PET Preform Screw L/D Ratio: Why 25:1 Beats 20:1 for Melt Homogeneity in Bottle Production
Two Pet Preform Injection Molding Machines sit side by side on a factory floor in Wuxi. Both run the same 28 gram preform, the same cycle time, the same resin. The preforms from machine A pass haze testing and AA testing; the preforms from machine B occasionally fall out of spec. The difference is not the resin, the mold, or the cycle. It is the screw L/D ratio: 25:1 on machine A, 20:1 on machine B.
Key Takeaways
- Screw L/D ratio directly controls the melt residence time available for plasticizing, which determines how uniform the PET melt is when it enters the injection stage.
- A 25:1 screw provides about 25 percent more residence time than a 20:1 screw, which translates to lower IV drop, lower AA generation, and fewer haze defects in the finished preform.
- For carbonated soft drink preforms, water preforms, and hot-fill preforms, 25:1 is the standard; for thin-wall packaging preforms under 12 grams, 20:1 is still acceptable.
- The longer L/D ratio does not extend cycle time if the screw design is properly optimized — it improves melt quality without productivity loss.
- SUCCESSOR's PET preform injection molding machines and SK-220PET preform system use 25:1 L/D geometry as the standard for carbonated soft drink and water preform production.
Table of Contents
This guide explains the engineering mechanics behind the screw L/D ratio, how a 25:1 screw produces more homogeneous PET melt than a 20:1 screw, and how to decide which L/D ratio fits a given preform application. The discussion is grounded in the SUCCESSOR PET preform injection molding machine family and the field experience of 40+ country installations.
What the Screw L/D Ratio Is and Why It Matters
The screw L/D ratio is the effective length of the screw divided by the screw diameter. A 25:1 screw is 25 diameters long; a 20:1 screw is 20 diameters long. For a typical 50 mm diameter PET preform screw, that is 1250 mm of effective length versus 1000 mm — a 250 mm difference in the plasticizing zone.
This length difference is not a passive measure. The longer screw contains more compression and metering zones, which gives the PET melt more stages of mixing, shear, and homogenization before it is injected into the preform mold. The longer the L/D ratio, the more opportunity the melt has to reach a uniform temperature and viscosity profile before injection.
Why Melt Homogeneity Drives Preform Quality
PET is a semi-crystalline polymer that reacts to thermal and shear history in measurable ways. Melt that is too cold in one region of the melt pool produces high-viscosity pockets that do not fill the mold cavity evenly, leading to preforms with wall thickness variations. Melt that is too hot or over-sheared in another region produces thermal degradation, IV drop, and AA generation that show up as haze or off-taste in the finished bottle.
A homogeneous melt eliminates both problems. The wall thickness is consistent. The IV drop is uniform. The AA generation is at its minimum. The preform blow-molds into a clear, dimensionally accurate bottle with consistent mechanical properties.
The Plasticizing Stage vs the Injection Stage
The L/D ratio specifically affects the plasticizing stage, which is the rotation of the screw that melts and mixes the PET resin before the injection stage pushes the melt into the mold. The injection stage is governed by different parameters (injection speed, holding pressure, packing profile) and is not directly affected by L/D ratio.
The plasticizing stage is where melt homogeneity is built or lost. A 20:1 screw has less room for the melt to reach uniformity; a 25:1 screw has more room. For high-quality preform production, more room is better.
How a 25:1 Screw Improves Melt Homogeneity
The 25 percent additional length on a 25:1 screw is not empty space. It is filled with functional screw geometry that contributes to melt quality.
Additional Compression Zone Length
The compression zone is where the resin transitions from solid pellets to a partially melted, partially compressed state. A 25:1 screw has more compression zone length, so the resin is compressed more gradually. Gradual compression produces less localized shear heating and a more uniform melt density entering the metering zone.
A 20:1 screw has the same compression (typically 2.5:1 to 3.0:1 ratio) over a shorter distance, meaning more abrupt compression and higher local shear that contributes to AA generation and IV drop.
