Hi,

I'm a quality Engineer of a connector manufacturing company.

We do plastic injection molding parts and this particular prduct that we are producing is from a multi cavity mold. As the parts are quite small, there bound to be short shot parts (imcomplete filling of the plastic resin) being produced. The problem is that the defect rate is very very low and now we are performing 100% screening before we derive with a solution. Out of 220K of goods, we only detected 4 pcs.

Is there a way to implement some form of control charts or sampling plan without the need to perform 100% inspection.

Please help.

Regards.

You are running at 5.6 to 5.7 sigma and it is at the good position. With your continous improvement, the next 3 lot you will produce, you will achieve 6 sigma.

There is a lot of sampling standard that you may can follow. Such as ANSI, JIS or DN sampling standard. You also can develop your own reasonable sampling standard as long as your confident level is high (base on your back history, observation, experience..etc).

Even, you have perform 100% with your part, can you ensure it was 0 defect? So to shorten your inspection period, use sampling standard. Normally people will accept ANSI.

When you are at such low levels of defects, throw your sampling plan away. It is not worth the electrons that it is stored on.

You have two choices:
100% inspect like you are doing now, or do zero inspection. The choice should be based on the economical risk of missing the defects.

Do you have any options of automating the inspection?

You are running at 5.6 to 5.7 sigma and it is at the good position. With your continous improvement, the next 3 lot you will produce, you will achieve 6 sigma.

There is a lot of sampling standard that you may can follow. Such as ANSI, JIS or DN sampling standard. You also can develop your own reasonable sampling standard as long as your confident level is high (base on your back history, observation, experience..etc).

Even, you have perform 100% with your part, can you ensure it was 0 defect? So to shorten your inspection period, use sampling standard. Normally people will accept ANSI.

Welcome Shafie,

Good to see your post and please continue with your active participation.

Hi,

I'm a quality Engineer of a connector manufacturing company.

We do plastic injection molding parts and this particular prduct that we are producing is from a multi cavity mold. As the parts are quite small, there bound to be short shot parts (imcomplete filling of the plastic resin) being produced. The problem is that the defect rate is very very low and now we are performing 100% screening before we derive with a solution. Out of 220K of goods, we only detected 4 pcs.

Is there a way to implement some form of control charts or sampling plan without the need to perform 100% inspection.

Please help.

Regards.

Thats not a problem - if I was a Customer, I would immediately ask your organization name and address and have my parts manufactured there ;)

When you are at such low levels of defects, throw your sampling plan away. It is not worth the electrons that it is stored on.

You have two choices:
100% inspect like you are doing now, or do zero inspection. The choice should be based on the economical risk of missing the defects.

Do you have any options of automating the inspection?

Not possible to automate the inspection as it is very costly to incoporate vision checking system.

What is the risk of shipping a short shot? What is the severity associated with short shot from your PFMEA? If risk or severity is high, you must continue to sort. If that risk is acceptable, then you may be able to alter your detection routine.

What have you done to prevent the short shots? Have you conducted a mold balance analysis? Has the tool been optimized? Has the process been optimized by using decoupled molding? Don't get hung up on improving detection if you haven't exhausted your options in fixing the root cause of the issue. Why is the process producing short shots?

Once your process is set up and is validated, you can do some machine monitoring to predict shots which contain suspect parts. This may reduce the population which needs sorted.

rmf180 gave an answer similar to what I was thinking.

If the consequence of shipping a short shot is not severe, see if your customer will accept a few extra parts per shipment to replace any potential short shots.

rmf180 gave an answer similar to what I was thinking.

If the consequence of shipping a short shot is not severe, see if your customer will accept a few extra parts per shipment to replace any potential short shots.

Short shot is a very severe issue at our cutomer side. It means the contacts will be exposed and may result in short circuit problem. This short shot issue is very tricky and there are many variables related to this. We can't really narrow down which area is giving the problem.

Below are the possible root causes:
1) Process
- Insufficient injection pressure/ injection rate insufficient material feed
- Excessive cooling of the melt

2) Mould
- Unbalanced multi-cavity mould
- Insufficient air venting blocks resins flow Runners, gates, or vents too small
- Material flow length too long.

3) Material
- Material viscosity too high
- Foreign material clogging nozzle and/ or gates
- Mould temperature too low

4) Machine
- Feed hopper blocked
- Barrel has no resins left
- Undersized cylinder heating capacity
- Material leaks/ back flow

Short shot is a very severe issue at our cutomer side. It means the contacts will be exposed and may result in short circuit problem. This short shot issue is very tricky and there are many variables related to this. We can't really narrow down which area is giving the problem.

Below are the possible root causes:
1) Process
- Insufficient injection pressure/ injection rate insufficient material feed
- Excessive cooling of the melt

2) Mould
- Unbalanced multi-cavity mould
- Insufficient air venting blocks resins flow Runners, gates, or vents too small
- Material flow length too long.

3) Material
- Material viscosity too high
- Foreign material clogging nozzle and/ or gates
- Mould temperature too low

4) Machine
- Feed hopper blocked
- Barrel has no resins left
- Undersized cylinder heating capacity
- Material leaks/ back flow

You are right. Short shots can be caused by various problems with process setting, tool balancing, machine ,...

