Plastic pellets look dry when they pour out of a bag. They are not. Most engineering thermoplastics — PET, PA, PC, ABS — absorb moisture from ambient air during storage and transport. A freshly opened 25kg bag of PET pellets can contain 0.2–0.4% moisture by weight. That is 50 to 100 grams of water that will travel through your injection barrel, flash into steam at 250°C+, and tear apart polymer chains before they ever reach the mold cavity.
The damage is visible and costly:
Splay marks / silver streaks on part surfaces — cosmetic rejects that cannot be sold
Internal bubbles and voids that weaken structural integrity and cause field failures
Brittleness from hydrolytic chain scission — parts that snap instead of flex
Dimensional instability as moisture-driven viscosity variation shifts fill patterns and shrinkage
Nozzle drool and gas burns from steam venting at the nozzle tip
For PET preform molding, moisture above 30 ppm (0.003%) causes catastrophic IV (intrinsic viscosity) drop, rendering an entire batch of preforms worthless. For PA6/PA66 automotive components, moisture-induced brittleness creates a safety liability. For clear PC parts, a single splay defect puts the part in the reject bin.
The economics are brutal. A molding cell running undried or improperly dried material can experience 5–15% scrap rates — entirely avoidable. Over one year, a single 200-ton machine running PET at 95% yield versus 85% is losing tens of thousands of dollars in wasted material, machine time, energy, and labor — all because of moisture.
The STG-U Series Hopper Drier from Chenxing Machinery eliminates moisture as a variable. By delivering precisely heated, dehumidified airflow through the resin bed directly above the feed throat, it reduces pellet moisture content to processing-safe levels before the material enters the barrel. And because the drier mounts directly to the machine's feed inlet, dried pellets drop straight into the screw — zero exposure to ambient air, zero opportunity for re-absorption.
Fourteen models span from 8-liter benchtop units for micro-molding to 1250-liter workhorses for large-tonnage extrusion and blow molding. Every model is built around the same proven hot-air circulation architecture, scaled to match your throughput.
| Model | Volume (L) | Capacity (kg) | Heater Power (kW) | Blower Motor (kW) |
|---|---|---|---|---|
| STG-U8 | 8 | 5 | 1.2 | 0.04 |
| STG-U16 | 16 | 10 | 1.4 | 0.06 |
| STG-U20 | 20 | 12 | 1.6 | 0.06 |
| STG-U25 | 25 | 15 | 1.8 | 0.06 |
| STG-U40 | 40 | 25 | 2.6 | 0.06 |
| STG-U80 | 80 | 50 | 3.9 | 0.12 |
| STG-U125 | 125 | 75 | 4.8 | 0.12 |
| STG-U160 | 160 | 100 | 5.4 | 0.25 |
| STG-U250 | 250 | 150 | 9 | 0.37 |
| STG-U315 | 315 | 200 | 12 | 0.37 |
| STG-U400 | 400 | 250 | 15 | 0.55 |
| STG-U500 | 500 | 300 | 18 | 0.55 |
| STG-U800 | 800 | 500 | 24 | 0.75 |
| STG-U1250 | 1250 | 800 | 36 | 1.1 |
| Machine Type | Typical Throughput | Recommended STG-U Model | Rationale |
|---|---|---|---|
| Micro-molding / lab | <5 kg/h | STG-U8 ~ STG-U25 | Small hopper volume sufficient for low-consumption applications; minimal energy draw |
| Small-tonnage injection (50–150T) | 5–15 kg/h | STG-U40 ~ STG-U80 | Enough residence time for proper drying of common materials like ABS, PP, PA |
| Medium-tonnage injection (150–350T) | 15–40 kg/h | STG-U125 ~ STG-U250 | Balances residence time with throughput for consistent moisture removal |
| Large-tonnage injection (350–800T) | 40–80 kg/h | STG-U315 ~ STG-U500 | Higher heating power to maintain setpoint at increased material flow rates |
| Extrusion blow molding PET preform | 80–200+ kg/h | STG-U800 ~ STG-U1250 | Maximum volume and heating capacity for sustained high-throughput production |
> Note on capacity: The "Capacity (kg)" column represents the approximate weight of plastic pellets the hopper holds when full. Actual drying throughput is a function of residence time — longer drying cycles (e.g., 4 hours for PET) mean lower sustained throughput for the same hopper volume. Factor in your material's required drying time when selecting a model.
The STG-U operates on a straightforward principle: heat the air, pass it through the resin bed, carry moisture away. It is simple by design, and its effectiveness comes from precise execution of that simplicity.
