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Gravimetric Dosing Systems


Brief Introduction of Gravimetric Dosing Systems 

In plastic production industries, many processors use two or more materials like virgin material, master batch, additive, filler or something else for a single product, and all the above materials need to be mixed based on certain ratios prior to processing in the molding machines like injection, extrusion, bottle blowing or film blowing machines.

Some processors prefer to mix them manually, but it takes much time and labor cost, moreover, people make mistakes, if they mix one of the ingredients by incorrect ratio, then the whole batch in the mixing tank will be ruined, thus factory loss money if the materials are expensive. And if the worker didn't notice the mistake and convey the ruined mixture to the molding machines, they will get defect products and that's another loss.  

Luckily, now more and more manufacturers are paying increasing attention to this issue, and they are looking for a suitable dosing device for their production.

There are many different dosing devices for the plastic production industry, for example, the gravimetric blender also called as gain-in-weight blender, volumetric blender, also called as a masterbatch feeder/doser, loss-in-weight blender, etc. We mainly focus on the gravimetric blender and volumetric blender. And I'd like to explain what they are, how they work, and how to select a suitable blender for your production.

What Is a Gravimetric Blender

Gravimetric blender also called as gravimetric dosing system or gravimetric batch blender, is an industrial device used in plastic production industry to dose or weigh two or more components and then mix them together prior to processing in the molding machines. It's a high-end technology for the discontinuous blending of materials at the highest level of accuracy. After years of exploring deeply in the industry and upgrading system many times, now Hengju's gravimetric blender can dose up to 8 materials at accuracy ±0.5%. That means, for example, if your recipe is comprised of virgin 80%, masterbatch 18%, filler 2%, and if we set the batch weight as 1000g, then it means that the amount of any ingredient could be off by 5g. That's very close to Europe- made blenders and with much lower cost. 

Gravimetric Blender’s Advantages:


Obviously, the gravimetric blender boasts of many advantages like the following:

*With the highest level of accuracy, for some factories or for some high-end products, the dosing accuracy matters a lot to the quality of the final products.

*Fully automatic operation, with one touch on the touch panel, you can sit back and watch the blender working, all you need to do is to check and refill the material silos.

*Easy setting for recipes, touch the recipe button on the panel and input the ratios of each material, then name it as Recipe A or whatever you like and save it at last.

*Saving labor cost, once having a gravimetric blender, factory owner only need one worker to refill materials and operate the blender for every shift. And you can calculate the ROI (Return on Investment) based on this.

It's expensive compared with the normal type of mixer, but you'll find it much worth it once you have one.

*Increasing productivity, with a throughput of up to 2000kg per hour, the blender can increase your productivity on a large scale. 

How Does a Gravimetric Feeder Work?


A gravimetric blender has two or more hoppers arranged above a weigh-pan which is controlled by a very sensitive load cell. And the hoppers will open and close under control of the slide-gate valve. Once the blender is turned on and the recipe is selected, it will dose and mix materials automatically.

For instance, if you select recipe A which is comprised of natural resin 90%, regrind 8%, color 2%, and you preset the batch weight as 1000g, then the PLC inside the blender will control the open/close of the hopper valves in order. First natural resin hopper open and resin flows down into the weigh-pan until it reaches 900g, then the valve closes. Secondly, the regrind hopper valve opens and regrind flows down into the weigh-pan until it reaches the amount of 80g, then the valve closes, and the same as the color. Once the batch has been weighed out, the contents of the weigh-pan are dispensed into a mixing chamber where they are blended. The resulting mixture exits the base of the mixing chamber into the processing machine. 

A look at Hengju’s HGB Series Gravimetric Blender


Over the past years, Hengju’s engineers put numerous time and effort into the upgrading and improvement of our blenders. We got many patents on blender hardware and software, and also we are honored with New High-Tech Enterprise by government.  

Hengju-Gravimetric Dosing Systems, Dongguan Hengju Machinery Equipment Co, Ltd
Hengju-Gravimetric Dosing Systems, Dongguan Hengju Machinery Equipment Co, Ltd-1

Right from the beginning of our establishment, we’ve always been adhering to the value of “customer-oriented, quality first”, beside the upgrading system software, we also use the best parts we can find around the world, for instance, the most important part-load cell, we buy from Mettler Toledo, which is the best for now. The air cylinder, we buy from Festo, Germany, and PLC & touch screen are from SIMENS, all the best parts guarantee the best performance of the machine.

