PET injection molding looks straightforward on paper—melt the pellets, fill the mold, cool the part. But polyethylene terephthalate is one of the least forgiving thermoplastics you’ll ever run through a machine. We supply bottle-grade and fiber-grade PET resins to manufacturers worldwide, and we hear about the same processing headaches over and over. Here are the five biggest mistakes we see—and how you can avoid them.
1. Skipping Proper PET Resin Drying
This is the number one mistake in PET injection molding, and it’s also the easiest to prevent. PET is hygroscopic, meaning it absorbs moisture from the air. If you run wet pellets through your barrel, you’re going to have a bad day. Moisture can cause hydrolytic degradation during the molding process, leading to weak and brittle parts. That’s because water molecules break the polymer chains at high temperatures, which drops the intrinsic viscosity (IV) and generates acetaldehyde—a compound that affects the taste of anything stored in your PET containers.
PET absorbs moisture like a sponge. Moisture levels need to be below 0.02% before processing. That’s not a suggestion. It’s a hard rule. The ideal moisture content after drying is approximately 10 to 40 ppm. Go above that, and you’ll see haze, cloudiness, bubbles, and reduced mechanical strength in your finished parts.
So how do you dry PET properly? Dry the PET resin at a temperature of around 160°C to 180°C for several hours. Most processors use dehumidifying dryers (desiccant dryers), not standard hot-air dryers. A hot-air dryer can’t pull the dew point low enough to get PET dry in humid conditions. You need a dryer that can hit a dew point of -40°C or lower. And here’s a tip we always share with our customers: use a moisture analyzer to verify your pellets are actually dry. Don’t just trust the timer. Dryers wear out. Desiccant beds degrade. The only way to know for sure is to test.
One more thing. Excessive drying will cause AA increasing and IV decreasing. So over-drying PET is also a mistake. Leaving pellets in the dryer at 170°C for 12 hours isn’t better than 4 hours—it’s worse. You’re degrading the resin before it even hits the barrel. Stick to the recommended time and temperature window, and check your dryer performance regularly.
2. Wrong Barrel and Melt Temperature Settings
PET has a narrow processing window compared to plastics like polypropylene or ABS. PET’s injection window is narrow. You’re walking a tightrope between incomplete filling and material degradation. Set the temperature too low, and the resin won’t melt fully—you’ll get short shots, poor flow, and visible defects. Set it too high, and you’ll thermally degrade the polymer, which shows up as yellowing, brittleness, or elevated acetaldehyde levels.
The machine’s barrel is heated to a temperature between 250°C and 280°C for unreinforced PET. Glass-fiber reinforced grades run a bit hotter. Usually, the barrel temperature is controlled at 240–280°C, and the barrel temperature of glass fiber reinforced PET is 250–290°C. Your barrel should be set up in zones with a gradual temperature gradient from rear to front. Set your barrel temperatures in zones. Start with 260–290°C at the rear and work up to 280–300°C at the nozzle. This creates the perfect temperature gradient for melting without degradation.
Here’s where many processors slip up: they pay attention to barrel set points but ignore actual melt temperature. Barrel temperature settings influence melting, but the actual melt temperature is driven mainly by shear, screw design, backpressure, and residence time. So your barrel might read 275°C, but the real melt temp could be 10–15°C higher because of shear heat. Always verify with a pyrometer or melt probe—don’t rely on the machine display alone.
Residence time matters a lot with PET. Avoid prolonged exposure of PET to high barrel temperatures to prevent thermal degradation and acetaldehyde generation. If your cycle time is long or you’re running a machine that’s too big for the shot size, PET sits in the barrel too long and breaks down. A good rule of thumb: your shot size should be 40–70% of the barrel capacity. Anything less, and you’re asking for trouble.
Key PET Injection Molding Temperature Parameters:
| Parameter | Recommended Range | What Goes Wrong Outside Range |
|---|---|---|
| Drying Temperature | 150–180°C | Under-dried resin → haze, bubbles; over-dried → AA increase, IV drop |
| Barrel Temperature (Unreinforced) | 240–280°C | Too low → short shots; too high → yellowing, degradation |
| Barrel Temperature (Glass-Filled) | 250–290°C | Too low → poor fill; too high → fiber damage, discoloration |
| Mold Temperature (Amorphous/Clear) | 10–40°C | Too high → crystallization, haze |
| Mold Temperature (Crystalline) | 80–120°C | Too low → poor surface, dimensional issues |
| Nozzle Temperature | ≤300°C | Too high → drooling, degradation |
3. Poor Mold Design and Cooling Setup
You can dry your PET perfectly and nail every barrel setting, but a bad mold will still ruin your parts. In mold design, the gate’s position is vital. If it is improper, the injection of liquid PET becomes a host of flow marks, air pockets, and uneven cooling. All these result in sink marks, weld lines, and warpage.