Step 1 — Air Intake & Heating: Ambient air is drawn into the blower housing and forced across an electric heating element. The digital PID temperature controller modulates heater output to maintain the setpoint with ±1°C accuracy, eliminating the temperature swings that cause inconsistent drying in on-off thermostatic systems.
Step 2 — Hot Air Distribution: Heated air enters the bottom of the hopper through a diffuser cone that spreads airflow evenly across the entire cross-section of the resin column. No channels form. No "hot spots" develop. Every pellet in the hopper sees the same drying conditions.
Step 3 — Counter-Flow Moisture Removal: As hot air rises through the descending pellet column, it absorbs surface and bound moisture from the resin. Moisture-laden air vents from the top of the hopper, carrying water vapor out of the system. The continuous counter-flow design means the driest air meets the driest pellets at the bottom — right above the feed throat — maximizing the moisture gradient that drives drying.
Step 4 — Direct Machine Connection: The STG-U mounts directly onto the injection machine or extruder feed inlet via a standard flange. Dried pellets fall straight into the barrel — there is no intermediate transfer bin, no open conveying section, no opportunity for the material to sit exposed to ambient humidity and re-absorb moisture. This direct-connect design is what enables the "avoid second moisture absorption" benefit that defines the STG-U series.
What This Means for Your Production
An STG-U16 drying 10 kg of ABS at 80°C for 2 hours consumes approximately 2.8 kWh. That is the energy equivalent of running a household hair dryer for two hours — to produce 10 kg of defect-free molded parts that might otherwise have a 10% reject rate from moisture defects. The energy cost of drying is a rounding error compared to the cost of not drying.
Feature: Optimized heater-to-hopper thermal coupling, high-efficiency blower motors across all models, and PID temperature control that eliminates overshoot and wasted heating cycles.
Advantage: Conventional hopper dryers using simple on-off thermostats cycle the heater between full power and zero power, wasting energy during the overshoot phase and allowing temperature to sag during the off phase. The STG-U's PID control maintains steady-state temperature with minimal power fluctuation — the heater draws only what is needed, second by second.
Benefit: Verified energy savings of 27–65% compared to thermostatically controlled dryers of equivalent capacity. For an STG-U250 running 16 hours per day, this translates to roughly 7,000–12,000 kWh saved annually — enough to pay for the drier itself within the first 18 months of operation.
Feature: The STG-U mounts directly onto the injection machine or extruder feed throat via a standard flange connection. Dried material exits the hopper bottom and enters the barrel with zero exposure to ambient air.
Advantage: In systems where the dryer and machine are physically separated, dried pellets must be conveyed or manually transferred to the machine hopper. Every second of exposure to factory air — especially in humid climates or non-air-conditioned plants — partially reverses the drying process. Pellets that left the dryer at 0.02% moisture can re-absorb enough humidity in 30 minutes of open transfer to push moisture back above the defect threshold.
Benefit: Consistent, predictable moisture levels from dryer to barrel. No seasonal variation in part quality between dry winter months and humid summer months. The drying investment is fully realized because none of the drying work is undone by transfer handling.
Feature: PID (Proportional-Integral-Derivative) digital temperature controller with LED display. Setpoint, actual temperature, and alarm indicators visible at a glance.
Advantage: Thermostatic (on-off) controllers allow temperature to swing ±5–10°C around the setpoint because they only respond after the temperature has already diverged. A PID controller continuously calculates the optimal heater output based on how far the temperature is from setpoint, how fast it is changing, and how long it has been off-target. The result is flat-line temperature stability.
Benefit: No risk of thermal degradation from temperature overshoots (critical for heat-sensitive materials like PVC and POM). No under-drying from temperature dips. Every pellet in the hopper experiences the same thermal history, so every shot fills consistently and every part meets specification.
Feature: Fourteen distinct models from 8L (5kg) to 1250L (800kg), with proportional scaling of heater power and blower capacity at each step.
Advantage: Many dryer manufacturers offer only 5–6 models with large gaps between sizes, forcing customers to oversize — and overpay — for a dryer far larger than their actual throughput demands. Oversized dryers waste energy heating empty hopper volume and impose unnecessarily long residence times that can over-dry some materials.
Benefit: You pay for the drying capacity you need, not the next size up. An STG-U40 running at its design capacity consumes 2.6 kW. The next common size up from competitors might be a 60L unit drawing 3.5–4 kW — a 35–50% increase in ongoing electricity cost with no throughput benefit.
