The machine fit in January.
Holes lined up. Covers closed. The slides ran free. The crew in North Dakota signed off, loaded the crate, and sent it west.
By July, it sat on a pad in Nevada, and nothing felt quite the same. A door that used to latch now needed a shoulder. A hole pattern that had been dead-on was off by enough to fight the bolts. Nobody had changed the drawing. The sun had.
That is thermal growth. Most engineers who already design around it know the story. A lot of first-time composite users do not — until a long metal frame leaves a cool shop and meets a hot climate.
Metals grow when they heat up. Pultruded composites usually grow less, especially along the fiber direction. On a 6-inch coupon, the difference is easy to ignore. On a 20-foot machine, it shows up as parts that no longer fit.
That is the conversation Tencom hears from some OEM and equipment customers. They are not only asking for a fiberglass part. They are asking whether a composite member can hold dimension while the metal around it moves — or whether a composite element can be used to manage that movement.
Coefficient of thermal expansion (CTE) is the change in length per unit length per degree.
Typical order of magnitude:
Work a simple case. A 20-foot member (240 inches) sees a 50°F rise — from shop temperature to a hot outdoor skin.
Approximate growth:
Sixteen hundredths of an inch does not sound like much until it shows up as a hole pattern that no longer matches, a cover that will not close, or a shaft that binds on one end of the machine.
Nevada sun on dark metal can be a larger swing than 50°F. A North Dakota winter-to-summer cycle is larger still. The point is not the exact number. The point is that length × ΔT × CTE is a real stack-up, and aluminum pays the biggest tax.
Pultrusion packs continuous fiber in the long direction. Those fibers restrain the resin, so the profile does not grow the way an unreinforced plastic or an aluminum extrusion does.
Glass fiber already keeps axial expansion in a useful range for outdoor equipment. Carbon fiber goes further. Carbon’s own CTE along the fiber is extremely low, sometimes slightly negative, so a carbon pultrusion can hold length through temperature swings that would walk an aluminum rail out of alignment.
That does not make carbon the default. It costs more, and you specify it when the application needs near-zero movement—long precision members, mixed frames that cannot tolerate aluminum growth, or parts that have to stay put while adjacent metal expands. For many industrial jobs, glass FRP is the practical step down from aluminum. Carbon is the option when “almost no change” is the requirement.
Two caveats still apply:
For outdoor machines, platforms, covers, and long reinforcements, the usual win is simple: less growth than aluminum, less corrosion than steel, less weight than both. Carbon adds the extra card: length that barely moves.
Some inquiries are not “replace this aluminum bar.” They are “the aluminum grows and we need a member that does not.”
That shows up as:
A pultruded glass or carbon bar, tube, or custom shape can stay closer to the as-built length while adjacent metal grows. It can also be used as a low-expansion reference or tie in a mixed frame — if the connections are designed for the difference.
Connections still need thought. If FRP is bolted tight to a long aluminum rail with no slot or spring, the aluminum will try to drag the composite with it. The composite’s lower expansion is only useful if the joint can do its job.
Do not bury a tight length tolerance on a 20-foot metal weldment and hope the field install matches the shop. Call out:
Tencom can hold tight profile tolerances — often ±0.005" on critical cross-section features — but length change in service is a material property, not a machining error. Design for the swing instead of inspecting it away.
If a machine, frame, or long reinforcement will be built in one climate and live in another, send those items. That is enough to decide whether a custom pultruded member is a substitute — or the part that keeps the rest of the assembly from walking out of alignment.
Does fiberglass expand less than metal with temperature?
Along the fiber direction, pultruded glass FRP typically expands less than aluminum and in a similar range to steel. Transverse expansion is higher than axial.
Does carbon fiber pultrusion change length with temperature?
Axial carbon pultrusions can have near-zero CTE, and some constructions are slightly negative. For many long parts, that is effectively almost no change across normal temperature swings.
How much can a 20-foot aluminum part grow in the sun?
With about a 50°F rise, a 20-foot aluminum member grows about 0.16 inches. Steel is roughly half that. Exact movement depends on alloy and actual temperature change.
Can pultruded FRP offset metal expansion?
Glass or carbon pultrusions can stay closer to as-built length than aluminum. Joints still need slots or other accommodation so the metal doesn't force the composite to move with it.