by Rick Unterlander and Ottmar Brandau Troubleshooting guides have suffered in the past from being locked away in offices and not being available to the people that actually need them. Considering the high cost of machine and tools high scrap rates can be a costly proposition eating away at already tight margins. PETplanet’s recently published “Rapid Guide to Perfect Preforms” takes a different approach. Bound in laminated paper that is easy to clean off, the small format of the book of 108 mm by 151 mm (4.25” by 6”) allows easy storage in back pockets or tool boxes. It has been written for the people on the floor of injection moulding companies to support their daily work in making the best preforms possible. The product of two industry experts, Rick Unterlander and Ottmar Brandau, it gives concrete advice on 35 of the most common problems in preform manufacture, most illustrated by pictures. From high AA levels to wavy bodies processors can find help when they need it.

Part 3: Air Hooks  / Streaks


Symptom:
Translucent streaks shaped as tiny hooks or small random curves on the surface of the preform. Usually near the gate area.

Cause:
The melt flow has been disturbed by air / gas in the gate / nozzle tip area causing poor formation on the moulding surface.

Solutions:
1.     Check nozzle tip heat settings. This area has a major impact on air streaks in the gate area. Settings too high may cause the melt in the nozzle tip to overheat resulting in gases to be mixed and conveyed into the cavity. Settings too low may cause plastic to build up in the nozzle tip and gate area, interfering with the melt flow, resulting in air being trapped in the melt. The settings should be adjusted to optimize melt quality and minimize the risk of air / gas entrapment.

2.    Check injection time isn’t too low (injection rate too fast) and adjust if necessary. Injection rate should be about
10 – 12g/sec/cavity.

3.    Check that the valve stems are not opening too early. This may cause melt to drool slightly and mix with air. Increase valve gate open delay timer as necessary.

4.    Check to ensure that the valve gate compressed air pressure is sufficient to open the valve stems properly. Typical pressures are in the range of 8 – 10bar (120 – 145psi).
5.    Check for adequate extruder back pressure. This must be high enough to squeeze out gases as the plastic pellets are being melted and compressed. Typical range is 15 – 20 bar (200 – 300 psi).

6.    Check that the cooling time is sufficient for the preform being moulded. This typically ranges from about 1 to 3sec depending upon the preform gate thickness. Adding time may better solidify the melt in the nozzle tip area and reduce the risk of drooling and mixing with air on the next cycle.

7.    Check that the hold (packing) time isn’t too little for the preform being moulded. Increase as necessary, especially in the last zone.

8.    Make sure the extruder screw cushion is sufficient to prevent the screw from bottoming out. This should be a minimum of 25mm to compensate for any fluctuations during recovery. (Two-stage machines only)

9.    Check for proper resin flow at extruder throat. If interrupted, air pockets may be drawn into extruder.

10.    Check that the decompression (pullback) time and position is not excessive. This should be set as low as possible to minimize the risk of drawing in air, yet enough to eliminate drooling into the cavity from excess melt pressure. Typical settings for valve gated moulds are: Time = 0 – 1sec;
Distance = 0 – 10mm. Non-valve gated moulds may require higher values to minimize gate stringing.

11.    Check for leaking core-air blow valve. Air pressure in the cavity during filling may restrict proper melt flow.

12.    Check hardware for worn, damaged or failed components, specifically the mould nozzle tips, tip insulators or PET filled space at tips, and clogged cavity and gate insert cooling channels.


Table of Contents


    i.    About the Authors
    ii.    Introduction

    1..    AA Level High
    2..    Accordion Preform
    3..    Air hooks / Streaks
    4..    Black Specks
    5..    Bubbles
    6..    Burn Marks
    7..    Colour Streaking / Peeling
    8..    Crystallinity
    9..    Drool Marks
    10..    Flash
    11..    Gate Depressions
    12..    Gate Peeling
    13..    Gate Voids / Pin Holes
    14..    Internal Gate Sink
    15..    Internal Sinks
    16..    IV Loss
    17..    Long Gate Nub
    18..    Milky Preform
    19..    Moisture Marks
    20..    Neck Finish out of Round
    21..    Parting Line Crease
    22..    Parting Line Deformation
    23..    Preform Yellowing
    24..    Short Shots
    25..    Sinks
    26..    Splay
    27..    Stretched Necks
    28..    Stringing
    29..    Surface Blemishes
    30..    Thread Flow Lines
    31..    TSS (Top Sealing Surface) Sink
    32..    Unmelts
    33..    Wall Thickness Variation
    34..    Wavy Body
    35..    Weld Lines

 

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