Feature: Adjustable temperature range accommodates the drying requirements of virtually all common thermoplastics: PET (160°C), PA (80°C), PC (120°C), ABS (80°C), PP/PE (70–80°C), PVC (65–75°C), and more.
Advantage: A molding shop running multiple materials does not need a dedicated dryer for each resin. Change the setpoint, allow the hopper to reach temperature, and switch materials. The digital controller stores no recipes, but its simple setpoint adjustment makes material changes a 30-second task.
Benefit: Lower capital equipment cost — one STG-U serves multiple materials and multiple machines when scheduled appropriately. Simplified spare parts inventory. Consistent operator familiarity with a single dryer interface across the shop floor.
Feature: Welded steel hopper body, industrial-grade heating elements, durable blower motor rated for continuous duty, clear LED temperature display, and integrated over-temperature safety protection.
Advantage: The injection molding environment is demanding — vibration from the machine, ambient heat from the barrel and mold, airborne dust and pellet fines, and 24/7 operating schedules in many plants. The STG-U is built to operate in these conditions without degradation. The over-temperature safety circuit prevents heater runaway if a blower fault occurs.
Benefit: Years of trouble-free operation with minimal maintenance. The only routine service item is periodic cleaning of the hopper interior and blower intake filter — no calibration, no consumable desiccant to replace, no complex mechanical components to wear out.
Feature: Consistent, controlled drying reduces moisture content in resin to processing-safe levels before the material enters the barrel.
Advantage: Moisture in the melt causes multiple defect types that compound each other: splay marks on the surface, internal voids that weaken the part, viscosity variation that shifts fill patterns and dimensions, and hydrolytic degradation that permanently reduces polymer molecular weight. A properly dried material eliminates all of these at the source.
Benefit: Documented scrap rate reductions of 5–15 percentage points across injection molding operations that switch from undried or inconsistently dried material to STG-U drying. For a mid-size molding facility producing 500 tons of parts annually at 15,000 per year in raw material alone — before accounting for machine time, energy, labor, and regrind reprocessing costs.
| Material | Recommended Drying Temp (°C) | Recommended Drying Time (hrs) | Max Moisture Before Processing | Notes |
|---|---|---|---|---|
| PET | 160–180 | 4–6 | <0.005% (50 ppm) | Critical: moisture >30 ppm causes IV drop in preform molding. Extended drying at lower temp preferred over short high-temp drying. |
| PC (Polycarbonate) | 120 | 3–4 | <0.02% | Over-drying at >130°C risks yellowing. Monitor color on clear parts. |
| PA6 / PA66 (Nylon) | 75–85 | 2–4 | <0.10% | Nylon absorbs moisture rapidly after drying. Direct machine-mount connection is essential. |
| ABS | 80 | 2–3 | <0.10% | Tolerant material; main defect from moisture is surface splay on cosmetic parts. |
| PMMA (Acrylic) | 80–90 | 2–3 | <0.05% | Over-temperature causes yellowing. Stay at or below 90°C. |
| POM (Acetal) | 80–90 | 2–3 | <0.10% | Over-temperature causes formaldehyde outgassing — a safety and quality hazard. Do not exceed 90°C. |
| PP / PE | 70–80 | 1–2 | <0.10% | Low moisture absorption; drying primarily needed for regrind or in humid climates. Often optional for virgin material. |
| PVC (Rigid) | 65–75 | 1–2 | <0.10% | Heat-sensitive. Do not exceed 75°C. Over-temperature causes HCl outgassing and degradation. |
| TPU | 80–90 | 2–3 | <0.03% | Hygroscopic; bubbles and surface defects if not dried. Monitor melt viscosity for consistency. |
| PBT | 120–140 | 3–4 | <0.02% | Similar sensitivity to PET; use upper temperature range for faster cycle times. |
> General rule: When in doubt, consult your resin supplier's technical data sheet for specific drying parameters. The STG-U's digital controller can be set to any temperature within its operating range.
The STG-U series delivers verified energy savings of 27–65% versus thermostatically controlled dryers of equivalent capacity. The savings come from three sources: PID temperature control that eliminates heater overshoot (the largest contributor), efficient blower motors that move more air per watt, and right-sized model selection that avoids the waste of oversized hopper volume. For a mid-range model like the STG-U250 running two 8-hour shifts, annual savings of 7,000–12,000 kWh are typical — at 700–1,800 per year in direct electricity cost reduction alone.
