SPC and Injection Moulding

[cyberjyothi]

Hi all,

we have injection molding process for producing container and covers, which is typically used in the battery manufacturing industry.

We are suppose to be implement control chart (Xbar -R chart) for the quality characteristic "container height".

For the above, we need ur support on setting up trial control limits for the said charts.

first of all,we already fix the subgroup size is 5,for that

How to calculate trial control limits & What is the frequency we follow when collecting the data for the trial limits.

Thanks in advance.

Jyothiswar

 

[Howard Atkins]

First thing before you set up the system ask these questions:

1. What is the shrinkage rate?
2. When are you going to measure in respect to the shrinkage?
3. What is the production rate?
4. How many cavities in the mold?
5. What are the size of your typical runs
6. Who requested the SPC?

After you have those answers come back and I will continue

 

[cyberjyothi]

Sir,

Pl. find my answers to ur questions.

1. What is the shrinkage rate?
1.40 to 1.70
2. When are you going to measure in respect to the shrinkage?
Nothing we can measure with respect to shrinkage. We measure the components (containers) height after 12 hrs of ageing
period.
3. What is the production rate?
We are producing about 720 containers per shift (cycle time is 80 sec).
4. How many cavities in the mold?
2 cavity mold.
5. What are the sizes of your typical runs?
We run up to 3 to 4 days continuously.
6. Who requested the SPC?
We only initiate to adopt spc method.

thanks in advance

 

[Howard Atkins]

Sir,

Pl. find my answers to ur questions.

1. What is the shrinkage rate?
1.40 to 1.70
2. When are you going to measure in respect to the shrinkage?
Nothing we can measure with respect to shrinkage. We measure the components (containers) height after 12 hrs of ageing
period.
3. What is the production rate?
We are producing about 720 containers per shift (cycle time is 80 sec).
4. How many cavities in the mold?
2 cavity mold.
5. What are the sizes of your typical runs?
We run up to 3 to 4 days continuously.
6. Who requested the SPC?
We only initiate to adopt spc method.

thanks in advance

In my mind the importance of SPC is to give an on-line warning of changes so that before the part goes out of tolerance then you can act.
If you are going to measure the part 12 hours after production then you are losing this aspect of SPC and any reaction that you have is going to be to late.
In my experience the dimensions on a well set up press vary very little after set up as long as outside parameters such as material, temperature etc do not change. The majority of problems are visual issues, flash, splays etc.
I would seriously ask yourselves why you want to do the SPC, what are the expected results of the implementation.

After you think about this then we can discuss sample size.

 

[cyberjyothi]

Sir,

First of all, we were measuring the Container height at the beginning (immediately after demolding i.e., within a 5 sec of time), for this only we want to initiate SPC, after then we again measure after 12 hrs of ageing period. We are having two spec (one is for demolding, other for ageing).

Sir, You are right on SPC is to give an on-line warning of changes so that before the part goes out of tolerance then you can act.

Why I'm giving this is, (When are you going to measure in respect to the shrinkage? Nothing we can measure with respect to shrinkage. We measure the components containers height after 12 hrs of ageing period) to tell about we are not going to measure the aging dimension with respect to shrinkage.

Thanks for reply.

Jyothiswar

 

[Howard Atkins]

Sir,

First of all, we were measuring the Container height at the beginning (immediately after demolding i.e., within a 5 sec of time), for this only we want to initiate SPC, after then we again measure after 12 hrs of ageing period. We are having two spec (one is for demolding, other for ageing).

Sir, You are right on SPC is to give an on-line warning of changes so that before the part goes out of tolerance then you can act.

Why I'm giving this is, (When are you going to measure in respect to the shrinkage? Nothing we can measure with respect to shrinkage. We measure the components containers height after 12 hrs of ageing period) to tell about we are not going to measure the aging dimension with respect to shrinkage.

Thanks for reply.

Jyothiswar

When I said "When are you going to measure in respect to the shrinkage?" I meant in terms of % shrinkage when are you going to measure. Not if you are going to analyse the shrinkage, sorry if I mislead you.
The measurement after demolding though means you must make a study because you are going to give a "virtual" dimension which only exists for a short period of time.
Question: How many dimensionally problems have you had?
How much has the rejects from dimensional problems cost?
Is it cost effective?

 

[Proud Liberal]

Additionally, I would add that molding exhibits autocorrelation and therefore is Not a candidate for X-bar and R charting? Try IX/MR charts instead.

 

[minudil2000]

dear sir,
i am a university student and doing a project on spc and sewing operation ( garment factory ) . don't you know how to apply spc to a garment factory. plz share your knowlegde

 

[gszekely]

Additionally, I would add that molding exhibits autocorrelation and therefore is Not a candidate for X-bar and R charting? Try IX/MR charts instead.

Hello Proud Liberal !
Could you explain please:
"molding exhibits autocorrelation", what do you mean by this ?
What type of chart is IX ?
MR_ I suppose is moving range,
When you say: "Not a candidate for X-bar and R charting" do you mean SPC on machine settings(parameters) or SPc on product dimesinon ?
Thanks in advance.
BR
György

 

[Tim Folkerts]

"molding exhibits autocorrelation"

This just means that the measurements are not independent and random. Knowledge about one measurment gives information about the next measurement.

For instance, suppose you draw 4 sequential parts from a molding operation. If the first is a little small, the others will also quite likely be a little small. If you draw 4 parts an hour later, they may all be a little large. The range for each subgroup will not accurately predict the variations expected in the Xbar values.

What type of chart is IX ?

I assume it is just an Individual chart (I chart), but I'm not 100% certain.


Tim F