The polyurethane foam insulation layer of the pipe body is molded in a factory setting, whereas the foam at the joints is poured on-site. A common question arises: are the foaming materials used on-site the same as those used in the factory?
In terms of the material itself, the foaming material used for joints and the material used for the pipe body should be chemically identical. Standards require that on-site foaming achieve performance and insulation effects equivalent to those of the straight pipe sections; thus, the joint foaming material must match the base pipe's foaming material. In other words, core indicators such as foam density, closed-cell content, and thermal conductivity should be comparable between the joint foam and the pipe body foam.
However, in actual production, the foaming conditions at the joints differ significantly from those in the factory. Factory foaming takes place on automated production lines where foaming temperature, raw material ratios, and injection pressure can be precisely controlled. The liquid polyurethane is injected in a single step into the annular space between the steel pipe and the outer casing at pressures of 10–20 MPa, ensuring the foam is uniform and dense. Before leaving the factory, the product undergoes multiple tests-including density, thermal conductivity, and closed-cell content-and is not shipped unless it meets specifications.
On-site joint foaming is a completely different matter. The jointing process takes place in a pipe trench, where ambient temperature, air humidity, and the cleanliness of the base surface all affect foaming quality. Excessively low temperatures reduce foam adhesion, while high humidity also compromises the foaming result. Manual on-site foaming is limited by human operation, making it difficult to match the material distribution uniformity achieved by machines. Furthermore, while factory-foamed pipes can be individually inspected, on-site foaming lacks systematic inspection methods, meaning quality issues often only come to light after the pipeline is in operation.
Therefore, while the formulations for joint foaming material and pipe body foaming material should be identical, the quality control challenges for on-site foaming are far greater than those in the factory. This is why pipe joints are considered the weak link of the entire pipeline: the materials are the same, but the construction conditions differ. When purchasing, it is advisable to check whether the manufacturer provides technical guidance and on-site services for joint foaming; experienced manufacturers will dispatch technical personnel to the site to oversee critical steps, ranging from the heating of heat-shrinkable sleeves to the actual foaming process.