Additional Metering and Mixing
The metering zone is where the melt is fully melted and pumped at a consistent rate. A 25:1 screw has more metering length, so the fully melted PET has more time at low shear to relax residual thermal gradients and viscosity differences. The longer metering zone is the single biggest contributor to melt homogeneity.
Modern PET preform screws include a mixing section (Maddock or pineapple mixer) near the metering zone end. The mixing section breaks up any remaining solid fragments and distributes melt temperature evenly. A 25:1 screw has more room before and after the mixing section for the melt to relax and stabilize, which is why the 25:1 design consistently produces lower haze and more uniform preforms.
IV Drop and How Screw Geometry Controls It
IV (intrinsic viscosity) measures PET molecular weight. Each thermal or shear event in the plasticizing and injection stages reduces IV slightly. Lower IV means the PET has degraded and the preform will not stretch as well during blow molding.
Acceptable IV Drop Ranges by Application
For carbonated soft drink preforms, IV drop of 0.02 to 0.04 dL/g from feedstock IV (typically 0.80 dL/g) is acceptable. For water preforms, 0.02 to 0.03 dL/g. For hot-fill preforms that must withstand 85 degrees C filling, IV drop should stay under 0.03 dL/g.
A 25:1 screw with properly tuned barrel heating zones typically achieves IV drop in the lower half of these ranges. A 20:1 screw typically achieves IV drop in the upper half, and can exceed the acceptable range at high screw speeds above 200 rpm.
How Screw Geometry Influences IV Drop
IV drop is driven by three factors: peak melt temperature, melt residence time, and shear rate. A 25:1 screw reduces all three compared to a 20:1 screw at the same shot size and cycle time.
- Peak melt temperature: The longer compression zone heats the resin more gradually, so peak temperature in the metering zone is lower. Lower peak temperature means less thermal degradation.
- Melt residence time: The 25:1 screw can have a lower peak residence time because the longer compression zone melts the resin earlier, reducing time in the high-temperature metering zone.
- Shear rate: The longer metering zone allows a shallower metering depth, which reduces shear rate at the same throughput.
| Metric | 20:1 Screw | 25:1 Screw |
|---|---|---|
| Peak melt temperature (typical) | 285 to 295 degrees C | 275 to 285 degrees C |
| IV drop (CSD preform, 28 g) | 0.04 to 0.05 dL/g | 0.02 to 0.03 dL/g |
| AA generation | 3 to 5 microgram per g | 1 to 3 microgram per g |
| Wall thickness variation | ±0.05 mm | ±0.02 mm |
The numerical differences are large in preform quality. A preform with ±0.02 mm wall thickness variation produces consistent bottles. A preform with ±0.05 mm variation produces bottles with weak spots failing drop or burst testing.
AA Generation, Sensory Compliance, and 25:1 vs 20:1
AA (acetaldehyde) is a thermal degradation byproduct of PET that produces a fruity off-taste in bottled water at concentrations above 3 microgram per gram. AA is generated by the thermal and shear history of the resin in the plasticizing stage and the hot runner system.
AA Generation and Screw Length
AA generation is closely tied to peak melt temperature and shear rate. A 25:1 screw with its lower peak temperature and lower shear rate generates 30 percent to 50 percent less AA than a 20:1 screw at the same shot weight and cycle time. For water preform production targeting sensory compliance, this is a meaningful difference.
For carbonated soft drink preforms, the AA threshold is higher (typically 5 to 7 microgram per g is acceptable) because the carbonation masks mild off-taste. For still water preforms, the threshold is lower (3 microgram per g) and AA control becomes a primary specification.
Real-World Production Tradeoff
The choice between 25:1 and 20:1 depends on the preform application. A 25:1 screw is the right default for high-clarity carbonated soft drink preforms, water preforms, and hot-fill preforms. A 20:1 screw is the right default for commodity packaging preforms where the preform is the final product, not the intermediate to a blow-molded bottle.
Buyers running a mixed product mix on a single machine should choose the 25:1 screw as the default because the longer geometry does not penalize cycle time and gives the flexibility to run higher-quality preforms when orders require it.
