Filling & Closing
Equipment For Filling And Closing
Much of the equipment used to move powdered, flaked or granular products within a manufacturing process can be adopted or modified to the sack packing process.
For granular products, the gravity feed method is frequently used. Other general methods used to accommodate specific products are fluidized bed, vibration feed, auger, air pressure, impeller, belt and vacuum. The basic handling characteristics of the product will determine the method of packing the product as well as the type of sack to use for the specific product. For example, granular or pelletized products flow easily when packed and can offer an opportunity to select either an open mouth or valve-type shipping sack. In contrast, powdery or dusty products will tend to complicate the packing process by requiring special equipment and limiting the type of sack to be used.
To further complicate the selection of packing equipment and sack type, there are hygroscopic products which require greater protection from outside moisture as well as other products which emit odors that may require a special odor barrier in the package construction and a secure heat seal.
Such special product considerations can be very important in deciding the packing equipment and type of shipping sack to be used. When evaluating a new packing system or a different type of sack, the technical department of the equipment manufacturer and/or the shipping sack manufacturer can offer immeasurable help, based on many years of cumulative experience with similar products and packing solutions for those products.
Open Mouth Sack Packers
For smaller or more basic packing operations to accommodate a free-flowing product, a gross weight open mouth bagger directly under a control hopper would normally be considered. An operator manually places a sack on a conveyor and the weighting unit fills and trims the weight of the package. Once filled, the operator releases the sack onto a sewing conveyor, transferring the filled package into the sewing head for closing.
From the end of the conveyor, the filled sack can then be inspected and palletized. Generally, this procedure outlines a basic system to accommodate nominal open mouth sack production.
To upgrade this basic system, it would be possible to use a pinch style sack that when filled, would feed into a pinch closing unit. The pinch style sack system greatly simplifies the sack closing operation because of the closing process itself. Where a sewing machine requires operator knowledge as well as frequent maintenance and adjusting, the pinch closing process is simpler and requires less maintenance. The simplicity of reactivating a pre-applied hot melt adhesive with hot air is a plus in any open mouth closing operation.
To further upgrade the open mouth sack packing operation, newer automatic weight scales can quickly weigh free-flowing products and “dump” a pre-weighed charge into a sack with remarkable weight accuracy. These high-speed weight scales can increase the rate of a packing operation considerably if the characteristics of the product lend themselves to the speed and accuracy of this equipment.
There is also some high technology equipment available for open mouth packing which employs robotics – selecting a sack, opening it for an automatic scale discharge, then releasing the sack for closing. Justifying this equipment would require very large volumes of product packing.
When considering any type of packing system – basic or more automated – the sack manufacturer should be consulted early in the decision-making process to avoid common errors.
Valve Sack Packers
All valve sacks are filled by passing the product into the package through filling tubes which vary in design according to the density and flow characteristics of the product. Again, as in open mouth sack packing, there are numerous ways in which to pack valve sacks: auger, belt, pressure, impeller, vacuum or fluidized bed. Flakes, pellets, crumbs or fine powders – products which may have been very difficult to handle – can be accommodated with very limited weight variation by selecting the proper type of valve packer.
The valve packer can be set to gross weigh a product or have a net weight scale above the packing unit which drops a pre-weighed charge into the unit.
Both types of units are common and are able to hold close weight tolerances. The units can be installed as a basic single unit or in banks of multiple units. While a tuck-in sleeve valve sack is popular for the basic type of packers, the automatic sleeve is used in the corner of a sack when high-speed packing is required. For this type of sleeve to function correctly, the product in the sack – as it approaches the top of the package – forces the sleeve closed. Accordingly, it is critical that the sack be sized properly since a shipping sack packed too loosely can sift the product. Again, properly sizing the sack to account for extremes in product density is a job for the sack manufacturer’s technical department. Robotics can also be utilized to some extent in packing valve sacks.
Robots with vacuum cup arms can index sacks, feed them onto a packer, transfer the sack off the packing unit once filled and even palletize the filled sack to programmed stacking patterns.
The valve-style sack lends itself to size adjustments in order to fit an interlocking or “brick wall” pattern on a shipping pallet (explained in detail in Chapter 9). Once again, a sack manufacturer’s technical department can provide valuable assistance in developing pallet stacking methods which best fit in a trailer or rail car to ensure the most economical shipping load.
Technology is keeping pace with demands for greater packing speeds, more accurate weights and more efficient warehouse handling of products. Accessories such as vibration settlers, sack flatteners, radiant and ultrasonic sealers and the increasing use of robotics are playing a vital role in providing the neat and clean interlocked pallet loads now coming from the valve sack packing station.
The Importance of Humidity
Paper shipping sacks are best packed and closed when the moisture content of the paper is between 5 and 8 percent. To maintain a level of moisture content, empty sacks should be stored in a fairly humid atmosphere. Optimal conditions are a temperature of 70 degrees (F.) and a 50-to-60 percent relative humidity level. Under these conditions, sacks will contain about six percent moisture and be flexible and strong.
As the moisture content of paper drops below four percent, paper tends to become progressively brittle. When brittleness occurs, bales or pallets should be opened and the sacks spread out in a humidified room (or on a loading platform in humid weather) for 24 hours to permit the sacks to absorb the moisture needed to restore their strength and flexibility. Keep in mind that moisture recovery of sacks containing a barrier ply will be slower than with sacks of an all-kraft ply construction.