I. Raw Material Formulation and Rheological Properties
CPVC Resin Itself: Due to its high chlorine content of approximately 67%, CPVC has a melting temperature significantly higher than that of PVC and exhibits extremely high melt viscosity. At the same temperature and shear rate, the flow pressure required for CPVC is typically 1.5 to 2 times that of standard PVC. If the resin has a broad molecular weight distribution or unstable polymerization degree, this can lead to pressure fluctuations.
Lubricant System: This is the key to regulating pressure.
Excessive External Lubricant: While reducing friction between the material and the mold walls can lower mold pressure, an excess can cause screw slippage and poor plasticization.
Insufficient External Lubricant: The material sticks to the mold, causing flow resistance to increase sharply, mold pressure to spike, and potentially leading to "mold burning."
Imbalance between internal and external lubrication: CPVC formulations typically require a more effective internal lubrication system to reduce internal melt friction, thereby controlling overall pressure.
Fillers and other additives: Excessive amounts of fillers such as calcium carbonate significantly increase melt viscosity, leading to greater resistance as the material passes through the mold and causing pressure to rise. At the same time, uneven dispersion of fillers can also cause localized pressure fluctuations.
II. Mold Design and Structural Parameters
Compression Ratio: This is the most critical pressure-influencing parameter in mold design. If the compression ratio is too high (resulting in a sharp reduction in the runner cross-sectional area), the material is compressed rapidly, causing a sharp rise in pressure. While this promotes compaction, it may lead to overheating and decomposition of the CPVC; conversely, if the compression ratio is too low, insufficient pressure will result in inadequate pipe density.
Length of the Straight Section (Shaping Section): The longer the straight section, the greater the resistance to melt flow within the die, and the higher the die pressure. This helps increase back pressure and improve plasticization; however, for CPVC, an excessively long straight section means the material remains under high temperature and pressure for a longer duration, increasing the risk of decomposition.
Flow Channel Smoothness and Streamlined Design: Dead corners, steps, or insufficient surface finish within the flow channel directly impede the flow of high-viscosity materials like CPVC, causing localized pressure spikes and stagnation-induced decomposition.
Manifold Support Structure: If the manifold support (the internal support structure within the die head) is not designed with a streamlined profile, the material will leave "weld lines" after passing through the support. These must be smoothed out by subsequent buffer grooves and compression sections, a process that results in additional pressure loss.
III. Production Process Parameters
Processing Temperature: The effect of temperature on CPVC pressure is nonlinear.
Excessively Low Temperature: The material is not fully plasticized; hard particles or highly viscous clumps are forced through the die, resulting in extremely high pressure and severe fluctuations, which can easily damage equipment.
Excessively high temperature: Although apparent viscosity decreases and pressure temporarily drops, CPVC is highly prone to degradation. Once degraded, it releases hydrogen chloride and produces char, which clogs the flow channels and causes pressure to spike abnormally.
Screw speed and feed rate: These are direct means of pressure regulation. Increasing the screw speed increases the extrusion rate, causing the die pressure to rise accordingly. However, CPVC is extremely sensitive to shear heat. If the frictional heat generated by excessive rotational speed cannot dissipate quickly enough, it will lead to uncontrolled temperatures and abnormal pressure within the die.
Pull-off speed: The relationship between pull-off speed and extrusion speed (draw ratio) affects the actual pressure inside the die. If the pull-off speed is too fast (resulting in an excessive draw ratio), the material at the die exit is drawn too thin, causing a drop in die pressure and affecting the density of the pipe.
IV. Condition of the Filter Screen and Perforated Plate
Filter Screen Clogging: The filter screen is used to filter out impurities and create back pressure. When impurities or gels in the CPVC formulation accumulate on the filter screen, or when a filter screen with an excessively high mesh count is selected, material flow is obstructed, and the pressure upstream of the mold continues to rise.
Perforated Plate Support: If the flow channels in the perforated plate become clogged or if the plate does not fit properly with the mold, it disrupts the uniformity of melt flow, resulting in uneven pressure distribution.
V. Equipment Status
Mold Temperature Control Accuracy: If heating elements are worn out or thermocouples malfunction, causing abnormally low temperatures in a specific section, the viscosity of the CPVC material in that area will increase instantly, forming a "lump" that blocks the flow channel and causes pressure fluctuations.
Carbon Deposits on Mold Walls: After a period of CPVC production, carbon deposits tend to accumulate on the inner walls of the mold. These deposits alter the dimensions of the flow channels, increase surface roughness, and gradually increase flow resistance, resulting in a slow rise in mold pressure over time.