Daniel Cykana has spent more than 40 years in profile extrusion and now is the director of Extrusion Solutions LLC in Sheboygan, Wis., focusing on solving extrusion die design and processing problems. Visit extrusionprocessing.com, call (920) 918-3250, or email email@example.com.
How To Size & Calibrate Profile Parts
Sizing is the term that refers to the handling of a profile as it exits the die and is subsequently shaped and cooled. Proper cooling is critical to maintain profile shape and size. In the early days of profile extrusion, the sizing techniques were crude, but over the past 40 years many good engineering principles have been applied to advance the technology.
One common sizing techniques used in profile extrusion is called free sizing. In free sizing, there are few or no constraints on the extrudate as it is cooled and shaped. A minimal amount of tooling is needed to size or hold the profile shape as it moves downstream. Elastomers such as flexible PVC, thermoplastic rubbers, TPOs, and TPUs are typically free sized. Polyurethanes, however, tend to stick and cling to ferrous metals, making string-up and sizing difficult. So the die designer must use very lubricous materials such as graphite, UHMW-HDPE, or some type of coating technology to overcome this issue.
Free sizing is typically used on simple profiles with equal walls, or on tubes and flat strips. As the profile exits the die lip, it runs into a water trough to help set the shape. The profile must enter the cooling trough quickly without dragging on the lip of the trough. The water trough should have linear bearings on the bed to allow for precise and smooth forward/backward motion to permit tank adjustment. Smooth up/down adjustment is also important for fine-tuning the water level on both the forward and aft sections of the tank.
In free sizing, water flow over the lip of the tank must be slow and steady. Water turbulence can cause the extrudate to vacillate in size and shape. Inlet water flow to the tank also should be steady and not create waves.
Most profile extruders utilize a series of water baths or divide the first water bath into compartments. The first compartment should have a steady flow of water to prevent waves, while the second compartment can have a more turbulent flow for enhanced cooling.
Free sizing for rigid thermoplastics employs techniques that differ from elastomers. Many thermoplastics, especially the higher-modulus, amorphous types, cannot tolerate high drawdown and orientation. The same is true for highly filled compounds, such as talc and calcium carbonate grades. Since high orientation (or drawdown) cannot be tolerated, the profile must set up as quickly as possible to prevent reduction in the overall dimensions and allow the desired shape and size to be achieved.
AIR-RACK & CALIBRATION SIZING
Two sizing techniques are common for simple shapes. The primary air-rack sizing system utilizes a series of support templates and convection fans located above and below the extrudate to cool and set the profile size and shape. Top templates and fingers or guides hold the shape while the profile travels downstream.
This system had been used for years on rigid thermoplastics, but was later replaced in high-volume production by calibration-style tooling. Today, use of air-rack sizing is confined to prototyping and short runs.
There are many disadvantages to the air-rack technique. Although inexpensive from a tooling-investment standpoint, this type of cooling compromises overall profile quality in production. The shape and size of the extrudate will change as the set point of cooling changes. The transition between the melt (or fusion) point and the set point is critical and must be maintained from a time and temperature standpoint, or the set point will change position. This will cause the shape and dimensions to change accordingly, creating scrap or defects. The critical set point to be reached to prevent further drawdown or size change is the Vicat softening point on engineering-grade thermoplastics, or the heat-distortion temperature (HDT) on amorphous resins.
Calibration sizing is the second commonly used technology. Here, the hot extrudate is shaped and cooled by surrounding static steel segments. The steel segments are internally cooled and designed to maximize cooling as the hot polymer passes across them.
The steel segments are also designed with a series of vacuum ports that pull the hot extrudate to the steel, enhancing the contact and thus the cooling.
There are three different styles of calibration: dry, wet, and combined style. Selecting the type that’s right for your process depends on the output rate you want to achieve.
Typical profile extrusions range in wall thickness from 1 to 3 mm. Line rates vary depending on the material, type and size of extruder, and the overall cross section of the profile.
Single-wall, non-hollow profiles can be cooled from both sides in a more efficient manner than hollow profiles. Hollow profiles require the use of wet calibration, and often differential calibration with a vacuum assist, to support the hollows and prevent collapsing.
Hollow profiles traditionally were sized and cooled with very long calibrators, sometimes exceeding 6 ft in length. But with escalating line rates it became impractical from a cost basis to size and cool this way. Plus, it required using long, robust pullers to overcome the drag forces.
