Short Molding Issue - Plastic Injection Molding from a Multi Cavity Mold

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.

Re: Short molding issue

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?

Re: Short molding issue

nurshafie said:

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.

Click to expand...

Welcome Shafie,

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

philiplim said:

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.

Click to expand...

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

Miner said:

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?

Click to expand...

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.

howste said:

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.

Click to expand...

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

philiplim said:

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

Click to expand...

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