Selecting and Using Bulk Bags - Part 1

Selecting and Using Bulk Bags - Part Two >>>

Bulk bags, also known as flexible intermediate storage containers, are inexpensive, safe containers that can be used to easily move and store from 500 to 8,000 pounds of bulk materials. Selecting a bag that's right for your application, filling it accurately, and discharging it completely will improve your operating efficiency. Part I of this article, the first of two, will help you select bags by introducing you to bulk bag design. Part II discusses how to fill, handle, discharge, and dispose of bulk bags.

Whether you're a material supplier or user, or a custom packager or shipper, you probably use bulk bags to safely and easily handle bulk materials and to improve your operating efficiency. However, to use bulk bags effectively, you should know how they're designed (Part I), as well as how they're filled, handled, discharged, and disposed of (Part II).

Bulk bag design

The most expensive part of your packaging system is the bulk bag itself, so selecting the right bag manufacturer and the right bag is important. Choosing a standard bag that's cheaper but doesn't fit your exact needs can raise labour costs, waste materials, and even cost you customers. To choose the right bag for your needs, consult a bag manufacturer and, together, consider several design factors: bag size, lifting loops, filling inlets, and discharge outlets; liners; and single-trip or multi-trip styles.

Bag size. A bulk bag's size depends on the bag's base width in relation to the internal dimensions of the shipping carrier, the weight and filled density of the material the bag will hold, and the material's net filled height in the bag.

To choose a bag that will fit into your carrier, you must consider the bag's base width. The base width of a bulk bag depends on the filled width of the bag, because the midsection of a bag with a square or rectangular top and bottom will round out. Measuring both the internal size of the carrier – such as a truck trailer or International Standards Organization (ISO) container – and the bag's filled width ensures that the bag will press fit into the carrier (most truck trailers have internal widths of 96 to 100 inches; ISO containers have internal widths of 92 inches).

To determine the base width for your bag when shipping bags side by side in a truck trailer or ISO container, use the formula:

internal trailer width x 3.14

8x stretch factor

To determine the rounded diameter or filled width of the bag, use the formula:

(base length + base width)

x 2 x stretch factor

3.14

The bag's stretch factor varies with the fabric weave's tightness, the fabric weight, and the material's filled density. For example, a bag with a 36-inch-square base and a stretch factor of 1.03 has a rounded diameter of about 47 inches.

Before ordering your bags, make sure your bag manufacturer offers the bag base size you'll need to fit your carrier.

The weight your bag must handle depends on your material's filled density and the weight you need to ship in each bag.

The height of your bag may be limited by the headroom you or your end user need to discharge the filled bag. Safe storage practices also affect bag height: Bags taller than 60 inches shouldn't be stacked; bags taller than 48 inches shouldn't be stacked more than two high, even if they're on pallets. Bags shouldn't be taller than twice their base size, either.

The material's filled height in the bag will vary, depending on the material's characteristics and the filling machine you use, so have your material tested on your filling machine before ordering your bags. Filling machines that densify the material during filling save bag height and make safer, more stable bags for stacking and handling.

Table I lists various bag sizes – including empty and filled sizes and capacities – and bag styles for three standard base dimensions.

Lifting loops, filling inlets, and discharge outlets. Selecting these components depends on your material, your equipment, and your hygiene requirements.

Lifting loops are usually located at the top four corners of the bag; they can also cross the corners. A bag's lifting loops must be strong enough to lift the bag when it's filled. Ten-inch loops are typical; 12-inch loops may be required if forklifts with fork tines will pick up the bags. Pop-up loops stand up to simplify inserting the fork tines into the loops, but are expensive. To avoid wear on all types of loops, smooth the sharp edges of the fork tines.

The bag's filling inlet is tailored to fit the filling machine. A filling spout is the most common type of bag inlet and is used with a filling machine that feeds material through a tube or pipe. While 14-inch spout diameters are common with unlined bags, a lined bag's filling spout diameter can be increased to 20 to 25 inches if the filling machine spout is clamped only to the bag's liner. This larger diameter allows material to fill the shoulders of the bag without bridging in the bag's neck, which is especially important with powders.

Bags with coned-top inlets accommodate cones formed by materials that don't flatten out at the top after filling, such as full-fat soya flour and some milk powders. A bag with a cone-top inlet can replace an oversized bag for a given weight of these materials as long as its lifting loops are extended and the filling spout's maximum diameter is used during filling. Bags with duffle-top inlets may also be used for this purpose. Both coned- and duffle-top inlets are uncommon.

Bags with inlet spouts are filled by filling machines with twin-tube filling heads. Bags with open tops or duffle-top inlets are filled by other filling devices, such as front-end loaders.

