This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

Short Shots to Correct Flash

The first step in troubleshooting flash is to determine whether it is a filling problem or a packing problem.


I have a part with flash. It ran many times in the past without flash, but recently we have been using a new machine and it is often creating flash. We have tried many adjustments including packing pressure, clamp tonnage, transfer pressure, etc.


The first step in troubleshooting flash is to turn off packing and determine if you have flash during first stage injection. This will tell you immediately whether the problem is occurring during 1st stage injection or second stage packing. Ultimately, this will focus your attention very quickly.

Additional Thoughts:

In many cases, you see technicians adjust parameters such as packing pressure and clamp tonnage when the problem occurs during injection. The problem with this approach is that the technicians almost never return the clamp tonnage or packing pressure to their original settings when they ultimately fix the flash. As a result, the plant gets lots of other defects such as shorts, sinks, and burns afterwards.

Documentation is the First Step

Documentation is the first step to good troubleshooting, it is the first step to continuous improvement, it is the first step to process improvement, it is the first step to better maintenance, it is the first step to getting your technicians up to speed faster, etc.


During a recent on-site training session, I was asked by the plant manager: What is the first step we need to take to get better?

My Response:

Better documentation is the first step you need to take to get better. With respect to processing, if you do not properly and accurately document the process that is being used to make a good part, there is nothing to use as a basis of comparison when troubleshooting a bad part. Everything that happens to a process, machine, mold, material, or a piece of auxiliary equipment should be documented. This includes everyone from tooling and processing to maintenance and die setters. Essentially everyone who works on the equipment or process must document what has transpired.

For example, if you don’t document the time and reason for the swapping-out of a failed water temperature controller during 2nd shift, the 3rd shift will inaccurate information when troubleshooting defects that might be found on that machine at the start of their shift and may not be able to locate the root cause of the problem in a timely manner.

Additional Thoughts:

When trying top get new process technicians up to speed faster, good documentation significantly reduces the learning curve. For example, let’s suppose a new technician notices delamination on the parts as well as an increase in transfer pressure, but no other significant deviations in the current process from the documented standard. In this case, he/she can place attention on why the pressure has increased. This approach puts the focus on what likely caused the pressure to increase which is much more efficient than trying to remember all the things that might cause delamination. If they correct the cause for the pressure increase and the delamination goes away, the new technician has a very high level of confidence that they corrected the problem and can move on to the next problem.

Correct Short Shot Size

The purpose of a short shot during 1st Stage Injection is to ensure that all cavities remain short at transfer during normal process variation.


I was told to short the mold around 98% based on final part weight, is that enough?

My Response:

Basing any short shot on the final part weight is an inaccurate and misleading process. This is faulty because there are so many factors which contribute to the percentage of material that is added to the mold between when the first cavity fills and the part is completely packed out. Aspects such as wall thickness, material shrinkage, filling imbalance, degree of semi-crystallinity, pressure loss, material consistency, packing pressure, etc. all make a general rule based on final part weight impossible. because of these factors, a short shot of 95-98% based on a fully packed part will likely be full in most if not all cavities at the time of transfer as well as show screw bounce.

The best approach is to use a short shot percentage based on 100% being equal to the weight of the shot when 1) only one cavity fills and 2) no packing is present. We typically recommend 90-95% short under these circumstances. If the packing is established correctly, the process will have enough pressure to keep the screw moving forward to fill and pack out the parts without flash. This will compensate for 1-2 percent check ring variation as well as 2-3 percent material viscosity variation.

Additional Thoughts:

If you are processing very unstable material such as post-consumer regrind having as high as 10-20% viscosity variation from shot-to-shot, then a short shot as low as 75% may be necessary to maintain a consistent final part weight.

Troubleshooting Sister Tools

When troubleshooting a problem which is unique to a particular tool, but not present in a sister tool, then the solution lies in locating the differences.


Company is encountering an unexpected reduction in part dimensions after conditioning where a growth in dimensions has been seen in all similar parts. The parts which are similar and produce expected dimensions are molded in ‘sister molds’ (term used for mold which is nearly identical in design and is molding parts of similar design). These parts have an expected dimensional growth across the entire part after conditioning.

Problem Solving Technique:

The solution to such a situation lies not in the new part alone, but in what differentiates the new system from the existing parts which produce expected results. This process involves comparing the new part, mold, process, & material until the difference is found. In theory two similar systems should produce similar results. If they do not, you need to find the differences whether it is a hotter core, reduced melt temperature, larger gate diameter, or a lower material drier residence time.

Once the differences are found, you can evaluate and test each to determine whether it is causing the unexpected result. For example, if a core is warmer, add an additional temperature controller to that cooling zone to reduce the temperature and determine if that causes a more expected result… Isolating and testing each potential cause is critical to determining the cause of a problem and its ultimate resolution.

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