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A standard cutoff is helpful when initially establishing your fill… but you ultimately want to target a percentage of shot size. When you target a specific cushion value, then you tend to have the same amount of material, and pressure loss, in front of the screw during pack.

Many companies who mold similar products in similar sized machines sucessfully target a specific cushion value. Most molders have the greatest success in targeting a percentage, such as 5% or 10%, of the full shot size.

The transfer position should not have a significant influence on screw and barrel wear… but focusing on a cushion rather than a specific transfer will improve your ability to maintain a solid and reliable injection molding process.

-Andy

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