Precise material feeding and consistent color blending are among the most consequential operational variables in any plastic processing operation. Whether you are running an injection molding line producing consumer packaging, an extrusion line manufacturing profile or film products, or a blow molding operation for containers, the accuracy of your additive and color dosing system directly determines product quality consistency, raw material waste levels, and ultimately your cost per kilogram of finished product.
This guide provides a systematic framework for selecting the right plastic doser and color mixer for your specific production requirements. It covers the two principal dosing technologies — volumetric and gravimetric — their respective advantages and limitations, the key selection criteria that matter in practice, and how to size and specify equipment for your particular application.
A plastic doser (also referred to as a color feeder, additive doser, or dosing machine) is a device that meters and feeds precise quantities of additives, colorants, masterbatch, or other processing aids into the main material stream of a plastic processing machine — typically an injection molding machine, extruder, or blow molding machine.
The doser sits between the material feed hopper of the processing machine and the main material supply (usually a virgin resin or recycled material hopper). It draws a measured quantity of additive or color concentrate from its own hopper and mixes it into the main material stream at a precisely controlled rate, expressed as a percentage of the main material throughput or as an absolute mass flow rate.
The quality of this dosing process matters for three reasons:
Volumetric dosers meter material based on the volume of additive drawn per unit time. The dosing mechanism is typically a rotating screw or piston inside a barrel — as the screw rotates at a set speed, it displaces a fixed volume of material per revolution. By calibrating the screw geometry and motor speed, the doser can be set to deliver a target mass flow rate of additive, expressed as kg/hr or as a percentage of the main material throughput.
Volumetric dosers are the more common type in entry-level and mid-market applications due to their lower upfront cost and simpler mechanical design. They require regular calibration to account for changes in bulk density of the additive or masterbatch — different material grades, particle size distributions, and moisture content can all affect the bulk density and therefore the actual mass flow rate delivered by a given screw speed setting.
Gravimetric (or weight-based) dosers measure and control the mass of material actually dispensed rather than inferring it from volume. The most common gravimetric doser architecture uses a loss-in-weight system: the additive hopper is mounted on a load cell assembly that continuously measures the weight of the hopper and its contents. As material is dispensed, the控制系统 continuously calculates the mass flow rate from the rate of weight decrease and adjusts the screw speed in real time to maintain the target dose rate.
Gravimetric dosers are preferred for high-precision applications where color consistency or additive concentration accuracy must be maintained across material batches, production shifts, and over time as the bulk density of the masterbatch may vary. They do not require the manual calibration that volumetric dosers need, and they automatically compensate for changes in material bulk density without operator intervention.
| Factor | Volumetric Doser | Gravimetric (Loss-in-Weight) Doser |
|---|---|---|
| Dosing Accuracy | ±1-3% under constant conditions | ±0.5-1% regardless of material variation |
| Bulk Density Compensation | Manual calibration required | Automatic, real-time compensation |
| Upfront Cost | Lower — simpler mechanical design | Higher — includes load cells and control system |
| Maintenance Complexity | Low — basic screw motor assembly | Medium — load cell requires protection from shock/vibration |
| Calibration Requirement | Regular calibration needed (every 2-4 weeks) | Self-calibrating via load cell feedback |
| Best for | Standard color dosing, stable material grades, budget-constrained applications | High-precision dosing, frequent material changeovers, pharmaceutical or food-contact applications |
| Dosing Range | Typically 0.1-25% of main throughput | Typically 0.05-25% of main throughput |
| Response to Material Changes | Requires recalibration for each new material batch | Automatically adjusts within one dosing cycle |
Before comparing doser types, clarify the specific parameters of your application:
The choice between volumetric and gravimetric dosing is fundamentally a question of how much color or additive concentration variation your product can tolerate — and how much that variation costs you in scrap, rework, or customer complaints.
For general-purpose injection molding and extrusion applications where color variation of ±2-3% between parts is acceptable — for example, in non-visible structural components, utility piping, or bulk packaging — a properly calibrated volumetric doser delivers adequate performance at a significantly lower equipment cost.
For applications where color consistency is tightly specified — consumer packaging with strict color tolerance, medical device components, automotive interior trim, or any product where color variation triggers customer rejection — a gravimetric doser with loss-in-weight control is the appropriate choice. The additional equipment cost is typically recovered within 6-12 months through reduced scrap and rework costs.
Undersized dosers are one of the most common selection errors. A doser that is sized for the average throughput of your machine will fail to deliver the required dose rate during peak production runs — resulting in under-dosing, product quality failures, and emergency procurement of additional equipment.
Size the doser for your maximum expected production rate, not the average. Add a 15-20% safety margin to the maximum dose rate requirement to account for production growth and material grade variations that may require higher concentration settings.
The purchase price of a gravimetric doser is typically 40-80% higher than an equivalent-capacity volumetric unit. However, when calculating total cost of ownership, factor in:
Beyond the doser itself, many plastic processing operations require a mixing step to ensure that color concentrate or additives are uniformly distributed throughout the base resin before the material enters the processing machine. This is particularly important when using high concentration masterbatches (typically 20-50% pigment) where achieving uniform dispersion requires mechanical mixing energy beyond what the dosing screw alone can provide.
Vertical mixers — also called plastic mixers or color mixers — consist of a vertical cylindrical mixing chamber with a central rotating shaft carrying fixed paddles or blades. Material is loaded into the chamber, the mixer is activated for a preset mixing time (typically 3-8 minutes depending on batch size), and the thoroughly mixed material is then discharged into the machine hopper or a storage bin.
