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Kenplas Forum ==> Injection Molding (Machine) ==> Trouble Shooting -- Injection Molding
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Dear Sir,

I wanted to know the acceptability criteria for plastic parts.
If there is a small black spot on part can we accept that part or should reject it.

Thanks
Lingaraj

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5/21/2012 6:09:49 PM By lingaraj
Olá, estou com problemas com prefórmas pet isto é, na hora do sopro estão furando muitas garrafas. Gostaria de saber se o Pigmento "coloração da prefórmas' pode estar danificando esse material , pois os furos são sempre com a cor verde, sendo furos na parede da garrafa fazendo forma de vinagre.

Marcelo A
Caieiras SP

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7/2/2011 6:39:18 AM By Marcelo Antonio Silva
we have one off the problem in pet preforms .mechine running time gate area cristality coming.how we r solve this problem

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11/25/2010 3:17:14 AM By vinoth
I would like to know more on avoiding weld line or warpage in PET Preform.
Are there any thub rules on Preform dimensions and fill speed.

Thanking you very much,

Yours truly,

Vijay BB

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10/17/2008 5:40:17 PM By VIJAY BB
hi
a have a problem in polly acetal. it shrinks after molding so what i have to do. plaese tell me

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8/25/2008 4:28:43 PM By sajid
Please give me all picture about trouble preform and solution. Thank''s

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9/25/2007 12:47:30 PM By hidayat
I HAVE SEEN THIS ISSUE DEALING WITH ACETAL AND OTHER OPAQUE VIRGINS. YOU MAY WANT TO MONITOR YOUR RESIDENCE TIME AND SPEEDS. ACCETAL WANTS TO BE EXTRUDED PRETTY FAST. THERE COULD ALSO BE COLORANT LEFT OVER FROM PREVIOUS RUNS WRAPPED AROUND YOUR SCREA

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1/24/2007 11:36:59 AM By WARREN THOMAS
PET preform molding,question what is the most % of regrind that i can use? using wellman resin.

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7/30/2006 2:29:09 AM By peter cummings
hi. we are some times facing flow line in the body of the preform in our hypet300 machine preform, plz tell it root causes & solutions. thx

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6/29/2006 4:13:40 AM By ali
we are facing colour variation problem in our machine. colour varies suddenly when the machine is smooth running.plz guide us how to resolve it

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6/29/2006 4:10:06 AM By ali
we are facing preform condensation problem . plz guide us what are its posible reasons & how to control it. thx

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6/29/2006 4:07:25 AM By ali
hello what o you have to dose colour in screw wihout mixing it in hopper are the small srews with hopper with variable speed is good to feed the color to big hopper nec
thanks

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12/4/2005 8:33:32 PM By firas
Hi,

Iam having colour problems extruding acetal pellets. Acetal pellets are supposed to be white in colour, but the are frequent occurrence of slightly brown coloured pellets produced. Hope you can help

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10/16/2005 9:54:38 PM By Wai
DIMENSIONAL VARIATIONS

Dimensional variations are caused by changes in the pressure distribution throughout the cavity. Dimensional variation in semi-crystalline materials can be caused by changes in the cooling rate in the post-mold cooling and stabilization environment. For proper analysis, the problem should be specifically categorized.

1. PART IS TOO SMALL
Plastic pressure in the cavity is too low.

Increase the packing pressure to obtain required mold pressures.
Increase pack time to obtain required mold pressures.
Perform a gate freeze analysis then increase pack and/or hold time as required.

2. Non-uniform mold cooling.
Ensure uniform cooling in the cavity.

3. Improperly sized mold. (The part has always been small and the mold must be over-packed to reach minimum mdimensions.)
Resize the mold dimensions to give you a bigger process window.

4. PART IS TOO LARGE
EPressure distribution throughout the cavity is too high.

Reduce packing pressure to obtain required cavity pressure.
Reduce pack time to obtain required mold pressures.
Ensure that switchover from fill to pack occurs when the mold is less than 100% full.

5. Mold is improperly sized. (The part has always been large and the mold must be under-packed to reach maximum dimension.)
Resize the mold dimensions to give you a bigger process window.

6. PART IS TOO SMALL NEAR GATE
Effective plastic pressure near gate is too low while the rest of the part is acceptable. Generally caused by an unsealed gate.

Perform a gate freeze analysis then increase pack and/or hold time as required.
Ensure sufficient plastic injection pressure during pack/hold.
Ensure cushion of plastic in front of screw during hold.
Ensure the actual melt temperature is within the manufacturer’s recommended range.

7. Poor cooling near gate.
Ensure uniform cooling through-out the cavity.

8. PART TOO LARGE NEAR GATE
EPressure in the mold near the gate is too high while the pressure in the rest of the part is acceptable.

Reduce packing pressure and/or pack time.
Change the pack rate if possible.
Allow discharge at the gate by reducing the hold time and/or hold pressure for gate unseal.
Ensure the actual melt temperature is within manufacturer’s recommended range.

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9/16/2005 12:38:33 PM By Kenplas
DIMENSIONAL INCONSISTENCY

Inconsistent dimensions are often caused by inconsistent pressure gradients throughout the part. These pressure gradients sometimes occur on a shot-to-shot basis. Other-wise, the inconsistent variations occur on a trend over time. Time trends indicate temperature or lot variation whereas shot-to-shot indicates packing fluctuation.

1. Gate discharge.
Perform a gate seal analysis then increase pack and/or hold time as required.

2. Screw non-return valve leakage.
Run a series of short shots to verify screw non-return valve function for consistent operation. Repair or replace as necessary.

