Testing Bags for Strength RICHA.RDW. LAHEY, Packing Editor
1
the more important considerations ONEin ofpicking the proper construction for a
bag is to decide how much strength is required for protection of the product against transportation, handling, and storage hazards. The only positive proof of the adequacy of any container is to make a sufficient number of shipments under commercial conditions to cover all eventualities in transportation. As this method is often expensive and takes much valuable time, it is sometimes supplanted by or supplemented with factory or laboratory tests. Although these laboratory tests can never be used for conclusive proof, they will usually give sufficient comparative data for the experienced shipper to decide on a bag with some degree of certainty. Because of the possible uncertainties in the factory or laboratory method of proving a bag construction, it is essential that all important "unknowns" be properly evaluated. Those which are necessary for consideration are: 1. Transportation hazards vary for each shipment, whether by express, by rail, carload or less carload, by truck load or less truckload, by water, or by a combination thereof. 2. Some dry materials require stronger containers because of their particle size and hardness. A finely powdered material, such as rubber sulfur or flour that has a tendency to pack rather than flow, cushions the forces caused by dropping. Opposed to this, hard pellets, lumps, or granular materials which flow freely, transmit these blows to the container with a minimum of cushioning. 3. Consignees handling and storing conditions vary. 4. Bag fabricators cannot lie expected to undertake complete responsibility for the choice of specifications because it is impossible for them to be familiar with the practices in use in the many chemical fields and the customs of the many industries which chemical manufacturers supply. For example, those bag constructions which are eminently satisfactory for transporting rock products or fertilizer could never be used as containers for fine chemicals. It is therefore evident that there is no positive source where construction problems can be solved. Large shippers who have had long experience in bag packing of a 1 Other considerations include contamination from foreign materials, permeability to water vapor, caking caused by compression of stacking, customer acceptance, filling and handling equipment at producing plant, etc A teat to determine transmission of moisture vapor was discussed in the April 25 issue of CHEMICAL AND
ENGINEERING NEWS, page 636.
VOLUME
2 2, N O . 14
wide variety of materials can often d r a w on this knowledge to decide o n a particular sack for a new product. If *be product is different in physical characteristics, o r if it is to be shipped to a new consuming field, -past experience will not prove to be of enough ihelp to solve the construction problem. I t would be a considerable h e l p in choosing bag constructions if there wer-e some standard laboratory test which wcxuld accurately provide comparisons of the aJbility o f sacks to withstand transportation and handling abuses. Among the advantages are: 1. Results of tests could b e compared between bag fabricators and tiaeir customers. 2. Such standard tests would m o r e accurately duplicate transportation conditions because they would be formulated b>y a group, and therefore be based on a broader background of experience. The natural result of the^se advantages would be to reduce the number o>f inadequate bags which are used to carry n e w products to their first customers—usually the most critical period in tho existence of these products. The Quartermaster Corps developed a test for subsistence bags thast has m u c h to recommend it and which has been regarded favorably by some bag fabricators a n d ship* pers. This is the "cycle" t e s t which consists of dropping bags from SL set aeighvt on each face, on both ends and b o t h edges eing increased by one foot increments after each cycle has been completed. T h e test is continued in this manner until the t>ags fail. All tests are made with dror> test -tables to eliminate the irregularities of lhand dropping. This is essential for true comparisons. This test might well be used as a basis for a standard with certain modifications t o fit it to more closely approximate civilian conditions. The height of the first cycle shtould be lowered as the hazards w i l l bo l e s s . Size is an important consideration. The slackfilled bag will stand more abuso, particularly from impacts. There a r e goner-ally- two types of bag failures. One is localized, being confined to small areas and caused by concentration of impact, pressure, or movesnent or a combination of all three. T h e other type might be termed "generalized", as it covers wide areas of the b a g surface. It usually results from transmission of stress
» » » JULY
2 5,
1944
of impact &>uch as is encountered in dropping. Localized failures, as snagging, are often more numerous than those broken by stress of impact, such as dropping or switching shtocks of freight cars. It is therefore in order to further modify this test so that half of the drops similate snagging hazards (localized damage) and the other half, straight drops (generalized damage). It has been assumed that the hazards of tnansporla-tion and handling abuses vary only with the t y p e of shipments. The degree of thtese hazards, however, increase as the length of trie haul increases. Thus a carload shipment o f a hundred miles will encounter time same hazards of car switching, vibration, swaying, etc., as a haul of over 1,000 miles, but the number of switchings and other hazards are greatly increased. This applies to less carload, express, and truck shiipments. Marine shipments encounter different conditions in that the abuses vary with the equipment and skill of the stevedores at t b e loading and discharging ports. Unloading by means of slings onto lighters in unprotected harbors with the lighter and the sbdp rolling with the swell of the ocean is a
Mechanically Operated Drop Test Table 119*
s t a r t i n g a t 2 t o 5 feet h e i g h t . After s u c c e s s ful c o m p l e t i o n of e a c h c y c l e , h e i g h t of d r o p shall be increased b y 1 foot i n c r e m e n t s u n t i l c o n t a i n e r fails.
