PP plastic ring

Product Model	Injection Molded Spigot Ring Series
Category	PP Molded Products
Reference Price	Price on request

Hot Customizable for Working Conditions 11 Technical Parameters

Product Overview

The PP coupling sleeve, also known as a direct coupling or sleeve connector, is a tubular fitting with sockets at both ends, specifically designed for the axial butt joint of two sections of same-diameter PP ducts. During installation, the two duct sections are inserted into each end of the coupling sleeve respectively; after centering, they are seal-welded at the sockets with welding rods of the same material, thereby extending the duct run. With its simple structure and low cost, the coupling sleeve is a standard fitting widely used for straight splicing of anti-corrosion ventilation ducts.

The coupling sleeve is formed in one piece by injection molding, with no weld seams and uniform wall thickness. Its inner wall joins smoothly with the duct's inner wall, avoiding obvious steps or protruding lips at the connection, resulting in low airflow resistance and less dust accumulation. Compared with flange connections, the coupling sleeve connection offers a cleaner appearance and lower cost, making it suitable for fixed duct sections that do not require disassembly. Compared with direct butt joint welding, socket positioning with the coupling sleeve is easier and delivers more consistent butt joint quality.

The PP coupling sleeves supplied by Xicheng Environmental Protection are matched with the diameter series of the molded ducts and are available in both standard PP and flame-retardant PPs materials. When used with PPs ducts, the flame-retardant coupling sleeve provides the same self-extinguishing performance upon flame removal at the joint, ensuring consistent fire protection throughout the entire duct run. The coupling sleeve can also be used to connect ducts to equipment interfaces equipped with sockets.

Working Principle

The coupling sleeve connection achieves sealing and fixing through socket fit combined with welding. The inner diameter of the coupling sleeve is slightly larger than the outer diameter of the duct, forming a socket fit. After the two duct sections are inserted, their ends align at the middle of the sleeve; the sockets restrict radial misalignment of the ducts, keeping the duct run straight. The socket gaps are seal-welded with a hot-air welding gun and PP welding rods. The weld seam closes around the circumference of the sleeve ends, fusing the three plastic sections into one integral piece so that gas cannot escape through the joint.

The socket structure improves the load-bearing capacity of the joint. When the duct is subjected to air pressure or slight vibration, the sleeve wraps around the duct ends, with the weld seam mainly serving the sealing function, while axial and radial loads are shared by the mating surfaces of the socket fit. This provides greater deformation resistance than simple butt joint welding. The PP coupling sleeve and duct have the same coefficient of thermal expansion, expanding and contracting synchronously with temperature changes, so the joint will not crack due to uneven expansion or contraction.

Structural Composition

The main body of the PP coupling sleeve is a short pipe section with a thickened socket at each end. The socket inner diameter fits the duct outer diameter, and the middle section features a positioning step or a straight bore of equal diameter. Annular reinforcing rings can be provided on the outer wall of the socket to improve rigidity, and chamfered ends facilitate duct insertion. The sleeve is integrally injection-molded with smooth inner and outer walls; its length increases with the diameter to ensure sufficient socket depth and proper weld seam position.

The flame-retardant coupling sleeve incorporates flame-retardant additives into the raw material. Its appearance is similar to that of the standard sleeve, but it offers self-extinguishing performance. The coupling sleeve itself contains no loose components such as bolts or gaskets; connection requires only matching PP welding rods and a hot-air welding gun. For sections that require maintenance and disassembly, the coupling sleeve is not suitable; flange connections should be used instead.

Specifications and Models

The table below lists the connection types and applicable positions of the coupling sleeves, with diameters corresponding to the molded duct series.

