Residence time can be impacted by rising or decreasing the amount of material the machine processes per hour or if the injection unit specs have modified, which is frequent when shifting molds amongst machines. The residence time is printed as a result of the time a resin pellet is bodily inside the barrel and uncovered to shearing or heating via conduction. Altering any time-related machine parameter — cooling time, packing time, holding time, mildew open/shut events, mildew security time, half ejection time and the addition or elimination of a robotic — will change the residence time.
This knowledge is important, as will most likely be used to schedule molds for specific machines, determine the optimum placement for thermally delicate provides to attenuate residence time, or assist a custom-made molder in shopping for a model new machine and specifying the acceptable barrel measurement for various provides and half weights.
As a plastics engineer, I sometimes uncover that among the many best questions in our space can be most likely probably the most tough to answer. One such question is determining what variety of photos of plastic are present inside the barrel and calculating the corresponding residence time. A usually accepted purpose for residence time inside the commerce is between 2 to 5 minutes. Nonetheless, this rule of thumb might be not related when working with high-temperature engineering thermoplastics, just like PEEK or thermally delicate provides like PBT or PVC. Furthermore, when PP and PE are utilized in packaging features, deviations from this guideline might also occur.
All via my occupation, I’ve observed that residence events for packaging features utilizing RS machines can at events fall beneath 1.5 minutes. This era is commonly inadequate for efficiently elevating semicrystalline plastics to their optimum processing temperatures. In positive circumstances, rising the barrel temperatures by 100°F to 200°F has been employed to spice up heat change through conduction, compensating for insufficient shear generated by the screw configuration. Although choices just like implementing barrier screws; utilizing screws with bigger L/D ratios and elevated compression ratios; or shopping for machines significantly engineered for packaging features might deal with these challenges, they sometimes present impracticalities referring to worth and machine functionality constraints.
I’ve moreover seen residence events as extreme as quarter-hour for thermally delicate provides working in semiautomatic mode. On this case, regardless of how rather a lot we tried to lower the exact plastic temperature, we wanted to intermittently purge the barrel to remove degraded supplies.
In all probability probably the most straight forward method to judge residence time for an current working utility is to note the utmost machine shot measurement, exact shot measurement and cycle time, and with these figures calculate the residence time using the following equation:
In determining your exact shot measurement, the cushion and amount of postdecompression must be eradicated to seek out out the exact amount of plastic displaced. For example, on a machine that has a most shot measurement of 12 inches with 1.5 inches of plastic being displaced that’s working in a 15-second cycle time, the residence time could be 2 minutes.
Nonetheless, if we take into consideration that true residence time is printed as a result of the time a pellet is bodily inside the barrel, the above calculation omits the amount of plastic that is being conveyed down the dimensions of the screw all through the metering, compression and feed zones as illustrated in Decide 1.
When determining residence time, molders ought to consider the completely completely different zones of a general-purpose screw. Provide: Orbital Consulting
Determining this consists of pulling once more the hopper or shot loader and noting when pellets are closing seen between the screw flights inside the feed zone after which allowing the machine to proceed to cycle. (Mandatory: DO NOT look down the feed throat nonetheless barely use a mirror and apply full PPE to steer clear of being burned by any scorching plastic that could be discharged from the feed throat).
For example, you rely 5 full photos in automated mode over a 30-second cycle with a 2-inch shot of exact displaced plastic, nonetheless on the sixth shot, the screw solely recovered to a calculated 1.5 inches of exact displaced plastic, chances are you’ll then calculate: 1.5 ÷ 2.0 = 0.75 inch, or three-quarters of a shot. On this case, you’d add three-quarters of a shot to the fifth shot, main to a whole of 5.75 photos. The residence time can then be calculated by multiplying the complete number of photos with the cycle time: 5.75 photos × 30 seconds equals a residence time of 172.5 seconds (or 2.9 minutes).
One different method could be in order so as to add a colored pellet of the equivalent supplies or a pellet from a batch of coloration focus into the machine and observe the second pellets are closing seen between the screw flights inside the feed zone by shifting the hopper or shot loader once more over the feed throat and noting when parts are first molded with coloration. In reality, in the event you’re working black supplies, this could be an issue.
John Bozzelli talked about this method in an article titled, “A Simpler Choice to Calculate Shot Measurement vs. Barrel Functionality,” first revealed in February 2017 and updated in June 2022. He locations forward completely different limiting parts, just like using the above method for an current utility in manufacturing and actually calculating the residence time for a machine purchase.
