EXPANDED POLYSTYRENE
( E.P.S. ) HANDBOOK
TEXAS FOAM INC.
1278 Highway 71 West
Bastrop, TX 78602
Phone: 512.581.7500
Fax: 512.581.7520
www.texasfoam.com
BASF
WYANDOTTE
CORPORATION
___________________________
Marketing Headquarters
100 Cherry Hill Rd.
Parsippany, New Jersey 07054
(201) 263-0200
•
Technical Center
Cranbury & South River Rd.
Jamesburg, New Jersey 08831
(201) 521-1600
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Styropor Protective Packaging
SECTION I
— PHYSICAL PROPERTIES —
Introduction
Styropor protective packaging offers a broad range of physical properties to the designer
and user. Described below are the characteristics of Styropor directly related to material
selection, package design and limitations of use.
Note — Within the context of this manual, Styropor refers to molded packaging
manufactured from Styropor expandable polystyrene.
Mechanical Properties
The mechanical properties of Styropor depend largely upon density; in general, strength
characteristics increase with density as illustrated in Table 1. However, such variables as
the grade of raw material used, geometry of the molded part and processing conditions
will effect package properties and performance. Therefore, Table 1 properties represent
typical values; variables inherent to any given test specimen may result in properties
10-15% from listed values.
It should be noted that compressive strengths listed in Table 1 are not ultimate values at
either a yield point or failure. Rather, because Styropor yields under compressive loads, as
illustrated in the typical stress/strain curves of Figure 1 below, Table 1 compressive strength
values are listed at 10% deformation, a level often considered to be a minimum value for
energy absorption under impact loadings. Maximum energy absorption in packaging
applications is reached at 55-65% deformation.
Compressive tests, such as those carried out in accordance with ASTM Standard D1621,
reflect strain or deformation under short term, increasing load conditions. If Styropor is
exposed to long term compressive or flexural loading conditions, deformation may increase
in the form of creep or cold flow. Compressive creep characteristics, with variables of
density, load and duration, are shown in Figures 2a, 2b and 2c.
In addition to material and processing variables previously listed, exposures below the
maximum recommended use temperatures of 165°F will marginally affect mechanical
properties. Figure 3 below relates exposure temperature to deformation under compressive
loading.
Thermal Insulation
Styropor is an effective, economical packaging medium for foodstuffs, pharmaceuticals and
other perishables which must be shipped and stored under thermally controlled environments.
Highly resistant to heat flow, its uniform, closed cellular structure limits radiant,
convective and conductive heat transfer.
The thermal conductivity (k factor) of Styropor varies with density and exposure temperature,
as shown in Table 2. Further, as with all thermal insulation materials, moisture within
the cellular structure will reduce thermal efficiency. For example, the k factor of Styropor
will increase approximately 3% for each 1% increase in moisture by volume.
Other Thermal Properties
Polystyrene is a thermoplastic material; therefore, high temperature exposure will cause
excessive deformation. Although use limitations will vary with load conditions as previously
illustrated in Fig. 3, it is recommended that intermittent exposures be limited to a maximum
of 180*F and long term exposure to 165' F.
The thermal coefficient of linear expansion of expanded Styropor is 3.5 x
is marginally dependent on density.
in/in/°F and
Water Absorption and Transmission
The cellular structure of Styropor is essentially impermeable to water and provides zero
capillarity. However, due to fine interstitial channels within the bead-like structure, moisture
may be absorbed under total immersion.
Although Styropor is relatively impermeable to liquid water, it is moderately permeable to
vapors under pressure differentials.
In general, neither water nor water vapor affects the mechanical properties of Styropor.
Electrical Properties
The low surface resistivity of Styropor permits the build-up of electrostatic charges on
moldings and may lead to excessive dust attraction. Water soluble anti-electrostatic agents
such as DuPont Zelac can alleviate this problem. The adhesion of the anti-static agent can be
increased by adding a polymer dispersion to the solution.
In the past, molders have also successfully used such simple household detergents as Joy,
etc., in the ratio of roughly one quart per 50 gallons of water for dip applications. While
these anti-static agents may not conform to certain military packaging specifications, the
results have generally been acceptable.
