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Basically, there is a way to optimize recovery speed… but it is a two step process:

1) First, determine the optimal feed zone temperature for your process by performing a feed zone temperature study. The purpose of this study is to determine the optimum feed temperature by graphing feed zone temperature versus screw recovery time. Starting with a low feed zone temperature, incrementally increase the temperature and document the screw recovery time at different increments. When graphed, the screw recovery time will drop and then rise as the temperature is increased. The optimal feed zone temperature is the temperature at which the screw recovery time is the lowest. This is the temperature where the polymer sticks best to the barrel, causing it to convey most efficiently.

2) Once the optimal feed zone temperature is determined, you should adjust the rotational speed of the screw so that screw recovery consumes 80 percent of the overall cooling time. Note that the back pressure used during screw recovery should be high enough to provide a consistent recovery time and consistent mixing. Your recovery times should not vary more than 5% from shot to shot, and 10% from material lot to material lot.

Many older molding machines cannot maintain consistent screw speeds at low RPM. In such a case, you may want to consider a longer delay before recovery to ensure the machine can maintain the desired consistency.

-Andy

I will address this question in a few parts…

Regarding Torque Ratings: Many bolt manufacturers will provide recommended torque values. You must remember that these bolts are not manufactured or designed specifically for injection molders. The same bolt you are using to hold your mold in place may also be used to secure the rafters in a stadium. The torque rating is based on what the bolt can safely sustain… not necessarily how it should be used. Since the machine platen is typically cast, the threads are significantly weaker than the threads on the bolt.

Regarding Torque Recommendations: Most injection molders using similar bolts use a torque value around 50-60 ft-lbs. For more on this, please read our past blog: Proper Torque Value for Clamping Mold to Platen

Regarding Lubrication: You should not need lubrication to remove the bolts from the platen unless you are using a small amount of anti-seize. If you are having problems removing the bolts from the platen, it is likely that your die setters are using too much torque on the bolts or your platen threads may already be damaged, burred, rusted, or dirty. If this is the case, you will need to repair or re-tap the platen holes to ensure proper mold clamping. Lastly, always ensure the platen is smooth, flat, and clean each time you change molds.

Your technicians will get much more support through the use of more clamps rather than using more torque. For more on this topic, I recommend reviewing the following post: How Many Clamps To Use?

-Andy

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