Injection Molding Setup: Resins, Drying, and Process Settings

A molded part is only as good as the setup behind it. The right resin has to be dry, the mold has to be hung and plumbed correctly, and each setting has to be a number you can explain and repeat next month. When a setup is written down well, the next run takes minutes to bring up. When it is not, somebody spends hours guessing. The resin maker's data sheet and your mold's setup sheet always beat the general numbers here.

What You Will Be Able to Do

• Tell amorphous resins from semi-crystalline ones and explain how each behaves.
• Set up and check resin drying, including dew point.
• Hang a mold and purge a barrel in the right order.
• Explain what barrel temperature, transfer, cushion, pack, back pressure and cooling each do.
• Run a gate seal study and record a process that someone else can repeat.

Know Your Resin

Plastics fall into two broad families, and they do not behave the same way.

Semi-crystalline resins take longer to cool for a reason. As they cool, their molecules line up into crystals, and forming those crystals gives off extra heat that the mold has to remove on top of the normal heat. Their narrower processing window and higher shrinkage also make them more demanding.

Before you load anything, check the resin name, grade, lot and color against the job. Regrind, which is ground-up runners and scrap fed back in, is allowed only up to the limit in the job's specification. Some parts do not allow it at all.

Drying Resin

Hygroscopic resins pull moisture from the air into the pellets. Nylon, polycarbonate, PET and ABS are common examples. At melt temperature, that water boils into steam and leaves silver streaks called splay, along with bubbles. In polycarbonate, nylon and PET, it also attacks the plastic itself, breaking its long molecular chains. That process is called hydrolysis. A part can look fine and still be brittle.

Typical drying settings:

Dew Point

A desiccant dryer dries air with a moisture-absorbing bed, then blows that hot, dry air through the pellets. Dew point is the temperature at which moisture in air would start to condense. The lower the dew point, the drier the air. A properly working desiccant dryer should hold a dew point of -20 F or lower, and many plants run colder, around -40 F. If the dew point climbs, the dryer is not removing enough moisture. The bed may be used up, a filter may be clogged or a hose may leak. Plain hot-air dryers use room air, so they cannot dry hygroscopic resin to this level.

Checking the Dryer

1. Confirm the drying temperature and time on the data sheet.
2. Read the dew point on the dryer or with a portable meter.
3. Check the temperature of the air going into the hopper.
4. Make sure the resin has had its full time in the hopper. New resin poured on top has not.
5. If your plant has a moisture analyzer, test a sample before you start.

Residence time is how long the pellets stay in the dryer hopper, and the hopper has to be big enough. Worked example: a job uses 40 pounds of nylon an hour, and the resin needs 5 hours of drying. The hopper must hold at least 40 x 5 = 200 pounds of resin. A smaller hopper sends out resin that has not dried long enough, even with a perfect dew point.

Drying too long can also hurt some resins, causing yellowing or loss of additives. Do not leave a dryer running hot on a full hopper over a long shutdown without checking the data sheet.

Hanging a Mold

Warning: Mold changes are done with the machine locked out when you are working between the platens, under your plant's energy control procedure. Only trained people rig and lift molds. A mold can weigh tons. Use rated eyebolts, threaded in fully, and never stand under a load.

1. Review the setup sheet: mold number, tonnage, resin, shot size, process settings and quality requirements.
2. Confirm that the press fits the mold: tonnage, tie bar spacing, mold height, opening stroke and ejector pattern.
3. Clean the platen faces and the mold's back plates. A chip or burr can tilt the mold.
4. Lift the mold with the crane, and seat its locating ring in the stationary platen.
5. Close the moving platen slowly to meet the mold, and bolt it to both platens with the required clamps.
6. Remove the mold's safety strap, the bar that keeps the halves together during lifting.
7. Connect the ejector linkage, then set ejector stroke and speed.
8. Connect the water lines by circuit and confirm the flow on a flow meter. Connect any hot runner and other services.
9. Set opening stroke and mold protection, a low-pressure close that stops the clamp if something is between the halves.
10. Cycle the mold slowly with no plastic, and watch the ejectors, slides and part removal.

