The most prominent advantage of plastic-coated steel pipes is their exceptional corrosion resistance. The construction industry typically involves complex environments where pipes are exposed to various corrosive media, such as acids, alkalis, salts, and chemical solvents. Coatings can effectively prevent these media from coming into direct contact, thus avoiding rust and corrosion of the steel pipes.

The coating of plastic-coated steel pipes not only prevents damage from the external environment, but also effectively resists corrosion inside the pipe. Because the coating material itself has strong chemical resistance and can withstand the erosion of various liquids, it is particularly suitable for conveying highly corrosive liquids or gases. With the protection of the coating, the service life of plastic-coated steel pipes is greatly extended, usually reaching several decades. When the coating is intact, it can provide protection for steel pipes for decades; however, once the coating is damaged during construction or use, localized corrosion can quickly spread from the point of damage. Rather, it depends primarily on the type of plastic material applied to the inner and outer surfaces.

The most common choices are epoxy resin (EP) and polyethylene (PE), each excelling in different areas.

Epoxy resin (EP) coating: the "top student" of chemical corrosion protection. Its resistance to strong acids, strong alkalis, and organic solvents is outstanding, and its adhesion to steel pipes is extremely strong. Therefore, it is the first choice for harsh corrosive environments such as the chemical and oil and gas industries. Its shortcomings lie in its brittle material, low flexibility, and poor impact resistance. It is prone to micro-cracks when subjected to drastic temperature differences or mechanical impacts.

Polyethylene (PE) coating: a "weather-resistant guardian" of physical protection. PE has excellent weather resistance and resistance to ultraviolet aging, good flexibility, and impact resistance, making it very suitable for municipal water supply and drainage pipes that are exposed or buried. Its main limitation is that its upper temperature resistance is relatively low (about 80℃), and as a non-polar molecule, its chemical bonding force with the steel pipe itself is not as good as that of epoxy, and it relies more on the hot melt process to ensure adhesion.

In practical engineering, a common configuration is "inner EP + outer PE": the inner layer uses epoxy resin to resist the chemical corrosion of the transported medium, while the outer layer uses polyethylene to resist atmospheric, soil and mechanical damage, achieving complementary performance.

Purchasing directly from steel mills: suitable for "large quantities, standard requirements, and cash payments".

Advantages: For large-volume, standardized long-term contracts, the price may be the best, and you can get the original manufacturer's warranty certificate, with the most direct quality traceability.

Limitations: Steel mills typically require payment before delivery, which can tie up significant cash flow. Furthermore, production schedules prioritize major clients; small-batch, multi-specification, or urgent orders are difficult to accommodate, and there are usually minimum order quantity (MOQ) requirements. If a shortage of materials leads to work stoppages on-site, the resulting losses could far exceed any savings gained from a lower unit price.

Purchasing through a trading company: Best suited for "diverse specifications, smaller volumes, and service-oriented needs."

Trading companies can consolidate resources from multiple steel mills, providing a one-stop solution for all your steel requirements—effectively overcoming the limitations of a single mill’s product range. More importantly, they offer value-added services such as cutting, processing, and delivery, ensuring materials are "ready for immediate use" upon arrival. Even if your individual order volume is small, a trading company’s overall scale may allow them to secure better pricing terms for you than you could obtain by dealing directly with a mill.

If your steel requirements involve a wide variety of dimensions and specifications, a trading company can help you source everything—even providing preliminary processing to your exact requirements before delivery to the site. In such cases, the total cost—factoring in time, capital, and management overhead—is often lower than dealing directly with a steel mill.

P20 mold steel has a carbon content of 0.40%, which is considered a medium-low carbon level. The carbon content of cold-work wear-resistant steel SKD11 exceeds 1.0%, while the carbon content of rust-proof mirror steel S136 is 0.41%, which is similar to that of P20, but the performance alloy ratios may be completely different.

If the carbon content of P20 mold steel is lower, the steel texture will be softer; CNC machining, grinding, and drilling will be smoother; tool wear will be less, processing efficiency will be higher; it will be easy to get started, and there will be fewer problems such as tool breakage and edge chipping. High-carbon steel is hard but brittle and prone to cracking under stress, while P20 has a moderate carbon content and sufficient toughness. During mold trials, mold locking, and minor impacts, it is unlikely to crack or chip and has good stability. Its heat-treatment deformation is relatively small. P20 is pre-hardened steel, and it has been heat-treated before leaving the factory. Subsequent processing will not cause much deformation. It does not require secondary quenching and tempering, making it more suitable for making some plastic molds with fast delivery and conventional precision.