Tag: valve gates and sequencing

Is this the correct Injection Molding Machine for your Tool?

By Bozilla
November 8, 2021

To start this discussion I’d have to first state that the size of the tool plays a large role when selecting an injection molding machine  More specifically, it is the projected area that is of concern and how the projected area, along with the pressure distribution over that projected area, creates clamp force

Selecting a machine based on clamp force (tonnage) is more common when you have a part with a large projected area; ie multi-cavity tools, bumper fascias, housewares and many other items

In today’s economic climate, it’s more important than ever to conserve energy  Many believe that using the smallest IMM is the best way to achieve this cost savings  However, there are reasons why a smaller machine isn’t always the most efficient machine

 Reason 1: If an optimum process is the objective, select a machine that does not allow the tool to exceed the clamp force and flash the tool (blowing open)  during an  ‘optimized’ process

We  have had many concerned customers consult with us about the process Their questions are directed at finding out why the part is warping or exhibiting cosmetic defects  Once I dig into the process, I typically find that the part is not packed sufficiently due to the tool blowing open  In order to keep the tool closed, they must pack with very little pressure for a very short time  Packing with too little pressure, too little time, or both can cause a loss of control with dimensional stability and/or cosmetic issues due to excessive shrinkage  These issues create problems that are caused because the tool is in an IMM that doesn’t have the proper clamp force requirement

In the image below the clamp force required to fill and make the part is 250 Tons  However, in order to pack the part out sufficiently and make a good part (meets tolerances, minimal cosmetic defects, minimal deflection, etc) the clamp force required during 2nd stage pack is 1,450 Tons  That’s a very big difference

clamp force plot

 

 Reason 2: You are able to make parts but the process window is so small that staying within the process window is difficult or impossible to maintain

The inability to stay within a process window could be caused by several issues, especially since there are so many variables in the molding process  However, if the machine does not have sufficient clamp force to stay closed during an optimum molding process, concessions will be made and

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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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