INJECTION MOLDING: Troubleshooting Bridging

During my seminars, I repeatedly state that there are hundreds of details to keep track of on the shop floor. The most important one isthe one that is holding up production, and it never ceases to amaze me how some of the simplest of things can bring everything to a halt. One of these is bridging in the feed throat—an aggravating problem that results in significant downtime.

A feed throat is considered bridged when resin is obstructed from feeding the screw flights. Before we go any further, a note on safety: Remember that moisture vapor and other volatiles need to vent out of the feed section of the screw through the feed throat. If pressure builds up due to bridging, resin granules and/or a hopper can actually blow off the feed throat. If you are going to work on the feed throat or look into the hopper, make sure you know what is in the barrel, that there is no pressure building up, and you have appropriate eye and face protection. Keep in mind that pressures inside the barrel can build to thousands of psi, and if this pressure releases, plastic granules may blow into your face, eyes or ears.

With safety concerns addressed, the first challenge to fixing bridging is to find out what is blocking the feed throat. Empty the hopper and vacuum out the feed throat. Please do not use an air hose to blast the pellets out of the hopper or feed throat. An aid to vacuuming the feed throat is to provide a tube or some type of air supply that allows air in so the vacuum picks up the pellets. Now you’ll get to see what is causing the bridge. Some of the more common possibilities include:

1. Contamination such as cardboard, paper, fabric, etc.
2. Large chunks of regrind.
3. Streamers or angel hair, long strands of plastic ribbons, which can be flat, thin or string-like.
4. Plastic granules lightly stuck together, relatively easy to break apart.
5. A ball of solid plastic, where there are few if any individual granules seen.

Let’s examine these one by one and suggest solutions.

•Contamination: First remove it then dig further and find its origin. The source has to be identified and eliminated. The nature of the contamination should provide a clue to its source.

•Regind chunks: They may occur if your grinder is not working properly. Check the grinder and repair it.  Usually the problem is that the blades are worn, chipped, or out of alignment.

•Streamers or angel hair: These are common when handling and air–conveying plastic granules. The longer the distance of travel the worse it gets. Again, the source must be identified and eliminated. Streamers not only bridge the feed throat but also wreak havoc with your process, especially screw recovery. Your conveying system may be at fault or streamers may come in with the resin directly from the supplier. Find the cause or use a deduster to clean the resin granules of streamers and fines before they enter the feed throat.

•Agglomerated plastic granules: Plastic granules lightly stuck together are not all that common. They occur when the granules somehow were heated to the point that they became tacky and stuck together. Typically, the processor lowers the feed-throat temperature; however, the rule is to find the cause before adjusting the feed-throat temperature. The cause could be the dryer running too hot or a more subtle issue like the desiccant bed cycled into the drying air too soon after regeneration and not cooled before introduction into the active air stream. The desiccant may be regenerated at 500 F (260 C), so a slug of superheated air hits the resin for a few minutes and the granules stick together. Another possibility is the dryer temperature is set too high. If it is one of these dryer issues, lowering the feed-throat temperature will not remedy the problem.

Take some time and check out the dryer. Then check the actual temperature of the feed throat. Is it operating at your setpoint? Is the cooling line open and controlling the temperature? In most plants, water quality is poor and it is common to see the water lines clogged with lime and rust. Bottom line, if your feed throat is operating below 150 F (65 C), I would not lower the feed-throat temperature for most resins. The feed throat needs to run warm to work properly as a vent. Few resins get tacky anywhere near 150 F. I recommend that the feed throat be PID temperature controlled just like the other zones on the barrel. This is not an expensive modification and it will provide a more robust process. My last choice is to reduce the feed-throat temperature, unless it is above 150 F.

•Ball or chunk of solid plastic: Bridging is almost always caused by a completely frozen, solid chunk or ball of plastic wedged in the feed throat, a real chore to break apart and clear (see photo). This chunk is not easy to reach and difficult to break up into smaller chunks to vacuum out. If you have this, you must think about how this plastic ball formed in the first place and wound up in the feed throat. If you check actual feed-throat temperature, my bet is that it is far below the melting point of your resin. For example, most polypropylenes melt well above 230 F (110 C). A feed throat at 70 to 150 F (20–65 C) will not melt your polypropylene. So how did this “bridge” of solidified polymer form?    

To understand the formation of the solid chunk of plastic, we need to review screw design. Assuming you have a “general-purpose” screw with a 20:1 L/D ratio, you have a screw with 10 flights of feed, 5 flights of transition (the melting zone), and 5 flights of metering (the pumping zone). The 10 flights of feed are deep and are meant to auger and compact the granules, while allowing the air between the granules to escape or vent out the feed throat.

Most of these 10 flights of feed can be seen by looking down the feed throat (remember, safety first!) as someone operates the screw pullback function on the machine’s controller.

Check to see if there is any plastic that melted and froze on these flights. The feed section is not designed to melt the granules, just to auger them forward, so there should be no solid polymer stuck in or on these flights.

If you see melted or solidified plastic other than the individual granules, then you have found the problem, and it is not the result of the throat temperature being too hot. Where did the melted plastic that froze to form the bridge come from? You could have the rear zone temperature set too high, so check that to make sure it is set and operating properly.

The only other answer is that molten plastic was pushed back to the feed section from the transition or metering zone. That means your non-return ball-check valve is leaking, or the barrel is worn so much that plastic is flowing backward over the slip ring during injection. This happens during repeated injections followed by screw rotation. Molten plastic gets pushed all the way back to the feed zone; then as the screw rotates it pushes the some of the melt into the feed throat, where it cools and solidifies. The remedy is not lowering the throat temperature but to repair or replace the non-return valve and/or the barrel.