Introduction to Pipe Sizing Methods
After the pipe exits the sizing sleeve, its temperature remains very high. To ensure dimensional accuracy, shape precision requirements, and good surface quality, cooling and sizing measures are essential. The purpose of the sizing sleeve is to ensure the pipe outer diameter or inner diameter meets the specified dimensional tolerance range, achieving a good surface finish after cooling.
There are two sizing methods: external diameter sizing and internal diameter sizing. In Chinese plastic pipe industry standards, the external diameter sizing method is commonly used.
External Diameter Sizing Method
The external diameter sizing method is suitable for straight die heads and micro-hole die heads. It can be divided into internal pressure external diameter sizing and vacuum adsorption external diameter sizing.
Internal Pressure External Diameter Sizing
During operation, compressed air is introduced into the plastic pipe, creating a certain internal pressure that presses the hot plastic pipe against the inner wall of the sizing sleeve. To prevent compressed air leakage, plugs can be used to prevent air escape. This sizing method is characterized by good sizing effects and is suitable for large-diameter pipes.
The internal pressure creates uniform radial force around the pipe circumference, ensuring consistent contact with the sizing sleeve wall for precise diameter control. This method is particularly effective for pipes with diameters exceeding 100mm where uniform pressure distribution is critical for dimensional accuracy.
Vacuum Adsorption External Diameter Sizing
In this method, vacuum is applied to the outside of the pipe while it passes through the sizing sleeve, drawing the hot pipe outward against the sleeve inner wall. The vacuum system must maintain consistent negative pressure throughout the sizing process to ensure uniform pipe dimensions.
Internal Diameter Sizing Method
The internal diameter sizing method is suitable for right-angle and side-sealing die heads. This method ensures the roundness of the system inner hole and the precision of the pipe inner diameter. Operation is more convenient since various pipes and the sizing sleeve make direct contact for cooling and shaping.
Internal diameter sizing is particularly useful when the internal bore accuracy of the pipe is the critical quality parameter, such as in applications requiring precise fluid flow characteristics or when the pipe interior must mate with specific fittings.
Sizing Sleeve Dimension Calculations
The sizing sleeve is the key component of the sizing device, directly affecting the quality of the sized pipe. The dimensions of the sizing sleeve include inner diameter and length. Different calculation methods yield different results.
The normal sizing sleeve inner diameter should be 0.8 to 1.2 percent larger than the pipe outer diameter, or 2 to 4 percent larger than the die inner diameter. The length of the sizing sleeve primarily depends on the plastic type, pipe dimensions, extrusion processing speed, cooling system effectiveness, and thermal conductivity.
For pipes with diameters greater than 100mm, the sizing sleeve length should generally be shorter, typically 3 to 5 times the pipe outer diameter. This proportional relationship ensures sufficient cooling contact time while preventing excessive drag that could affect surface quality or cause dimensional instability in the finished pipe product.
