Sep 13, 2026Manufacturing & Processing
Internal and External Epoxy Powder Coating: Common Process Problems
Internal and external epoxy powder coating runs at tightly controlled temperatures, and most process problems trace back to preheating, curing behavio
Quick answer: Internal and external epoxy powder coating runs at tightly controlled temperatures, and most process problems trace back to preheating, curing behaviour and the way the spray gun and the pipe move relative to each other. PE powder preheats at up to about 300 C and EP powder at about 200 C, while both cure at about 200 C, so the coating method must match the powder chemistry.
Preheating and Curing Temperatures
Preheating and curing set the window for the whole coating line. PE powder coating allows a steel pipe preheating temperature of up to about 300 C. EP powder coating uses a steel pipe preheating temperature of about 200 C. The curing temperature for both EP and PE powders is about 200 C. In practice the preheating temperature varies slightly with the degree of powder tempering, so the setpoint is not a single fixed figure.
The Chemical Difference Between PE and EP Powder
PE powder undergoes a physical change during solidification and can be repeatedly heated, melted and solidified. EP powder undergoes a chemical change during curing, and that change is irreversible and cannot be repaired once curing has taken place. That single difference explains why the two powders cannot be treated as interchangeable.
Why PE Powder Coats Inside and Outside Together
Because of the high temperature involved, PE powder coated steel pipes can only be coated on the inside and outside simultaneously. The production line is therefore equipped with internal and external coating mechanisms, an internal coating button powder system, an internal coating push-pull rod spraying system, and fluidized bed immersion coating of PE powder in the preheating furnace and surface drying furnace. Several internal coating methods can be selected for different customer needs.
Where Coating Happens on the Line
The coating of steel pipes is carried out between the flat drying furnace and the bridge curing furnace on the production line. A connecting device between the two furnaces is a lifting and translating device that transfers the steel pipe from the flat drying furnace to the external coating position.
Small Diameter and Larger Diameter Methods
For coating method, DN15 to DN300 small-diameter steel pipes use pipe rotation, with clamping rotating seats at both ends. The electrostatic powder spray gun moves back and forth along the longitudinal direction of the pipe to complete the external coating.
DN15 to DN500 larger-diameter steel pipes use a non-rotating pipe, with an electrostatic powder spray gun installed in the external coating powder spray box that reciprocates along the longitudinal direction of the pipe.
The Pipe Mouth Quality Problem
Small-diameter pipe from DN15 to DN100 can be lightly sprayed by hand, and after solidification the appearance quality of the pipe mouth can meet the requirement.
For DN100 to DN500, if the pipe is clamped and rotated with a clamping seat, a powder rotating device inevitably forms at the coated pipe mouth and the appearance quality after solidification is poor. The recommended method is therefore a non-rotating pipe with a spray gun that swings longitudinally. Because the pipe is suspended directly on the bridge curing furnace, the coated pipe mouth has no clamping seat; after external spraying it enters the curing furnace directly and the mouth appearance is good.
The history matters here. Around two decades ago the non-rotating direct spraying method was dropped because the gun swing problem could not be solved, which left the powder coating non-uniform. Over the past decade most pipe coating has used clamp rotation with an electrostatic gun spraying along the pipe length, but this method still struggles with pipe mouth quality. A swing mechanism improves coating uniformity, because the pipe and gun can rotate and move longitudinally relative to each other, so powder is sprayed evenly.
Pipe mouth quality is where rotating-clamp coating repeatedly falls short; a non-rotating pipe with a longitudinally swinging gun sidesteps the clamping seat that causes the defect.
FAQ
What preheat temperature is used for PE and EP powder?
Up to about 300 C for PE and about 200 C for EP, with both curing near 200 C. The exact figure varies with the degree of powder tempering.
Why can epoxy coating not be repaired after curing?
EP powder cures by an irreversible chemical change, so a cured coating cannot be re-melted or corrected in the way that PE can.
Which diameter range uses pipe rotation?
DN15 to DN300 small-diameter pipe uses rotation with clamping rotating seats. The larger range is coated with a non-rotating pipe and a reciprocating gun.
Why is manual spraying used for DN15 to DN100?
A light manual spray followed by solidification can give a pipe mouth appearance that meets the requirement.
Send your pipe diameter range, coating requirement, standard and destination port to the SPAI STEEL team for a confirmed recommendation.
