drooling

Possible Cause

Suggested Remedy

1.  Nozzle temperature too hot

2.  Wrong nozzle design

3.  Moisture in material

a.   Reduce nozzle temperature

b.   Decrease melt temperature

c.   Reduce back pressure

d.   Increase screw decompression

e.   Use enough screw RPM’s to allow screw to recover using approximately 90% of the cooling time

Use reverse taper nozzle

a.   Review material handling to eliminate moisture pick up

b.   Dry material prior to moulding

c.   Use hopper dryers

Excessive Cycle Time

Possible Cause

Suggested Remedy

1.  Poor mould cooling design

2.  Platen speeds excessively slow

3.  Melt temperature too high

4.  Mould temperature too high

5.  Screw recovery time excessive

a.   Increase mould cooling in hot spots

b.   Ensure fast turbulent flow of water through cooling channels

a.   Adjust clamp speeds to safely open and close quickly

b.   Low pressure close time excessive, adjust clamp positions and pressures to safely and efficiently open and close mould

Decrease melt temperature

Decrease mould temperature

a.   Check machine throat and hopper for blockage or bridging

b.   check for worn screw and barrel, especially in the feed zone

Flashing

Possible Cause

Suggested Remedy

1.  Material too hot

2.  Injection pressure too high

3.  Excessive packing of material in cavities

4.  Projected area too large for available clamping force

5.  Mould clamping pressure not properly adjusted

6.  Uneven or poor parting line and mating surfaces

7.  Non-uniform cavity pressure due to unbalanced filling

a.   Decrease melt temperature

b.   Decrease mould temperature

c.   Lengthen cycle time

a.   Decrease injection pressure

b.   Decrease boost time

c.   Decrease injection rate

d.   Increase transfer position (to lower injection peak pressure)

Decrease packing pressure

Use larger tonnage machine

a.   Increase clamping pressure

b.   Check mould parting line for obstruction

c.   Check platen parallelism

a.   Remove mould, carefully inspect and repair parting lines, cavities and cores which do not have positive shut off

b.   Add support for mould cores and cavities

a.   Balance/increase runner and gate sizes to obtain uniform filling

b.   Properly balance cavity layout for maintaining uniform cavity pressure

Part Sticking in Mould

Possible Cause

Suggested Remedy

1.  Overpacking material in mould

2.  Improper finish on mould

3.  Insufficient draft on cavities and sprue

4.  Knockout system poorly designed

5.  Core shifting and cavity misalignment

6.  Undercuts in mould and possible surface imperfections

7.  Non-uniform cavity pressure in multi-cavity mould

8.  Moulded parts too hot for ejection

9.  Moulded parts sticking to stationary half of mould

a.   Decrease first stage injection pressure

b.   Decrease boost time

c.   Decrease injection forward time

d.   Decrease packing pressure

e.   Increase injection transfer position (to lower injection peak pressure)

Draw polish mould to proper finish

Polish and provide maximum allowable draft

a.   Redesign knockout system for balanced ejection forces

b.   Review operation of knockout system, plates not opening in proper sequence

a.   Realign cavities and cores

b.   Add interlocks to mould halves

a.   Repair and polish

b.   If undercut is intentional, redesign or reduce

Redesign runners and gates for balanced filling of cavities

a.   Increase cooling time

b.   Decrease melt temperature

c.   Decrease mould temperature

a.   Redesign sprue puller

b.   Apply mould release

c.   Increase nozzle temperature

d.   If parts remain on wrong side of mould, undercut other side or try differential mould temperatures

Short Shots

Possible Cause

Suggested Remedy

1.  Melt temperature too low

2.  Mould temperature too cold

3.  Injection pressure too low

4.  Insufficient feed

5.  Insufficient injection forward time

6.  Entrapped air causing resistance to fill

7.  Restricted flow of material to cavity

8.  Unbalanced flow to cavity in multi-cavity mould

Increase melt temperature

Increase mould temperature

a.   Increase first stage pressure

b.   Increase boost time

c.   Increase injection speed

a.   Increase shot size to maintain a constant cushion

b.   Inspect non-return valve for wear

a.   Increase injection forward time

b.   Increase injection rate

a.   Provide proper venting

b.   Increase number and size of vents

a.   Increase gate size

b.   Increase runner size

c.   Use nozzle with larger orifice

a.   Increase gate size

b.   Redesign runner to provide balanced flow

Sprue Sticking

Possible Cause

Suggested Remedy

1.  Improper fit between nozzle and sprue bushing

2.  Insufficient taper on sprue bushing

3.  Rough surface of sprue bushing

4.  Sprue puller design inadequate

5.  Overpacking material in sprue

6.  Nozzle temperature too low to provide clean break

Nozzle orifice should be smaller than sprue bushing orifice

Increase taper

Eliminate imperfections and draw polish surface

a.   Redesign sprue puller and increase undercut

b.   Increase sprue diameter if too small for strength

c.   Decrease sprue diameter if too large for cooling

a.   Decrease packing pressure

b.   Decrease injection forward time

c.   Use machine sprue break

a.   Increase nozzle temperature

b.   Use reverse taper nozzle