Tag: frozen layer

End Part Failure due to Mold Filling Imbalance

By Bozilla
June 7, 2023

Is Part quality and performance of your injection molded part important? Do you enjoy spending that extra time and money having your tool reworked? How about the pleasure of explaining the faulty part to your OEM? Let’s get real Balance your tool analytically before mold steel is cut-NO EXCEPTIONS! If you ignore this step, there is a decent chance of experiencing an unfavorable result on complex or seemingly simple tools

Let’s discuss mold balance

When can a mold-filling imbalance occur? These imbalances may be due to gate location(s) on the part, part geometry, or a combination of both Unless it is analyzed in flow simulation software, it is extremely challenging to determine how a part will fill

What creates an imbalance?

In a single cavity tool, an imbalance can occur when one location of the cavity finishes filling while another has yet to fill

In a multi-cavity tool and family tool, the same imbalance may occur within each cavity, but an imbalance may also arise from cavity to cavity

 How can imbalances cause problems with part quality and performance? 

 It is crucial to understand polymer flow A plastic engineer excels at possessing polymer knowledge Applying this expertise during the virtual optimization stage of your injection molded part will provide significant insight into an imbalance, which can affect part quality and performance

Where can an imbalance occur?

As the cavity fills, the temperature of the polymer flowing through the tool must not fluctuate to keep the properties of the polymer consistent throughout the cavity If flow velocity isn’t uniform in any region of the cavity, hesitation can occur and cause the polymer to cool down As the polymer cools, a frozen layer will form on the mold walls This frozen layer forms more rapidly in slower-moving regions and exceptionally fast in areas where the flow has stopped Once the cavity produces enough pressure to continue filling these hesitating regions, the flow will begin to move again However, the polymer is now cooler and will create tremendous shear stress as it continues to fill the remainder of the cavity THIS SHEAR STRESS WILL CREATE THE POTENTIAL FOR PART DEFLECTION AND EVEN PART FAILURE

If imbalances occur in a multi or family tool, the entire cavity experiencing the hesitation is at risk for this increased shear stress The imbalance also causes the hesitating region of the cavity to become non-uniformly packed, which translates into non-uniform shrinkage, another precursor to part warpage

shear in polymer

How does

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Part 2: Can Moldflow Validate the Quality of your Part?

By Bozilla
September 30, 2022

Part 2: Can Moldflow Validate the Quality of your Part?

In Part 2 we will continue our discussion of utilizing the Moldflow software to ensure the part does not fail in the field (Part 1: Can MoldFlow Validate the Quality of Your Part?)

Is the gate in the best location?

In many cases, the customer provides a part with a suggested gating location The gating location is sometimes determined during the part design process and is designed based on the assumed gate location However, the pre-determined gate location may not always be the best location for several reasons:

  • The gate location may not provide a balanced filling pattern which can cause severe velocity changes to the flow front or an undercut scenario where the flow direction changes after the frozen layer on the wall has been established which causes tremendous shear within the part laminae Both of which can cause part deformation or even failure
  • If the gate location creates an imbalanced filling pattern it can create a pressure spike which will also create a clamp tonnage spike Both of which lead to issues mentioned in aforementioned bullet
  • The gate location may not be in the best location with regards to the material freezing back to the gate which is essential for proper part packing Some regions of the part may not be able to be packed due to a poorly selected gate location This can cause excessive volumetric shrinkage in isolated regions of the part which can lead to sink marks or even voids In the images below there is a wheel for a drawer carrier which is failing due to internal voids There is a cut-away showing the voids which are revealed in the analysis where the high volumetric shrinkage shows the regions where the voids can potentially occur:injection molding voids

    Has the tool design been verified?

    This may seem like a redundant question but there are times when the tool may undergo some design changes which may affect the outcome of the part design such as:

    • Cooling inserts added
    • Rerouting of cooling lines
    • Cavity orientation changes
    • Feed system changes (perhaps due to cavity orientation changes)
    • d possibly other tool design changes

    Many businesses integrate a final tool review before or at the initial trial run, so if this is not standard procedure it is highly recommended If anything HAS changed, the mold filling analyst will need to re-optimize the process based on those changes

    Has the material been qualified/verified? (Keep the material

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Can Moldflow Predict Part Quality? (Part 1)

By Bozilla
September 2, 2022

This subject contains a significant amount of important information so it will be broken into two parts

Part 1: Can Moldflow Validate the Quality of Your Part?

The short answer is YES Validating your plastic injection molded part before it enters the field is the purpose for which the Moldflow software was created

A well trained and seasoned user of the Moldflow software should be able to take a part through the entire injection molding optimization process and validate each of the factors that will ensure the part meets the design criteria ie quality of the part

Injection molding design validation

For most parts, the criteria is very similar:

  • The part must meet design tolerances and must have minimal deflection/warpage
  • Weld lines must form in acceptable places or perhaps weld lines are not allowed on specified surfaces
  • Air traps must be kept to a minimum and/or be vented properly
  •  The process must have a large molding window so that any minor variations will not cause the part to fall out of specification
  •  The design of the part must meet injection molding standards (correct draft, not have any regions that are die-locked, must have correct thickness ratios, etc)
  • The design of the mold must allow for adequate filling and cooling along with many other criteria such as proper ejection (that will not deform the part) and so-on

Why is injection molded part failing

 

What happens if you believe your part design and process is optimized properly only to find out that it fails in the field?  This is a very common situation

The purpose of the Moldflow software is to mitigate such a situation I’ll explain

In order to understand if a part is going to be molded correctly, the analyst must utilize the software to correctly optimize, at a minimum, each one of the following points

1 Is the part filling at the right speed? 

It is critical to fill the part with the proper flow front velocity (screw velocity profile) As the cavity is filling, the flow front of the polymer can cool off or heat up; both of which are critical with regards to how much stress is imparted in the polymer as it fills Too much stress during the molding of the part can cause excessive post-molding stress relaxation and deflection

2 Is the feed system designed correctly? 

An improper feed system design can control cycle time or cause the pressure requirement to fill the cavity

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