In this pag, we will go over the most common processing problems found in RSBM. There is often more than one possible cause—and more than one solution to a particular problem even from the same cause. Not all machines have the features that may be mentioned from time to time, so other solutions may have to be found. Because solutions in single stage are often quite different than in two stage, they are added to the end of the discussion.
- Internal Folding in the Neck Area
A number of processors with older machines have this problem, which usually shows up as ring of thick material at the start of the bottle shoulder.
Causes
- Insufficient heat in the area underneath the NSR.
- Preblow pressure too late or too low.
Solutions
- Increase heat in zone #1. If that leads to overheating and haze, increase fan cooling.
- Move oven bank slightly lower.
- Push lamp #1 closer to the preform.
- Reduce heat in weak areas especially the base. This strengthens these areas of the preform allowing more material to be pulled out of the neck.
- Reduce preblow pressure delay in combination with.
- Increase preblow pressure: while proceeding in this way, occasionally turn high-pressure off, ensuring that the preblow pressure is not creating too big a bubble.
- Excessive Material in the Base of the Bottle “Candle Stick”
This defect consists of unsightly accumulations of material in a ring or half-ring shape around the inside center of the bottle. External base folding has the same causes and solutions.
Causes
- Preblow pressure too late or too low: Material is allowed to gather around the stretch rod, cooling down as a result, and becoming too cold and thick to blow out during high pressure blow.
- Preform base too hot.
- Combination of blow pressure too low and base too hot.
Solutions
- Increase preblow pressure: While proceeding in this way, occasionally turn high-pressure off, ensuring that the preblow pressure is not creating too big a bubble.
- Decrease preblow delay: If the delay is already at zero it might indicate that the preblow valve is opening late. Try replacing it.
- Move the switch indicating the end position of the stretch rod away from the bottle base until the gate goes off-center, then move it back a little. It may be taking too long for the high-pressure air to reach the bottom of the preform.
- Decrease heat to the base of the preform.
- Increase blow pressure to a maximum of 40 bar (580 psi).
- Off-Center Gate
Whenever the preform gate is not exactly in the center of the bottle base, the wall thickness of the bottle becomes uneven (Fig. 10.10). If for example, the stretch rod tip is skewed to the left, the material on the left will reach the mold wall earlier and more material will harden there even with a perfect temperature profile around the circumference of the preform.
Causes
- Preblow pressure too high: This pressure can become high enough to blow the preform off the stretch rod. Minute temperature differences around the circumference of the preform drive the preform toward the cooler side.
- Preblow pressure too early: If preblow pressure commences before the stretch rod is firmly engaged in the preform bottom, the gate may wander off the center.
- High-pressure air too early: The switch indicating the end position of the stretch rod may be not close enough.
- Stretch rod incorrectly set: Stretch rods should be 1/2–1 mm (0.020–0.040in.) higher from the base insert than the preform gate wall thickness. As that distance increases, the preform may slip to one side.
- Stretch rod bent: As neck finishes become smaller, as is often the case for custom containers, stretch rods have to be smaller too. The smaller in diameter they become the easier they bend when they hit a cold preform, for example. This is easy to see: the gate will always be skewed to the same side. Check several bottles and see where the gate is in relation to the recycling symbol or some other engraving. Other causes will push the gate randomly.
- Preform intrinsic viscosity (IV) too low: When preforms are underdried or overheated during injection their IV may drop significantly and they may blow off the stretch rod.
Solutions
- reduce preblow pressure,
- increase preblow pressure delay,
- move stretch rod switch closer to end of stretch rod or increase blow delay,
- readjust stretch rod,
- take stretch rod out and roll over a plane surface. This will show any distortion, and
- try preforms from a different gaylord or batch. Check IV if necessary.
- Haze in Bottle Walls
Cloudiness or haze first shows when temperature induced crystallinity reaches around 3%. It should not be confused with gate crystallinity, which is always a preform defect whereas haze can be created in both injection and blow molding. Whitish rings or streaks right around the preform gate indicate gate crystallinity whereas haze can occur anywhere on the preform, with prevalence toward the bottom. Haze usually shows as a milky coating on the outside of the bottle.
Causes
- Haziness already present in the preform: It is not unusual to
see these defects in preforms. - Preform overheats in the blow machine oven: When preform temperature comes close to 120°C (248°F) preforms may crystallize during equalization as they cool down.
- Mold temperature may be above 65°C (149°F).
- If haze happens randomly and is not found in the preforms, preforms with a higher initial temperature may have become mixed with the colder ones for which the process was adjusted.
Solutions
- check preform supply first
- reduce lamp settings, increase fan cooling, or speed up machine
- reduce mold temperature to 60°C (140°F) or less
- ensure all preforms are at the same initial temperature
- Pearlescence or Stress Whitening
Also referred to as stress whitening, this defect shows up as whitish rings not unlike pearls, hence the name. They are actually microcracks in the PET molecule structure. They are always on the inside of the bottle and show as a milky coating. If there is doubt whether whitening observed in the bottle is haze or pearlescence a simple test can be done: if the affected areas can be scratched off with a finger nail on the inside of the bottle, it is always pearlescence. This can be understood from the knowledge that the inside of the preform has to stretch further and therefore also breaks first.
Causes
- Preforms are overstretched during blowing. They are either too cold or too thin. The difference can be determined by checking the wall thickness of the affected areas. If they are very thin, the preform may be too hot in this area while other areas are too cold. In this case lamps pointing at the pearlescent part(s) may be at too high a setting. If the affected area is of normal or above normal thickness the preform was too cold before blowing.
- Preforms are too cold overall.
- If pearlescence happens randomly preforms with a lower initial temperature may have become mixed with the warmer ones for which the process was adjusted.
Solution
- Reduce relevant lamp settings to areas that are too thin while at the same time increase lamp setting to areas adjacent to the affect areas. This will move material into the overstretch parts of the bottle leading to thicker walls.
- Increase overall lamp settings if wall distribution is acceptable.
- Ensure all preforms are at the same initial temperature.
- Reduce fan cooling and thus increasing oven temperature often does not lead to success as this measure mostly effects the outside preform wall.
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