Tag: gate location

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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Can Gate Location Really affect Part Warpage?

By Bozilla
August 17, 2021

Based on part geometry, gate location(s) will determine how the polymer fills the cavity  If the cavity doesn’t fill in a balanced/uniform fashion, the internal stresses will be anisotropic- meaning non-uniform properties  So it is important to place a gate in a location such that the polymer flowfront fills the cavity at a uniform rate and reaches the end of the cavity at all locations, including weld line locations, simultaneously

With simple part geometry, identifying an ideal gate location may be possible by using experience and examining the part  However, with more complex geometry and gating limitations (cooling line interference, ejector pin interference, slides, etc), it is nearly impossible to determine the appropriate gate location(s) without using FEA(flow simulation)  Not only can FEA(flow simulation) produce actual deflection results(warpage), it can also provide data that is a precursor to warpage-such as volumetric shrinkage and frozen-in stress which is typically due to a response from forcing the material into the cavity while the material is trying to freeze

gate location and part warpage courtesy of sciencedirectcom

Gate location(s) will determine polymer orientation  Based on that location, it will ultimately determine polymer shrinkage  Also, different regions of the part will cool at different rates(regions of the cavity near the gate that were first to fill will cool before regions furthest from the gate)

 

Why is this important?  Because there are 3 major components that contribute to warpage:

 ✔ Polymer Orientation

✔ Polymer Shrinkage

✔ Cooling Effects

 

Shrinkage and orientation are both directly correlated to injection location(s) on a part as it relates to processing conditions  Warpage due to cooling effects is  based on the rate of how the polymer cools on one side of the cavity relative to the other side Non-uniform cooling through the thickness will create warpage

Because gate location(s) directly correlates to the contributors of warpage, gate location is therefore extremely important in the tool creation process and ultimately the quality of the part

The injection molding professionals at Bozilla Corporation have over 20 years of experience assisting OEM’s, Tier 1 & Tier 2 suppliers, and Tool Shops to create quality parts that meet timing and goals

wwwBozillaCorpcom

 

About the author

Chris Czeczuga President Bozilla Corporation

Chris Czeczuga is a Plastics Engineer, Injection molding expert, Military Veteran and the President of Bozilla Corporation He has proven success with many OEM’s Tier

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