Hollows can be only cooled from the outer walls at a specific rate. Heat transfer from the inner section reheats the outer wall, requiring additional sections to maintain the profile shape.
While the overall costs of calibration exceed other types of sizing for rigid profiles, that cost is saved by producing profiles with higher overall quality and less scrap.
To Troubleshoot Profile Extrusion, First Get to Know All the Variables
To troubleshoot any type of processing problem, you need a thorough understanding of the multiple variables that constitute a given process. In profile extrusion, the number of variables is increased by a factor of two, since the basic process of extrusion is complicated by the complexity of unique die design for each profile as well as a calibration segment for cooling the extrudate.
Start off by reviewing the complete profile extrusion “complex,” defined as a five-segment process consisting of extruder, tooling, sizing/cooling, takeoff, and ancillary downstream. Each segment of the profile extrusion process itself can create unique problems, resulting in poor extrudate quality and/or customer rejects. In some cases, a defect could be attributed to multiple segments of the process. This article will provide an overview of the process. Subsequent articles will deal with specific “critical-to-quality” attributes of profile extrusion.
Three main categories of problems in profile extrusion plague operators and technicians:
•Aesthetic flaws, such as pits, black specs, pinholes, drag marks, die lines, sink marks, undulations, “turkey-tracks,” and the like. We will cover these in a separate article.
•Size variance, which can be contiguous or intermittent. Depending on its severity, several segments can contribute to this problem.
•Dimensional variations, or flow shifts, which result from a change in the rheology of the plastic, which can be affected by a change in operating conditions.
Solving each of these requires a deep understanding of the process, of each component of the extrusion complex, and of its synergistic effect on the other components.
‘HARDWARE OR SOFTWARE’?
In looking at each segment of the extrusion process, you can identify two distinct areas in which to search out the root cause of a pressing problem. So-called “hardware” issues relate to equipment. On an extruder, the screw, barrel, drive, motor, breaker plate, and any mechanical portion that cannot be readily changed are considered hardware.
“Software” refers to the operating conditions that an operator or technician can readily manipulate, such as screw rpm, temperature settings on the barrel, and screw temperature (see table above).
When a problem occurs, the operators should first look to the software to validate the operating conditions as described on their setup sheets. Are the proper temperatures set for the barrel zones? Are any of the zones underheated? Are any barrel zone temperatures overriding? These are definite signs that the process has been altered or is out of control.
Most importantly, nothing should be changed before validating that the original operating conditions have been checked. All too often, people have opinions on what conditions—such as temperature profile—work best. But using experience alone—without a theoretical basis —can lead to more problems. Case in point: Some operators prefer an “ascending barrel-heat profile,” where the temperature is colder in the feed section and higher in the metering zone (last zone), while others prefer a “reverse profile” where the temperatures in the feed zone are warmer than the metering zone.
While each has its place, the choice must be based on the thermoplastic being used and the screw type at a given output rate and melt temperature. The reverse profile is often used to offset an improper screw design that is creating excessive frictional heat.
In any case, setting the proper barrel temperature profile should not be based on guesswork or preference. It should be based on the hardware and software of the operating setup sheet. The equipment (screw, screen pack, tooling) and operating conditions must be followed in detail where the tool was originally developed.
DISSECT THE PROCESS
Problems of aesthetics, dimensional variation, or flow shifts can be caused by one or more of the five segments of the profile extrusion process as outlined above. The key is to understand the function of each of these components and decouple them for good “root cause analysis.” This is referred to as “dissecting the process.” Focus on one component, and rule it out before moving to another.
Start with the extruder; this is the primary piece of the complex and where most of the action takes place in the process. The screw conveys, then melts, and finally “meters” the thermoplastic, so it plays a key role in the overall process. Are you using the right screw—the one for which the profile die was originally developed? Comparing the setup sheet to the actual screw being used will validate this factor.
Each screw design will give a different output at a given rpm, as well as a unique melt temperature, which impacts melt flow and dimensional control. Next, check the screw temperature. Was the profile die developed with a “neutral temperature” (no screw cooling) or with internal screw cooling by water or oil? What is the listed temperature?
After verifying the temperature profile (per the setup sheet) and validating that there are no heat overrides, look at two or three of the critical indicators of the process: pressure and motor load. Is the pressure range similar to that listed on your setup sheet? What is the range of motor load (amps)? These are very important indicators to tell you what is going on in the extruder.
Troubleshooting requires patience, logical thinking, and a methodical approach to assess each unique problem and solve the problem in a timely fashion.