Vertical mixers are suitable for batch sizes from 25 kg to 500 kg and are the standard solution for pre-mixing color concentrate with base resin before feeding into the processing machine. They deliver thorough distributive mixing — the even dispersion of color throughout the batch — but not necessarily intensive dispersive mixing for the hardest-to-disperse pigment agglomerates.
Match mixer capacity to your typical batch size, not to your largest batch. A mixer that is permanently underfilled — operating at 30-40% of rated capacity — delivers poor mixing quality because the material forms a dead zone at the bottom of the chamber where it is not contacted by the mixing blades. As a rule, select a mixer with a rated capacity that is no more than 60-80% above your typical batch size.
ZILLION offers a complete range of volumetric dosers and vertical plastic mixers covering throughputs from 0.3 kg/hr to 700 kg/hr, suitable for injection molding, extrusion, and blow molding applications across all production scales.
| Model | Type | Dosing Output | Screw Size | Hopper Capacity | Weight | Application |
|---|---|---|---|---|---|---|
| ZL-SM-86-12 | Volumetric Single Color | 0.3–3 kg/hr | Screw 12 | 12L | 27 kg | Small injection molding, compact machines, lab use |
| ZL-SM-86-16 | Volumetric Single Color | 0.7–7.2 kg/hr | Screw 16 | 12L | 27 kg | Medium injection molding, general purpose color dosing |
| ZL-SM-86-24 | Volumetric Single Color | 1.4–14 kg/hr | Screw 24 | 12L | 27 kg | Large injection molding, extrusion lines, high-output dosing |
| ZL-DM (Double Color) | Volumetric Double Color | Output x2 vs single color | Dual screw | 12L x2 | 38 kg | Multi-color products, dual additive dosing, film extrusion |
| ZL-PM Series Mixers | Vertical Mixer | 50–500 kg/batch | N/A | 50–500 kg | 95–500 kg | Pre-mixing color concentrate + base resin, batch blending |
For high-volume consumer packaging production running 24/7 with frequent color changeovers, ZILLION recommends the ZL-SM-86-24 paired with a ZL-PM100KG vertical mixer. The 1.4-14 kg/hr dosing range covers most medium-to-large injection molding machines, while the 100 kg batch mixer provides enough pre-mixed material for 2-4 hours of continuous production before requiring a reload — minimizing changeover downtime.
For technical injection molding applications such as automotive components or electrical housings where color tolerance is tight and additive concentrations must be precise, the ZL-DM double color doser enables simultaneous dosing of color masterbatch and functional additives (UV stabilizers, flame retardants) from separate hoppers — eliminating the need to pre-blend additive concentrates and providing maximum formulation flexibility.
Film and sheet extrusion lines typically run at high throughputs with consistent formulations over long production runs. The ZL-SM-86-24 at the high end of its dosing range (14 kg/hr) can support extrusion lines with throughputs up to approximately 600 kg/hr of main material, while the continuous mixing action of a ZL-PM200KG vertical mixer provides a buffer of pre-mixed material that insulates the extrusion process from minor variations in color concentrate flow rate.
For small injection molding machines up to approximately 150 tons clamping force, where the material throughput is relatively low and changeover frequency is high, the ZL-SM-86-12 with its 0.3-3 kg/hr dosing range provides the fine control needed for small-dose applications without the risk of over-dosing that can occur when a large-capacity doser is run at its minimum speed setting.
The doser should be mounted as close as possible to the machine hopper, and the feed tube connecting the doser discharge to the machine hopper should be as short and straight as possible. Long or convoluted feed tubes create material hang-up points where color concentrate can accumulate and cause batch-to-batch color carryover — particularly problematic when changing between dark and light colors.
Establish a calibration schedule based on production volume rather than calendar time. For high-volume operations (above 500 kg/hr throughput per machine), calibrate the doser every 2 weeks or 40 production hours, whichever comes first. For lower-volume operations, monthly calibration is typically adequate. Keep a calibration log — recording the target setting, the actual measured output, and the correction applied — to identify trends in doser performance over time.
Masterbatch and additive concentrates are sensitive to moisture absorption, which can affect both their flow characteristics (compromising volumetric doser accuracy) and their functional performance. Store all masterbatch and additive materials in a dry environment, and do not leave hoppers open to ambient air for extended periods during production changeovers. Using a hopper dryer ahead of the mixing and dosing system provides an additional safeguard against moisture-related flow variability.
The selection of a plastic doser and color mixer is fundamentally a decision about your product quality requirements weighed against your budget constraints and operational complexity tolerance. For most general-purpose injection molding and extrusion applications, a well-sized volumetric doser from the ZILLION SM range — correctly calibrated and maintained — delivers adequate dosing accuracy at a compelling price point. For high-precision applications where color consistency is a quality specification, the additional cost of a gravimetric or loss-in-weight system is justified by the reduction in scrap, rework, and customer complaints.
Regardless of which dosing technology you choose, pairing the doser with an appropriately sized ZILLION vertical mixer ensures that the color concentrate or additive is fully and uniformly distributed throughout the base resin before it enters the processing machine — eliminating a significant source of batch-to-batch variation that no dosing system alone can resolve.
Need help sizing a doser and mixer system for your specific application? Contact the ZILLION engineering team for a free production audit and equipment sizing recommendation.
La serie de productos de nuestra empresa es bien conocida en la industria de maquinaria auxiliar de plástico por su excelente rendimiento y alto costo.
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