Bozzelli moreover elements out a typical error in calculating the barrel functionality — sophisticated soften density with specific gravity. The barrel is commonly specified by ounces or grams of general-purpose PS or at events PE. A regular misunderstanding is that parts can be produced with any supplies on the required barrel functionality. We should always convert the barrel measurement to exactly replicate the material that we’re molding, whether or not or not or not it is roughly dense than GPPS, by using soften density versus specific gravity. It can consequence inside the following equation:
For example, we’re inquisitive about working PC with an L/D ratio of twenty-two:1 and a 60-ounce barrel, the place the soften density of GPPS is 0.97 g/cm3 and 1.02 g/cm3 for PC. This would possibly yield a barrel functionality of roughly 63.1 ounces of PC.
Conversely, if a a lot much less dense supplies is molded — just like PP with a soften density of 0.75 g/cm3 — this yields a barrel capable of processing roughly 46.4 ounces of PP.
Because of PP is far much less dense than GPPS, if a scheduler or anyone providing quotes assumes that 60 ounces of PP can be processed, they’ll uncover the exact amount is barely 46.4 ounces. This misunderstanding leads to an inaccurate calculation of the barrel’s utilization proportion, which can impact the anticipated residence time. In some situations, this will likely find yourself within the utmost machine shot measurement being inadequate to fill and pack the machine and mildew combination, which ends up in temporary photos or the need for the molder to take advantage of intrusion to compensate.
As talked about earlier, to comprehend a further appropriate residence time, it’s moreover essential to account for the material being plasticized inside the screw’s feed, compression and metering zones. A means I’ve been using comes from a info revealed by Spirex, titled “Plasticating Requirements.” Initially authored by Paul and Illene Colby in 1979, the data has been updated with further knowledge in variations launched in 1985, 1992 and 2000.
Inside this info, they counsel a barrel subject based on the work of Bernie A. Olmstead and Martin E. Davis by which a component of 1.4 is used to calculate the residence time if the machine injection stroke is 4 diameters or further. If the machine screw stroke is three diameters, the multiplication subject is 2.0. This assumes the screw has a 20:1 L/D ratio with regular perform compression parts and flight depths.
Thus, Equation 1 is modified with a barrel subject of each 1.4 or 2.0 to yield Equation 3.
Barrel parts based on bore diameters. Provide: Wise Injection Molding, Olmstead and Davis
As with Occasion 1, in calculating the residence time, we’d each have 2 minutes x 1.4 or 2 minutes x 2.0 to yield a residence time of each 2.8 or 4.0 minutes, counting on the machine injection stroke and its screw diameter.
I’ve used a barrel subject of 1.4 all via the vast majority of my occupation with good outcomes. What I did not take into consideration is the provision of these barrel parts. In reviewing Olmstead’s and Davis’ info of their information, “Wise Injection Molding,” they developed a barrel subject based on the bore diameter of the barrel which is cited beneath in Desk 1.
As a means to make use of this info, we must always first calculate what is named Screw Inventory (SI) which considers a L/D ratio aside from 20:1 and take advantage of a barrel subject from Decide 2. The equations are as follows for Screw Inventory and Residence Time:
The place:
Thus, for a machine with a 60-ounce barrel, bore diameter of 75 mm (1.30 subject from Decide 2), L/D ratio of twenty-two:1 working 23 ounces of PC, at a cycle time of 30 seconds our screw inventory and residence time is as follows:
Evaluating the screw inventory of 90.2 ounces versus the sooner calculated screw inventory of 63.1 ounces of PC (see occasion 2), the barrel functionality is elevated by 27.1 ounces of PC. An increase of virtually 43%, this may clearly impact the residence time for this occasion.
If one chooses to calculate the residence time based on the utmost machine shot measurement or to include the amount of plastic that is between the screw flights, various residence events can be calculated. In my humble opinion, Equations 4 & 5 put forward by Olmstead and Davis current an alternate calculation considering a particular L/D ratio along with the bore diameter of the barrel.
Furthermore, using specific gravity to remodel barrel capacities for numerous provides is often a mistaken technique that assumes the injection unit is rated in grams or ounces of GPPS and that rating applies to all provides molded on a machine.
ABOUT THE AUTHOR: Umbert Catignani is president of Orbital Plastics Consulting Inc., a consulting company that has higher than 100 years of combined experience in scientific injection molding, teaching, supplies selection, half design evaluation, in-mold instrumentation, problem administration and instruments selection. Umberto is a earlier president of the Southern A part of SPE and has higher than 29 years of hands-on injection molding experience. He has licensed and expert a complete bunch of plastic professionals inside the commerce and has served as an skilled witness. Earlier employers embody IBM, Frequent Motors, Delphi Automotive and Husky Injection Molding Strategies. Catignani earned a grasp’s diploma in polymer engineering from The Faculty of Akron and a bachelor’s diploma in provides engineering from the Faculty of Cincinnati. Contact: 404-849-6714; umberto@orbitalplastics.com