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This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

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How Much Back Pressure is Too Much?

The screw does most of the melting, but the barrel temperature should be maintaining the melt temperature.

Question:

How do I know if there is too much back pressure?

My Response:

The quick method is to make a significant drop in the back pressure, such as reduce by half (ie. 1000 psi to 500 psi) and see if the melt temperature shows a significant drop (>10°F or >5°C). If so, it is likely too high.

The best method for this is to plot back pressure vs. melt temperature to see the trend. For example, you might see results such as 445°F @ 1500 psi, 427°F @ 1000 psi, 425°F @ 500 psi, 422°F @ 250 psi,  420° @ 0 psi. This would indicate that any back pressure 1000 psi or less is likely acceptable, though lower back pressures tend to provide better long-term processes.

Additional Thoughts:

If you need a higher melt temperature to disperse a colorant or fill the mold, you should add heat using barrel temperature. Using high back pressure is a very unreliable way to add heat to a polymer and will introduce inconsistency to your process.

Pressure Setting During Mold Open and Close

The molding machine does it’s best to achieve the set mold open and close speed as long as there is enough pressure available.

Question:

What is the pressure and speed relation for clamp open and close? How can I make sure that I have enough pressure to satisfy the set speed?

Answer:

During clamp movement, the molding machine adjusts the pressure (or power when using electric machines) to the clamp in an attempt to achieve the desired speed. If adequate pressure is not available, the machine will not be able to achieve the desired open or close speed because the movement will be pressure limited… This will result in inconsistent clamp movements and part removal times.

It is easy to determine whether you are using enough pressure 1) Time the current part removal time. 2) increase the pressure and time the part removal time. If your part removal time decreases with a higher pressure (2), then you had inadequate pressure.

Additional Thoughts:

If you want to determine the optimum pressure: 1) Use the steps above to verify you are not pressure limited 2) incrementally reduce the pressures to determine the minimum pressure necessary to maintain your part removal time 3) once the minimum is determined, add 10% to each pressure to compensate for normal variation. These steps will ensure you have adequate pressure to maintain cycle time, but not excessive pressure… this will reduce the amount of mold and machine damage which might occur in the case of complications.

Note:

The above discussion does not include mold protect. Mold protect intentionally use a low-pressure, pressure limited clamp movement to protect the mold during the final stages of mold closing.

Watch Your Back Pressure

The fun of molding PP is that it can take a significant amount of abuse during molding and end use, but only if it has been melted properly.

Concern:

In the past week, two different customers consulted us for assistance in troubleshooting failures with their molded parts. They were both running PP and were both running back pressures around 1500psi plastic pressure. Not to get into specifics, the parts were exhibiting brittleness during end-use testing. This was not necessarily the only issue, but significantly high back pressures are common issue in this industry.

My Recommendations:

There tends to be a misunderstanding of the proper use and setting of back pressure. This value is typically set too high and is adjusted far too often during troubleshooting. In general, you want to use enough back pressure for mixing and melt consistency, but not much more. This pressure should not have to be changed during production unless a significant change such as the composition of the material, new additives, new screw, or the screw or barrel have been modified in some manner (lot to lot changes in material do not qualify for a back pressure change).

Essentially, i you need more heat for the colorant to properly disperse, then add barrel temperature… likewise, if you need more melt temperature for the material to melt, mix or inject properly, then add barrel temperature as well. This is why it is critical to know the melt temperature which is used to mold a good part as it is critical to troubleshooting melt-related problems.

Additional Comments:

You should never use back pressure to add extra heat to the material during recovery as this breaks polymer chains & fibers as well as degrades many of your additives, colorants, & processing aids. All of this will reduce the strength of your part and make it more brittle.

New Online Courses Teach 5S Methodology

Routsis Training announces the release of five new online training courses, entitled The 5S System (Steps 1-5).

The 5S system is a workplace organization method that describes how to manage a work space for efficiency and effectiveness. The five steps – Sorting, Straightening, Sweeping, Standardizing and Sustaining – are covered in detail and will ensure that any work environment will be clean, organized, consistent, properly maintained and up to date.

Participants will learn the required practices and habits that the world’s most efficient molders perform on a daily basis to establish and preserve workplace organization. Each ‘Production SkillSet’ course is accompanied by a worksheet so that your employees can immediately integrate the 5S System into your plant.

“Cleanliness and organization are essential components of a highly profitable molding operation and customers demand proven systems in today’s competitive marketplace,” says Andy Routsis, President of Routsis Training.

The new online courses, and all existing Routsis Training courses, are cross-platform and can be played using any web browser and on any tablet or smartphone.

Burn Marks & Weld Line Issues

It is always critical to check your first stage short shot since it affects both mold filling and part packing.

Request:

Please provide reasons & remedies of burn-mark or weld line problems in plastic (HDPE) moulded product during the injection moulding process.

My Response:

There are many possible causes, but injection should be checked first:

First, the mold must not be full during 1st stage injection.

If the mold fills all the way during 1st stage injection:

  • The gas will be trapped in the mold and may cause burning
  • The screw will bounce back which will cause:
    • Melt pressure loss in the cavity
    • Reduced pressure at the melt front
    • Reduced weld or meld line strength

If you short shot the mold, verify the burning is not present, and then finish filling the mold cavity with packing pressure, you may get better weld line and and burn marks.

You should also consider reducing the clamp tonnage to allow the mold to vent better.

Additional Comments: You can also download our reference guide for more assistance.

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