a shaft provided with a helical channel used to convey from the feed hopper to end of barrel.

Surface Speed:

The plasticating rate given by the machine manufactures is either in GPPS or HDPE and is calculated at the maximum speed in RPM that a machine shall run. RPM, revolution per minute is a typical setting for screw rotation, and the universal setting would be to convert this RPM to surface speed per mm/in/ft./meter whichever system one would wish to use.

In processing whether injection or extrusion we are conveying the material though a barrel and screw combination, and shearing the material over the flites of the screw. Thus by calculating the speed of the surface such as a point that rotates at 100 RPM, which with a 45 mm diameter would equal 14,137 mm of travel per minute. Now if we used a 60mm diameter screw and the same RPM we would be traveling at 18,849 mm minute, or 33% faster.

If one wishes to maintain the same speed on the surface, thus maintaining the shear effect than it would be, than by math, using the 14,137 mm/sec as our speed it would be divided by the result of Pi x 60 which than equals a RPM of 74.99 for the 60mm diameter screw to equal the 45 mm diameter screw in surface speed.

In calculating a formula to use say mm or any measurement in a time of 1 second one could than take Pi and divide it by 60 (seconds in a minute) and this equates to a constant of 0.0523599 which one could use as in the following formula.

Screw surface speed (mm/sec) = dia. of screw (mm) x RPM x 0.0524

Now from data the ideal screw surface speed of HDPE and GPPS are 750 and 800 mm/sec respectively

Acetal for example has a listing of 100 mm/sec for a homopolymer and 200 mm/sec for a copolymer. Note that this data has to be gathered from the material manufacture and there are some reference materials about with this data that have been published.

Where this now comes into play is the plasticating rate for the machine and job based on ideal or optimum plasticating rates for homogenous melt uniformity.

By using a previous formula to calculate out the plasticating rate of material (vol. 5 no 15, 14) one comes up with what the machine is capable of, that result would then be multiplied by the result of the following:

                                Screw surface speed value of the material to be process  

                                                Screw surface speed value for GPPS or HDPE***

***Whichever one was used by the machine manufacture, for volumetric capacity of barrel.

An example would be going to PC from GPPS. If our machine has a capacity to plasticate 30 g/s in GPPS and we divide by 0.8916 (melt density of GPPS) this equates to 33.65 cm³/s. If we than multiplied this by 1.051 g/cm³ (melt density of PC) it would yield a rate of 35.67 g/sec for our machine. But the screw surface speeds for optimum homogeneity are different for the materials, GPPS being 800 mm/s and PC being 400 mm/s.

(400/800) x 35.37 g/s = 17.69 g/s plasticating rate

If in fact the shot weight is 100 grams than a minimum screw recovery time would be 5.65 seconds (100g/17.69 g/s). A shorter screw recovery rate could be established by increasing the RPM of the screw but this may lead to inconsistent melt uniformity.

Though in the above example the machine is capable of yielding 35.67 g/second in PC ideally we would want to only have a recovery rate of 17.69 g/second so as to have a quality melt of the material. It is true some screws may yield higher rates and it may also be true that one may find that the inconsistency one gets is not an issue with the quality that one desires, that determination has to be made by the processors and functionality of the part in the application.

SL Silvey

silveysplastics@hotmail.com