Custom Plastic Extrusion Terms – Part Four
In part one of the series we discussed extrusion industry terms you may hear during conversations and in printed materials related to extruders and tooling for the extrusion industry. In part two of our series, we tackled the top 10 thermoplastic extrusion terms relating to ancillary, or downstream equipment used to manufacture custom plastic profiles, tubing and other hollow profiles. In part three of the series, we addressed material terms you may hear extrusion suppliers talk about when they discuss plastic materials.
In the final blog on plastic extrusion terminology, we are going to dive into processing terms you will likely hear a plastic extrusion supplier use when talking with you about the process. Read our top ten extrusion processing terms below:
Weld Line or Sink Marks– During the design of a plastic extruded profile, areas where intersection legs occur are prone to a visible line opposite where the legs intersect. First, what causes the line. If you take a close look at a drawing for a plastic profile with an intersection leg, you will see there is more mass in the area of the intersection. Since thermoplastic materials shrink some during the cooling process, the intersection of two legs cools at a slower rate than the rest of the profile, causing it to shrink more due to the mass of the profile. As extrusion tooling is being designed, there are “tricks of the trade” used to minimize this line, or sink mark in the final part. A tool designer would typically separate the flow of the two legs within the die, and allow them to come back together as the extrudate is being compressed to the front of the extrusion die. This can in some cases create a weld line where the two melt streams are brought back together so careful tuning of the extrusion tooling is critical to minimize this line. Another option is to turn the potential issue into a design feature, by designing indentations on the side opposite the intersection leg. The drawing below shows some options for designers to consider when dealing with a custom profile extrusion with intersecting legs to create features on the exposed surfaces.
Camber & Bow– As extruded profiles are being conveyed down the extrusion line, the cooling process begins. Common methods of cooling plastic extrusions include water, air, and even a combination of both depending on the product. In some cases during high speed extrusion, warm water may be used so the surface of the product is not shocked, which could cause physical property issues. In the case of multi-hollow extruded plastic profiles, cryogenic cooling using nitrogen may be used to rapidly cool the inner legs of the extrude product to maintain profile integrity and increase line rates. Even, consistent cooling during the conveying period is critical to maintaining straightness of the extruded profile. There are two terms used by plastic profile extrusion companies; bow and camber, to define lack of straightness. An extruded profile is considered to have bow when the part exhibits side to side warping. Camber is a term to define up and down warping of the profile. Generally, tolerances are set-up during the design and tooling development phases, and then checked as part of the ongoing quality assurance checks performed during production of the plastic profile extrusion.
Cross Section– The easiest way to define cross-section is to look at the end of the product in a two-dimensional view. The above illustrations demonstrate the cross section of the four examples showing ways to design around sink marks. In this case each of the four drawings show different cross sections of the extruded profiles. Cross sectional drawings of profiles are used by the Formtech quality assurance group and our production operators for quality assurance measurements and checks. The drawings are dimensioned, with tolerances to allow multiple quality operations to be performed using a variety of equipment such as digital calipers, micrometers and optical comparators.
Back Pressure– If you look the overall extrusion process, thermoplastic materials are under some type of compression from the time the material enters the hopper on the machine, until the material exits the front of the extrusion die. The compression area just before the extrusion die and flowing through the extrusion die is measured in pounds per square inch (PSI). Too little backpressure during this process will result in loss of profile integrity and physical properties. Too much backpressure can cause safety concerns as well as potential equipment damage, so it is important for the extrusion operator to monitor back pressure during the process using pressure gauges on the extruder and inserted into the die.
Tolerances– The custom plastic extrusion process involves melting plastic, forcing it out of the extrusion die, and then controlling the plastic profile shape and size at the part is cooled. As with any manufacturing process, the extruded profile will vary slightly during the process necessitating tolerances on key dimensions. Tolerances on plastic extrusions impact fit and function, and the best time to start looking at tolerance requirements is during the initial design phase. By working with Formtech early in the process, we can provide design for manufacturing assistance including tolerance requirements. Typically, a control print will be developed by our quality assurance group to include dimensions and tolerances for critical dimensions, along with a control plan for use by our manufacturing and quality assurance groups. For a more detailed explanation about how tolerances impact pricing, and a general tolerance guide, please click here.
Bulk Density– Bulk density is defined as the weight per unit volume of the plastics purchased by Formtech for use in custom extrusion applications. Bulk density impacts flow and feed rates on the extruder, and variations in bulk density of the plastic will cause variations in size so it is an important consideration for custom extrusion manufacturing operations
Die Swell– We talked about back pressure above, and the importance of having the appropriate amount of back pressure during the extrusion process for physical property control. The back pressure is caused by compression inside the extruder and extrusion tooling. Once the plastic compound exits the extrusion die in a compressed form, the compound will relax, or expand slightly. This is referred to as die swell. Die swell is material specific, meaning for example a vinyl (PVC) material will experience a different die swell than a Polyethylene or Polypropylene compound. Experience tells our tooling engineers the amount of die swell we typically get by material, and this is a critical consideration in tooling design.
Purging– In the extrusion process cleaning materials out of barrels, screws and plastic extrusion tooling takes time and increases cost. For instance, if material changes are being made on the line from one job to the next, this could involve removing and cleaning the die, pulling the screw on the machine to clean, and brushing out the barrel of the machine while the screws are out. Ultra-stabilized compounds have been developed that allow cleaning all these items, and flushing the system using these purging compounds. In fact, the purging materials are so stable they can even be left in the extrusion die during storage, put back on the extruder the next time the job needs to run, and started without time consuming cleaning. The normal compound will push the purge out of the die when it exits. Another use for purge compounds would be during shutdown for weekends or holidays. Formtech extrusion technicians use purge materials when shutting some lines down for these periods to clean and protect the extruder and die. For some materials like Polyethylene and Polypropylene this is not required, but for PVC for instance, the material will degrade within the tool if it is left to cool and then be reheated for start-up.
Starve Feeding– When operating an extruder, materials are fed to the extrusion screws and barrel to make the plastic extrusion. There are really two methods that can be used to accomplish this feeding process. First, the material can be contained in a hopper directly above the feed-throat of the extruder and fed by gravity. The screws will then take in the maximum amount of material they can with every revolution of the machine, and this is called flood feeding. An alternative to flood feeding is to add a metering device at the feed-throat to control the amount of material introduced to the screws. The flights of the screw are not completely filled with every revolution of the machine. This is called starve feeding of the extruder, and offers some advantages to the extrusion technician in certain situations. By not filling the screw channel completely, it offers more control to the technician, and removes some other variables that can occur with flood feeding.
Surging– Surging is bad! Surging has always been one of the most common process variation on a single screw extruder. For every revolution of a single screw machine, there is some slippage back over the flights of the screw. This slippage is advantageous for mixing of materials and colorants, but it creates an opportunity for some surging and variation with the screw revolution. With this being the case, differing pressures as material comes off the screw can cause this processing issue for the extrusion technician. Surging means variation, and this can impact size & tolerance control. Proper screw design for the type of plastic compound being extruded is extremely important to the process to minimize surging. Perhaps we will do a future blog on screw design, outlining the different zones of the screw and the function of each.
We hope our four-part series covering extrusion industry terminology review will help you better understand the extrusion process.
We are anxious to work with you and your team on your next plastic extrusion requirement. Give us a call at 614-531-9954, contact us, or email at email@example.com for more information or a quotation.