Welcome!

I will first define the two concepts in practical terms, and then address the differences between the two…

Molded-In Compressive Stress – As the polymer fills the mold and cools, the polymer begins to shrink. As the polymer shrinks, additional polymer is then forced, or packed, into the mold to compensate for this shrinkage. As additional polymer is packed into the mold, internal pressure can build up causing general compression. Some molders will refer to a part with too much molded-in compressive stress as over-packed. These stresses most-often occur when the mold temperature, melt temperature, or packing pressure is too high allowing additional packing to occur. 

Molded-In Orientation Stress – As the polymer fills the mold, the long polymer chains tend to orient in the direction of flow. Basically… more of the polymer chains are aligned in the direction of flow than are aligned perpendicular to the direction of flow. Immediately after the polymer fills the mold, the polymer chains try to orient themselves randomly (their preferred state). Since the polymer chains themselves tend to shrink more along the length of the chain, than perpendicular, the differential shrinkages tend to cause stress within the part. Additionally, the polymer chains are not in the preferred random orientation, so there will always be some internal stress due to orientation. Even though these stresses will always exist, they are made worse by factors such as low mold temperatures and high injection speeds.

Basically, compression stress is created during packing while orientation stress is created during mold filing. As a result, you can have a part with both molded-in compressive stress as well as molded-in orientation stress. Although both can cause a bad part, a good molding process is typically a balance of minimizing these affects and maximizing productivity.

There are many applications where a molded-in compressive or orientation stress is beneficial to the products performance. For example, hinges and cable ties get their strength and longevity from a high degree of molded-in orientation. Likewise, many molders improve the useful life and performance of shock absorbing components when they have internal compressive stresses.

In some engineering applications, the molder will heat treat, or anneal, the molded parts to allow the polymer chains to align themselves in a more random state to reduce the internal stresses.

-Andy

Since a clamp tonnage reduction helps your part quality, you have correctly determined that it is a venting issue. Additionally, it is most likely that you only need venting on the parting line to correct the issue.

The easiest way to determine the vent location is to mold a short shot which will allow you to see where the end of the flow front is located. In most cases, this is where the venting should take place.

Since you are molding PC, your application may be optimized with a large vent encompassing most of the parting line.

In any case, your mold should have a deep channel behind the vents to ensure the venting can leave the mold easily and quickly.

-Andy

You are correct in believing that the melt is not properly attaining the shape of the texture. Basically, proper surface texture reproduction is the result of good interaction between the polymer melt and the mold surface during mold filling. If the melt cannot properly reproduce the mold texture, it obtains a shiny appearance. As a result, the primary factors which improve this are an increase in melt temperature or mold temperature at the point of contact.

Regarding mold temperature: There is really only one way to increase mold temperature… by increasing the coolant temperature, or reducing the coolant flow.

Regarding the melt temperature: Since we are only concerned with the temperature of the melt as it contacts the mold surface, there are five ways to improve this: 1) increase barrel temperature 2) increase screw recovery speed 3) increase back pressure 4) increase injection speed and 5) decrease the gate size.

Options 1-3 are most helpful if your melt temperature is too low when measured by a melt pyrometer.

Options 4 & 5 are often the most helpful because they avoid excessive heating and degradation inside the barrel, and only provide heating through shear at the time of injection.

If you strongly believe venting is the cause, you can always seal the parting line and add a vacuum pump to investigate that prospect further.

-Andy

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