I received a question accompanied by a newsletter published by from a prominent author in the plastics industry…
In the article, the author suggests that a Full-Taper Nozzle should be used for ABS, and a GP (General Purpose) Nozzle is not appropriate for that material. Should we be using Full-Taper nozzles for all our engineering resins? We currently use GP and reverse taper nozzles.
note: In the article Jim referenced, the author correctly identified a situation where the sprue, runner and gates were significantly undersized, and then made a large point about how the molder used a GP nozzle rather than a full-taper nozzle.
Although there are a variety of nozzles available to the industry, most have a large diameter opening where they attach to the barrel with a significant reduction to the final orifice where it meets the sprue bushing.There are three common ways the internal dimensions are constructed at the sprue bushing end of the nozzle:
1) GP (General Purpose) Nozzle – This nozzle typically uses a straight land area where the polymer enters the sprue bushing. For example, if the nozzle orifice diameter was .100″ or 2.54mm the orifice would maintain that diameter for the length of the land. The benefit to this is the polymer tends to be pulled from the nozzle during mold opening, providing a small area for material to drool between cycles. One disadvantage to this design is that the amount of material that is removed from the nozzle can often be inconsistent. The other disadvantage to the long land area is the increased shear rate that occurs in this region. The general purpose nozzles are often helpful for molding materials which tend to exhibit small amounts of drool.
2) Full-Taper Nozzle – These nozzles have a nozzle orifice diameter which is smallest where the nozzle meets the sprue bushing. Unlike the GP nozzle, the polymer gets the least resistance to flow. The advantage to this design is it provides the most flow with the least shear. The disadvantage to this design is that any drool has the potential of causing cycling issues. Most molders use such nozzles for amorphous materials such as PC and ABS.
3) Reverse-Taper Nozzle – This nozzle uses an orifice which has an opening larger than the inner dimensions of the nozzle. The purpose of this reverse-taper design is to promote the removal of material from the nozzle as the mold opens. This can be very advantageous for low-viscosity semi-crystalline materials such as nylon and PP which are prone to drool.
To answer your specific question, GP nozzles are bad for some materials, great for certain applications and OK for others. It may be a good idea to educate your employees on the differences, and start using more appropriate designs for applications which could benefit from reduced shear and increased flow.
Whenever possble, I opt for a nozzle which is best suited to the application. This decision would incorporate some of the following factors:
1) What is the maximum nozzle orifice diameter I should use?
2) What is the shortest nozzle I can use?
3) Would a full, straight, or reverse taper be best?
4) How long of an orifice land do I need?