Purging the Barrel

Purging clears the old resin out of the barrel before you start a new one. It keeps the last job's material and color out of your part. It also keeps incompatible plastics from reacting, or a heat-sensitive resin from degrading when it meets a hotter one.

1. Put on heat-resistant gloves and a face shield, and close the purge guard.
2. Pull the nozzle back from the mold.
3. Check that the barrel is at the right temperature for what is in it.
4. Run the old material out in short shots. Use the purge material your plant calls for, such as a commercial purge compound or a compatible resin.
5. Keep going until the shots come out clean, in the new color and with no streaks.
6. Put the hot purge in a metal container or on a tray to cool.

Warning: Check compatibility before any resin change. Acetal and PVC must never meet in a barrel, because acid from the PVC breaks the acetal down quickly into gas. Pressure can then blow plastic out of the nozzle or hopper. Follow the plant's rule, which may call for a neutral purge resin or for pulling and cleaning the screw.

If a nozzle is frozen, do not just crank up the heat and stand in front of it. Trapped gas can blow out when the plug lets go. Ask your process tech.

Process Settings

Temperatures

Barrel zones are normally set to climb from the feed end to the nozzle, so the resin warms up in steps. For an ABS job, that might be 430 F, 450 F and 470 F, with the nozzle at 475 F. The feed throat is cooled with water, so pellets do not melt early and block the opening. A few resins and screws do better with a reversed profile, but use one only if your process sheet calls for it.

Set points are not the real melt temperature. Friction from the screw adds heat. Measure the melt with a hand pyrometer in a purged air shot, meaning a shot into open air with the nozzle pulled back. After the barrel reaches its set points, let it heat-soak for about 20 to 30 minutes before you trust it.

• Melt too cold: thick flow, short shots, poor surface.
• Melt too hot: degraded resin, stringing, too much shrinkage.

Mold temperature comes from the water or oil temperature and flow rate. Check it with a surface pyrometer on the mold face. It affects finish, shrinkage, warp and cycle time, and in semi-crystalline resins it changes how much crystal forms.

Shot Size and Transfer

Shot size is how far the screw backs up to measure enough plastic for one cycle. As a rule of thumb, a shot should use about 20 to 80 percent of the barrel's capacity. Too small a shot leaves resin cooking in the barrel for many cycles, and too large a shot may not melt evenly.

The screw fills the mold under speed control. At a set screw position, called the transfer point or V/P switchover, the machine changes to pressure control for pack. Set it so the cavity is about 95 to 98 percent full when the switch happens. If transfer is too late, the mold is hit with a spike that causes flash. If it is too early, the flow can hesitate, leaving short shots.

Fill Speed and Pressure

Injection speed is usually set as a profile: slower at the gate, faster through the part, slower at the end of fill so that trapped air can get out. The injection pressure setting works as a limit. Watch the actual peak pressure. If it reaches the limit, the machine cannot hold the speed you asked for, and the fill changes from shot to shot.

Know which pressure you are reading. A hydraulic machine's gauge shows oil pressure. The screw multiplies that into plastic pressure by the intensification ratio, which is often about 10 to 1 and is listed for each machine. So 800 psi hydraulic is about 8,000 psi on the plastic. Electric machines show plastic pressure directly.

Pack and Hold

Pack pressure comes on after the cavity is nearly full and pushes in more plastic to make up for shrinkage. It is commonly set at 50 to 80 percent of fill pressure. How long it should last depends on when the gate freezes.

Running a Gate Seal Study

The gate is the first part of the flow path to freeze. Until it does, you can keep adding plastic, and if pack ends too early, plastic can flow back out. A gate seal study finds the right hold time.

1. Get the process stable with a long hold time.
2. Set a short hold time and run a few shots.
3. Weigh the part, without the runner, on a good scale.
4. Raise the hold time by a second, run a few more shots and weigh again.
5. Keep going until the weight stops rising.
6. Set the hold time at the point where the weight levels off, plus a small margin.

