Home » Injection Molding Troubleshooting
Silver Streaks / splay is usually caused by moisture, but can also be caused by high filling rates and poor vents which can cause entrapping of gas either from the environment, mold water leaks, etc. A single line that appears like a silver streak, I refer to as jetting. There are many causes but some to look at are the following. Silver Streaks
  • The filling process should be such that the speed is not so high until the gate is reached and material will solidify causing less shear heat as the injection speed is ramped up. If you are blasting the material through a small gate, random shear stress can cause burning and jetting.
  • In the case of a nozzle, if small amounts of material solidify on the nozzle tip and then subsequently get injected into the mold/gate, the cooler material will tend to hang at the gate and free flowing material will flow around this point. Where the injected material meets can cause what looks like a silver streak but can actually be a single line that is actually a flow or knit line.
  • Having a cold slug well at the end of the sprue/runner can trap this initial material from the nozzle before it gets to the gate and cavity.
  • Gate thickness variation can cause the same issue if the tooling/ gate dimension is not equal.
I will not go down the road around the tooling or processing assumptions in this matter but concentrate on the fact that it's a black colored part, and the black color has been introduced by using a masterbatch. There are a number of theoretical possibilities linked to the coloration as such and around the formulation of the masterbatch. The black colorant is likely a carbon black but could potentially also be a back iron oxide.

Establish qualitative knowledge of the details of the masterbatch used linked to above mentioned factors. By a good co-operation with your masterbatch supplier, this should not be a problem to discuss.
Investigate by DSC if the colorant introduced influence the Tc of the resin vs the neat resin. May be complemented by actual measurements on shrinkage between colored and non-colored parts as especially LDPE is difficult to measure statistical Tc differences on.
An understanding of why the product does not break while functioning is helpful. If you know specifically what in the formulation makes the part not break when rotating then you can focus on that ingredient. Perhaps the original formulation development notes can shed some light. Why talc is used? Is it added to reinforce or physically strengthen the material or for some other property like opacity or just to cut costs?

If I saw your part I'd go down a checklist:
  • Is the break point random or consistent?
  • If consistent is it weld line related? Or vent related?
  • If part of an assembly, does the mating part affect it somehow? Inserts can cause cracks in mating parts.
  • Failure during rotation: Does the load change or is it erratic or continuous?
I have seen the problem of an incremental change in cushion each cycle for about 20 shots and then begin to decrease and return to nominal once before on a different make of injection molding machine. The shot size was minimal, 20% of the max. It was an amorphous resin, (I believe PC/ABS Blend). As the cushion would change we would go from making good parts to parts that were over packed and showed ejector marks. The pressure at cutoff changed with the cushion, as you would expect.
We plotted all of the TC outputs by cycle, barrel zones, feed throat, thermolator, and thermocouples we attached to the tool. All of the zones appeared to be working normally. We spent a lot of time watching the feedback from the transducer tracking the screw position. We even hooked it up to a PC to monitor.

The more information that you have to look at, the better your troubleshooting will be. RJG is a great way to take a look into the mold and see what is truly happening relative to injection, pack, hold and recovery (your injection process). Without the look into the mold, it really becomes more of a game of trial and error.
The solution:
Step 0 - train everyone - there is nothing magic in injection molding. This is a skill, neither an art nor a science. From the initial qualification run you have created the 'recipe'. From there on out, folks follow the recipe. If you can't do that yourself, contact me. I got books/training program and spreadsheets. NOT EXPENSIVE
Step 1 - work on efficient setup - Training
Step 2 - make sure your machines and molds are properly maintained - Training - do you have a program?
Step 3 - work on standardize process guides where only a little algebra (via a spreadsheet) will give you the conditions for a different sized machine. - Training Step 1
Step 4 - learn the 'method' for troubleshooting. I train this also. Troubleshooting can make a lot of scrap if you don't know how to do it efficiently.
Regarding your stringing issue you can check these points:

Melt temperature too high:

=> Decrease melt temperature.
Decrease hot runner temperature.
Decrease screw rotation speed.
Decrease hot runner temperature (avoid nozzle

Gate area too hot:

=> Decrease mold temperature.
Check contact tightness between nozzle tip and
Check mating between nozzle tip and mold.
Check isolation cap thickness.
Use colder nozzle / cone point insert.
(A) "Why is the cooling time THAT long"
Because that is what the shrink rate built into the injection mold will allow for dimensionally acceptable parts. I have optimized gate seal, cooling rate, fill rate and melt temp. Unfortunately the injection mold maker missed the shrink on the steel.

