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Technical Articles
Home >> Technical Articles >>Blown film process requires careful considerations to prevent polymer wastage
Blown film process requires careful considerations to prevent polymer wastage
Time: 2008-12-30
Blown film process has thickness variation ranging up to 15% which means that a significant amount of polymer gets wasted due to higher thickness in thickness variation. For highly automated extrusion lines, waste should amount only to the safety margin built into the product (set point plus machine variation). For those without automation, significantly more wastage gets generated. In addition to the loss of the costly raw material it also involves loss of other variable cost of power, machine time, and manpower and also of overhead fixed cost. Loss occurring in blown film manufacturing has two main sources: chronic minute thickness variations - either slow or fast, and uncontrolled film width.

Thin spots in blown film are a processor's and converter's nightmare. Bag making, sealing, and printing all suffer when film thickness is not uniform. No blown film die produces a perfect thickness profile, but a chronic thin spot forces operators to give away polymer to meet minimum gauge specifications. Correction of dies helps in reducing thin or thick spots. Frequent or regular cleaning of dies helps in controlling the wastage by maintaining thickness uniformity to a much higher level.

If internal bubble cooling is added to a die that already has a minute gauge problem, it results in the material running much faster. Blown film dies are often manufactured with pre-drilled IBC passages, so IBC can be added later if not needed immediately. When the upgrade is made later, there exists a danger of forgetting the fact that the die is no longer new. The extra 20-40% increase in production rate can result in laxness in scrutinizing for thickness variation. Compensating for a small chronic thin spot in IBC mode that has 30% higher productivity, costs 30% more in wastage .Hence dies should be inspected and refurbished before installing IBC.

On blown film lines without IBC or automatic lay flat-width control, operators maintain bubble size by periodically re-inflating the bubble to a size larger than required. This compensates for the bubble drifting smaller as air leaks through the primary nip rollers. Depending on the condition of the upper nip rollers, the time between periodic inflations could be anywhere from 30 min to 4 hr. Manual re-inflation results in the product being over-wide. A blast of air into the bubble to inflate it can cause a vacuum zone (venturi effect) around the entry passage, pulling the bubble away from the air ring locally and reducing the size of the bubble. Operators instinctively slow down the inflation process to prevent this effect, but this also extends the over-width period. This situation results in wastage on two fronts. First, the product is periodically wider than it needs to be. Second, when the larger size is set up, the material will be slightly thinner than it should be so operators will add more material to reach the minimum thickness.

An automatic lay flat-control system that constantly checks lay flat against set point and adjusts it when necessary reduces wastage because it eliminates extra material added for inflation of the bubble to correct the downward size drift. Also the control system should provide a method for pulling air out of the bubble and not venting the air. This is because the very low pressure inside a blown film bubble provides no impetus to push air out the open vent. Non-IBC dies have a passage through the die to supply inflation air and to remove air when the bubble gets too big. Furthermore, a control system that can handle quick inflation without introducing a venturi provides more production per shift. A 30-day study by a blown film processor of can liners showed that automatic lay flat control made up to an 8% improvement in production time because of faster start-up and bubble-size changes compared with manual bubble inflation that creates a venturi.
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