PET preform production is one of the most demanding branches of plastic injection molding. On the same machine 48, 72 or even 144 cavities run at once; the cycle time is measured in seconds, and the smallest deviation in the moisture of the raw material is reflected directly in product quality.
In this guide we explain where preform production departs from standard injection molding, what you need to look at when choosing a machine, and why drying comes before every other parameter.
Why is PET different from standard injection molding?
| Standard injection molding | PET preform | |
|---|---|---|
| Cavity count | 1 – 16 typical | 48 – 144 |
| Cycle time | 15 – 60 s | 8 – 15 s |
| Mold temperature | 40 – 120 °C | 5 – 15 °C (cold) |
| Drying tolerance | Flexible | Mandatory and narrow |
| Quality criterion | Dimensions, surface | Dimensions, surface + IV, AA, clarity |
The most striking difference is the mold temperature: the preform mold is deliberately kept cold. The aim is for PET to freeze in an amorphous (transparent) state before it crystallizes. A preform that cools slowly crystallizes; it becomes hazy and brittle and does not blow properly at the stretch blow molding stage.
1. Drying — comes before everything else
PET is hygroscopic: it chemically binds the moisture in the air. If damp PET is processed, hydrolysis occurs — the polymer chain breaks and the material's IV value (intrinsic viscosity) falls.
The consequences of IV loss cannot be reversed:
- The preform becomes hazy and loses its clarity
- The wall thickness is distributed unevenly during blowing
- The pressure resistance of the bottle falls, creating a risk of bursting with carbonated drinks
- Silver streaks appear on the surface
Drying conditions
| Parameter | Value |
|---|---|
| Drying temperature | 160 – 180 °C |
| Drying time | 4 – 6 hours |
| Dew point | −40 °C |
| Target moisture | below 0,005% (50 ppm) |
These conditions cannot be achieved with an ordinary hot air dryer. A dry air (desiccant) dryer with a high-temperature version is needed — the EasyDry HT series is designed for exactly this range (50 – 170 °C).
The critical point: Dried PET must not pick up moisture on its way to the machine. It must be fed from the dryer to the machine through a closed line; dried PET standing in an open hopper takes the moisture back within a few hours. That is why on preform lines a central conveying system is not a matter of preference but a necessity.
More on the subject: Plastic resin drying and hygroscopic plastics
2. AA (acetaldehyde) value
When PET is processed at high temperature, acetaldehyde is released. This substance gives water a sweetish taste; in water bottle production the AA value is one of the most critical quality criteria.
Factors that increase AA formation:
- A melt temperature higher than necessary
- Excessive shear in the screw — a wrong screw design or very high speed
- Material being held in the barrel for a long time
- Dead zones and stagnant material in the hot runner
That is why preform machines use a PET-specific screw geometry and precise temperature control. In water bottle applications the target is usually below 3 ppm.
3. Tonnage and cavity count
Because the preform mold has many cavities, the total projected area grows quickly:
Required tonnage = Projected area of one preform × Cavity count × Cavity pressure factor
For preforms the factor is taken at the high end (0.6 – 0.8 ton/cm²), because the thin, long flow path requires high injection pressure.
A guiding range in practice:
| Cavity count | Typical tonnage |
|---|---|
| 24 – 32 cavities | 250 – 350 Tons |
| 48 cavities | 350 – 450 Tons |
| 72 cavities | 450 – 550 Tons |
| 96 cavities and above | 550 – 650 Tons and above |
The final value varies with preform weight and neck type; the projected area given by the mold maker should be taken as the basis.
4. Plasticizing capacity — the real bottleneck
In preform production, what limits the machine is usually not tonnage but plasticizing capacity (kg/h). Filling 96 cavities in an 8-second cycle means the screw must melt and prepare the required material in that time.
The calculation is simple:
Hourly requirement (kg/h) = Shot weight (g) × 3,600 / Cycle time (s) / 1,000
Example: 48 cavities × 25 g = 1,200 g shot, 10-second cycle → 432 kg/h.
The plasticizing capacity of the machine must be above this value; running at the limit raises the screw speed, which in turn increases the shear heat and the AA value.
5. Screw and injection unit
A PET screw differs from a standard screw:
- L/D ratio of 22 – 24 — for sufficient melting and homogeneity
- Low compression ratio — prevents excessive shear heat
- Barrier or PET-specific geometry
- Large injection volume, to cover the multi-cavity shot in one go
The melt temperature is typically kept in the 275 – 295 °C range. The upper limit of the temperature is set by the AA value, and the lower limit by the risk of short shots and freezing. For values by material: Material processing values table
6. Hot runner and mold
In a 96-cavity preform mold each nozzle is a separate temperature zone. A deviation in a single zone affects every preform coming out of that cavity — and this is only noticed at the blow molding stage.
At this scale the choice of control unit is critical: individual monitoring and alarms per zone are essential. The JETmaster TP series meets this need in the 24 – 240 zone range.
On the mold side, the balance of the cooling circuits is decisive. If there is a temperature difference between cavities, the preform weights and wall thicknesses diverge. Mold temperature control units keep this balance constant.
7. Color and additive dosing
In preform production the color (blue tint, UV barrier) and additive ratios are very low — usually in the range of 0.1 – 1%. At such small ratios, volumetric dosing deviation turns directly into color differences.
Gravimetric dosing systems eliminate this deviation because they measure by weight; they also bring masterbatch consumption down to the level actually needed.
8. The DKM PET Series
The DKM PET series are third-generation machines designed for preform production:
| Model | Clamping force |
|---|---|
| DKM-250PET | 2,500 kN |
| DKM-350PET | 3,500 kN |
| DKM-450PET | 4,500 kN |
| DKM-550PET | 5,500 kN |
| DKM-650PET | 6,500 kN |
Common features of the series: a PET screw with L/D 24, a low AA value with PID temperature control, high plasticizing capacity and wide tie bar clearance suited to multi-cavity molds.
Checklist
When setting up a preform line, verify in this order:
1. Dryer capacity — does it cover your hourly consumption with a 4 – 6 hour residence time? 2. Dew point — is it measured, and is −40 °C achieved? 3. Closed feed line — does the dried material reach the machine without picking up moisture? 4. Plasticizing capacity — is it above the kg/h value you calculated? 5. Tonnage — has the cavity count × projected area calculation been done? 6. Hot runner zone count — does it cover all the nozzles of the mold? 7. Mold cooling — has the temperature difference between cavities been measured? 8. Dosing precision — is it gravimetric for low-ratio additives?
For machine and auxiliary equipment selection for your preform line, you can consult our technical support team.
Once tonnage and cavity count are clear for preform production, you can move on to the machine side: the PET-specific series and alternative tonnages are listed among the plastic injection molding machine series.