Six Sigma Project on Reducing Plastic Injection Molding Rejects



Recently, i just started a six sigma project on reducing plastic injection molding rejects. Any reference or example to share? Thanks
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Bev D

Heretical Statistician
Staff member
Super Moderator
well, what are the defects? what is the defect rate? ow many different product types what is the defect pareto? how many different pieces of equipment? are you working on one defect type? one product type?
what is the time series of defect rates by defect type?

we need more details. this isn't a cookbook exercise.


Hi, Thanks for your response. Currently my project statement is to reduce wheel assembly rejection rate from 30% to 5%. This wheel assembly process contain of the overmold process (two molding process). involving a lot of handling activities from process 1 to 2. This wheel assembly is TPE material that so soft and easily cause the defects. The current process performance in the pareto chart showed the dent and scratch were the highest defects of 10.2% and 7.3% representive. Any format and example to start off this project?
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Bev D

Heretical Statistician
Staff member
Super Moderator
before I respond - I must ask what training you have had on six sigma? the questions you ask are very basic, whether you are "doing a six sigma project" or have simply been tasked to improve the process.

If you are in a six sigma training program your instruction in the first week shoudl have covered your questions


If you are in a six sigma training program your instruction in the first week shoudl have covered your questions
What i referring is the powerpoint format to start off my project. This is the purpose of the presentation to my top management. Currently take off the six sigma black belt program. Just beginning of the program and hopefully can acquired as much knowledge as can. Any powerpoint format available? Thanks;)

Bev D

Heretical Statistician
Staff member
Super Moderator
my recommendation would be to provide a Pareto-Trend of the defects currently experienced.
you might look at the last 6-12 months of data (whatever makes sense) and pareto by quantity of occurence (assuming that cost to fix is the same for each defect type - regrind? if cost isn't the same, then you might also pareto by cost for each defect type).
then take each of the top 5-10 defect types, trend the occurence rate for the same period of time as the pareto. (Do not trend cost only occurrence rate, monthly or weekly depending on volumes.) This show demonstrates whether or not the defects are stable, increasing, decreasing or if a spiek occurred and has since been corrected. it adds info to the pareto that will help with prioritization of which defects you should pursue.

I would aslo repeat this for each of the top product types - there are likely differences in some defect occurrences based on product types.

you would then select the top defect/products as the target for your project

In my handouts I use a portrait format with a tall pareto that has the trend charts positioned to the right of their pareto an overhead I show the pareto on one slide then the trend charts in pareto order on a second slide...
for project on Defect reduction you first have to define the defect or give the defect definition, type or Data is it defective type or defect type.
also gather data for atleast two month in measure phase for finding out the DPMO.
then try to do brainstorming and form FDM and also make Process FMEA for Injection moulding rejections.
in Analyze you do Y analysis for finding the variation of defects in diff model or mould, also find the time series graph for rejection for monitoring the the rejection pattern on time line. do hypothesis testing and chi square testing.
also try to use DOE for the optimized parameter in injection moulding process.

i have done three projects on thermoforming machine as defect reduction, cycle time optimization, sheet thickness reduction in samsung india



also gather data for atleast two month in measure phase for finding out the DPMO.
This sounds very much like a typical blind follower of six sigma. 30% rejects and he suggests collecting data for 2 mths to calculate DPMO. Yeh sure !

The first step in Scientific Method is observation. Take a step back and observe what is happening to your process. Careful observation may make some of your problems obvious. I have found this very powerful.
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