Chemical Properties
Packaging materials may be required to protect contents from chemical exposures. Although
Styropor is not affected by most common chemical solutions, most organic solvents will
have a harmful effect. The resistance of Styropor to various classes of chemicals and solvents
is summarized below:
All substances of unknown composition should be tested for compatibility before use in
contact with Styropor. Accelerated tests may be carried out by exposing Styropor to the
substance at 120-140°F.
UV radiation has a slight effect on expanded Styropor, causing superficial yellowing and
friability, but otherwise not affecting physical properties.
Biological Properties
Styropor has no nutritive value for any known organism and does not provide a breeding
ground for fungi, bacteria or insects. However, it is not resistant to penetration by rodents
and insects.
SECTION II
PACKAGING DESIGN GUIDELINES
A preliminary list of detailed requirements is often necessary before the design of. a package
can be determined:
1) Dimensions and weight of the merchandise.
2) Shock sensitivity of merchandise.
3) Must the package be ventilated or leakproof?
4) Must the package offer thermal resistance; how much?
5) Maximum drop height.
6) Maximum stacking height of packages, etc.
Once these and other requirements are known, preliminary designs can be tested. The final
choice of design will ultimately depend on the functional requirements, final price per unit
and moldability of the part. Prototypes will usually be constructed and tested with Styropor
parts being fabricated from block or boardstock.
Fragile Articles
Fragile articles such as glassware require a rigid pack to prevent excessive movement of the
articles as well as to protect from external pressure forces.
Rigid articles that are sensitive to both shock and external pressure can be supported in a
container by the use of molded Styropor inserts (Fig. 4 & 5). The inserts together with the
outer container form a type of suspension pack.
Shock sensitive articles with moderately durable housings, such as radios and television
sets, can usually resist moderate pressures and require only cushioning. Cushioning against
shock can be provided by molded Styropor containers with ribbing, or by molded Styropor
inserts. Cushioning design is covered in a later section.
Designing for Strength
Molded Styropor containers must be adequately dimensioned for strength but can be reinforced
by the use of ribbing, enlarged sections, etc. Interior intersections of surfaces should
be well rounded. Large radii and gradual transitions are essential in highly stressed regions of
containers. All internal radii should be as large as possible to reduce stress concentration
with a minimum internal radius of 3/8". Abrupt changes in profile should be avoided.
Longitudinal or diagonal ribs may be used to stiffen containers (Fig. 6). Often these ribs
are approximately semi-circular in cross-section: Interior or exterior cross-ribbing can
also be an effective method of package reinforcement.
If dropped on edge, heavy articles can deliver a severe blow to the side of a container
(Fig. 7). The article may well break through the end of the container or cause rupture at
the neighboring edge. Reinforcing the sections near the container edges both strengthens
the container in the regions of maximum stress and cushions the fall.
If a package is dropped flat, the goods are most likely to be damaged; if a package is dropped
on a corner or edge, the container is more likely to be damaged. The edges and corners of
molded Styropor containers should therefore be strengthened and any ribs in the vicinity
should be strategically located to avoid rib fracture.
SECTION III
COMPRESSIVE STRESS - STACKING OF BOXES
Styropor protective packaging may be subject to severe compressive loads when stacked.
In practice, the compression resistance of Styropor may be used as a guide, but final design
features must be worked out empirically.
The compressive load that can be tolerated by Styropor packaging depends upon several
factors: density and grade of raw material, molding conditions, design geometry, and the
length of time over which the load is applied. Therefore, it is recommended that design
values not exceed two-thirds of the laboratory determined values. Typical maximum design
values are given below:
These values represent the stress that molded Styropor can tolerate under direct compression
without excessive deformation, and are used for design calculations.
Wall Thickness Design
If the walls of a container are excessively high in relation to their thickness, they may bend
or fail under compressive loads. An approximation of the maximum permissible ratio of
wall thickness to height can be derived from the theory of loaded columns.
Sections subjected to compressive loads may be classified as either compression blocks,
short columns or long columns. If a section behaves as a compression block it will compress
under load, and any bending is considered negligible. A long column is the other extreme —
it bends under load and any true compression is negligible. If the stress on a long column
exceeds a certain value, it will fail by buckling.