Type	Connection Method	Applicable Position
Standard PP coupling sleeve	Socket welding	General acid and alkali exhaust
Flame-retardant PPs coupling sleeve	Socket welding	Indoor fire-protection ducts
Thickened coupling sleeve	Socket welding	Large-diameter main ducts

Product Features

The PP coupling sleeve provides straight connections and quick installation, making it an economical connector for straight sections of fixed ducts. Its main features are as follows:

Injection-molded in one piece, no weld seams, overall corrosion resistance
Socket positioning at both ends keeps ducts centered and straight for fast installation
Smooth inner wall transition, low air resistance, resistant to dust and dirt accumulation
Dual protection of socket fit plus welding, excellent sealing, crack-resistant
Simple structure and light weight, lower cost than flange connections
Specifications matched with molded ducts, flame-retardant material available
Expands and contracts synchronously with the duct, excellent temperature adaptability

Material	PP / PPs Polypropylene
Molding Process	Integral injection molding
Connection Method	Two-end socket welding
Applicable Pipes	PP Duct with Same Flange
Corrosion resistance performance	Acid-alkali-resistant salt
Flame Retardant Options	PPs self-extinguishing
Inner wall features	Seamless connection with ductwork
Removability	Fixed connection, non-detachable
Function	Tube splicing reinforcement
Matching Port	Multiple size options available
Sealing Features	Welded Seam Connection Sealing

Application Industries

Linear extension and splicing of straight ducts in acid-alkali exhaust systems of electroplating workshops
Fixed pipe section connections of exhaust air ducts in chemical enterprises
Direct pipe connections in acid-alkali exhaust systems of semiconductor factories
Segment connections between laboratory exhaust shafts and horizontal main ducts
On-site splicing of corrosion-resistant exhaust ducts in metallurgical pickling lines
Extended connections of odor collection pipelines in wastewater treatment plants
Fixed section connections of tail gas emission ducts in pharmaceutical factories

Typical Process Locations

Collars are used for straight duct sections in ventilation systems that do not require disassembly, commonly found in horizontal main ducts along supports and vertical shafts. Due to the limited single length of standard formed ducts, collars are used for point-to-point connections over long distances. The collar is positioned between two hangers or near the support, forming a continuous and integrated pipeline after connection. Collars are not used for equipment inlets/outlets or areas requiring frequent maintenance.

In exhaust collection systems, the collared pipe sections are located in the transport paths between hood branch ducts and main ducts, as well as between main ducts and purification equipment. During installation, one end of the duct is fixed, the collar is inserted, and the other end is connected after alignment and leveling, followed by sealing along the flange circumference. Removable sections near fans and equipment are connected with flanges, while fixed transport sections primarily use collars to balance sealing performance and cost-effectiveness. Before collaring, verify that the outer diameters of both duct ends match the collar inner diameter, ensure uniform insertion depth and consistent annular gaps, and clean/dry the flanges before symmetrical welding around the circumference to avoid offset caused by unilateral buildup. Collared sections are positioned between two hangers and temporarily fixed, with sealing integrity checked after welding. Areas near fan vibration zones and those requiring disassembly are not connected with collars but instead use flanges or flexible connections.

The sleeve connection has low cost, a simple design, and excellent weld seam sealing, but it is non-detachable after welding, making it suitable for fixed, non-maintenance required straight pipe sections and long-distance dry pipe transportation. The flange connection has a slightly higher cost and larger dimensions, but it can be disassembled at any time, making it suitable for equipment inlets/outlets, valve and muffler ends, and areas requiring regular cleaning and maintenance. In engineering, sleeve connections are typically used for fixed transportation sections to control costs, while flanges are used for equipment and valve connections for maintenance. The two methods are used in combination based on the location. Detachable sections near vibrating equipment such as fans should be equipped with flexible connections and should not be welded shut with sleeves. When selecting, the connection type is determined based on whether the section requires disassembly, whether it is near equipment, while considering sealing performance, cost-effectiveness, and future maintenance.

