Injection Mold Design Shrinkage

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					Injection Mold Design Shrinkage

        Jeremy Baker 2 posts since
Jul 10, 2009 First, I am not an engineer or a daily SW user. I am researching solutions to help improve the mold
design in my company.

Second, I am not asking for details of how to do this but rather can it be done accurately.

Can (or should) injection molds be designed using shrinkage factors that are accurate enough to put into production
without what is called in my company as a "dialing in" process?

We make small fittings (1/16 to 1/2) and screw covers in a variety of materials with most tolerances at +/- 0.005.


Jeremy Baker

         Jeff Mowry 275 posts since
Dec 11, 2007 1. Injection Mold Design Shrinkage Jul 10, 2009 1:33 PM
I'm an industrial designer, but I've helped tool-makers with my 3D CAD output in this sort of thing before, and yes--
the output can be done as accurately as you need--at least in terms of the fairly common tolerances you mention.

The big factor is selecting a resin and sticking with it, since the degree of shrink is dependent most largely on your
material type. So if you move from ABS to Nylon, you can expect some issues with differing shrink rates (not to
mention gating, sprues, etc.).

So after a material has been selected, I often apply the scale factor to my base part and then build the core and
cavity around my part (there's a tutorial within SolidWorks as to how to do this--along with plenty of other "old-school"
ways of creating mold components manually).

          Jeremy Baker 2 posts since
Jul 10, 2009 2. Injection Mold Design Shrinkage Jul 10, 2009 1:58 PM
Thank you Jeff. Your input is very helpful.


Jeremy Baker

         Kelvin Lamport 2,322 posts since
Dec 2, 2006 3. Injection Mold Design Shrinkage Jul 10, 2009 2:04 PM

           Kelvin Lamport 2,322 posts since

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Injection Mold Design Shrinkage

Dec 2, 2006 4. Injection Mold Design Shrinkage Jul 10, 2009 2:07 PM
Not being nasty with the following link ... I just like using it.

         Andrzej Soporowski 24 posts since
Oct 17, 2007 5. Injection Mold Design Shrinkage Jul 12, 2009 8:40 AM
I have been a mold designer for over 30 years and I can tell you that "Yes", molds can be design very accurately
using "shrink factor". Having said that I have to add that "shrink factor" calculation is only the first step in the process.
Molds are designed to a specific resin and most likely will not produce dimensionally accurate part when other resin
is used. Every mold has "core" (male) and "cavity (female) component. Molten resin will not shrink the same way
over the core as in cavity. Resin shrinks "into" core, so you will notice less shrinkage than on cavity side where resin
shrinks away from the steel. Under normal circumstances, standard shrinkage factor, for given resin, is enough to
achieve sizes to ±.005. Some polyolefines use (due to their nature) will require dual shrinkage (across and with
the flow) calculations. For more accurate size control, specific cavity size calculations will be required. I would
recommend you to get "Injection Molding Theory and Practice" book written by Irvin I. Bubin book published in 1972.
Other, critical part of mold design is cooling. Proper cooling of the mold is important not only to part sizes but it will
determine (to large extent) you molding cycle time which equate to $.
The second part of controlling part sizes is molding process control, but that is totally new discussion.
I hope, that this will answer most of your questions. If not, please do not hesitate to ask.

Best regards

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