PVC does not arrive as a compound
Every other material in this series arrives at the extruder as ready-made pellets. PVC does not. It arrives as a fine white powder — suspension PVC resin, K-value 57 to 70, with a particle size of 100 – 160 µm — and the processor has to build the compound around it.
That compound is a dry blend: resin, stabiliser, lubricants, impact modifier, filler, pigment and processing aid, mixed hot until the additives are absorbed into the porous resin particles, then cooled. It is still a powder when it goes into the hopper, and it is still a powder when it enters the screw. It only becomes a melt inside the extruder.
The number that governs the whole process: the dry blend's hot-mix temperature. Typically 115 – 125 °C. Below 110 °C the additives are not fully absorbed and the powder is dusty and uneven; above 130 °C the particles start to gel prematurely and the blend becomes lumpy and unprocessable. Fifteen degrees is the entire window.
The mixer: hot and cold, in sequence
| Stage | Equipment | Temperatur | Time | What happens |
|---|---|---|---|---|
| 1. Charge | Hot mixer (high speed) | Ambient | 10 – 30 s | Powders are dispersed mechanically |
| 2. Heizung | Hot mixer, friction | Ambient to 80 °C | 2 – 4 min | Frictional heat; additives start to melt and absorb |
| 3. Absorption | Hot mixer | 80 to 115 – 125 °C | 3 – 6 min | Lubricants and stabiliser penetrate the resin pores |
| 4. Discharge | Hot mixer to cold mixer | 115 – 125 °C | 10 – 20 s | The blend is dumped before it gels |
| 5. Kühlung | Cold mixer (low speed) | 125 to 40 – 50 °C | 5 – 12 min | Prevents agglomeration; sets the free-flowing structure |
| 6. Sieving | Vibrating screen | — | — | Removes lumps and foreign matter |
| 7. Ageing | Storage silo | Ambient | 12 – 24 h | Homogenises; some formulations improve |
Table 1 — The dry blend cycle. Discharge temperature is the single most important control point.
The hot mixer is a high-speed, self-friction machine: a large impeller at 400 – 900 rpm generates the heat, and the jacket can add or remove some. There is no external heating in most designs — the friction does the work, which is why the batch time depends on the formulation's lubricant level. A heavily lubricated formulation generates less friction and takes longer to reach temperature, which is one of the ways a formulation change shows up in production.
The cold mixer is a large, slow, jacketed vessel. Its job is to bring the blend down below 50 °C quickly so the particles do not stick together. A blend discharged at 120 °C and left in a drum will cake into a solid block overnight.
What goes in, and why the order matters
| Komponente | Typical phr | Added when | Reason for the order |
|---|---|---|---|
| PVC resin | 100 | First | The reference; everything else is per hundred of it |
| Stabiliser (CaZn) | 3 – 5 | Early, with resin | Must be distributed before any heat is generated |
| Titanium dioxide | 3 – 6 | Early | Hard to disperse once the lubricants are in |
| Filler (CaCO₃) | 3 – 10 | Early | Abrasive; add before the mix gets sticky |
| Impact modifier | 4 – 12 | Mid | Needs shear to disperse, but not excessive heat |
| Processing aid (ACR) | 1 – 3 | Mid | Promotes fusion later, in the extruder |
| Internal lubricant | 0.5 – 1.5 | Late | Melts early; adding it too soon coats the particles |
| External lubricant | 0.3 – 1.0 | Last | Forms the outer film; must be on the surface, not absorbed |
| Pigment / masterbatch | 0.5 – 3 | Late | Heat sensitive |
| Blowing agent (foam only) | 0.4 – 1.2 | Very last, cool | Decomposes above 170 °C; must not see hot mixer temperature |
Table 2 — Addition order. The lubricant sequence is what separates a good blend from a bad one.