Properly installed ring flanges with spigot welding will not leak air. The key lies in ensuring the two ends of the ducts are inserted correctly and the axes are aligned. The annular gap of the socket should be continuously welded around using a weld wire of the same material, without any breaks, pores, or incomplete welds. Before welding, clean the oil and oxide layers from the socket. During welding, ensure the weld wire fully fuses with the base material, and no stress should be applied to the weld before it cools. For systems with high sealing requirements, air leakage tests or segmented pressure checks can be conducted after welding. Air leakage typically occurs when the weld is not closed, the weld wire does not fully fuse with the base material, or the socket is not inserted properly. These are all construction quality issues. As long as the connection is properly inserted according to the process, fully welded around the circumference, and post-weld checks are performed, the sealing performance of the ring flange joint can meet the requirements of corrosion-resistant ventilation systems.

The inner diameter of the sleeve socket is designed according to the standard outer diameter of the ductwork. As long as the outer diameters of the two connected sections of ductwork are consistent, even with slight variations in wall thickness, they can generally be spliced together. However, significant differences in wall thickness can result in one end being too loose and the other too tight, affecting alignment and weld quality. Excessive looseness can also increase the amount of welding filler required. The outer diameter tolerances of ductwork from different series or manufacturers may vary. Purchasing matching ductwork and sleeve sockets from the same manufacturer ensures better dimensional fit. When the outer diameters are inconsistent, do not force the connection, as misalignment and stress can cause air leakage. Instead, replace with a matching sleeve socket or use machined parts or flanges for transition. Standardizing the outer diameter standards and tolerances of ductwork during ordering can prevent mismatched connections on-site.

The ring itself is merely a connecting piece and does not bear the weight of the pipeline. Support brackets should be independently installed based on pipeline weight, diameter, and span requirements, ensuring welds are not subjected to long-term load. The location of the ring joint should be near the support bracket arrangement to avoid placing the joint exactly at the position of maximum deflection between two brackets, preventing long-term bending stress on the joint. For vertical pipelines, fixed brackets should be installed below the ring joint to support the weight of the upper pipeline section. The spacing of horizontal pipeline brackets is determined based on pipe diameter and wall thickness, with appropriate densification for larger diameters. Proper bracket arrangement can reduce bending moments and vibrations at the joint, extending the service life of welds. During installation, brackets should be fixed first, followed by welding the joint, to prevent misalignment and additional stress caused by welding under suspended pipeline conditions.

The sleeve is a fixed joint for spigot welding, non-removable after welding, with a simple structure and low cost, used for extending and splicing large-diameter ventilation straight pipes. The slip joint consists of a nut and a seal ring, forming a detachable joint that can be sealed by hand or tool tightening without welding, suitable for small-diameter piping requiring frequent disassembly. Ventilation and exhaust pipes generally have large diameters, with welding being the primary method on-site, predominantly using sleeves and flanges; small-diameter water supply, drainage, chemical dosing, and instrumentation piping typically use slip joints. The sleeve excels in reliable sealing and low cost, while the slip joint excels in ease of disassembly. They differ in diameter range and application scenarios. Selection should be based on pipe diameter, the need for frequent disassembly, and connection method, with slip joints generally not used for large-diameter corrosion-resistant exhaust pipes.

The concentric sleeve has symmetrical structures at both ends and no installation direction requirement. However, it must ensure that both sections of the ducts are inserted to the same depth and aligned centrally in the sleeve. Avoid inserting one end fully while leaving the other end hanging, as this will cause uneven weld stress and potential air leakage. Before splicing, clean the pipe ends of burrs, debris, and oxidation layers. Align the duct axis and level it before welding circumferentially. For special sleeves with flow direction requirements or joints with internal flow guides, install them according to the shell's marked airflow direction and never in reverse, as this will increase resistance and affect flow guidance. When adjacent to eccentric components or flow guide fittings, verify the overall direction as well. Before installation, confirm the fitting type and flow direction against the drawings. After welding, recheck the alignment gap and centering.

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