(B) "in plain language how do you duplicate the process in machine "A" when you put that mold in machine "B" that has a different size/capacity injection unit and clamp and not start from scratch?"

NO I duplicate the plastic conditions from machine A to machine B, Melt temp, Flow rate, pack/hold pressure (calculated by knowing the difference in Ri) and cooling rate (calculated using Reynolds # and coolant temp)

(C) even better "How do you know that waterline hookup is (1) optimal for that mold and (2) identical to the last five times you ran it?"

Reynolds calculation and/or delta P as for pattern I consult the water line diagram.
These are some cheap things to try before getting more involved with modifying the injection mold with added mechanics, coatings or adding cooling. Assuming you have covered the processing side.

Troubleshooting the problem this way. First eliminate thing which cause the part to stick. Look for undercuts, insufficient draft, vacuum, texture, etc. Then provide draw polish. If there is still a problem you should then provide positive pull on the moveable. If the other side is clear of problems very small undercuts should suffice. In an extreme case you can shorten KO pins and undercut. The vestage can be cut or broken off.
The flow lines are related to the one side of the injection mold, not necessarily the geometry of the part or the composition of the plastic. Follow the instructions below to find the problem.

If the injection mold is in the press, check the following:
There is usually an offset between the injection mold steel that comes in contact with the plastic and the mold base. This is usually between .004" and .010", (.10mm to .25mm). With the injection mold closed and locked up, insert a feeler gage into the gap between the A & B sides. The gap should be very close from the left side of the mold to the right side of the mold. The right side of the injection mold will have a smaller gap. This makes the shut off surfaces of the tool not parallel.
Voids can appear in two areas and both are cause by the same phenomenon. Plastic flows laminar "period". As the plastic enters the cavity depending on the shape, there will be volume changes. The material as it flows will go changes in velocity which has a direct bearing on the material viscosity. Sharp corners create the biggest changes where the material wants to continue to flow straight, but laminar flow forces a portion of the material to wrap around the corner, thus forcing a separation of the material and a void is formed. At the end of fill you can repack this void if the material remains molten enough to do so. Valve gate sizes, venting, injection speed, moisture, shrinkage, hot runner bore channels, decompression, and whatever else you want to throw at it, has no bearing on this phenomenon. What does correct it is possibly increasing the injection mold temperature, but you may have to go so high that it sacrifices cycle time and part dimensional's. The other is to change the plastic part geometry to minimize sharp corners, or move the gate location so the part is filled differently minimizing the filling voids.
Increase the venting of the plastic injection mold. From my experience in the connector industry, venting is the number one culprit for variations in plastic parts quality. I have had many injection mold makers tell me, the plastic injection mold can not be vented anymore. Then after another review of the injection mold, we manage to increase the venting. Sometimes the vents have no channels out to the atmosphere, ejector pins/blades are vent channels (is the injection mold maker greasing the ejectors, results in no venting) Sometimes, you may need to split an insert into several inserts as a way of increasing the venting. Vent the runners.

In the case of the beginning of a run, the holding fixtures have acceptable warpage. Then the warpage increases after 2 days. This may be an indicator the vents are plugging and need cleaning. Make sure to vent to channels before going to the atmosphere as a way to help maintain the venting condition. SPC part weight during the production as a measuring tool. You may discover that your processes are not under control to begin with.
The gloss level of glass filled nylon is mostly controlled by the injection mold heat. Because the resin shears away from the fiberglass with the fountain flow effect you get a resin rich surface. Higher mold heats allow the resin to mirror the tool surface best.

Surface interruptions that disrupt the fountain flow will create dull spots. If you shoot past a pin, as the plastic moves past the pin, you will see a duller surface on the opposite side. You can reduce the effect of pins that do not go through the entire wall by generous radii. The same holds true with ribs.

Lastly, as you increase the injection mold temperature, you will also increase the crystallinity of the plastic part. This will change the plastic properties, most notably, the shrink rate. Your plastic part will get smaller.
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