Technical Profile: Open-Mouth or Valve Bags?

by Thomas Ziolko
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Buhler
An operator stands next a bagging machine manufactured by Bühler AG, Uzwil, Switzerland.
Photo courtesy of Bühler.
 
The decision whether to use open-mouth bags or valve bags when bagging product in the grain processing or feed industries is influenced by several factors.

In Europe, North America and Australia, grain milling products mostly are filled into tank trucks in bulk, while in Latin America, Africa and Asia, bagging into open-mouth or valve bags is predominant. However, there is a clear trend away from valve bags toward open ones in recent years. This is driven above all by the international availability of less expensive open-mouth bags, the easier filling process and the option of dust-tight closures that parallel the trend toward greater food safety.

Not all bags are alike

Compared to the open-mouth bags, the upper part of the valve bags is almost completely closed. The valve bags have just one small opening, the “valve,” in which to fill the bag. As soon as the bag is full, the interior pressure of the bag closes the valve to stop product from spilling over.

Open-mouth bags are closed along the lengthwise side and on the bag bottom. They may be made of paper or plastic. They are easily filled and may be sealed by welding, sewing or gluing so nothing trickles out.

A wide variety of bag types may be used. The side-fold bags fill out in the shape of a block due to the inserted side fold, which makes them easy to stack. The cross-bottom bags are particularly easy to stand alone or stack after filling because of the bottom. They are also effective for advertising because print may be used on the bottom and sides of the bag. Laminated bags of woven polypropylene also may be printed in good quality and are available as a pinch bag.

Valve bags have been used for bagging milling products since the mid-1920s. Even then they were more expensive than open-mouth bags. There was a perception that they could not be sealed well and that a lot of dust resulted during packing, which is why open-mouth bags were always more popular. Even if the quality of the valve bags has improved in recent years, many grain processing companies still prefer open-mouth bags compared to valve ones.

A recent survey of some large grain processing operations confirmed this trend toward open-mouth bags. One of the largest flour milling companies in the United States gave several reasons for this: 1) Open-mouth bags are largely available in the various production countries. 2) The sealing for open-mouth bags, whether made of paper or woven polypropylene, is tight and may be reproduced even at high throughput. 3) Ultimately, open-mouth bags are selected because they are easy to handle in bagging stations as well as in the downstream closing station.

Sanitation and costs

Buhler
The automatic bagging station Maia for open-mouth bags with a pinch closing station fulfilled the requirements fro the flour milling family Molino Quaglia of Vighizzolo D'este PD in Italy.
Photo courtesy of Bühler.
Open-mouth bags ensure compliance with the high safety requirements in the food and chemicals industries through the various sealing options. Depending on the product requirements, open-mouth bags may be sealed by being sewn, glued, heated or a combination of these methods. In addition, a resealable closure may be used, which is a big benefit for the customers of the bagged product.

The cost of valve bags compared to open-mouth bags depends on the region. Feed producers in Chile have emphasized that polypropylene valve bags can really only be permanently sealed with an ultrasonically welded principle. This increases the price of the investment and the purchasing costs of each bag by about 50%. If a valve bag is not welded, the risk of the content leaking out increases because the bag is not completely sealed. This company prefers open-mouth bags for sanitation and safety reasons.

Today’s bagging stations are characterized by higher capacity and lower bag weight. Palletizers are used for optimum stacking of bags and containers. For palletizers, open-mouth bags often were considered to be harder to stack. However, a leading processor of agricultural products in South America finds the opposite to be true. Thanks to the pinch bottoms, open-mouth bags are stable when stacked on pallets. And, it’s possible to bleed the air out and seal the bags. This guarantees good stacking quality and safe transport.

Furthermore, open-mouth bags may vary in terms of protrusion while valve bags are less flexible on this issue.

Summary

The choice of the right bag ultimately depends on various factors such as product properties, packaging rate and speed, the food safety requirements and the costs of the system and the individual bag. Bühler has decided to go exclusively with the trend toward open-mouth bags for its bagging stations. The main reasons for this decision are that open-mouth bags are easy to open, internationally available without any problems, and both paper and woven PP bags may be handled in the same bagging station.
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