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While the previous articles describe the screw components and some notes this section shall explain further some tips or notes to consider in the construction and understanding of the screw.
Heat into the plastic materials is on a two front basis, that of the barrel temperature and that of the shearing action involved within the mechanical action of the rotating screw. There is a balance between the two, meaning that in all cases there should not be 100% heat from the barrel or 100% heat for the shearing action.
Thus in looking at the two families of plastics, amorphous and semi-crystalline they each could use their own screws in a general sense given that amorphous do not like shear, and semi crystalline do. ( in most plants a general purpose screw is in the machine, and it must run all types of materials, for cost factors.) Thus the RPM, temperature profile must be checked for each to optimize on the same screw. Further in watching the heaters on the machine the time of cycle for heater on and off should be monitored so as to see approximately how much heat is being imputed via the heaters. If staying on for 50% of time than approximately that is the heat impute for the barrel, 50% (this is approximate). If the heaters are not cycling than the screw (shear) is providing the heat to the material, and this is not always advisable in all materials.
L:D ratio:
The length to diameter ratio is good to understand, as is the length of each of the section in the screw. Since the L:D ratio is length to diameter and the general screw flite is 1 diameter long (generally speaking ) if the screw is a 20:1 than one might find that the screw is 10:5:5 meaning that the feed is 10 diameter in length and the compression is 5 diameters in length and the metering is 5 diameters in length. Having this understanding may help in troubleshooting when the mold is placed in a new machine which has a 24:1 or maybe 18:1 L:D ratio.
In many plants it is common that no-one knows what is in the machine, but in many cases they can tell you that when running that mold in that machine it produces great parts but when placed in the other machine it does not, though both machine may be of 100 ton clamp. Yet the screw diameters may be different, and thus the L:D ratio and thus the amount of metering, transition and feed section which may have an effect on the melt quality of the plastic material be processed. Many manufactures supply 3 and sometimes 4 different screw diameters for a specific tonnage machine, so that there is a choice between a/B/C which could have 22/20/18:1 L:D ratios approximately.
Note:
A longer feed zone creates a greater potential throughput
A longer transition zone results in less shear heat, thus is gentler on the materials while a shorter transition applies higher shear and is or can be harsh on the materials
A longer metering zone allows more uniform pumping in extrusion process, more uniform melt quality while shorter yields less uniform melt quality.
Channel depth:
The depth of the channels/screw flites affects shearing of the material and exposure to barrel temperature if shallow, while deep channel depths have the opposite effect in that less shear and exposure to barrel temperature. Residence time is greatly effected by this depth condition and may be an overlooked factor of the actual screw construction.
Compression ratio:
While basically how much one works the material by how much it is compressed, two screw of same compression ratio may in fact have different depths (channels) and thus different effects on the material quality and through put. Also if the L:D ratio is different and the compression ratio is the same of two screws there is a difference in the melt quality and processing.
Mixing:
While a GP screw is not known as a mixing screw one can determine the effects by testing. Typical test is to drop a colored pellet in the throat of the screw while processing natural resin and note the results, i.e. in that if streaked than not good mixing quality while if no streaking but solid in effect a good mixing screw.
The other issues with mixing are based on shot size, particle size of additives, back pressure, screw RPM and profile temperature settings. An extruder is much better at mixing given that it is continuous in process while in molding it is segmented, stop start type of arrangement.
Mixing screws exist and with their use comes caution as to how much shearing action is imputed into the material and the compatibility to the materials being processed.
Shot size;
A general guideline to shot size should be based on the diameter of the screw, in general using 1 diameter should be a minimum size in stroke, and about 3 diameters may be the largest in stroke to avoid issues. What is necessary to understand is that when creating the shot, where the material is coming from, meaning which section of the screw is is used to compose this shot. In general 1 diameter may be all contained in the metering section, while 2 to 3 include the transition section. Greater than 3 now encompasses the feed section, which may induce air into the mix and or require the use of higher backpressure to compress the air, achieve a higher shearing action and or longer time to heat the plastic.
It is critical to monitor and know, so as to achieve repeatable consistent parts.
Most screw manufactures have excellent references to check for types of screws, design of components/sections of screws and their particular brand names, as do the machine manufactures.
SLSilvey
07062015.01
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