The behavior of short columns falls between that of blocks and long columns — the maximum
permissible stress is less than that of a block because it will bend, but failure due
to buckling under a given load is less likely than with long columns. When designing Styropor
packaging, all sections to be subjected to compressive stress should be designed to
perform as short columns.
The transition from a short to a long column occurs when the "slenderness ratio" H/R
(R = least radius of gyration of the cross-section, H = height of section) exceeds a certain
value, dependent upon the strength of the material. For molded Styropor this value is
about 30-45 and in the case of a rectangular Styropor wall, the corresponding minimum
permissible ratio of wall thickness to height (T/H), in order to avoid "long column"
behavior, is 0.08.
In general, boxes more than 6" in height may be subject to wall buckling. It is therefore
recommended that if a box is to be 6" or greater in height, the wall thickness T should
be such that
T > 0.08 H
where H equals the box height.
A basic design problem follows:
Transmission of Stress
Molded Styropor packages intended for stacking must be designed so that stresses are
transmitted properly from box to box.
The base of a stacked box should not be significantly smaller than the top as this reduces
the effective wall thickness in relation to vertical stresses (Fig. 8). The base should rest on
the entire rim of the box below. Figure 9 shows an ideal stacking arrangement, which may
not be easily attained due to mold draft. If a tongue-and-groove arrangement is necessary,
the groove should be in a package base, and not in the rim, as shown in Figs. 10 and 11.
The longer sides of boxes tend to bend outwards under stacking loads. This tendency can
be reduced by gradually reducing their height towards the center (Fig. 12); this transfers
much of the load to the corners which will compress without bending. The height reduction
should not exceed 0.1 inch.
Tongue-and-groove arrangements can also prevent the sides of packages from bulging. In the
event it is necessary to place a ridge along the package base, the arrangement shown in
Fig. 13 is more suitable than in Fig. 14, since the projection from the base of the lower
box is not excessively loaded.
Stresses During Transportation
Stacked packages are subjected to dynamic stresses during transportation. These stresses may
have both horizontal and vertical components and are superimposed on the normal static
stresses. Test procedures such as those developed by the National Safe Transit Association
may be used to determine package performance during simulated transportation conditions.
Package failure due to splitting is likely to occur as the result of crack initiation in regions of
stress concentration. Splitting can be reduced by avoiding abrupt concave sections of the
package. In most boxes internal radii should be 3/8" or greater to avoid corner failure and
it is often beneficial to increase the wall diameter near corners as shown in Fig. 15.
Cut-outs in package walls are always a source of weakness, even when the internal radii are
as large as possible. Therefore, the handhold arrangement in Fig. 16 can be improved as
shown in Fig. 17.
Effect of Openings
Certain packaging applications require openings in the sides or bottom for ventilation or
drainage. Openings in the base of a box have little effect on strength if the total area of
the openings is less than 3% of the base area.
Openings in the sides of boxes can be molded as shown in Fig. 18. Again, there is minor
strength reduction unless the opening area is excessive.
Methods of Reinforcement
Moderately flat boxes holding less than 20 lbs. of merchandise seldom need reinforcement if
corners are properly rounded and some type of tongue-and-groove arrangement is used.
However, high boxes and boxes containing over 20 lbs. of material may require more
sophisticated design to enhance strength without excessive material usage.
One useful method of reducing package weight without sacrificing wall strength is shown in
Fig. 19. A method of increasing wall strength with minimal gain in container weight is
illustrated in Fig. 20.
Material properties can also be a factor in package design. Product molded from large beads
will flex further without failure than product molded from finer Styropor grades; molding
at higher steam temperatures may also increase flexibility.
SECTION IV
CUSHIONING WITH STYROPOR
During transportation and handling, merchandise is often subjected to a variety of jolts and
drops and must be adequately protected from damage. The amount of cushioning protection
required in package design is largely determined by the value of the packaged item as well as
its fragility or "G-Factor". An item's G-Factor is a measure of its ability to withstand impact
deceleration, based on one "G" equaling the acceleration due to gravity i.e. 32.2 ft/sec. 2 .
