You can't judge a book, or a hollow metal door, by its outside appearance. The core within the door gives it unique characteristics that may set it apart from another door that appears identical on the surface. Hollow metal doors can be ordered with many different types of cores, each with features that may be seen as advantages or disadvantages, depending on the application. Successful application of hollow metal doors depends on choosing the right type of core, made of the right materials, properly constructed, finished and installed within the door by the manufacturer.

The right core adds the optimum combination of desired properties to a hollow metal door. These may include such qualities as structural integrity and rigidity, sound deadening, fire protection (20 min. to 3 hr.), insulating properties ("U" value or "R" factor), thermal control (temperature rise), and flatness or shape.

Sorting Out the Cores

While various combinations of doors and different types of cores are available, these are some of the most prevalent core types on the market today.

Honeycomb was developed as a structural core for military aircraft wings during World War II and first used in standard metal doors by Steelcraft in 1957. Among its advantages are a high strength-to-weight ratio, uniform crushing strength, high shear strength, and excellent impact resistance. It is durable, can be treated to resist decay and insects, and also provides sound deadening and insulating properties. The rigid honeycomb structure is integrated with the door to form hundreds of small I-beams with the door, with a uniform thickness and flat surface that makes it easy to add lites, louvers or other features. It reinforces the full width and height of the door.

To make the honeycomb (Figure 1), a heavy kraft paper is bonded together into hexagonal cells. The core is impregnated with enough phenolic resin to resist moisture, mildew and vermin without creating brittleness. It must also be sanded to the proper thickness to keep the door flat and create a broader base for the contact adhesives used to bond it to the inside of the door panels. Although they cannot be seen once the door is assembled, all honeycombs are not created equal. For example, cell sizes may vary from one manufacturer to another, although they typically range from _" to about 1" in size. Honeycomb-core doors may be used in exterior or interior applications.

Polystyrene slab core is viewed primarily for its insulation value, rather than its structural value. It is used mainly for exterior applications or where a greater temperature differential from one side of the door to the other is needed. Its lower service temperature (approximately 165 F) may prevent a manufacturer with a baked-on enamel paint system from applying a factory finish, since the elevated temperature during the drying (baking) operation could melt the core.

Polyurethane is also mainly an insulator. It is used where extreme protection from frigid cold is a priority. It offers the lowest "U" value (approximately 0.09) and the highest "R" factor (approximately 11.1) of all the cores. It also has a relatively low service temperature, which may prevent factory painting. This core is usually not found as a fire-rated product.

Steel Stiffened doors are used mainly for exterior doors, where rigidity is important. They are available in varying degrees of strength and quality. While the thickness of the stiffeners can vary, the majority are made of 20 gage steel. Heavier gages sometimes are used, particularly on security doors. Spacing between stiffeners may vary from 4" to 6". They are usually welded to each other at the top and bottom, and to the inside door skins on 4" to 5" centers. The cavities in between the stiffeners are usually filled with fiberglass insulation.

Temperature Rise doors are constructed with additional insulation and other features to minimize their heat transfer characteristics so they will protect occupants on the other side of the door for a specified period of time during a fire. They include a special core that is similar to gypsum board. As a result, such doors can be considerably heavier than standard doors and may require heavy-duty hardware. Typical uses are in stairwells of high- rise buildings if designated by building or fire codes. Ratings are based on a half-hour of exposure and specify the surface temperature in excess of ambient temperature on the non-fire side of the door. From best to worst, they are 250 F, 450 F and 650 F. By contrast, a standard core would reach approximately 1400 F in the same time period.

Other Core Characteristics

Cores affect several characteristics of doors, including weight and fire resistance. These in turn impact the selection process.

Weight of a door is a combination of the gage of metal used to fabricate it, the type of construction, and the core material. Heavier doors may need stronger hinges and other hardware to provide their full service life. Based on a standard 18 gage, 1-3/4-in thick 3070 door, the following table shows comparative weights of doors with various types of cores:

Core Material     Approximate Door Weight
Polystyrene     97 lbs.
Honeycomb     97 lbs.
Polyurethane     97 lbs.
Steel-stiffened     126 lbs.
Temperature rise     136 lbs.

Positive Pressure compliance is playing a growing part in fire door selection, as new codes incorporating this requirement are being adopted in a growing number of jurisdictions. In the positive pressure test, the neutral pressure plane is lowered to a standard 40" from the floor, which can cause smoke, hot gases and potential flames to be pushed out around the perimeter of the door assembly. Manufacturers certify that their specific door constructions pass the positive pressure requirements, and these are typically available in honeycomb-core, steel-stiffened, or polystyrene doors. Generally, doors with polyurethane cores do not pass this test.