I recomend you to perform an analysis to find out when short shots occures and at the same time to find out a route cause (by checking process,material,drying,machine) of the failure
Based on the analysis you can set up a priority list.

- 80% of short shots are caused by process setting. Short shots shout occure when you start/restart the machine - first 5 shots are ussually directly sorted out (manualy, by robot or sorting flap)
If you have short shots during the production then something else is wrong or someone changed the process :(
- Mould balancing: you can perform "filling study" to check if this is your case.
- Material: its up to your incomming inspection and purchasing - material certificates (I suppose that you use the correct material)
- Machine: machine maintanence

So the solution it not easy. You need to get your process, machines,... under control then you can minimize short shots

Not possible to automate the inspection as it is very costly to incoporate vision checking system.

I am not familiar with this process, but would a simple weighing of the parts reveal a short shot? To me this would seem to be quicker and more reliable than a visual inspection.

Short shot is a very severe issue at our cutomer side. It means the contacts will be exposed and may result in short circuit problem. This short shot issue is very tricky and there are many variables related to this. We can't really narrow down which area is giving the problem.

Below are the possible root causes:
1) Process
- Insufficient injection pressure/ injection rate insufficient material feed
- Excessive cooling of the melt

2) Mould
- Unbalanced multi-cavity mould
- Insufficient air venting blocks resins flow Runners, gates, or vents too small
- Material flow length too long.

3) Material
- Material viscosity too high
- Foreign material clogging nozzle and/ or gates
- Mould temperature too low

4) Machine
- Feed hopper blocked
- Barrel has no resins left
- Undersized cylinder heating capacity
- Material leaks/ back flow

If shorts are serious as indicated, you need to keep detection efforts in place. 100% inspection is probably not good enough. Adding a second 100% can be a good short term effort, but automation is really the only option.

1) Process
- Insufficient injection pressure/ injection rate insufficient material feed
- Excessive cooling of the melt

Both of these conditions are signs of poor tooling. Tool should be optimized to eliminate pressure drops or flow stall points.

2) Mould
- Unbalanced multi-cavity mould
- Insufficient air venting blocks resins flow Runners, gates, or vents too small
- Material flow length too long.

Mold balance studies should be a standard part of any validation process. If your tool is not balanced, your tool needs fixed. Same is true for insufficient venting. Mold flow analysis is a great tool to predict ability to fill parts and determine gate location(s).

3) Material
- Material viscosity too high
- Foreign material clogging nozzle and/ or gates
- Mould temperature too low

Material should be fairly consistent once you have validated your process. If you are seeing wild ranges of material properties, you need a better resin supplier. If your purchasing department buys from the lowest bidder, you should start looking for a different job. Mold temperature is a critical parameter and should be controlled by your control plan.

4) Machine
- Feed hopper blocked
- Barrel has no resins left
- Undersized cylinder heating capacity
- Material leaks/ back flow

Machine inputs should be controlled by an effective PM system. Machines should be sized appropriately when the project is started up. Material leaks and back flow are caused by broken screw tips and or check rings.

Your list is accurate related to potential causes, but these causes should not be an integral part of your operation. These items must be taken care of and eliminated from future concern if you are running critical components. Sooner or later your customers are going to figure out that your systems are not mature enough and will send the business somewhere else.

I would suggest:

1) Evaluate your machine and repair all issues noted.
2) Perform a mold balance study (using part weight as noted by Craig). All cavities should be within %10 of each other (maximum).
3) Perform CV study. This will show you consistency (or lack thereof) from shot to shot.
4) Complete a viscosity curve study to determine correct injection speed. Done correctly, this will allow the machine to "compensate" for viscosity changes from lot to lot.
5) Perform gate freeze study to determine correct fill time. This insures that we are not over or under filling cavities.
6) Tool pressure drop study to look for signs of poor runner/sprue/gate design or locations.

Document your settings from these settings and ensure that the process is always run in this way. Many molders think that molding is black magic. In fact one can take a very scientific approach which results in greatly reduced variation. It takes a little longer up front, but we don't have short shots!:D

Is there a way to implement some form of control charts or sampling plan without the need to perform 100% inspection.

Please help.

Regards.

Presuming you are dealing with the root causes as many have suggested, for sampling plan we use an old school method here as a guide.

Hi,
I've been thru exactly this issue (with board to board connectors)
1. 100% inspection doesn't work - forget it
2. short shots are not inevitable, Taguchi to find the critical process parameters (in our case holding time & clamping force)
3. impose process control on those critical parameters, most modern machines have data logging, use it and stop setter / operators tweaking the process.
4. we also monitored piece part weight very closely (.01 grammes) by selecting heaviest and lightest cavities (plotting on x/r chart) you can quickly identify trends, with your current level of defects you should be able to impose very tight control limits and when you see trends within those limits you can tighten the process control further.
5. I found that closing off "dodgy" cavaties was a complete disaster as it unbalanced the process totally

To achieve what you aspire to is very hard work, educating mould shop personnel, using controls which will "appear" draconian, and your tooling guys will squeal like hell when you identify cavity to cavity variation.

Good luck, its a tough task but it can be done, but only by applying the tools and discipline.