3. Machine hydraulic variations.
Ensure sufficient injection fill pressure by checking consistency of fill time. Fill pressure (boost or first stage setpoint should be approximately 300-400 psi greater than peak pressure at transfer. This is known as delta P.
Ensure consistent pack and hold pressure and/or velocity. Repair as necessary.
Check for delay between fill and pack. The response time should be approximately 0.1 - 0.4 seconds.

4. Temperature variations.
Ensure consistent melt and mold temperatures.

5. Material variations.
Try a different lot or container.

6. Melt pressure in cavity.
Increase the fill pressure available to maintain the selected velocity of most runs. If there is enough pressure, lot to lot changes will show little or no change in dimensions. As the variations increase, the dynamic pressure losses that occur during the cavity fill may increase or decrease to a greater degree. In many cases abundant pressure will handle the change. You may find as the material fills the cavity higher pressures and or velocity settings may be needed to maintain a consistent volume of fill into the cavity.


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9/16/2005 12:37:16 PM By Kenplas
DELAMINATION

Delamination of plastics generally involves contamination with an incompatible material. Delaminated parts and contaminated materials should be discarded and not put into the process stream.

1. Incoming material contamination.
Try a new lot of material.

2. Material contamination after receipt.
Run 100% virgin material.
Properly re-clean the granulator, loading and /or dryer systems.
Try another container of material.

3. Incompatible color concentrates.
Run natural material to verify then change to a compatible concentrate.

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9/16/2005 12:34:35 PM By Kenplas
CRACKING AND CRAZING

Cracking and crazing are breakages of the material often concentrated in the last areas to cool. The major criteria for solving this problem are to determine whether or not it is a mechanical problem (part removal and/or handling) or degradation of the material’s properties.

1. Contamination – (Material viscosity almost always shows a change)
Check material viscosity.
Change to different material lot.
Eliminate regrind as a possibility.
Remove colorant if being used.

2. Chemical problem due to contamination by an additive or foreign substance such as mold spray, cleaners, body oils, solvents, etc.
Clean the mold.
Change the part-handling procedures.
Remove the substance.

3. Part removal (Ejection, part sticking; could be due to overpacking).
Look at improper post-mold handling.
Check mold temperature for hot spots – excessive shrinkage.
Reduce the amount of packing pressure and/or time.
Check for adequate draft on the part.
Look at the part for undercuts.
Look for distortion before ejection.

4. Plasticizers
Look for contact with other materials that may contain plasticizers which are incompatible with many amorphous materials.

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9/16/2005 12:31:30 PM By Kenplas
BURNS AT THE GATE
Burns at the gate are a rare occurrence and generally occur with shear-sensitive materials such as rigid PVC or acetal. If streaks exist near the gate, look for a rough spot or sharp corner at the gate. Reducing the injection rate, and subsequent change in viscosity, may be used as a last resort. Commodity resins are generally not as shear-sensitive when subjected to high injection rates.

1. Burrs or sharp corners at the gate.
Polish the gate area to eliminate rough areas and sharp edges.
Change gate type.
Decrease the gate land length.

Last Resort:
Decrease injection rate – increases the material viscosity.
Increase gate size – increases the material viscosity.

2. Gate size too small.
Gate size should be an average of 40-60% of the nominal wall thickness.

3. Hot runner tool.
Check for dead spots/hang-up areas near the tip.
Check probe placement at gate insert area.
Check valve gate operation, if applicable.
Check thermocouple location.

4. Color concentrate. (Some color concentrate aditives and carriers are shear-sensitive and will burn at the gate.)
Remove color concentrates and run natural material to verify.

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9/16/2005 12:28:16 PM By Kenplas
BURNS AT THE END OF FILL

Burns at the end of fill are generally caused by trapped air dieseling, which ignites the air/plastic at the end of fill causing char on the molded article. There is never too much venting — only improperly cut or designed vents.

1. Plugged vents.
Clean the vents.
Check for over-clamping; Reduce the clamp force if needed.

2. Inadequate vents.
Add more or re-machine the vents.
Vent around cores and core pins – 180 degrees from the gate location.

3. Vents hobbed shut.
Resurface the parting line and re-machine the vents into the mold.
Reduce clamp force to prevent additional hobbing.

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9/16/2005 12:24:45 PM By Kenplas
BRITTLENESS

Brittleness is generally caused by some sort of polymer degeneration (degraded polymer) and will often be accompanied by a marked change in plastic viscosity. To check material viscosity, compare peak plastic pressure at switchover multiplied by the fill time to a non-brittle lot.

1. Excessive heat time, or the presence of a chain-breaking substance such as moisture in nylon, polycarbonate, polyester or polyurethane.
Check the moisture content of the material to see if it is dry.
Check the actual melt temperature.
Check residence time.

2. Contamination and excessive regrind in a virgin/regrind blend.
Run 100% virgin material.
Change to another material lot number.
Verify correct colorant and loading ratio.

3. Excessive heat or energy.
Check the barrel temperature settings.
Check actual barrel temperature with a pyrometer. (Remember: except for when current has recently been on the heater, the barrel surface temperatures should be 25-30º F. lower than the set temperature.)
Check the actual melt temperature.
Check that screw design is correct for this material.
Check the screw for fit and wear.
Decrease the backpressure.

4. Moisture content too high.
Properly dry the material. Check dryer temperature settings and actual air temperatures. Type of dryer; (Dual or multiple desiccant bed dryers are best.) Hopper design; (Most dryers do not have he proper taper to prevent funnel flow. Because of this, fresh material introduced at the top of the dryer may funnel through the center with insufficient drying time. Check desiccant beds.

5. Moisture content too low.
Nylon is one material that needs a certain amount of moisture (H2O) to maintain its molecular integrity when heated.

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9/16/2005 12:17:45 PM By Kenplas
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