Definition: Screw: In extrusion, the shaft provided with a helical channel that conveys the material from the feed hopper through the barrel, working it vigorously and causing it to melt.

In various manufacturing processes for plastics a screw is used to convey melt or soften the plastics so as to ultimately form our products. These products can be injection molded, extruded and or blow molded.  The plasticizing unit of a machine consists of a barrel, screw and heating elements.

The screw is described in many publications and typically only those within the extrusion industry and extruders in general can actually convey what the data is about their screws. In other industries typically the molding area most cannot describe what they have within their machines.  This is a sad fact and can result in issues.

The screw is designed to feed, compress and meter material to the front of the plasticizing unit. In extrusion this is to a die, in blow molding this is to die and mandrel system and in molding to the nozzle and or past the check ring / valve assembly.  The screw is typically described in term such as the L: D ratio, compression ratio and types. Other variations can lie within the 3 basic section of the screw, (feed, compression, metering) as to how many flights and other design parameters.

The L:D ratio is the relationship of length (L) to the diameter (D). Thus by dividing the diameter of the screw into the overall working length of the screw and come up with the relationship. This is of course related to the working area of the screw, where the flights are located.

In extrusion and or blow molding which are using screws the L:D ratio can be up to and over 32:1 or greater. This is not to say they cannot and are not shorter. While in injection molding they can start at 16:1 and go up from there. In recent years there has been a trend to increase the L:D ratio for molding machines.  From this standpoint alone it should be noted that if one wants to mix material better in a molding process than a longer L:D  ratio would help in this.

It is important to know also that when selecting an injection molding machine and you have your choice of shot sizes that as you go up to larger volume per machine you are going down in the L:D ratio of the screw for the corresponding shot volume.  An example per one manufacture was that from the smallest shot volume to largest on a certain machine frame the L:D was 23/20/18 :1 for the 3 shot sizes.

Why is this important, because on the smallest shot size we have a longer L:D ratio and mix the material longer while on the larger shot size the mixing is less though it has a larger volume of material working through it.

The compression ratio of the screw is how much we compress the material between the feed section of the screw and the metering sections of the screw. Thus dividing the area volume of one flight of feed by the volume of one flight of meter well yield what the compression ratio is. This is really how much we squeeze the material (work) and the ratio can be low at 1.5:1 to possibly 5:1 with the nominal being around 3:1 +/- for a GP screw.

In extrusion and blow molding the screw well be pulled and one for that material would be used and this is standard practice. Thus they have material specific screws or screws that well work on a range of materials.

In injection molding the typical shop would use what is called a general purpose screw and run all materials through it. It is not common to have a specific screw for a specific material in the shop. The exception to this is high volume shops whom may specify a screw for one material such as nylon, or PVC as that is all they run on that machine.  In certain cases the OEM may specify that a certain screw design be used.

The importance of knowing the L:D ration and compression ratio is that it is how the material is worked per that machine. It is a bit of knowledge that allows for the understanding of why the melt uniformity with a particular machine may be different than another or why certain material process better in one machine than another. The knowledge than would allow for variation in parameters to adjust for these machine differences and or why to avoid certain machines in processing certain materials.

SL SILVEY

360-882-3183