Dried plastic pellets are hygroscopic — they start re-absorbing moisture from ambient air the moment they leave the dryer. In humid factory environments or non-air-conditioned plants in tropical regions, this re-absorption happens quickly. A pellet dried to 0.02% moisture can climb back to 0.08–0.12% within an hour of open-air exposure — above the defect threshold for materials like PET and PC. By mounting directly on the machine feed throat, the STG-U ensures dried pellets drop into the barrel within seconds, not minutes. There is no transfer bin, no conveying line, no waiting. What you dry is what you mold.
Match your machine's hourly material consumption to the hopper volume, accounting for required residence time. A 200-ton injection machine molding ABS at 15 kg/h needs roughly 30–45 kg of material resident in the hopper at any given moment (ABS requires 2–3 hours drying time). The STG-U80 (50 kg capacity) or STG-U125 (75 kg capacity) would be the appropriate choice — providing sufficient residence time with a safety margin. Contact Nicole at ceo@cxsljx.com with your machine tonnage, material type, and cycle time, and we will recommend the optimal model.
Any hygroscopic polymer — one that absorbs moisture from the air — requires drying before processing. This includes PET, PC, PA6/PA66, ABS, PMMA (acrylic), PBT, POM (acetal), TPU, and rigid PVC. Non-hygroscopic materials like PP and PE generally do not require drying for virgin material, but regrind, recycled content, or material stored in humid conditions may benefit from drying to eliminate surface moisture. The rule of thumb: if your parts show splay marks, bubbles, or surface defects that disappear after drying, you need a dryer.
PET requires 160–180°C with 4–6 hours of residence time. The key parameter is not just temperature — it is the combination of temperature, time, and airflow that determines final moisture content. For preform molding where IV retention is critical, use the lower end of the temperature range (160–165°C) with longer residence time (5–6 hours) to minimize thermal degradation while still achieving <50 ppm moisture. The STG-U's PID controller holds this temperature to ±1°C, which is essential because PET drying at 155°C is ineffective and at 185°C causes yellowing and IV loss.
The consequences cascade from cosmetic to structural to economic. First, surface defects — splay marks, silver streaks, bubbles — appear on parts, driving up visual inspection reject rates. Second, internal voids and porosity weaken the part mechanically; a structural bracket that should hold 200 kg snaps at 150 kg because microscopic steam bubbles created stress concentration points. Third, for condensation polymers like PET and PA, water in the melt causes hydrolysis — a chemical reaction that permanently breaks polymer chains, reducing molecular weight. An undried PET preform can lose 20–30% of its IV, making it unfit for carbonated beverage containers. Finally, moisture-related scrap compounds: rejected parts represent wasted material, wasted machine time, wasted energy, and wasted labor — costs that a properly operated STG-U drier prevents entirely.
Routine maintenance is minimal and straightforward. Clean the hopper interior when changing between incompatible materials or colors — simply empty the hopper, wipe down interior surfaces, and vacuum any residual pellets or fines. Inspect and clean the blower intake filter monthly in dusty environments, quarterly in clean environments — a clogged filter reduces airflow and drying efficiency. Check the heater element connections annually for tightness and signs of oxidation. The digital temperature controller requires no calibration under normal use. Total annual maintenance time for an STG-U in typical service is under 2 hours. There are no desiccant beds to regenerate, no rotating drums to service, no compressed air filters to replace — the simplicity of hot-air drying translates directly to low maintenance burden.
Adding an STG-U Hopper Drier to your injection molding or extrusion line is one of the fastest ROI improvements available in plastics processing. Here is how to get started:
Step 1 — Define Your Requirements: Material type, hourly throughput, and required drying time. Use the material drying guide above for temperature and time parameters.
Step 2 — Select Your Model: Match your throughput and residence time requirements to the specification table. Contact Nicole if you need assistance with sizing — provide your machine tonnage, material, and cycle time for a precise recommendation.
Step 3 — Request a Quote: Reach out with your model selection and quantity. We will confirm availability, provide pricing, and advise on lead time and shipping options.
Step 4 — Install and Start Saving: The STG-U mounts directly to your machine's feed inlet using the standard flange. Installation typically takes under 30 minutes. Set your drying temperature on the digital controller, load material, and begin producing moisture-free, defect-free parts from the first shot.
For complete material handling solutions, Chenxing Machinery also supplies screw feeders for automated hopper loading, high-speed mixers for material blending, CJ magnetic frames for ferrous metal removal at the hopper inlet, and industrial chillers for mold temperature control — ask Nicole about integrated system packages.
Contact Chenxing Machinery Today:
Contact Person: Nicole
Phone / WhatsApp: +8615951187228
Email: ceo@cxsljx.com
Company: Zhangjiagang Chenxing Machinery Co., Ltd.
Website: www.chenxingmachinery.com