The higher the G-Factor of an item, the more deceleration shock can be withstood without
damage. Table 4 can be used as a guide if the G-value of an item to be packaged is not known.
The force acting on a package of mass m that is brought to rest in a distance D after falling
through a height H, is equal to G times its own weight. Since G = H/D, the G value (and
shock force) to which the package will be subjected can be reduced by increasing the
deceleration distance D. Cushioning material placed between the packaged item and the
impact surface will increase this deceleration distance and protect the item by absorbing
shock force through forced dissipation of air from the porous cushioning material. High
energy absorption may be obtained when a cushioning material is irreversibly deformed;
however this provides a "once-only" absorption of shock (i.e., corrugated cardboard inserts;
automobile "crush zones").
Styropor protective packaging efficiently absorbs shock forces and resists irreversible
deformation when compressed to roughly 50-60% of its original thickness. The compressive
stresses listed in Section III provide for this optimum compression. It should also be noted
that oversizing the thickness of cushioning material is as ineffectual as undersizing — excessively thick cushioning will not properly "give" during impact and can transmit abrupt
shock and excessive "G's" to the falling item. On the other hand, insufficient cushion thickness
may completely compress and rupture during impact, also resulting in high shock force.
Cushioning design is also dependent upon the maximum height that a package may be
expected to fall. Obviously, as the drop height of a package increases, more cushioning
is required to protect the packaged goods from impact damage. It is often difficult to
predict the exact height a particular package will fall throughout its usable life. However,
general package weight/drop height relationships have been developed through experience
and are shown in Table 5.
Cushioning Curves for Expanded Styropor
Cushioning curves can be used to derive the effective area and thickness of cushioning
material required for a particular packaging application. Cushioning curves give the G-Factor
provided by the molded Styropor as a function of the static load F, for given values of drop
height (h), thickness of the Styropor (T) and the Styropor density (p). Cushioning curves
for Styropor are shown in Figs. 21-23. These curves are based upon "second-drop" test
data, and represent realistic cushion design values.
If the highest value of G that can be tolerated is G max, the thickness T and cushioning
area A of the molded Styropor must be such that for a given drop height h, the impact load
factor is less than G max. In practice, of course, the thickness should generally be as low
as possible for economic reasons.
The following example illustrates the use of Styropor cushioning curves for the determination
of the appropriate values of Styropor thickness and area for a particular cushioning
application:
Example 2
Using the data given in Example 1, we wish to increase the Styropor thickness by 20%:
T = 1.8 x 120% = 2.2 inches
h/T = 36/2.2 = 16
From cushioning curve A, the curve for h/T equal to 16 has a minimum G value of about 40
and G is below 50 for all values of F between 0.75 psi and 2 psi.
Since the necessary cushioning area = m/F, the acceptable cushioning area range is 22-59 in2 ,
and Styropor requirements are now 48 in3 to 130 in3 depending upon the cushioning area
used.
Similar cushioning thickness calculations may also be made using the BASF cushioning
calculator, which can be found inside the front cover of this binder. Knowing the approximate
&value and weight of the merchandise to be protected, and the maximum allowable
drop height, the calculator can be used to determine the cushioning thickness and area
necessary to protect an item with Styropor protective packaging.
The BASF calculator will be used to solve the following cushioning problem:
Rib Cushion Design
Very often the packaged merchandise has a total surface area that is considerably larger than
the cushion area required for impact protection. Overcushioning must be avoided — the
Styropor cushion will not compress upon impact and the packaged goods will be subject to
rapid deceleration and high G's. In these cases, ribbed or knob-shaped elements are used
to reduce the contact area. However, since dimension calculations and cushioning design
are based on drop tests with square-shaped items, the data provided by these tests have
to be corrected for ribbed cushions by increasing cushion thickness "d" by 10%. The
calculated total cushioning area F is shown in Fig. 27 as the shaded cushioning areas of
all the ribs provided for a definite impact direction and situated at half the rib height H.
Further, the following guidelines and the recommendations below should be followed
to achieve proper rib design.
