Corner Pads for Double Case Packages

By Paul Marcon

Figure 1 - A double case package

Corner pads (see Figure 1) can provide highly effective protection for fragile items, and can greatly assist in their safe shipment. The easy design method described here will allow you to create corner pads for double case packages without consulting cushion performance data or carrying out design calculations. Simply weigh the inner case (including its contents) and then refer to Table 1a (metric units) or Table 1b (imperial units) at the end of this document to choose the right corner pad size. Corner pad construction is then a matter of cutting the individual pad components and gluing them together.

Tables 1a and 1b are based on cushion performance data for polyurethane ester foam with a density of 33 kilograms per cubic metre (kg/m3) [2 pounds per cubic foot (pcf)]. Polyurethane ester foam can be obtained from a number of sources(see the section "Polyurethane Ester Foam") and has excellent shock and vibration isolation properties when used correctly.

Figure 1 shows an example of a double case package. Fragile items are first packed inside the inner case. The four lower corner pads are placed inside the shipping case followed by the inner case and the upper set of corner pads. The outer case cover is then closed and the package is ready for shipment.

Corner pads made using the method described here should be suitable for most cube-shaped or elongated cube forms. However, heavy items with a narrow dimension may require a different approach.

The Inner Case and Its Contents

Figure 2. Inner case construction detail.

Before selecting the corner pads for your application, check the following points to help ensure that your package will perform well.

1. Make sure that the inner case can sustain multiples of its weight without bending, straining, or deforming. In other words, the inner case needs to be reasonably stiff; however, because it's a cushioned item, it does not have to be exceptionally strong. Figure 2 shows an example of a suitable inner case made of ½-in. (12-mm) plywood, assembled with glue and screws without framing.
2. If multiple objects are to be packed in the inner case, make sure that each one is held firmly in place and that the objects will not collide with each other or the inner case walls. If possible, try to position the individual objects in a way that will distribute their weight evenly in the inner case so the corner pad cushions will deflect uniformly on impact.
3. If the object (or objects) to be packed in the inner case contains any loose or vibration-prone parts, gently (but not forcefully) restrain them. If you are using a transit mount, ensure that the object is held firmly on the mount. One way to achieve a firm hold is to increase the contact area between the object and the mount. This will also minimize the load per unit area on the object surface. If small projections are present, a small void can be carved in the mount around them.

Careful attention to points 2 and 3, combined with the use of properly designed polyurethane ester foam cushions, will result in a cushioning system that is effective for both shock and vibration isolation.

Corner Pad Selection/Construction

Cushion Performance

Assuming that polyurethane ester foam (density 33 kg/m3, 2.0 pcf) is used, a cushion thickness of 50 mm (2 in.) can limit shocks to 50g or less from a drop height of 75 cm (30 in.). This thickness should be satisfactory for many different object types in medium to large cases. However, smaller packages weighing 7 kg (15 lb.) or less, or packages containing extremely fragile items, should be fitted with thicker cushions for additional protection. This is especially important if the packages will be travelling by parcel post or courier networks. The weight ranges indicated in Tables 1a and 1b can also be used for thicker cushions as long as they are made of the same polyurethane ester foam (density 33 kg/m3, 2.0 pcf). Predicted cushion performance for a pad that is 75 mm (3 in.) thick is 40g or less from a drop height of 75 cm (30 in.); for a pad that is 100 mm (4 in.) thick it is 30g or less from a drop height of 75 cm (30 in.).

Package Performance

Shock isolation values are only a prediction. The actual performance of your package will depend on how everything works together. For best results, keep the following points in mind.

1. Movement of the inner case on its cushioning system is necessary for effective shock and vibration isolation. Therefore, make sure that the pads do not fit so tightly as to restrict movement. It should also be possible to assemble all of the package parts and close the cover without excessive force.
2. Although some movement is necessary, it is important to avoid excessive looseness. Note that the cushions at the bottom of the inner case (in the package's ordinary orientation) may lose about 10% of their thickness over time. This is normal and should be expected. The goal is a close but not tight fit on the top and bottom surfaces.
3. It is essential that the corner pads do not fall apart, so the individual components need to be securely bonded. Hot melt adhesive generally produces good results.
4. If the cushions are thicker than 50 mm (2 in.), ensure that the pad components are not taller than they are wide to avoid buckling (see Note 2 in the tables).
5. Make sure that nothing is placed inside the sway space (the space between the inner case and the inner surface of the outer shipping case, see Figure 1). Effective shock and vibration isolation requires that the cushioned inner case be able to deflect into this space.

