Definitions: Plasticating Capacity: the maximum rate at which an extruder or molding machine can melt room temperature feedstock and raise it to the temperature suitable for extrusion or molding.
Plasticate: to render a thermoplastic more flexible, even molten, by means of both heat and mechanical working.
In selecting a molding machine and or extruder there are a few things about both which are critical in production of parts. One is the plasticating rate, or basically how much material can one extrude / process per hour, the other is the configuration of the screw / barrel. While in extrusion the screw shall be specific in most cases to a particular material and through put rate, molding has been not so selective given the nature of trying to do all with one type of screw barrel combination. Also in molding the shot capacity of a machine is critical in its selection process again a screw and barrel combination.
As the definition above states the plasticating rate is number / amount and is the maximum amount of molten/ softened / melted material that can be delivered by the unit in a given time. The other important function here is that it is delivered in a uniform manner.
In injection molding the plasticating rate is typically given in grams per second (g/s), but may also be given in other units such as oz. /s or kg/h or lbs. /h of PS and or if in HDPE per EuroMap 19. Extruders are typically given in rates per hour.
Why is this data important for injection molding? It is the rate at which the machine in use can produce our material in a uniform manner. If we do not have an adequate plasticating rate with our screw and barrel combination it may cost us cycle time and profits as we wait for the screw to recover. In the majority of setups for parts production it is advisable for the screw to get back 1 to 2 seconds prior to cooling time ending. In this manner there is a bit of play for non-uniform screw recovery and less of a chance of effecting uniform cycle time and all that that may entitle. In some cases increasing screw rpm to meet cycle time can result in non-uniform plastic materials.
Note that if the rate is given in pounds per hour or KG per hour this is fine for an extrusion process because this process is continuous. In the molding field the screw only rotates typically while the machine is in the cooling timer segment of the cycle, thus even though there may be a rate of lbs. /hr., Kg/hr. the screw is not rotating continually so one would have to convert this to a function of lbs. /sec or kg/sec so as to apply to screw rotation time. This is possibly one of the overlook factors in specifying a machine, or in placing a mold in one of the plants existing machines.
An example is a client whom could not meet cycle time, though injection, pack and motion were optimized to the minimal time required to produce good parts; the screw recovery was the issue. This lead to a discussion of using shut off nozzles, (allowing screw to continue to recover while molds opened and closed) and or to place mold in larger barrel capacity machine, which would reduce recovery time, but also would reduce injection pressure available, limit control of injection due to shorter stroke of shot. The sad part was this machine was brand new having been bought for this particular project.
When it is understood what the plasticating rate is for a machine (based on PS/HDPE) than one can by math covert this rate to the material that one runs in the machine. This than gives the rate in the materials that one uses. This is done in two steps: (starting point of grams/ second, g/s)
1st: convert the current g/s rating to a volumetric figure.
2nd: Multiply the volumetric results from the 1st step by the melt density of the new material which shall result in the plasticating rate in the new material for that machine in a g/s figure. (Based on starting with a g/s unit in 1st step)
To be continued-
SL SILVEY
Silveysplastics@hotmail.com
1-360-882-3183
