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The opening is the theme of this month's column. I'm talking about mold openings or mold gaps. Our extruded plastic comes from mold gaps, not the political struggle of an open economy that is now raging with the virus. I am one of those fragile seniors, so I stay at home to write these articles and try to understand the politics of plastic.
Some mold gaps can be adjusted manually or automatically. If it is automatic, you have a permanent watchdog, but it needs to be calibrated and maintained. This should be scheduled and not "managed" through the "call-X-if-it-breaks-down" strategy. Even automatic devices depend on the starting point. Flat mold products are a good example, because they are designed to produce a uniform product for a specific resin—viscosity is a function of speed and temperature, not just melt index—at a specific mass flow rate, the following can be passed Mix the extruder thoroughly and cool it properly after leaving the die.
In addition to built-in and automatic thickness control, start-up may require uneven adjustment of mold bolts and choke rod bolts, as well as mold heater control. The latter is usually the easiest to adjust, but sometimes it is not enough. Insulating all or part of the mold and selective local heating may help. If there is a problem with the bolt connection, a magnet or stainless steel shackle can be used to maintain the insulation.
Some molds of specific sizes cannot be adjusted at all. Some molds are most easily pre-set with centering bolts, while other molds may not be adjusted during operation, but have lip inserts that allow different ranges. Some products will also get further thickness control during the cooling process, but if the product is heated later, it may relax enough to push the thickness back to where it was before cooling/squeezing. The well-known science of metal annealing also applies to plastics.
Regardless of whether it is adjustable or not, three "SSD tools" can be used for more thickness management-expansion, contraction and contraction.
Plastic molecules are long chains, and when they are removed from the mold, they line up like logs drifting along a river. In the extruder and some molds, they are not strongly oriented in any direction; when they reach the die lip, they are fixed to the side, but can move freely in a linear direction. Once they leave the mold, they can move in all directions, which usually increases the thickness. This is expansion. Depending on the mold design, it can vary from close to zero to 100% or more (gradual narrowing means less expansion); molecular structure of the resin; melt temperature; and time—that is, the melt is confined in the mold The last part of the time. Running slower and hotter will relax the molecules, so the expansion is smaller.
Shrinkage is the change in volume from melting to solid and is resin-specific-the shrinkage of semi-crystalline can be as high as 20%, while others can be as low as 5%. The filler further reduces this value. Other additives usually have little effect, except for blowing agents, which add gases (nitrogen, carbon dioxide) to increase the thickness again. The performance and appearance depend on the type and amount of foaming agent, temperature and formula (nucleation), which in itself is a big technical field.
Expansion and shrinkage can be measured and studied in the laboratory, but production is usually not affected because the product is pulled down to the required thickness by the third factor. Characteristics such as impact strength and surface appearance may be affected by the degree of draft required to obtain the final desired thickness, and some melts are easier to draft than others. Stretch resonance in flat films and coatings is a unique situation. At a specific stretch ratio—usually estimated as gap: final thickness—the product vibrates and displays the corresponding cross line. Running faster or slower will exceed the critical range and solve the problem. Since stretching rate and quantity are both important, another remedy is to change the distance from the mold to the first cooling, which is easy to do if it runs down into the water tank.
More drops will increase anisotropy, which is a technical term when one direction is different from another. Orientation is a more ambiguous word for the same thing. It depends on temperature and stress to affect characteristics, just like ordinary oriented films, filaments, sheets, and bottles. In extrusion, there is always a machine direction, and anisotropy is manifested as the MD/TD difference or ratio in the final product, tensile strength, elongation at yield, and/or modulus (rigidity). This difference may be required in tear tape or strapping, but it is undesirable if it reduces impact strength or shrinks differently on cooling.
Thickness control is essential for cost control, because producing more than you need wastes material and money, and sometimes affects performance-pipes that are too thick will not enter the fittings. Too thin may mean failure on site, few obstacles, and too flexible. This means determining the acceptable thickness, usually in several places on the product. Try to define the range instead of the average. It is not easy, but without it, you will fly in the fog.
I mentioned temperature many times, but the amount of heat used as a control is limited because the plastic chain will be destroyed by the combination of temperature x time. Stabilizing additives (often called antioxidants) prevent this. Thermal stability is an important material characteristic. Discoloration to yellow may be the first sign, but if the product is black or black, it is useless. If you color it yourself, you can test the uncolored resin. Vacuum ovens are better (there is no air in the extruder), but ordinary ovens may have to do this.
Last but not least is safety. Adjusting the mold gap during operation can be dangerous, especially on wide flat molds. We are told to never stand in front of a hot and full mold because the hot melt will blow out the opening. However, we also know that the gap when cold is different from the gap when hot. This is not as big as it seems: although the gap when hot controls the result, it depends on the setting when cold, so once we know how to set it to cold, on an empty/clean mold, we can do In this way, even if we know that it will change when it is hot. As long as we run the same material at the same speed, this is fine. Speed is important because faster pushing will change the center-to-end distribution on the flat mold. In addition, even for products with the same number, the viscosity of the material will be different, and the percentage of recycled/scrap will also change, and there are always people who want to try other heating settings. Therefore, we need to be able to safely adjust the flow, such as a well-maintained and calibrated automatic system, an extension arm on a manual bolt turner, and changes in area temperature (barrel and mold). For medium to heavy paper, when the paper enters the cooling stack, a light in the nip will safely tell us our situation.
That's enough for now. If you have other questions, please leave a comment or email me. Remember, no plastics are toxic by themselves, but people want to believe that they are toxic. For more information, see my "Plastics Today" article on May 12.
As for on-site seminars, there are no plans for the near future-you know why-but you can still "participate" online through my new audiovisual seminar, even better than on-site: no travel, no waiting for on-site dates, the same PowerPoint slideshow Film, but my audio explanation and written guide. Watch at your own pace; group attendance is offered at a single price, including the right to ask questions and get full answers via email. For more information, please call 301-758-7788 or send an email to [email protected].
Allan Griff is a senior extrusion engineer. He initially provided technical services for a major resin supplier. Now he has been working independently for many years as a consultant and an expert witness in legal cases, especially as an educator through webinars and seminars. , Both are public and internal, and are now in his new audiovisual version. He wrote the first practical extrusion book in the United States, "Plastic Extrusion Technology", and the "Plastic Extrusion Operation Manual", which is updated almost every year, in Spanish, French and English. Learn more on his website www.griffex.com or send an email to [email protected].
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