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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?

-Andy

Although there are a lot of questions, I think it can be handled from a couple different angles.

1) Pressure is created by resistance to flow – During injection, a significant amount of pressure is being created in front of the screw. Since polymers are compressible, there is a counteracting pressure pushing the screw backward. If your hold pressure is less than the polymer pressure in front of the screw, then the screw will move backward.

2) Excessive mold filling during first stage – Ideally, your mold should not be completely full during first stage injection. If you completely fill the mold during first stage, then there is a spike in injection pressure as the polymer begins to pack out the mold cavity. When this spike in pressure is followed by a lower packing pressure, there is often a backflow of material out of the mold cavity… resulting in screw bounceback. Overall, this is not a good approach to processing because changes such as material viscosity or melt temperature will cause significant variation in your process. 

Your best approach is to:

1) Increase your transfer position to obtain a short shot during first stage.

2) Increase your hold pressure until you obtain appropriate part quality.

3) Perform a gate seal study to ensure the 8 sec. hold time is appropriate for the adjusted hold pressure.

-Andy

I was wondering if you help me locate some references on Processing HDPE and PP with 1% chemical foaming agent? All the information I find focuses on Structural Foam which is a low pressure process. We primarily use high pressure injection molding, using the the chemical foaming agent for weight reduction and cosmetics.

Since the banning of foaming agents containing CFC’s, the use of chemical blowing agents has obtained an inappropriate stigma in the marketplace. Believe it or not, their use is not as rare as it may seem. Many injection molders will use a small amount of blowing agents to eliminate sinks, lower material costs, and even enlarge the part to help meet dimensional requirements.

In your case, there are many places to find information… when searching online, use terms such as:

or get more specific:

There are also some good articles available online:

or

Since structural foam molders use the largest volume of additives, much of the literature is focused on their needs… but you will find that many of these resources will also provide great high-pressure molding information… click for an example of this from Bergen.

You may have to register to see some of the materials from the Suppliers, but this is typically free.

Keep in mind, your material providers can be great resources if you ask the correct questions. Some additive providers provide agents specifically formulated for your type of application.

Basically, when you are conducting a high pressure molding process with blowing or foaming agents, you would melt and inject the material in a manner similar to a traditional process, but apply many of the packing and cooling strategies of structural foam. Ultimately, the more blowing agent you add… the more the polymer will behave like a structural foam after it is injected. 

-Andy