The lubricant order is the part that most people get wrong. Internal lubricants reduce the viscosity of the melt and promote fusion; external lubricants form a film on the outside of the particles and delay fusion. If the external lubricant is added at the start, it coats the resin particles before the stabiliser and impact modifier can be absorbed, and those additives never get in. The result is a blend that looks fine, runs badly, and produces a profile with poor impact and plate-out on the die.
Judging a dry blend
| Test | Methode | Typical target | What it predicts |
|---|---|---|---|
| Bulk density | Graduated cylinder, tapped | 0.52 – 0.62 g/cm³ | Feed consistency and output |
| Dry flow | Funnel flow time | Formulation-specific | Hopper bridging |
| Plasticiser absorption | Hot-mix time to temperature | 4 – 8 min | Additive absorption quality |
| Gelation (DCM) | Dichloromethane soak | No gel in the blend | Premature gelling — a bad batch |
| Volatiles | Oven at 105 °C, weight loss | < 0.5 % | Moisture — causes bubbles and voids |
| Particle size distribution | Sieve set | < 3 % over 500 µm | Feed surges and unmelt |
| Fish eyes | Pressed film, visual count | < 20 per 100 cm² | Unfused resin particles |
| Thermal stability | Congo red or static oven | > 25 min at 200 °C | Scorch risk in the extruder |
Table 3 — Dry blend tests. Bulk density and fish eyes are the two that correlate most directly with profile quality.
Fish eyes are worth explaining because they are the most visible symptom of a bad blend. A fish eye is an unfused resin particle that has survived the extruder and appears as a small lens in the surface. In a white profile they show as slightly glossy spots; in a painted or laminated profile they telegraph through. They come from a blend that was under-mixed, or from resin with a high proportion of "glass" particles — very dense, low-porosity resin that absorbs additives poorly.
Blend problems and their symptoms
| Blend problem | What you see on the line | Ursache |
|---|---|---|
| Under-mixed | Fish eyes, plate-out, poor impact | Too short a hot-mix time, or too low a discharge temperature |
| Over-mixed | Lumps, surging feed, hard spots in the profile | Discharge temperature too high; blend started to gel |
| Wet blend | Bubbles, voids, silver streaks | Resin stored in humid conditions; insufficient drying |
| Lubricant imbalance | Die build-up, rough surface, or poor fusion | Wrong internal/external ratio, or wrong addition order |
| Low bulk density | Output lower than expected; feed surges | Resin grade, or excessive filler |
| Segregation in the silo | Batch-to-batch variation within a run | Particle size distribution too wide; no ageing |
| Kontamination | Black specks, colour streaks | Mixer not cleaned between colours; sieve bypassed |
Table 4 — Blend problems. Note that three of the seven are visible only as extrusion defects.
The segregation problem is the one that produces the most confusing complaints. A dry blend with a wide particle size distribution will segregate in a silo or a hopper: the fines stay at the top, the coarse particles sink. The first part of a run uses one part of the distribution and the end of the run uses another, so the profile changes character halfway through a shift with no change to any setting. Ageing the blend for 12 – 24 hours in a silo with a proper discharge helps, but the real fix is a tighter particle size distribution in the resin.
Buy compound or blend your own?
| Faktor | Buy ready dry blend | Blend in house |
|---|---|---|
| Capital | None | €80,000 – €400,000 for a mixer line |
| Material cost | +8 – 20 % (compounder margin) | Raw material cost only |
| Formulation control | Supplier's | Yours |
| Flexibilität | Limited to the supplier's range | Any formulation, any batch size |
| Quality risk | Supplier's responsibility | Yours — and blend faults look like extrusion faults |
| Minimum order | Often 1 – 5 t | One batch, typically 200 – 500 kg |
| Am besten für | < 300 t/year, or a single stable product | > 500 t/year, or a wide product range |
Table 5 — The make-or-buy decision. The crossover is around 400 – 600 tonnes a year.
The quality risk row is the one that surprises people. When you blend in house, a blend fault appears as an extrusion fault — fish eyes, plate-out, poor impact — and the extrusion team will spend days adjusting temperatures before anyone suspects the mixer. Compounders who supply dry blend carry that diagnostic burden; processors who blend their own need to be able to test the blend, not just the profile.
