Adding DOP at 80C is the easy part. Knowing what the blend should look like at 80C, how the motor load should behave, and what to do when the blend still feels wet at discharge — that is where batches go wrong. Temperature alone does not make a good dry blend. Without the right physical indicators between setpoints, you end up with a sticky mess that clogs your feeder. Here is the complete parameter set: temperatures, RPM, timing, and the floor-level signs that tell you absorption is actually happening.
Step-by-Step Mixing Procedure
Fill the hot mixer to 55-75% of net volume. Overfilling reduces fluidization and extends cycle time; underfilling causes uneven heating.
Load PVC Resin and Start Heating
Charge the PVC resin at room temperature and run the mixer at high speed (1200-1500 RPM for a 200-500 L unit). Blade friction heats the resin. At this stage you are driving off surface moisture and bringing the powder to the DOP addition window.
Add Stabilizer and Lubricants at 60-65C
Once the batch reaches 60-65C, add your heat stabilizer and lubricant package. These need time to coat resin particles before DOP arrives. Stabilizer selection affects DOP compatibility during processing, so confirm your pairing before the first batch.
Add DOP at 70-80C
DOP has a fusion temperature of 84C, meaning absorption accelerates fast once both resin and plasticizer reach the 80-84C zone. Start addition at 70C to give the plasticizer time to wet grain surfaces before hitting that acceleration window.
Spray DOP over the swirling mass rather than pouring it in one spot. DOP diffuses faster than DIDP or TOTM, but dumping the full charge overwhelms surface absorption capacity and creates temporary agglomerates. A steady spray over 60-90 seconds distributes the load evenly.
Reduce blade speed to 800-1000 RPM during addition. This two-speed approach gives diffusion time to keep up with heating. At full speed, frictional heat outruns absorption and you get a partially wet compound. Return to high speed after the DOP charge is fully introduced.
Some operators add DOP at 50C. For low-PHR formulations (under 20 PHR) where surface adsorption alone suffices, this works. For standard flexible PVC at 30-50 PHR, adding at 50C means DOP sits on grain surfaces without meaningful pore diffusion. You lose cycle time and risk wet-band formation.
Add Fillers at 85-90C
After the DOP absorption zone, add calcium carbonate or other fillers at 85-90C. Adding fillers too early competes with DOP for resin surface area. The correct DOP-to-filler ratio depends on your filler loading — higher filler PHR absorbs some plasticizer and may require dosage adjustment.
Discharge at 110-120C
The processing window for this formulation is 110-120C. Below 110C, plasticization is uneven. Above 120C, you consume stabilizer and risk degradation. At 115C, PVC particles consolidate into larger, uniform agglomerates — that is your target discharge point.
How to Tell When DOP Absorption Is Complete
The motor amperage curve is your most reliable real-time indicator. As DOP wets the resin surface, motor load increases — agglomerating powder puts more drag on the blade. Then load drops as agglomerates break apart. A secondary peak follows as absorption progresses and particles swell. The final load drop signals the free-flowing state. Those two distinct breaks in the amperage curve mark initial and final absorption completion.
Alongside the amperage, watch for these physical signs:
- The powder transitions from wet and clumpy to dry and free-flowing. Grab a handful from the observation port — it should flow through your fingers without sticking.
- Batch temperature stabilizes or rises more slowly in the final phase, because plasticizer diffusion briefly slows the heating curve.
If your hot mix cycle completes in under seven minutes, the blend did not have enough absorption time — add a hold interval. If the cycle exceeds 12 minutes, check blade condition, thermocouple accuracy, and mixer seal integrity. A normal cycle for 30-50 PHR DOP runs 8-11 minutes. Higher PHR loading extends the cycle — adjust your dosage and formulation targets to match.
Common DOP Mixing Failures and How to Fix Them
Three failures account for most bad batches I have seen.
Adding DOP Too Fast
Dumping the full charge in under 30 seconds overwhelms surface absorption capacity. Wet clumps form and persist through the entire cycle — more mixing time will not fix them. Spray over 60-90 seconds minimum for a 200 kg batch. If your mixer lacks a spray bar, install one. Pouring from a bucket through the lid port is the single most common cause of wet-blend complaints.
Wrong Addition Temperature
Adding DOP below 65C for a 30-50 PHR formulation means resin pores are not open. The plasticizer coats the surface but cannot diffuse inward. The blend looks dry at discharge but produces plate-out and fish-eyes during extrusion because DOP redistributes under heat rather than being pre-absorbed. If you see these defects, check your addition temperature first.
Premature Discharge
Operators watching the clock instead of the amperage curve sometimes dump the batch before the final load drop. Squeeze a handful of the blend — if it compacts into a ball instead of crumbling, absorption is incomplete. Hold until the motor load completes its final descent. An extra 60-90 seconds at temperature costs far less than reprocessing a caked batch.
Ribbon blenders with straight walls trap unabsorbed plasticizer along the vessel wall, stretching cycles to 45-60 minutes. Plow-type or high-speed mixers with fluidized-bed action eliminate this entirely. If you run a ribbon blender for plasticized PVC above 30 PHR, the equipment is working against you.
Cooling and Post-Mixing Procedure
Transfer the hot blend to a cooling mixer immediately. Cold mixer volume should be roughly three times the hot mixer to handle batch accumulation. Run cooling water at 13-15C through the jacket and discharge the cooled blend at 40-45C. Above 45C, the blend can cake in storage silos.
Let the cooled blend rest 12-24 hours before extruding. Residual plasticizer continues diffusing into the resin core during this period, and apparent density measurably increases after a full 24-hour rest — better bulk flow and more consistent extrusion feed. If you have inconsistent extruder output despite solid mixing, check this variable first.
The Step Most Operators Skip
The procedure runs on three pillars: correct addition temperature (70-80C, not 50C), controlled pour rate (spray, never dump), and verified absorption (motor amperage, not the clock).
Track your cycle times batch to batch. A creeping cycle time — 9 minutes today, 10 next week, 11 the week after — signals blade wear, thermocouple drift, or resin porosity changes long before defects reach the extrudate. Catch it at the mixer.