iim"h greater hazard for the containers t h a n dock s i d e unloading. W i t h all of these fa»*tor* in mind, t h e foll o w i n g test h a s been prepared as a suggested s t a n d a r d . It should IK» considered merely a s a proposal, subject t o improvement. It is offered in t h e hope that enough interest will be s t i m u l a t e d t o carry t h e woi k to its goal of a s t a n d a r d t e s t that will prove acceptable t o b a g fabricators as well a s shippers. 1. S e v e n ba;n* of e a c h const ruction s h o u l d l»e t e s t e d . In tabulating the results, e l i m i n a t e high a n d Ou e x t r e m e s and average res u l t s of t e s t s . remaining five ba«s. 2. M u l t i w a l l p a p e r hags shall c o n t a i n not less t h a n ô per c e n t nor more than 7 per c e n t m o i s t u r e . T e x t i l e bags ane packed for test u n d e r t h o s e conditions. In addition, tests s h o u l d be m a d e w i t h the product cooled to room t e m p e r a t u r e before filling test hags. 3 . B a g s will be filled with exsu-t w e i g h t t o be packed and o u t a g e shall be uniform. C o n d i t i o n s should represent packing condit i o n s of regular factory runs except for the t e m p e r a t u r e of product (see 2 a b o v e . * If hags are parked at repulnr fartorv packing temperature, bags will be weakened by «lehy· I ration. This dehydration procès is progressive over a certain period, the length an«l «iejrree of which is controlled by the temperature, rate of cooling, and characteristics of the product. The process is then reversed and the container absorbs moisture until it is in balance with the conditions of the surrounding atmosphere. Tests conducted 2 hours after packing with tx>t material in addition to tests of containers parked with cool material should provide datu comparable with other tests and show changes, if any. «uused by high temperature of contents.
New Local Section Publication T h e Del-Chem Bulletin is about t o b e l a u n c h e d a s t h e official organ of the D e l a w a r e S e c t i o n of t h e A C S . T h e editor is Charles J. Krister, former e d i t o r of 1 so ίο pics, regional bulletin of 14 cooperating sections in O h i o ,
a Drop on one face. b Drop on opposite face across 4 X 8 inch baffle. c Drop on one edge (gusset). d Drop 3 on opposite edge (gusset) across 4 X 8 inch baffle. e Drop on one end. / Drop 3 on opposite end across 4 X 8 inch baffle. (The above constitutes one cycle.)
Procedure of Tests
H a n d - O p e r a t e d D r o p Test Table 4. P a c k e d h a g s shall be s t a c k e d o n floor o n their faces. 7 high for I hour. B a g s will t h e n b e r e s t a c k e d . reversing order of s t a c k i n g s o t h a t t o p bags will h e b o t t o m , etc. After a n hour s t o r a g e of t h e s e c o n d s t a c k i n g ar r a n g e m e n t , ba«, · are r e a d y for ' .*>. B a g s r^iuild be t e s t e d UDOC. mditions of n o t less t h a n 4 0 ° F. nor 4 0 percent R . H . ti. Bair» will he dropped m e c h a n i c a l l y b y d r o p -able or o t h e r a p p a r a t u s to insure uniform t e s t s . 7. B a g s will be d r o p p e d o n a c o n c r e t e floor in a c c o r d a n c e w i t h t h e following c y c l e .
Drop·* shall be so arranged t h a t t h e s e c o n d c y c l e d r o p s arrows the baffle will be m a d e o n t h e o p p o s i t e face. edge, a n d ox\r\ t h a n similar d r o p s in the first.