Myth: "dry blend is just mixing powder — how hard can it be?" The blend determines fusion, and fusion determines impact strength, surface and dimensional stability. A profile made from a blend discharged 15 °C too hot will fail a low-temperature impact test that the identical formulation, mixed correctly, passes. The mixer is the least understood piece of equipment on a PVC profile line and it has more influence on the product than the die.
Storage and handling
| Issue | Effect | Kontrolle |
|---|---|---|
| Moisture pickup | Bubbles, voids, silver streaks | Sealed silos; dry air; use within 5 days |
| Caking | Hopper bridging, feed surges | Cool below 50 °C before storage |
| Segregation | Batch variation within a run | Age 12 – 24 h; avoid tall silos with a single outlet |
| Temperature cycling | Condensation inside the silo | Insulate; avoid filling a warm silo with cold blend |
| Cross-contamination | Colour streaks | Dedicated lines per colour, or validated clean-down |
| Age | Additive migration, reduced flow | Use within 30 days; first in, first out |
Table 6 — Storage. Caking and moisture are the two that cause visible defects.
Moisture is a smaller problem for PVC than for polycarbonate or nylon, because PVC absorbs little water — but "little" is not zero, and a dry blend stored in a humid plant can pick up 0.3 – 0.8 %. At the extruder that becomes bubbles and voids, and the natural conclusion is that the formulation is degassing. Drying PVC dry blend is not standard practice, but keeping it dry is.
Frequently asked questions
What is the right hot-mix discharge temperature?
115 – 125 °C for a standard rigid profile formulation. Below 110 °C the additives are not fully absorbed and you get fish eyes and plate-out; above 130 °C the blend starts to gel and becomes lumpy. The exact figure depends on the formulation's lubricant level and on the resin's porosity.
Why does my blend cake in the silo?
Because it was stored too hot. A blend discharged at 120 °C and put straight into a silo will fuse into lumps as it cools slowly. It must go through the cold mixer to below 50 °C first, and the silo should not be filled with hot material.
What is a fish eye and what causes it?
An unfused resin particle that survives into the profile and appears as a small glossy lens in the surface. It comes from under-mixing, from resin with a high proportion of low-porosity ("glass") particles, or from a blend that was discharged too cold.
Can I use a ribbon blender instead of a hot/cold mixer?
No, not for rigid PVC. A ribbon blender mixes powders but generates no frictional heat, so the additives are never absorbed into the resin particles. The result is a dusty, segregated blend that produces a profile with poor impact and heavy plate-out. Hot/cold mixers are not optional for rigid PVC.
How long can I store dry blend?
30 days in a sealed silo, 5 – 7 days if the silo is not sealed and the plant is humid. Beyond that the additives migrate and the flow properties change. Ageing for 12 – 24 hours before use is beneficial; ageing for two months is not.
Does the blend need drying before extrusion?
Usually not, if it has been stored dry. PVC absorbs only 0.2 – 0.4 % and most of that leaves in the feed zone. But a blend stored in a humid plant can reach 0.8 %, which produces visible bubbles — so keep it dry rather than drying it.
Why does the blend affect impact strength?
Because impact strength depends on fusion, and fusion depends on how well the additives are distributed and absorbed. A blend where the impact modifier is not properly dispersed produces a profile with unfused regions, and cracks start at those boundaries. The impact modifier can be present at the right loading and still not do its job.
What is the difference between a one-step and a two-step blend?
In a one-step process the whole formulation is mixed in the hot mixer. In a two-step process the additives are pre-blended or masterbatched first, then added to the resin. Two-step gives better dispersion of small-quantity ingredients and is common where the formulation has many components at under 1 phr.
Buying PVC dry blend, or setting up your own mixing?
Tell us the profile, the output and the product range. We will tell you whether in-house blending pays at your volume, and what the blend specification should be.
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