Plastic Injection Defects: Troubleshooting & Correcting Flash

There is a solution to troubleshooting any molding defect you may encounter. Before you begin.. if you’ve made good parts before, the only reasonable cause for any defect is “something has changed.” Run the parts with lower pack/ hold pressure until a short occurs first! If the flash is still there, inspect your mold for build-up, damage, etc. Flashing under these conditions generally point to a tooling issue.

Flash is probably the simplest and most common defect to understand. Molded parts are actually created in the same way as flash: Molten material enters an area, cools, and becomes solid prior to ejection. Simply put, if there’s a hole and molten plastic can get into it.. it will do so.

Here are the primary areas that a processor will need to analyze when they are diagnosing the root cause of the symptom, flash:

Environment

This is the plant’s 5S cleaning procedure, temperature, operating procedures,etc. The only cause of flash that can be considered in this section is some kind of maintenance failure. I’ve seen screwdrivers or channel-locks used to remove parts or clean molds. Make sure that your personnel have been properly trained in how to clean and care for your tooling. The first step in dealing with nearly any defect is mold cleaning and inspection. This can save you alot of time when it becomes common practice.

There are a number of factors that could require the next shift to change the setup:

If your process conditions have not been optimized, even a few degrees difference in the plant temperature can cause changes. During the day it’s hot.

A burned-out heater may be being compensated for by overheating their contributing partners. Watch for changes in your melt temperature.

When plants fail to standardize their watering procedures, the result is multiple watering approaches. Changes in mold temperature or turbulent flow in your water circuitry can cause variances in your material flowfronts.

The plant cooling water can change by a few degrees from day to night.

Equipment:

Machine

Pressure is resistance to force. If the plastic force is greater than the pressure holding the platens together, the mold can be blown open. Valve leaks can be causing this, variance in head pressure and so on. These issues require a maintenance solution. Watch for pressure fluctuations in clamp pressure, peak pressure, etc.

Poorly squared platens will close up very firmly on one side while the other will be loose. This can cause a drop in clamp pressure. This also requires a maintenance solution. Inspect the platen bushings and tie bars for signs of wear and make sure they are lubing correctly.

Overtightening mold clamp bolts will begin to pull the threads out of the platen, leaving high spots. While the actual platens might be square to each other, the high spots will cause molds to close improperly. This requires a maintenance approach. Add checking platens for high spots and burring from molds bumping the platen as they are set and pulled to your set-up procedure. Each time a mold is hung, scrape the platen with a straight edge. Any high spots discovered should be stoned flat prior to clamping. It may add a couple minutes to your set-up, but may save you hours of troubleshooting.

Mold

Components: check the part and the runner for an artificial vent—e.g., a paperclip, duct tape, metal shim

Look at the front and back sides of the mold before clamping it in. Are there any loose bolt heads stopping the mold from hanging square?

Use a torque wrench and tighten all the bolts on both halves of the mold to the same amount. Different torque can cause poor tool seating.

Are there enough pillar supports so that the ejector plates aren’t bending under the pressure of injection?

Are the ejector-half plates thick enough to not bend? Do they have enough bolts holding them together?

Have you blued the parting lines? (If you don’t know what this is, ask a moldmaker.) Put it on one half of the mold; then, under full clamp pressure, check vent shut offs. If not, machine away material beyond the shutoff to increase the tonnage as well as rework it.

With a flashlight, look into the leader pin holes. Remove all the crushed and impacted pellets/parts/crud. If holes are filled with foreign material, the mold can’t close regardless of pressure.

Did somebody close up on something? Or did they close the mold on a part and bend or damage something?

Is there a bent leader pin?

Are the ejector pins flush when eject retract is in the back position?

Did you clean and lubricate the mold?

Was the die height and low pressure close set properly (older presses)?

Process Variables:

Heat

Check the mold heat. Make a comparison of setpoint to actuals.

Check material melt teperature. Where are you in comparison to the manufacturer MSDS window? Where are you at in comparison to your melt temperature data from previous runs?

Pressure

Pressure will blow the mold open after it is filled. You can blow open the mold if several cavities are filling early, put too much material under high pressure, or if packing pressure is too high. Fill pressure and fill time are related. You can also weigh each cavity, and then make a comparison of part weight to establish whether you are overpacking due to a poorly balanced runner system. If you are running a hot runner mold, you may be able to adjust drop temperature to establish a more equal filling pattern.

Time

Fill time should be established at 95-98% of mold fill without pack/ hold. The last 2-5% of the time will be utilized for going from fill to pack. If you slam fast-moving, low-viscosity melt into a full mold, flash can easily be the result.

Packing time should be based on gate seal requirements. It should have little to do with flashing.

Speed

The speed of fill is what allows viscosity to drop. Fast fills with adequate venting and appropriate melt temperatures fill molds... not flash them. Switching to packing mode slows velocity, thus increasing viscosity and reducing the possibility of flash.

Heat is a minor component of inherent viscosity; therefore, the speed of screw recovery (also backpressure) should be considered. As a general rule, the screw stops turning and is fully decompressed 1-2 seconds before the mold opens.

Position

The screw must return to the same position every time. Cushion must be maintained. Cushions are designed to pressurize plastic in the cavities to offset shrinkage of cooling material. Cushions too large don’t transmit cavity pressure; too small and there’s not enough material to create pressure. Cushion will be best determined by the return position of the screw. Since most processes are "transfer by position,” if the screw picks up too much material, the mold can be filled before it switches to packing mode. Also check the decompression after rotation position. If the screw shows signs of struggling, your decomp aft. position, pressure or heat profile could be incorrect.

By monitoring your process and evaluating your machine, you will be able to correct the condition or at a minimum develop an action plan to remove the problem.