Gate design and placement need special attention with PET. The material flows fast and cools quickly, so you need gates that let the resin fill the cavity before it starts solidifying. Runners should be short and smooth, avoiding sharp bends or dead zones. Runners, gates, and cavities must be polished to prevent flow marks. For high-volume production—especially bottle preforms—hot runner systems are the standard. Hot runner systems are recommended for efficiency, especially in bottle preform molding. Hot runners keep PET molten right up to the gate, which reduces material waste, cuts cycle times, and gives you better gate vestige control.
Cooling is where things get tricky for PET. Excessive moisture in the resin or improper cooling conditions affect crystallinity. PET is a semi-crystalline polymer, and the cooling rate directly controls whether your part comes out clear or cloudy. If you want transparent parts (bottles, food packaging, cosmetic containers), you need rapid, uniform cooling to keep the PET amorphous. If the mold temperature is low and uniform, and the cooling speed is fast, the product will be transparent with little crystallization.
Cooling channels circulate coolant throughout a mold so that injected molded parts can cool properly. As a rule, cooling channels should be as close as possible to the mold’s interior space. Avoid cooling channel dead-ends that form air traps, and use larger cooling channels with thicker walls. Poor cooling channel design can lead to sink marks or depressions in the surface. With PET, uneven cooling doesn’t just cause cosmetic defects—it causes crystallization in hot spots, which shows up as white marks or cloudy patches that you can’t fix without re-molding the part.
If you’re making parts that need to be crystalline (like oven-safe trays or some industrial components), then you actually want a mold temperature in the 80–120°C range to promote controlled crystallization. The mold temperature must be within 80–120°C. If the mold has temperatures outside that range, the manufacturer experiences slow cooling, shrinkage, permanent solidification, and improper filling. The point is: know what you need, and design the cooling system around it.
4. Using the Wrong Screw and Machine Setup
PET doesn’t behave like polyethylene, polypropylene, or ABS in the barrel. PET injection molding comes with its own set of challenges. If you’re used to working with materials like ABS or polypropylene, you’re in for a surprise. And one of the fastest ways to mess up PET processing is to run it on a general-purpose screw that was designed for less sensitive resins.
Because PET is heat sensitive material, easy to make high temperature molecular chains, so that molecular weight reduction, produce acetaldehyde, reduce the quality of preform; the screw compression shear heat generated is too large, the thermal cracking of PET may occur, therefore, PET injection screw shall have low shear design. In plain terms: PET needs a screw that melts the pellets gently, without generating too much friction heat. Use a specially designed screw with a low compression ratio (2.0:1 to 2.5:1) to avoid excessive shearing and degradation of the PET.
A general-purpose screw with a 3:1 or 3.5:1 compression ratio will over-shear PET and create hot spots in the melt. That extra shear heat degrades the polymer, raises acetaldehyde, and drops IV—all of which show up as defective parts. Keep your screw speed low—around 50–80 RPM. High speeds create shear heat that can degrade your material.
At CBRHK, we always tell our customers to match their machine to the material. PET has a very short stable time after the high melting point. Therefore, an injection system with a multi-stage temperature controller and less self-friction heat generation during plasticization is required. If you’re processing PET in serious volumes, invest in a machine with a dedicated PET screw, accurate multi-zone temperature control, and the right shot-size-to-barrel-capacity ratio. It’s not the place to cut corners.
Back pressure also needs attention. Too much back pressure generates excessive shear heat in the barrel. Too little, and you get an inconsistent melt with air trapped in it. For PET, back pressure is typically set between 400–600 psi (plastic pressure). This gives you a homogeneous melt without overheating. If you’re seeing silver streaks or splay marks on your parts, back pressure and screw speed are two of the first things to check.
5. Ignoring Acetaldehyde and Crystallization Control
If you’re molding PET for food or beverage packaging, acetaldehyde (AA) is a number you need to track. Acetaldehyde (AA) is a byproduct of PET thermal degradation. Even at low concentrations, it gives water a slightly sweet or fruity off-taste. Beverage manufacturers typically require AA levels below 8 ppm for water bottle preforms. For still water applications, the requirement is even tighter—often below 3 ppm. If your processing conditions are too aggressive (high temperatures, long residence times, excessive shear), AA levels spike and your preforms fail quality checks.
Every mistake we’ve talked about so far—wet resin, high barrel temps, wrong screw design, extended cycle times—feeds into higher acetaldehyde generation. That’s why AA control isn’t just about one parameter. It’s the result of getting everything else right. A dedicated PET preform injection molding machine controls AA through optimized screw geometry, lower processing temperatures, and reduced residence time.
