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Retaining Defects…

I was recently visiting a molder who was having a specific problem…

Supervisor
We are having trouble with our employees properly identifying and naming part defects… any suggestions as to how we can improve this?
note: After a couple questions, it was determined that they do not retain defective parts as they are all discarded, reworked, or reground.
My Response
It is critical that you retain current samples of each part defect your company encounters. These defects should be placed in a visible area to ensure the employees will walk by and see them on a routine basis. This make a great training tool and will help provide a common ground when defect-related questions arise.
Additional Thoughts
Please review your defect board a few times a year to ensure the parts are up to date and still defects. I have seen many parts that are so old that factors such as dust and dimensional creep will make the intended defect invisible.
-Andy

The Affect of Moisture on Injection Pressure…

The other week, I received a multiple part question regarding many aspects of materials, this is one part…

Nathan
Although the 1st stage injection time is the same, the virgin PC/ABS (<0.06% moisture) has a 1RMPa higher peak injection pressure than virgin PC/ABS (0.08% moisture). Is this change in pressure common?
My Response
Moisture in the polymer causes hydrolysis to occur. As the water molecule is heated, one of the hydrogen atoms breaks away and both the H+ and OH- ions look for pairs along the polymer chain. In hygroscopic polymers such as PC/ABS these ions break up the long polymer chains resulting in a lower average molecular weight. This drop in molecular weight results in a drop in overall material viscosity.
Additional Thoughts
Because water has a molecular weight of only 18, it alone acts as a plasticizer which reduces the overall viscosity of the material. This is one reason we often recommend molders of non-hygroscopic materials ensure they remove surface moisture from their materials.
-Andy

Determining Intensification Ratio Using Specifications…

I was asked this question during an on-site visit earlier this week…

Technician
I know this machine is rated with a maximum injection pressure of 38,000psi, but I am not sure how to convert this to an intensification ratio.

note: The intensification ratio is a factor which is used to
convert the hydraulic pressure being applied to the hydraulic injection
cylinders into the actual pressure being applied to the polymer melt at the
front of the screw.

 

My Response
If you know the maximum plastic pressure (Pp-max) along with the maximum hydraulic pressure (Ph-max) for your machine, you can estimate the intensification ratio (Ri) for your machine. /  = 

Pp-max / Ph-max = Ri

 -Andy

The Importance of Consistency…

Many processors often experiment in training without a clear-cut goal. One year, they might send some people to a seminar, another year, they might buy some products or try making something themselves. 

Develop a Structured Approach
Although most industry training has some merit, you need to develop a structured approach. For instance, if one form of training turns out to be beneficial for a technician, then it should be evaluated as part of an overall structured training plan for all technicians, both current and future.
-Andy

Attribute Data vs. Variables Data…

I have often encounter confusion on these two terms when talking to molders…

Attribute Data
Attribute data has two different outcomes. An example of such an outcome would be a GO / NO-GO gauge which is used when testing parts. This test results in only two different outcomes, Pass or Fail.
Variable Data
Variables are those aspects measured using a scale. An example of this would be a dimension measured using calipers. This device provides a wide variety, or scale, of results based on the accuracy of the instrument and ability of the person taking the measurements.
-Andy

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