Polyurethane Ester Foam

Polyurethane ester foam is an open-celled material that provides very good shock and vibration isolation when used correctly. It is reasonably stable; some material is still in good condition after 10+ years of storage. However, it will deteriorate with time (and very rapid deterioration has occurred in some lab environments) so it should not come into direct contact with artifacts. For best results, store the foam in a dark location or, even better, place it in black polyethylene wrapping or bags. Off-cuts or assembled corner pads should be stored in dark green or black garbage bags.

Working with polyurethane ester foam is easy. It can be cut with a band saw or an electric carving knife, and it bonds well with hot melt adhesive. For neat results, trim a small amount of foam from the assembled corner pad edges after the corner pads are glued together. The slight loss of material should not affect performance. Do not trim the large flat surfaces as this will change the pad thickness.

Polyurethane ester foam does not go by a particular brand name. One source of supply is manufacturers of road cases (laminated panels held together with riveted aluminum extrusions) for electronic equipment or other fragile items. These manufacturers can be found in most major cities and they may be able to provide sheets of foam or off-cuts. Another option is hardware companies that supply road case manufacturers with fabrication materials, including foam. One such company is Trans Continental Hardware (http://www.tchweb.com/cgi-bin/TCH2.storefront/), tel. 1-800-465-6281 in Canada and the United States, which supplies 2-in.-thick polyurethane ester foam in sheet form (2 in. x 54 in. x 72 in.) as "2 lb. ester foam, product number 509-4520905 gray". Firms that repair furniture and upholstered items may also be able to supply foam materials.

Basic Terminology

Acceleration units (g) and object fragility — Shock intensity is expressed in g, which are multiples of the force due to earth's gravity. An object that is subjected to a shock of 10g will "feel" a force that is 10 times its weight for a short period of time. Many small- to medium-sized objects can sustain this amount of force without damage. Small- to medium-sized greenware (unfired slip-moulded clay) items and delicate glassware can sustain up to 45g depending on their shape. Small oil-on-canvas paintings can sustain 65 to 90g (depending on their condition), and even higher forces when equipped with backing boards.

Foam specifications — Foam specifications start with the material type (e.g. polyurethane ester). Density is the weight of a given volume of the foam; it is expressed in kilograms per cubic metre (metric units) or pounds per cubic foot (imperial units). The final specification is thickness. Foam cushioning material is usually available in thicknesses of 25 mm (1 in.), 50 mm (2 in.), 75 mm (3 in.), and 100 mm (4 in.), and is supplied in sheets measuring 1371 mm x 1828 mm (54 in. x 72 in.). The 50-mm (2-in.) thickness is a good choice for general purpose use.

EXAMPLE

SOLUTION

1. Refer to the "Pad thickness" line near the top of the table to determine the required thickness for the cushion pad. In this case, a thickness of 50 mm (2 in.) will limit shocks to the required level (50g or less).
2. Search the "Weight range" column of the table to find the first line that corresponds to the weight of the crate to be cushioned. In this example, the crate weight of 17 kg (38 lb.) can be found in line 4, i.e. 5 to 18 kg (10 to 40 lb.).
3. Pad 1 dimensions are always independent of thickness, so move on to the "Pad 2 dimensions" and "Pad 3 dimensions" columns and look under the desired thickness heading — in this case 50 mm (2 in.) — to find the proper dimensions for the pads. In this example:
   Pad 1: 125 mm x 125 mm (5 in. x 5 in.)
   Pad 2: 175 mm x 175 mm (7 in. x 7 in.)
   Pad 3: 125 mm x 175 mm (5 in. x 7 in.)
   
4. Confirm that the pads will fit on the narrowest side of the crate to be cushioned with plenty of space between them. In this example, with 50 cm (20 in.) on all sides, the cushions will fit easily
5. Construct the pads by cutting eight Pad 1's, eight Pad 2's, and eight Pad 3's and assembling them as indicated with hot melt adhesive.

Weight Range (kg)	Pad 1 Dimensions (x,y mm)	Pad 2 Dimensions (x,y mm)			Pad 3 Dimensions (x,y mm)
Pad thickness ->	t= 50,75,100 mm	t= 50 mm	t= 75 mm	t= 100 mm	t= 50 mm	t= 75 mm	t= 100 m
1 to 3	50 x 50	100 x 100	-	-	50 x 100	-	-
2 to 7	75 x 75	125 x 125	150 x 150	-	75 x 125	75 x 150	-
3 to 12	100 x 100	150 x 150	175 x 177	200 x 200	100 x 150	100 x 175	100 x 200
5 to 18	125 x 125	175 x 175	200 x 200	225 x 225	125 x 175	125 x 200	125 x 225

