Plastic Parts Mass - Plastic part weight to be +/- 0.5% from PPAP parts

I removed this post but will try to rewrite.

Customer asks for plastic part weight to be +/- 0.5% from PPAP parts. I have 4-cavity tool and variation for each cav. is ca. 0.002 g. Tolerance will be around 0.01 g. Problem is that difference between average weigth of each cav. is 0.02 g. Question is... should I set nominal weight for each cav. separately? It is not possible to modify tool in order to 'add' weight to some cavities.

Re: Plastic parts mass

Sorry if I am exaggerating, but atleast in auto industry, I have never seen Customers giving tolerance and leaving the Specification to the decision of the Supplier. May I know the application of the parts?
Thanks.

Re: Plastic parts mass

Chennaiite said:

Sorry if I am exaggerating, but atleast in auto industry, I have never seen Customers giving tolerance and leaving the Specification to the decision of the Supplier. May I know the application if the parts?
Thanks.

Click to expand...

I wanted to remove this thread but since you've answered ...

Calculated mass was given on drawing but final weight is based on actual result. Application is a part of fuel supply unit.

How much is the customer willing to pay for this, would be my first question. Appears to be a new requirement (a very tight one).

Thinking about shot size, material variation, process variation: how can you control this on a daily basis?

Yes, I would control by cavity.

Kales Veggie said:

How much is the customer willing to pay for this, would be my first question. Appears to be a new requirement (a very tight one).

Thinking about shot size, material variation, process variation: how can you control this on a daily basis?

Yes, I would control by cavity.

Click to expand...

I can't tell you the price and margin ;-) but believe me it's standard OEM price, nothing special.

To spice things up Ppk is >= 2.00 and Cpk >= 1.67. How can we control? I am not molding expert but what we will do is... We are going to control this taking 5 piece sample every hour, maybe more frequently and check weight.

Remember this is in Europe and what I have been exposed to is (180) from same standards in the US.

1. I agree on cavity control.

2. I would do a 30 piece study on each cavity by weight and demensional.

3. As long as the study shows capability and processes stability and meets all
testing, stack studies for mating parts, then accept.

4. Remember is the material from the same lot all the time, which is
impossible with potrolem products, and the temp, area, facility, weather.
Is it from the same supplier???.

5. I would do a study with different supplier material and show the customer
all conditions and acceptence and come to a agreement.

From my experience, the customer is concerned about part voids. Voids are usually caused by slow inject times, low inject pressures and low pack pressures. With insufficient cavity fills, as the part cools and shrinks and since the outer skin has hardened, the lack of material “forms a bubble” in the interior of the part. While developing the molding machine settings, the part weight is plotted vs a flash dimension or vs observed flash. You want to push as much plastic in without flashing the part.

Part weight is directly related to the compound’s density. What does your supplier publish for variation? The customer may have used an average value or past experiencefor their calculation. Ask them.

Part weight will need to be collected by cavity. The cavities should track together or in a similar fashion. Your control charts WILL probably go out of control with a change of compound lot number.

As for “Ppk is >= 2.00 and Cpk >= 1.67”, this is not possible. Maybe, Ppk >= 1.67 and Cpk >= 2.00, but Ppk maybe even lower. (To be confidant that these values are correct, your gage R&R will need to be about 5%.) In my experience the customer only wanted Cpk >= 1.67 across a lot of compound as Ppk was not relevant.

UNLESS, your customer is trying to control weight in order to control float balance. In this case your customer has a poor design and has elected to nominate part weight as a "Critical" specification for the manufacturer to meet. Ask to see the portion of the DFMEA which made part weight critical. If this is the case and your price is already set, then ... (another plan will need to be developed).

world quality said:

Remember this is in Europe and what I have been exposed to is (180) from same standards in the US.

1. I agree on cavity control.

2. I would do a 30 piece study on each cavity by weight and demensional.

3. As long as the study shows capability and processes stability and meets all
testing, stack studies for mating parts, then accept.

4. Remember is the material from the same lot all the time, which is
impossible with potrolem products, and the temp, area, facility, weather.
Is it from the same supplier???.

5. I would do a study with different supplier material and show the customer
all conditions and acceptence and come to a agreement.

Click to expand...

I did a study for 125 parts/cavity and if I adjust nominal value for best cpk I get at least 4,00 for each cavity.

Yes, material is from same supplier (nominated by customer) - always .

As for “Ppk is >= 2.00 and Cpk >= 1.67”, this is not possible. Maybe, Ppk >= 1.67 and Cpk >= 2.00, but Ppk maybe even lower. (To be confidant that these values are correct, your gage R&R will need to be about 5%.) In my experience the customer only wanted Cpk >= 1.67 across a lot of compound as Ppk was not relevant.

Click to expand...

I give you values from cutomer's spec. That's how it is written. As I understand Ppk is for 125 pcs (min. qty for this study) and Cpk a long term value. Is that right?

I don't have the MSA for the gauge (weight) so I can't write what's R&R value.

Thank you for your answers.

ario80 said:

As I understand Ppk is for 125 pcs (min. qty for this study) and Cpk a long term value. Is that right?

Click to expand...

No, Cpk is for 125 pcs (min. qty for this study) and Ppk a long term value.

Why would anyone put specifications on part weight? The part weight by itself is very seldom a critical parameter, but impact strength is another matter. Most impact tests destroy the part, and impact tests take more time to run and require much more expensive equipment than weighing parts.
As ADAMSJM explained, poor packing leads to voids in the center of the part. Voids tend to be more frequent problem in semi-crystalline plastics like nylon and acetal resins than in amorphous plastics. Since this is a fuel supply unit, and that usually requires good solvent (gasoline, diesel, methanol, ethanol, etc.) resistance, the resins is likely to be semi-crystalline .
Voids in parts can lead to part failure. Running impact tests, X-ray imaging, cutting the part in half, etc. can detect voids, but weighing the part is an inexpensive way of trying to detect under-packing.
Different average weight for different cavities could be caused by differences in cavity dimensions or in de-gating, but different average weights could also reflect deferent degree of packing. If I were the customer the parts were molded for, I would ask for 10 of the lowest weight parts (-3 sigma) from each cavity, and run impact tests and cut these low weight parts in half; impact test results OK and acceptable voids, then I would agree to different weights for different cavities.
Ernie7