The bag's discharge outlet should be large enough to discharge the material easily, which depends primarily on the discharger used. The most common type of outlet is an outlet spout. Outlet spout diameters can range from 10 to 25 inches. Typical outlet spouts are 14 inches in diameter and 18 inches long. The outlet spout must be long enough – at least 15 inches – to be tied off (usually by a drawstring) and, in most cases, goosenecked; if the outlet is too long, though, it can get caught in the discharger's feed system.

Table 1 Various bulk bag sizes and capacities for three standard base dimensions

Other bag outlets work with particular materials or dischargers that standard outlet spouts can't: hygiene outlets, super hygiene outlets, total-opening bag bottoms, and flat or plain bag bottoms.

Bags with hygiene outlets have coated hygiene flaps that cover and protect that bag's base to protect it from dirty pallets or floors; these flaps can be pulled back before discharge. A sewn-in polyethylene membrane inside the bag or a full sealed liner covers a 15- to 18-inch-diameter hole, rather than an outlet spout, in the bag's base. Bags with super hygiene outlets are similar, but have either tailored polyethylene or tubular liners, allowing the liner to be clamped to the discharger before emptying. Hygiene bags require a special discharger and are used for foods, pharmaceuticals, and other materials that must be protected from contamination.

Bags with total-opening bottoms, which open like bomber doors, can be discharged by releasing a holding cord. This type of outlet works with very low density materials, such as chopped straw or fiber, as well as with chicken manure and wood pulp, as long as the material in the bag weighs 1,500 pounds or less.

Bags with flat or plain bottoms discharge material when the entire bag bottom is cut manually or with a fixed-knife discharger; these outlets are obviously for single-trip bags. Because cutting the bag can leave shreds of bag fabric in the material, this outlet is best for materials like base minerals that can't be harmed by the shreds or for applications where the shreds can be screened out after discharge. Coating the bag base fabric will minimize shredding when the bottom is cut.

A bag with a plain bottom must be slit open to discharge. A bag with a drawstring outlet is easy to untie and controls dust; a bag with a lined drawstring outlet may be used when the bagged material must be protected by a polyethylene liner.

Liners. Liners protect the bagged material – primarily fine powders, foods, pharmaceuticals, and chemicals – from moisture and air. Liners are typically made of linear-blend polyethylene because it's stronger than low-density polyethylene and is less likely to crumb or shred if cut during discharge. It's also safe to use when filling with materials with temperatures below 80°F; high-temperature liners (safe to 120°F) must be used with materials over 85°F, but are more expensive. Linear-blend polyethylene liners are also rate moisture-proof, not waterproof, but will admit moisture over a long period.

Liners are extruded in tubes and may be gusseted immediately or left ungusseted. (Using ungusseted liners is often more efficient than using gusseted liners: Pockets of material can collect in a gusseted liner's sealed gussets, and shaking this material out by hand and controlling the dust created is time-consuming). The extruded tube's circumference should equal the bag base's perimeter plus a 2 percent stretch factor. The liner's thickness can be from 2 to 8 mils, depending on the amount of protection the bagged material requires.

Single- or multi-trip styles. Sing-trip bags can be used once; multi-trip bags can be used several times. Almost any bag design can be either single- or multi-trip, as long as the bag isn't damaged during discharging, such as by cutting the bottom.

The fabric and construction of single-trip bags should provide a safety factor of 5 to 1; for multi-trip bags, the safety factor should be at least 5 to 1.

Whether you choose single- or multi-trip bags depends on several factors. Single-trip bags are cheaper on a cost-per-bag basis, though not necessarily on a cost-per-ton-moved basis. Lined bags with outlet spouts must be single-trip, because relining and retying off the outlet spout on multi-trip bags is too expensive. Single-trip bags are also more practical in many situations because returning multi-trip bags is time-consuming and costly – you must carefully handle and fold the bags before they're returned.

Multi-trip bags have stronger fabric and loop material than single-trip bags. You can use multi-trip bags as often as 50 to 100 times if you carefully fill, handle, and discharge them. Multi-trip bags are best for in-house use (as long as bagged materials aren't cross-contaminated or such contamination doesn't matter) and for regularly shipping one material between one company's plants.

However, to be cost-efficient, your multi-trip bags should have a return rate of 50 percent: on average, base your cost estimates on using the bags three times. The costs of returning and inspecting the bags and retying or resecuring their outlets – often underestimated – also make the bags expensive. Multi-trip bags that must be relined or that have discharge outlets that don't refasten easily raise operating costs, too. Finally, the further apart the filling and discharging locations for multi-trip bags are, the less efficient the bags will be.

Selecting and Using Bulk Bags - Part Two >>>