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Immediately after 2nd stage pack, there is typically a large amount of pressure present in front of the screw. If this pressure is not relieved, it will increase the torque applied during screw rotation. This additional force can quickly weaken, twist, or even break the screw within the barrel when screw recovery starts.

This concern is even greater with electric injection molding machines, since the servo-motor can apply a great amount of torque instantaneously…. especially on direct-drive machines where the minute flex of the belt is non-existent.

To relieve the pressure that’s at the front of screw and to prevent unnecessary screw damage, the ‘screw delay’ or ‘screw decompression’ option should be used. The screw delay option allows you to add a delay after 2nd stage packing to relieve the plastic pressure on the screw before recovery. The decompression will actually back up the screw to relieve the pressure… similar to decompression after recovery.

Hydraulic molding machines often avoid this issue since they have an inherent buffer due to the slight buildup of pressure as the valve opens, and a period of pressure stabilization that occurs before the screw reaches full speed. Adversely, the electric servo-motors apply a high amount of torque to the screw virtually instantaneously. People who are are familiar with hydraulic molding machines often have difficulty adjusting to the instantaneous response of an all-electric molding machine.

-Andy

Each process and company has a different tolerance for wear. A company molding medical devices needs much more control over melting and residence time than a company molding commodities.

The benefits to regularly measuring your screw and barrel is that you can determine what level of wear tends to indicate inconsistent processes. Ultimately, an excessively worn screw will increase residence time, decrease melt consistency, and reduce melting efficiency.

Many companies will have a couple ‘junk’ machines which are not worth replacing the screw and barrel… but can run a handful of jobs well enough. Other companies keep every machine in perfect shape. Remember, you don’t need a race car to get your groceries… but you also don’t want to enter a race with a Yugo.

-Andy

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