Reduce parison sag by using an HDPE grade with adequate melt strength, controlling actual melt temperature, shortening unsupported hanging time and programming extra wall thickness where drawdown is greatest. For large drums, tanks and technical parts, an accumulator head is often the decisive equipment choice because it discharges the shot quickly instead of allowing a long, hot parison to hang during continuous extrusion.
Do not start by adding part weight or changing many program points. First stabilize the resin, melt temperature and machine sequence. Then measure the blow molding parison over time and adjust one variable at a time.
What Is Parison Sag?
The parison is the hot hollow tube extruded before the mold closes around it. While it hangs unsupported, gravity stretches the melt. This time-dependent elongation is also called parison drawdown. It can produce a thin upper or shoulder region, excessive bottom weight, inconsistent pinch-off and poor wall distribution.
Sag is not the same as die swell. Sag is gravitational deformation after the melt leaves the die; die swell is elastic recovery that increases parison dimensions. Both affect length, diameter and wall thickness, and both change when resin, temperature, die gap or extrusion rate changes.
1. Use an HDPE Grade With Sufficient Melt Strength
Large-part HDPE blow molding normally requires a high-molecular-weight blow molding grade capable of supporting a heavy parison. Melt index is useful for comparing materials under specified test conditions, but it does not fully describe extensional viscosity, molecular-weight distribution, die swell or long-term sag behavior.
Compare candidate resins at the same melt temperature, shot weight and discharge time. Ask the resin supplier for blow molding recommendations, swell and sag data, environmental stress-crack resistance and any limits on recycled content.
When regrind is used, control its source, particle size, contamination, moisture and blend ratio. Variable regrind rheology can make a previously stable parison program drift from batch to batch. Do not introduce an unknown modifier solely to reduce sag without revalidating impact strength, chemical resistance, weld strength and regulatory suitability.
2. Control Actual Melt Temperature
Hotter HDPE generally has lower melt viscosity and sags faster. Reduce unnecessary thermal load while maintaining complete melting, stable pressure and acceptable surface quality. The relevant value is the temperature of the polymer leaving the head, not only the barrel-controller setpoints.
Memeriksa:
- Actual melt temperature with a suitable measurement method
- Barrel, adapter and head-zone stability
- Thermocouple position and calibration
- Heater-band contact and insulation
- Screw speed, motor load and shear heating
- Soak time after a large temperature change
A circumferential hot zone can create one thin side and look like general sag. Correct heater or flow imbalance before modifying axial parison programming. Avoid lowering every zone until unmelt, high motor load, melt fracture or weak welds appear. The target is the lowest stable thermal condition, not the lowest possible setpoint.
3. Minimize Unsupported Hanging Time
Sag depends on both temperature and time. Record the interval from first melt exiting the die until the mold captures or pinches the parison. Slow extrusion, carriage travel, mold closing and control-sequence delays all add drawdown time.
For smaller continuously extruded parts, a controlled increase in output may shorten the interval. However, increasing screw speed can add shear heat and make the result worse. Large products require a heavy shot, so equipment architecture matters more.
An accumulator head plasticizes material during other parts of the cycle and then discharges the stored shot rapidly through the die. The JWELL JWZ-BM160/230 blow molding machine uses an accumulating head for large industrial containers.
4. Measure Parison Drawdown Before Adjusting It
Create a repeatable sag test at stable production conditions:
- Use the same resin lot, regrind ratio and shot weight.
- Mark a fixed reference below the die.
- Record parison length at equal time intervals.
- Record total discharge time and mold-capture time.
- Measure upper, middle and lower parison diameter or layflat where practical.
- Mold parts and map wall thickness at fixed locations.
A useful comparison is the change in parison length per unit time. Video with a visible time reference can show when elongation accelerates, when the tube swings or whether mold motion creates an avoidable delay. Repeat the same measurement after every major resin, temperature or head change.
5. Apply Parison Wall-Thickness Programming
Parison programming changes die gap during the shot. It places more material in regions that will stretch farther in the mold or lose thickness through drawdown and removes excess material from low-stretch areas.
Build the program from finished-part measurements:
- Start with a conservative, nearly uniform profile.
- Allow the process to reach thermal equilibrium.
- Map minimum and average wall thickness at repeatable points.
- Link each part location to the corresponding parison time.
- Adjust only a limited group of program points.
- Use smooth transitions and repeat the trial.
An aggressive step change can create flow instability, fold-over or a visible thickness transition. Confirm that the die-gap actuator follows its command under actual head pressure. For related checks, see JWELL’s guide to wall-thickness uniformity in blow molding.
6. Balance Die Gap, Diameter and Shot Weight
A wider die gap produces a thicker parison, but it also increases shot weight and the gravitational load carried by the upper section. A narrow gap increases shear and can change swell. Therefore, die gap is not an independent sag correction.
The die diameter must provide sufficient circumference for the mold without excessive inflation or flash. Center the mandrel, inspect die surfaces and verify uniform heating. If one side remains thin, correct die centering, temperature or flow distribution before trying to hide the defect with an axial program.
7. Coordinate Mold and Machine Timing
The mold should capture the parison at a repeatable length and position. Check mold-close speed, carriage timing, pre-pinch devices, blow-pin motion and exhaust. Air movement can make a long parison swing; shielding may help if it does not create unsafe heat buildup.
Pre-blow can keep the tube open, but excessive pressure or poor timing can distort it. Very long or asymmetric parts may need parison supports, robotic handling, suction preforming or three-dimensional blow molding rather than a freely hanging vertical parison.
Parison Sag Troubleshooting Table
| Gejala | Periksa dulu | Avoid as the first response |
|---|---|---|
| Parison becomes too long | Melt temperature, resin grade, discharge time | Adding total shot weight |
| Thin upper wall, heavy bottom | Drawdown time, program timing | Changing every program point |
| One side consistently thinner | Die centering, heater uniformity | Axial compensation |
| Shot-to-shot variation | Regrind, melt temperature, accumulator control | Raising screw speed blindly |
| Fold-over during closing | Diameter, swell, mold timing, pre-blow | Lowering all temperatures |
| Thin molded corner only | Local stretch ratio and program | Treating it as sag alone |
Pertanyaan yang Sering Diajukan
Does lower melt temperature always reduce parison sag?
Usually, but only within a stable processing window. Excessively low temperature can cause poor melting, high motor load, rough surfaces and weak weld lines.
Can parison programming completely compensate for weak melt strength?
No. Programming redistributes material; it does not remove the instability caused by an unsuitable resin, excessive melt temperature or a slow shot.
Is an accumulator head necessary for every HDPE product?
No. It is most valuable when a large shot must be delivered faster than a continuously extruding screw can produce it without excessive hanging time.
Configure the Process Around the Product
Reliable parison control combines resin rheology, uniform melting, fast shot delivery, correct head geometry, repeatable mold timing and evidence-based programming. Review the JWELL blow molding machine range or hubungi JWELL with the product drawing, HDPE grade, virgin/regrind ratio, target weight, dimensions, wall-thickness map and cycle-time goal. These inputs allow the head, accumulator capacity and control system to be sized around the actual product.