The weight stops rising at about 6 seconds. That is where the gate seals, so a hold time of about 6.5 to 7 seconds is a sensible setting. Time past the seal point adds nothing to the part and only adds to the cycle.

Cushion

Cushion is the small amount of melt left in front of the screw at the end of pack. It is usually 0.1 to 0.5 inch. It is what the screw presses against to transmit pack pressure into the mold.

If the cushion reaches zero, the screw has run out of travel. It stops against the end of the barrel, so it cannot press on the plastic and pack pressure stops reaching the part. The result is light parts with sink marks. Cushion should hold steady from shot to shot. A cushion that wanders usually points to a leaking check valve. Fix the cause, then adjust shot size if needed.

Recovery and Back Pressure

During cooling, the screw turns to build the next shot. Screw speed should be set so recovery finishes shortly before the mold opens. Higher RPM recovers faster but adds friction heat.

Back pressure is the resistance applied against the screw as it backs up. It packs the melt more tightly, drives out air and mixes it evenly. On a hydraulic gauge, a typical range is 50 to 150 psi. Machines that show plastic pressure read much higher, around ten times that.

• Too little: Uneven shot weight, poor mixing.
• Too much: Longer recovery and overheated resin.

Cooling Time and Clamp

Cooling time is the time after pack until the mold opens. It has to be long enough for the part to be stiff enough to eject without bending. Cut it only when the process and parts can prove it is safe to do so.

Set clamp tonnage to what the mold needs, from the setup sheet. Enough clamp stops flash. Much more than needed crushes vents and wears out the mold.

Starting a Job

1. Confirm that the mold is hung and the press matches the setup sheet.
2. Check the resin and verify the dryer has run its full time at the right dew point.
3. Set barrel temperatures and allow for the heat soak.
4. Purge the barrel until it runs clean.
5. Enter the saved process or the settings from the setup sheet.
6. Start with short shots and work up to a full part. This protects the mold from overpacking.
7. Inspect the first shots and make small adjustments, one at a time.
8. Run a sample, often about 30 minutes or 50 shots, and have quality inspect and approve it.
9. Record the approved process.

Recording the Process

A setup sheet that lists only set points is half a record. The machine's real outputs are what prove it is the same process as last time.

Write down the mold number, resin grade and lot, and any changes made. Keep the sheet where the next person can find it. Proven settings save hours of troubleshooting on the next run.

Common Setup Mistakes

Check Your Understanding

1. A job runs 25 pounds an hour of a resin that needs 4 hours in the dryer. Your hopper holds 75 pounds. Is that enough?

No. It needs at least 25 x 4 = 100 pounds. With 75 pounds, resin only gets about 3 hours of drying at that rate.

2. The dryer's dew point reads +10 F. Should you run?

No. The target is -20 F or colder, so the dryer is not doing its job. Find the cause, such as a saturated bed or a bad filter, and hold the resin until it is proven dry.

3. Your gate seal study shows part weights of 18.2, 18.9, 19.3, 19.4 and 19.4 grams at 2, 3, 4, 5 and 6 seconds. What hold time do you set?

The weight levels off at about 5 seconds, so set around 5.5 to 6 seconds. More time does nothing for the part.

4. You have just finished a job in PVC and the next job is acetal. What do you do?

Stop and get your process tech or lead before you change anything. These two must never mix, and the plant's rule may need a neutral purge or a full screw pull.

5. A cushion that was steady at 0.25 inch is now bouncing between 0.05 and 0.3 inch. What do you suspect?

A leaking check valve, which lets melt slip back during injection. Report it, because a wandering cushion means shot-to-shot variation.

Key Takeaways

• Semi-crystalline resins shrink more and cool slower than amorphous ones, because forming crystals gives off extra heat.
• Dry hygroscopic resin at the right temperature, for the full time, at a dew point of -20 F or lower.
• Barrel zones normally rise from feed to nozzle. Measure melt temperature on an air shot.
• A gate seal study finds hold time: weigh parts as the time goes up until weight stops rising.
• Keep a steady cushion. At zero, pack is lost.
• Record real outputs as well as set points, so the process can be repeated.