What is dosing? Precision dosing in industrial production

What is dosing?

Dosing is the measured, repeatable feeding of each raw material in a recipe into a production process, in a defined quantity and a defined order. Measurement can be by mass (gravimetric) or by volume (volumetric), and a dosing system can work in batches or continuously at a set rate. The goal is the same composition in every batch, or every hour.

Dosing is used across plastics, chemicals, food, building chemicals and coatings. This article uses PVC mixing lines as the example, because PVC formulations show very clearly why dosing matters.

Why dosing matters so much in PVC compounding

A PVC formulation is never a single material. Alongside the PVC resin it contains a heat stabiliser, lubricants, fillers such as calcium carbonate, an impact modifier, processing aids, pigment and, in flexible PVC, plasticiser. Some of these make up a large share of the recipe, others are used in small amounts. A small error in a minor ingredient can still have a visible effect:

• Too little stabiliser and the compound can yellow or degrade during processing.
• An unbalanced lubricant package causes pressure and torque swings in the extruder and poorer surface quality.
• A drifting filler level changes both mechanical properties and cost per kilogram.
• A pigment error shows up straight away as a colour difference.

That is why dosing is the first step that decides PVC dry blend quality. However good the mixer is, it cannot fix a batch that was weighed wrongly.

Powder dosing vs liquid dosing

A PVC line doses two kinds of material, and each has its own difficulties.

With powders, the same volume can weigh more or less depending on moisture and the supplier lot. With liquids, viscosity and density shift with the ambient temperature, so winter and summer batches differ if you measure by volume. For plasticiser and liquid stabiliser handling, see our automatic liquid dosing system.

Gravimetric vs volumetric dosing

Volumetric dosing measures by volume, for example a set number of screw revolutions or a fixed chamber size. It is simple and inexpensive, but it goes wrong whenever the material density changes.

Gravimetric dosing weighs the material directly and stops the feed once the target mass is reached. Density changes do not affect it. In PVC recipes, with fillers that vary from lot to lot and minor additives that matter a lot, this is usually the preferred method.

We compare the two in detail in gravimetric vs volumetric dosing.

Batch vs continuous dosing

Dosing systems also differ in how they run.

• Batch dosing: ingredients are weighed one after another into a complete batch, which is then discharged in one go. PVC dry blend lines built around heating-cooling mixers use batch dosing, because the mixer itself works in batches.
• Continuous dosing: material is fed without interruption at a set rate. This is typical at the extruder throat, for example on compounding or masterbatch lines, using loss-in-weight feeders.

The right choice depends on the process. On a dry blend line, the key requirement is that every mixer batch is correct and ready on time.

How dosing connects to the mixer

On a dry blend line, the dosing system and the hot mixer work as one unit:

1. Select the recipe. The product's recipe is called up in the control system.
2. Weigh the powders. Resin, filler and powder additives are fed from their own storage into a weigh hopper.
3. Weigh the liquids. In flexible recipes, plasticiser and liquid stabilisers are prepared on a separate liquid weighing unit.
4. Charge the mixer. Powders and liquids go into the hot mixer in the order the recipe defines.
5. Prepare the next batch. While the mixer runs, the dosing system weighs the next batch, so the mixer never waits.

Order matters here too. The stage at which each additive enters the mixer has a direct effect on the dry blend, which we explain in PVC additive addition order.

Manual vs automatic dosing

Many plants start with manual weighing: open the bag, weigh it in a bucket, tip it into the mixer. At small volumes that can work, but as output grows the problems add up:

• Weighing results that vary between operators and shifts
• Bad batches from the wrong bag or a forgotten ingredient
• Dust loss, a dusty work area and heavy bag handling
• No record of what went into which batch

Automatic dosing turns these steps into a closed, repeatable process, with the recipe held in the system rather than on a paper sheet.

Erk PVC Makine dosing systems

Erk PVC Makine's dosing systems are designed for PVC mixing lines:

• Automatic powder dosing system: weighs PVC resin, filler and powder additives gravimetrically to the recipe and transfers the batch to the hot mixer, a silo or, where the line needs it, the extruder side.
• Automatic liquid dosing system: weighs plasticisers and liquid stabilisers gravimetrically and runs under the same recipe as the powder dosing.

Both can be planned as one line together with the mixer, storage and feeding equipment.

What to check when choosing a dosing system

• Number of ingredients and their shares: how many powders and liquids, and which is the smallest?
• Batch size and batch frequency: dosing has to keep up with the mixer cycle.
• Raw material form: big bags, sacks or silos?
• How often recipes change: in multi-product plants, recipe management and easy cleaning matter more.
• Traceability: do you need a record of actual weights per batch?

Bottom line

Dosing is where mix quality begins. The right measuring method, the right operating mode and a dosing system matched to the mixer make it far easier to get the same recipe in every batch. If you are planning dosing for your line, send us your ingredient list through the quote form.