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Is it 90 or 95% Short…?

I often see many people getting hung up on the specific percentage fill for first stage short shots…

Student
In a 4 cavity mold, I usually have to make the part 90% full to create a short shot in all cavities. Is that OK, or does it have to be exactly 95% full?
My Response
It is more important to have a short shot in each cavity than it is to have the mold 95% full. Personally, I am less concerned about the percentage being 90 or 95% than I am about the cavities being short shot. If you have a short shot in each cavity, then you can properly compensate for variation during fill. If you are already full during 1st stage, then you have no buffer for variation, and are likely to encounter shorts and flash over the course of your run.
Additional Thoughts
For more about this and related topics, please feel free to read the following:
-Andy

Documenting Coolant Temp. without a Temp. Controller…

I encountered this question the other day in a training session…

Student
Our production facility does not use a temperature controller. Is it still important to document the coolant temperature?

My Response
In this case, it is even more important. If a temperature controller is present, then you can at least assume that the coolant temperature approximates the setpoint. If you do not have a temperature controller, then you can only guess the temperature of the water being supplied without a measurement.
Additional Thoughts
If you do not have a mold temperature controller, then it is also important that you document the mold surface temperature after the mold stabilizes. When doing this, you should always provide a diagram of the actual location being measured.
-Andy

Molding of Magnesium…

I received this question during NPE…

BD
We have a customer who wants magnesium parts. What is the basic principle to metals injection molding?

My Response
Essentially, materials such as magnesium are effectively alloyed to create a thixotropic material. This alloy can be shear heated like a polymer to create a semi-solid slurry which can be injection molded. The tonnage is approximately 1 ton per cm^2. This process does require specialized equipment and should not be done on a standard injection molding machine.
Additional Thoughts
Like injection molding, the parts should have good design features such as draft angles and fillets. The final part has approximately 0.5% shrinkage.
-Andy

Polishing Actually Removes Metal…

I heard a die setter make this statement recently…

Setter
Mold makers get all bend out of shape whenever we touch the mold surface. It’s no big deal to polish out a scratch.

My Response
First off, polishing takes a long amount of time and is a very labor-intensive process. This person has obviously never polished a mold himself. Second, every time you polish out a nick, scratch, or dent, you are actually removing steel. Any metal removal will ultimately affect the dimensions and appearance of the final molded part.
Additional Thoughts
Personally, I like the idea of having every employee who’s job function is to touch the mold surface to spend at least one day of polishing. These type of exercises teach the employees the importance of protecting the mold surface.
-Andy

Mold Temperature and Weld Lines…

I was discussing a processing issue with a technician and encountered this question…

Tech
The cause of the poor weld line was mold temperature, but I don’t really understand why since the melt flows from the center and should be hot when the flow fronts meet.

My Response
This is a true, but the strength and appearance of the weld line is often dependent on the ability of the two flow fronts to interact after meeting. When the flow fronts meet, the polymer near the mold surface will stop flowing and interacting faster when the mold surface temperature is lower. This can cause the weld line to freeze before the intersection of the two flow fronts makes contact with the mold surface, thus reducing the appearance.
-Andy

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