Crystallization control is the other half of this equation. PET can exist in both amorphous (clear) and crystalline (opaque) states, and the processing conditions decide which one you get. The material crystallizes over a temperature range from 120–220°C (248–428°F). If any part of your process allows the PET to sit in that crystallization window too long—whether it’s slow cooling in the mold, uneven temperature distribution, or a hot spot near the gate—you’ll get localized crystallization that ruins transparency.
If the gate area cools too slowly or the mold temperature is uneven, crystallization occurs at the gate—showing up as a white spot. We see this all the time with preform molders. The fix involves optimizing the cooling circuit near the gate area, using valve-gated hot runner nozzles with proper thermal insulation, and keeping mold temperature uniform across all cavities. For multi-cavity molds running 16, 32, or 48 cavities, maintaining thermal balance is one of the biggest engineering challenges.
For processors who work with multiple thermoplastics, it’s worth noting that crystallization behavior is very different across polymers. If you also run polypropylene, for example, you can learn more about how PP grades behave in different processes in our polypropylene resin guide. The processing logic is similar—match the material properties to the right conditions—but the specifics for PET are much less forgiving.
How to Get Better Results from Your PET Molding Line
All five of these mistakes come back to one thing: not respecting PET’s sensitivity. It’s a high-performance material that rewards precision and punishes shortcuts. PET injection molding sits at the intersection of material behavior and high-throughput production. Getting the fundamentals right, especially resin handling, drying, temperature control, and mold cooling, often determines whether a line runs smoothly or struggles with waste, downtime, and customer complaints.
Here’s a quick checklist we recommend to our customers:
Start with verified-dry resin (below 50 ppm moisture, confirmed by testing—not by timer). Set barrel temperatures in zones with a gradual profile that stays within the recommended range for your specific PET grade. Use a PET-specific or low-compression-ratio screw and keep screw speed moderate. Design molds with uniform cooling and polished surfaces. Track acetaldehyde levels if you’re in food or beverage packaging.
And if you’re still troubleshooting defects, here’s a quick reference:
| Defect | Most Likely Cause | First Fix to Try |
|---|---|---|
| Haze / Cloudiness | Moisture in resin, uneven cooling | Re-dry pellets; check dryer dew point; verify mold cooling |
| Yellowing | Barrel temp too high, long residence time | Lower barrel temps 5–10°C; reduce cycle time |
| Brittleness | Moisture, degradation, or crystallization | Check all three: moisture, temps, cooling rate |
| Short Shots | Low injection pressure, incomplete melt | Increase pressure/speed; verify melt quality |
| White Spots at Gate | Uneven cooling, crystallization at gate | Improve gate-area cooling; check hot runner insulation |
| Sink Marks | Low holding pressure, early gate freeze | Increase holding pressure and time |
FAQs
What temperature should PET be dried at before injection molding?
The drying temperature should be maintained between 160°C and 180°C. Drying time is typically 4–6 hours in a dehumidifying (desiccant) dryer with a dew point of -40°C or below. Moisture levels need to be below 0.02% before processing. Always confirm with a moisture analyzer rather than relying on time alone, because dryer performance can degrade over time without showing obvious signs of failure.
Why are my PET injection molded parts cloudy or hazy?
This issue is typically caused by excessive moisture in the resin or improper cooling conditions that affect crystallinity. When PET is not sufficiently dried before processing, hydrolysis can occur, leading to reduced clarity. Inconsistent mold temperature may also increase crystallization, resulting in a cloudy appearance. The fix starts with verifying your drying parameters. If the resin is dry and you still see haze, check your mold cooling setup for hot spots or inconsistent temperature zones across cavities.
Can you use recycled PET (rPET) in injection molding?
Yes, but it requires adjustments. Compared to virgin PET, recycled PET has its own characteristics: the melt flow rate is different and, being reused, its appearance is inevitably affected by impurities. rPET typically has lower IV and different flow behavior, so you may need to tweak barrel temperatures, injection speeds, and holding pressures. rPET must always be tested with the formula, especially when dealing with a mix. Many processors blend 20–50% rPET with virgin material for a balance of sustainability and performance.
What causes yellowing in PET injection molded parts?
Temperature’s too high or residence time’s too long. Lower your barrel temperatures by 5–10°C and reduce cycle time. Yellowing happens when PET thermally degrades in the barrel. If you’re running a machine where the shot size is too small relative to barrel capacity, the resin sits at high temperature for too long between shots. Also check for dead spots in the barrel or hot runner where material can stagnate and degrade.
What’s the right mold temperature for clear PET parts?
For transparent or amorphous PET parts like bottle preforms and clear food packaging, mold temperature should be between 10–40°C.This rapid cooling prevents crystallization and keeps the part clear. For crystalline PET applications that need heat resistance (like oven trays), mold temperature should be 80–120°C to promote controlled crystallization and achieve the right balance of strength and thermal stability.