Guidelines for the Design of Ribbed Cushions
1. Establish which section of the packaged item is least vulnerable to the force resulting
from the impact retardation (weight x G = impact force).
2. The supportable area of the article to be packed should be approximately double that
of the cushioning area of the corresponding outer ribs. Otherwise, the article will embed
itself into the sidewalls upon impact, without the cushioning benefit of rib compression.
3. Should the item to be packed not offer a sufficiently large support area, or if certain
parts of the item can accept virtually no impact force without breaking (i.e., thin-walled
coverings or sensitive individual parts), load distributing inserts should be provided
(Fig. 24).
4. In the case of an eccentric position of the center of gravity, the total cushion area is
determined and then distributed to the supporting areas in proportion with the weight
distribution profile.
5. Particular care must be taken with internal cushion supports to see that free deformation
is possible (Fig. 25).
6. The radii at the base of ribs must be at least 3/8 inches.
Calculation and Design
Therefore, the areas that actually cushion the article on impact must be reduced by contouring
either the inner or outer faces of the package. The usual procedure with molded
cushioning units is to form ribs or bosses on the outside of the package since the fit around
the packed article is then unaffected by any compression under dynamic loads. Ribs generally
run the length of a side of the package, while bosses are individually placed triangular or
knob-shaped protrusions.
For cushions with ribs or bosses in the form of truncated cones or pyramids, as shown in
Fig. 27, the effective cushioning area A is the total area in a plane cutting the ribs at exactly
one-half their height. Further, rib height H is generally designed to be between 50%-65% of
the total cushioning height required.
In this example the tape recorder package will be cushioned with 8 bosses on both the base
and the lid, and 3 ribs per sidewall, as shown in Fig. 26.
The complete rib design calculation is carried out as follows:
Bottom & Lid Design — Column A
Lines 1-4
Data supplied by manufacturer or from Tables 1 & 2.
Lines 5-6
Calculated above.
Line 7
Arbitrarily chosen.
Line 8
Cushion area for each boss is simply the total cushion area required divided
by the number of ribs or bosses chosen.
Line 9
As mentioned above, the thickness of a rib or boss cushion dR is taken as
10% greater than the cushion thickness T calculated from the appropriate
cushioning curve.
Line 10
The height of each boss or rib is 50-65% of the rib cushion thickness d R . !n
this case,
Line
Line
Line
Line
11
12
13
14-15
The effective cushioning area F is calculated at one-half the rib height.
Chosen such that rib length (I) times width (b) = rib cushion area.
As shown.
Check to assure that rib head (b') is greater than or equal to H/3. If not,
the distribution of the total cushion area must be changed.
Side Wall Design — Column B
Utilizing 3 ribs per sidewall, the calculation is made in a similar manner.
SECTION V
THERMAL PROTECTION WITH STYROPOR PACKAGING
Styropor protective packaging can provide the added benefit of thermal insulation. Articles
packed in enclosed Styropor containers may be maintained for long periods of time at
temperatures above or below ambient conditions, and can also be protected from sudden
temperature fluctuations.
The ability of a material to conduct heat is measured by its thermal conductivity, or k-factor.
A material with a low k-factor does not conduct appreciable quantities of heat and can be
effectively used as a thermal insulation material. Depending upon the density of the Styropor
and exposure temperatures, typical Styropor k-values range from approximately 0.19-0.26
. The "R" value of a product is simply the reciprocal of the k-value and
measures the products' ability to resist heat flow. The higher the R value the greater the
resistance to heat flow. Typical Styropor k and R/in values are shown in Table 6. For
comparative purposes, Table 7 lists the R/in of several other materials.
Rate of Temperature Change
The flow of heat into or out of a container will eventually change the temperature of its
contents, in turn causing the temperature differential across the walls of the container
to steadily decrease. Since the rate of heat flow Q is proportional to the temperature
differential, it also decreases.
If the heat capacity of the packaged article is C and the heat capacity of the container is
negligible, the following relationship holds true:
____________________________________________________________________
Heat Flow Calculations
Although this example is purely hypothetical, it illustrates the use of the basic heat flow
equations.
Example 3
Given:
Same data as in previous example, but thickness of Styropor is increased to 2".