Core Problems and Solutions

Thermal Bow occurs mainly with polystyrene and polyurethane cores when the ambient temperature differs greatly from one side of the door to the other. This may occur in exterior doors located in hot climates. When air conditioning creates a large temperature differential, the core may swell, causing the door to bow toward the sunlight. As a result, the lock can bind in the strike and fail to latch properly. Adjustments to the strike, latch or weatherstripping in the field are a temporary and inadequate solution. Honeycomb-core and steel-stiffened doors are the least susceptible to thermal bow. Using a lighter color finish also may help prevent thermal bow.

Weld Marks may sometimes show on steel-stiffened doors, because manufacturers generally do not dress the welds before finish painting. These marks will be more apparent with high gloss finishes, so a paint with lower gloss should be selected to minimize this effect. Although some doors may require a galvanized zinc coating such as A-60 for rust protection, caution should be exercised with steel-stiffened doors that require greater protection for highly corrosive applications. Many manufacturers will not produce steel-stiffened doors from the G-90 zinc-coated material normally used under these conditions because the welding can cause crater marks on the surface of the door.

Insulation Effects may include settling and condensation. The mineral wool used to insulate some doors may settle, causing uneven insulation. Other types of insulating cores, such as polystyrene, polyurethane, or foamed in place maintain uniform insulating properties because they do not settle. In cold climates, condensation may collect on the skins of exterior doors, highlighting the locations of the stiffening ribs inside. While some types of insulation may minimize this tendency, it may not be possible to eliminate it completely in extreme climates.

"Oilcanning" may sometimes occur on steel-stiffened doors if the panels bow, causing them to look wavy.

A Word About Tests & Standards

As with many types of architectural products, there are various standards and tests that demonstrate compliance to them. For steel doors, ANSI 250.4 (formerly ANSI 151.1) test standards are widely applied to steel doors and are incorporated in several door industry specifications. For reference, here are some of the specifications that can be reviewed for further information when specifying steel doors:

SDI 100: "Recommended Specifications for Standard Steel Doors & Frames" (Steel Door Institute)

NAAMM 860-92: "Guide Specifications for Hollow Metal Doors & Frames" (National Association of Architectural Metal Manufacturers)

NAAMM 861-92: "Guide Specifications for Commercial Hollow Metal Doors & Frames"

When someone specifies a door, they expect it will have certain qualities and perform certain functions. It needs to be straight, to fit the opening properly, to have certain characteristics of strength and durability. It may have to provide sound deadening, insulating or fire-resistant qualities as well. However, whether using SDI specifications or NAAMM specifications, there is really only one physical endurance test standard that deals with the structural integrity of metal doors, other than fire and paint durability. That test standard is ANSI 250.4.

Among the tests performed on steel doors, cycle tests and twist or deflection tests are the most common. Most tests use a standard 3070 door. To meet ANSI 250.4, which is based on SDI 100 standards, a 20 gage door must withstand 250,000 cycles; an 18 gage door must withstand 500,000 cycles; and 16 gage or 14 gage doors must withstand 1 million cycles.

In the twist test (Figure 2), a door is clamped at three corners, while an increasing force is applied to the fourth corner in 30 lb. increments until the door fails or a maximum of 300 lbs. is reached. To pass, a 16 gage or 14 gage door cannot push more than 1-1/4" out of the opening, while an 18 gage or 20 gage door must not exceed 2-1/2". The remaining portion of the door that extends out of the opening after the pressure s removed is known as residual deflection. It may not exceed 1/8" for doors of all gages.

Test results sometimes bring out surprising facts. For instance, many architects specify doors with 20 gage steel stiffeners where they feel greater strength is needed due to heavy usage or possible abuse. In most cases, however, a honeycomb-core door is actually stronger, as shown in the table below:

Door Construction     Deflection Test Results
Steel Stiffened     9/32" to 29/64"
Honeycomb (3/4" cells)     19/64" to 21/64"

In addition to the ANSI 250.4 tests, some manufacturers perform other tests to verify the strength or durability of their doors and cores. Here are a few examples that have been performed on honeycomb doors:

Compression tests (Figure 3) apply pressure to the face of a door or test panel until failure. When one manufacturer subjected several 1-3/4" thick 12" x 12" honeycomb panels with 18 gage cold rolled steel face sheets to a uniform pressure until the honeycomb core failed, the average ultimate load was 5126 lbs. per sq. ft. The weight of one sq. ft. of the honeycomb core was only 3 oz.

Shear tests (Figure 4) are applied to a specimen made up of two honeycomb cores laminated to three steel plates in an offset condition. In one series of tests, a force of 1147 lbs. Per sq.ft. was applied before the honeycomb core failed.

Manufacturers may perform other tests as well. In a security test, per ASTM 476, a door is mounted in a frame and latched, then impacted with a 100 lb. ram at mid-position. The door must survive three impacts per rating while remaining in the opening, and the weight is increased at each rating level.

While there may be more to cores than meets the eye, manufacturers have a wealth of information available on tests and applications. To obtain the best service from a hollow metal door, be sure to take advantage of all the information available to help you make the best choice of door construction and core materials.