Tag: valve gates

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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Valve Gates and Sequencing-for injection molding

By Bozilla
August 24, 2021

Valve Gates are invaluable as they relate to their primary design purpose and have many important functions

 They can:

✔ Eliminate waste that cold runners create

✔ Eliminate vestige

✔ Be sequenced

✔ Eliminate weld lines

✔ Control filling patterns

However, users should be aware that there are a few potential issues that could come with using valve gates and sequencing

valve gates for injection molding

 Vestige v Witness Marks

Valve gates can minimize or completely remove vestige by direct gating on the part  They do leave witness marks on the part where the valve gate tip seats into the cavity but with proper grinding or surface finish, it can be minimized or completely hidden

 Controlling the Fill Pattern v Machine Stroke Programming

When multiple valve gates are used to fill a part, it may be necessary to time the sequencing in order to create a more uniform filling pattern  It is extremely important to understand that if the valve gates are sequenced, then the flow rate input must also match the demand of the feed system

For example: If your tool has four valve gates and you initially open two valve gates, then open the next two valve gates, the IM machine must deliver twice the flow rate when the two additional valve gates are opened in order to maintain equal flow rates through all nozzles in the feed system

If the machine stroke is not profiled to compensate for the flow rate demand, the properties of the polymer will change in the cavity due to different filling rates  This could translate into non-uniform shrinkage and stress which directly translates into warpage  It can also cause surface finish variations as shown in the picture below

Nozzle and machine pressure for injection molding

Cascade Sequencing (eliminate weld lines) v Machine Stroke Programming

If the intention is to sequence the valve gates as the flow front passes by in order to remove weld lines, then the same concerns arise if the machine stroke is not programmed to compensate for the additional flow rate demand as additional nozzles are opened

Cascade sequencing can also create back-flow and uneven packing along with uneven stress even if the machine stroke is profiled to compensate for flow rate

Cascade sequencing removes weld lines, therefore the potential problems that accompany it must be weighed  Cascade sequencing should be used as a last resort when trying to eliminate weld lines

Valve Pin Control

Hot runner manufacturers have now developed controllers to

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