Why 25:1 Does Not Slow Cycle Time
A common concern about longer L/D ratio screws is that they extend plasticizing time. In practice, this is not the case for properly designed PET preform screws.
Plasticizing Time Is Set by Throughput, Not Screw Length
Plasticizing time for a given shot weight is determined by screw rotation speed and throughput per revolution. The longer screw does not rotate more slowly. At 100 to 250 rpm typical for PET preform, a 25:1 screw delivers the same shot weight in the same time as a 20:1 screw, assuming similar compression ratios and metering depths.
The longer screw has slightly more mass, adding less than 0.1 second to the screw recovery time during acceleration — well within normal cycle-to-cycle variation.
The Real Cycle Time Bottleneck
Cycle time for PET preform injection molding is set by the cooling time in the mold, not by the plasticizing time. For a 48-cavity CSD preform mold, the cooling time is 8 to 12 seconds, and the plasticizing time is 4 to 6 seconds. The plasticizing time runs in parallel with the cooling time, so the cycle is set by whichever is longer — usually the cooling time.
Extending the L/D ratio from 20:1 to 25:1 adds less than 0.2 seconds to plasticizing time. This 0.2 seconds is invisible within the cooling time and the overall cycle time. The 25:1 screw gives better melt quality at no productivity cost.
When 20:1 Is Still the Right Choice
Despite the technical advantages of 25:1 for high-quality preforms, 20:1 L/D ratio is not obsolete. Two application categories still favor the shorter screw.
Thin-Wall Packaging Preforms Under 12 Grams
Preforms that become thin-wall packaging containers (yogurt cups, deli containers, single-serve food containers) typically weigh 5 to 12 grams. The melt volume is small enough that the shorter screw achieves adequate homogeneity. The cost saving on the shorter screw is meaningful for high-cavitation molds running 72 to 96 cavities, where per-cavity machine cost is sensitive.
High-Cycle Commodity Preforms
Preforms that prioritize cycle time over absolute quality (commodity water preforms for non-premium markets, single-use packaging preforms) can use a 20:1 screw. The 20:1 design has a lower machine purchase cost, attractive for buyers running commodity preforms at scale.
For new machines targeting high-clarity preforms, 25:1 is the modern default.
Procurement Checklist for PET Preform Buyers
When evaluating a PET preform injection molding machine for purchase, the following checklist covers the screw L/D ratio and adjacent melt quality specifications.
- Confirm the screw L/D ratio. Request the screw L/D ratio specification in writing. A 25:1 screw is the standard for high-quality preform production; 20:1 is acceptable for commodity applications.
- Confirm the screw geometry details. Ask for the compression ratio (typically 2.5:1 to 3.0:1 for PET) and the metering depth. A well-designed screw has a metering depth matched to the throughput target.
- Confirm the barrel heating zones. PET requires at least 4 barrel heating zones to maintain melt temperature uniformity. Some machines offer 5 or 6 zones for finer control. A higher zone count helps the 25:1 screw deliver its full melt quality advantage.
- Confirm the shot size capacity. The maximum shot size should be 1.5x to 2.0x the typical preform weight to avoid running the screw near its minimum shot limit, where melt quality degrades.
- Request IV drop and AA test data. Ask the machine supplier for measured IV drop and AA generation at the target preform weight and cycle time. Compare across suppliers to identify the machines that deliver the lowest degradation.
- Confirm the hot runner system. The hot runner is a separate source of IV drop and AA generation. A well-designed hot runner with no dead spots is essential. The screw and hot runner should be specified together.
- Request field references. Ask the supplier for references from buyers running similar preform applications. Verify the IV drop and AA performance in real production, not just in the supplier's lab.
For buyers targeting the 25:1 screw design, the SK-220PET preform system from SUCCESSOR is a representative example. The 220 ton clamping force supports 48-cavity CSD preform molds, and the 25:1 L/D screw is standard. Reach the engineering team through the contact page for application-specific specifications.
The 220 ton clamping force supports 48-cavity CSD preform molds, and the 25:1 L/D screw is standard. Reach the engineering team through the contact page for application-specific specifications.