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Plastics/internal preassure


QUESTION: I am trying to stop a long core from moving during injection.  I want to capture the end of the core with a slide that telescopes out of the cavity until the part is 90% full.  It would then retract .060in. allowing the part to finish filling and pack out. The projected surface area of the slide subject to injection pressure when extended over the core would be .5sq. inches and when it is retracted back 1.5sq. inches.  How can I calculate how much pressure it will take to hold the slide forward until the right time?  When during the injection cycle is the greatest force applied to internal surfaces of the mold?

ANSWER: Hi Dave,
A friend of mine lives in Georgia just outside Atlanta he is also called Dave.

It would help me if you could give an better description of the core, the wall thickness and some more details regarding the mould and material so that I can picture what is in front of you.
What you are looking to do is capture the core end while it fills evenly and then remove the slide so the core can continue to finish filling the core but because the material fills the core end first you may not be able to do what you require.

Is the construction of the mould in production yet, if not you may be able to changed the construction so core deflection is not a problem get back to me on this.

If you wish to attempt your solution then you need to treat the slide section as you would a sliding core pin or other slide if it is possible and use as much locking force as can be generated via the tool faces to keep the slide in place during injection or use hydraulics to hold the pin in place.
I would use the calculations used for sliding core sections and add 10% but you will need to use more than the injection force that is applied to the core or it will be pushed back.

It sounds like you need to apply the solution that is used for caulking tubes.


---------- FOLLOW-UP ----------

Do you have E.S.P.?  I am talking about a caulking tube, it is valve gated at the tip and after trying several different core tip configurations, gate sizes and wall stock adjustments it has yet to mold a good part. I have told my boss what I want to do however he is not on-board with me yet.  Will you tell me the solution others have used?   
Please make me a hero,

Hi Dave,
Too late for the other solutions mould tool is already made and you would have to start all over again.
Where was the tool made?
Is the gate central or off set to the core if its central try below.?

What I suggest you can try is the following which may improve the quality of the part if your machines have the capability of more than two stages of injection, reduce the injection pressure and speed so that the base of the part fills about 1/2" to 1" up the core then increase speed and pressure to fill rest of core, once plastic has flowed round the base of the core it may stabilise it during the rest of the filling stage and prevent it from deflecting.
By using trial and error you may end up with a reasonable product without loosing too much cycle time.

Let me know how you get on.

kind regards,


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Robert Tomlinson


Tooling design,design for manufacture,tooling procurement,machine design,machine procurement,
Processes injection moulding,injection blow moulding,extrusion blow moulding, process control,productivity improvement,Quality,Decoration,Assembly,Automation.Thin wall moulding expert.


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