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.