Servo Valves for Extrusion Blow Molding & Parison Control in Injection Molding

Extrusion blow molding depends on tight parison control, the wall thickness profile of the extruded tube must match the final part’s requirements or you risk weak sections, excess material, and unstable processing. Domin servo valves help extrusion blow molding machines in parison control by providing fast linear flow response and predictable, repeatable motion.

How Servo Valves Impact Parison Control

In extrusion blow molding, the parison is a continuous tube of molten plastic that will become the end product by being inflated inside of a mold. Different parts of this plastic tube have different thicknesses depending on if the material needs to stretch more or carry more load. This thickness, or extra material, is determined as the extrusion is happening via a servo valve. For applications where parison control involves velocity-to-pressure behavior, see our overview of P/Q control in injection molding.

When the servo valve lacks proper accuracy, the extrusion thickness becomes varied, resulting in excess material in the wrong places and thin material where more stability is needed. For applications where injection speed consistency is the main concern, visit our article about velocity control. For example, consider a plastic water bottle. If parison control is not accurate and there is less material being extruded for the base than intended, this high traffic area may thin, wear, and bust.

Plastic manufacturers are demanding better precision and repeatability from their hydraulic systems and, thus, the servo valves within them. Domin’s valves are created with this demand in mind with high positional accuracy and low hysteresis. For parison control, this means that the valve can follow the program closely and without inconsistencies from cycle to cycle.

Our Recommended Valves for Extrusion Molding

Domin’s servo valves are ideal for applications that require high precision and repeatability, and have been independently tested for their highly stable motion, linear response from command to flow, and overall strong performance. These characteristics contribute to a valve well suited for parison control systems.

For extrusion blow molding and parison control applications using NG06-sized axes, Domin’s S6 Pro Servo Valves are a strong starting point, as they provide the dynamic performance and close-loop spool feedback. For applications with different mounting sizes or flow requirements, the larger models in the same S Series provide the same level of high performance.

FAQ for Parison Control & Blow Molding

Because the parison profile is executed through a hydraulic actuator or cylinder, any delay, drift, or hysteresis in the valve directly affects how accurately the die gap follows its program. This, in turn, affects wall thickness and part material distribution. For a more in-depth explanation at how valves switch from velocity to pressure and why this matters for accuracy, read our article here.

Domin’s valves use onboard electronics to close the loop on spool position, delivering high positional accuracy, low hysteresis and responsive control. This combination makes the parison control loop easier to tune and more stable over time. Similar control benefits apply to injection speed regulation.

Manufacturers using Domin valves can see improved precision and repeatability, reduced scrap rates, better energy efficiency due to lower leakage, and even a reduced price tag depending on which model customers are planning to replace.

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