Weight Range (lbs)	Pad 1 Dimensions (x,y in.)	Pad 2 Dimensions (x,y in.)			Pad 3 Dimensions (x,y in.)
Pad thickness ->	t= 1,2,3,4 in	t= 2 in	t= 3 in	t= 4 in	t= 2 in	t= 3 in	t= 4 in
2 to 6	2 x 2	4 x 4	-	-	2 x 4	-	-
4 to 14	3 x 3	5 x 5	6 x 6	-	3 x 5	3 x 6	-
6 to 26	4 x 4	6 x 6	7 x 7	8 x 8	4 x 6	4 x 7	4 x 8
10 to 40	5 x 5	7 x 7	8 x 8	9 x 9	5 x 7	5 x 8	5 x 9

TABLE 1a: Corner pads by inner case weight (Metric units)

Weight Range (kg)	Pad 1 Dimensions (x,y mm)	Pad 2 Dimensions (x,y mm)			Pad 3 Dimensions (x,y mm)
Pad thickness ->	t= 50,75,100 mm	t= 50 mm	t= 75 mm	t= 100 mm	t= 50 mm	t= 75 mm	t= 100 m
1 to 3	50 x 50	100 x 100	-	-	50 x 100	-	-
2 to 7	75 x 75	125 x 125	150 x 150	-	75 x 125	75 x 150	-
3 to 12	100 x 100	150 x 150	175 x 177	200 x 200	100 x 150	100 x 175	100 x 200
5 to 18	125 x 125	175 x 175	200 x 200	225 x 225	125 x 175	125 x 200	125 x 225
7 to 26	150 x 150	200 x 200	225 x 225	250 x 250	150 x 200	150 x 225	150 x 250
9 to 36	175 x 175	225 x 225	250 x 250	275 x 275	175 x 225	175 x 250	175 x 275
12 to 47	200 x 200	250 x 250	275 x 275	175 x 225	175 x 250	175 x 275	200 x 300
15 to 59	225 x 225	275 x 275	300 x 300	325 x 325	225 x 275	225 x 300	225 x 325
15 to 59	225 x 225	275 x 275	300 x 300	325 x 325	225 x 275	225 x 300	225 x 325
18 to 73	250 x 250	300 x 300	325 x 325	350 x 350	250 x 300	250 x 325	250 x 350
22 to 88	275 x 275	325 x 325	350 x 350	375 x 375	275 x 325	275 x 350	275 x 375
26 to 105	300 x 300	350 x 350	375 x 375	400 x 400	300 x 350	300 x 375	300 x 400
31 to 123	325 x 325	375 x 375	400 x 400	425 x 425	325 x 375	325 x 400	325 x 425
36 to 143	350 x 350	400 x 400	425 x 425	450 x 450	350 x 400	350 x 425	350 x 450
41 to 164	375 x 375	425 x 425	450 x 450	475 x 475	375 x 425	375 x 450	375 x 475

TABLE 1b: Corner pads by inner case weight (Imperial units)

Weight Range (lbs)	Pad 1 Dimensions (x,y in)	Pad 2 Dimensions (x,y in)			Pad 3 Dimensions (x,y in)
Pad thickness ->	t= 1, 2, 3, 4 in.	t= 2 in	t= 3 in	t= 4 in	t= 2 in	t= 3 in	t= 4 in
2 to 6	2 x 2	4 x 4	-	-	2 x 4	-	-
4 to 14	3 x 3	5 x 5	6 x 6	-	3 x 5	3 x 6	-
6 to 26	4 x 4	6 x 6	7 x 7	8 x 8	5 x 7	5 x 8	5 x 9
10 to 40	5 x 5	7 x 7	8 x 8	9 x 9	5 x 7	5 x 8	5 x 9
14 to 58	6 x 6	8 x 8	9 x 9	10 x 10	6 x 8	6 x 9	6 x 10
20 to 78	7 x 7	9 x 9	10 x 10	11 x 11	7 x 9	7 x 10	7 x 11
26 to 102	8 x 8	10 x 10	11 x 11	12 x 12	8 x 10	8 x 11	8 x 12
32 to 130	9 x 9	11 x 11	12 x 12	13 x 13	9 x 11	9 x 12	9 x 13
40 to 160	10 x 10	12 x 12	13 x 13	14 x 14	10 x 12	10 x 13	10 x 14
48 to 194	11 x 11	13 x 13	14 x 14	15 x 15	11 x 13	11 x 14	11 x 15
58 to 230	12 x 12	14 x 14	15 x 15	16 x 16	12 x 14	12 x 15	12 x 16
68 to 270	13 x 13	15 x 15	16 x 16	17 x 17	13 x 15	13 x 16	13 x 17
78 to 314	14 x 14	16 x 16	17 x 17	18 x 18	14 x 16	14 x 17	14 x 18
90 to 360	15 x 15	17 x 17	18 x 18	19 x 19	15 x 17	15 x 18	15 x 19