Our blenders can dose up to 8 materials with capacity ranging from 50kg/hr to 2000kg/hr.

With dosing accuracy 0.5 %, our blender ranks top 3 among all suppliers from China.

Herewith a specific datasheet: 

Hengju-Gravimetric Dosing Systems, Dongguan Hengju Machinery Equipment Co, Ltd-4

Now more and more customers are introducing gravimetric blender to their production for LED, auto parts, non-woven bag, film, bottles & containers, all injection parts, cables, and wire, etc. And the areas are expanding.  

Hereafter are some application cases for your reference:

1) 4-components gravimetric dosing system for drinking bottle production:

Hengju-Gravimetric Dosing Systems, Dongguan Hengju Machinery Equipment Co, Ltd-5

Material: HDPE, regrind, masterbatch.

Customer's blowing machine has two layers, and one layer uses mixture material of HDPE, regrind, and master batch, the other uses HDPE virgin.  

2)  5-components gravimetric dosing system for 3-layer extruder:

Hengju-Gravimetric Dosing Systems, Dongguan Hengju Machinery Equipment Co, Ltd-6

The customer has many sets of 3-layer extruders, and they make PP/PS sheet which will be made into disposable food containers. They use five materials, PP, PS, filler, etc., each layer uses the same recipe comprised of 5 materials, but they have up to 20 different recipes.  

To do the dosing and mixing job manually is a huge job for their workers, now they just need one worker to monitor the five silos and refill the materials.  

3) 6-components gravimetric blender on single-layer film blowing machine 

Hengju-Gravimetric Dosing Systems, Dongguan Hengju Machinery Equipment Co, Ltd-7

Customer uses LDPE, LLDPE, additive, filler, and other 2 materials for his film production, and before having this gravimetric blender, he mixed the 6 materials manually which was quite common around his area but slow and less accurate, but right now he loves the blender very much and considers to purchase more for his new plant.

To make good gravimetric blender and offer gravimetric blending system to customers, it’s not just about technique, it’s more about the experience and also engineers. For example, when we make blenders or select models, we’ll do that based on material bulk density around 0.65kg/L, that’s the normal B.D. for most resins. But if the resin bulk density is much less than 0.65kg/L, say 0.35kg/L, then the model we selected should be twice bigger as it should be. Moreover, if the bulk density of the materials differs too much, then it’s better to place the blender right above the feeding inlet of the molding machine to avoid the delamination problem which could increase the rejects.

Gravimetric Dosing Unit VS Volumetric Dosing Unit

A volumetric dosing unit is a dosing device for one main material and one side ingredient, either color master batch or additive. It's also known as synchronous dosing stations, as it can meter all ingredients simultaneously. The mass flow corresponds to the recipe at all times, and the dosing station's control system ensures that the right amount of each ingredient is metered, on the basis of calibrated values and in the ratios required by the recipe. The material is fed directly into the feed throat of the processing machine, if the side ingredient is in a very small ration, then no need to mix the components prior to the processing machine, otherwise, it's necessary to mix the components. 

With injection molding, the mass flow is constant during the cycle time of the molding machine or to be precise, during the plasticizing time of the IMM, the doser's control system calculates the speed and cycle of the dosing modules on the basis of shot-weight, additive ratios, and calibrated values. The injection molding machine gives a signal to start the dosing cycle.  

When you select a suitable model, the following aspects should be considered:

1) Cycle time or plasticizing time of the injection molding machine

2) The ratio of color master batch or additive

3) Shot weight of the injection molding machine

But for extruder or blowing machine, you only need to care about two aspects:

1) Output capacity required per hour

2) The ratio of color master batch or additive

Herewith a specification sheet of our HSD series volumetric doser

Hengju-Gravimetric Dosing Systems, Dongguan Hengju Machinery Equipment Co, Ltd-10

Here comes a question, which dosing unit is suitable for you? We may get some ideas after we compare these two dosing devices from the following aspects:

1) Cost

Gravimetric blender is much more expensive than volumetric dosers, nearly twice the price.

2) Operation

Gravimetric blender is with an easy setting for recipes and easy cleaning. If you use many recipes or change color quite often, it's better to use a gravimetric blender.  

But if you use only one or two recipes and you don’t change color quite often, then the volumetric doser is a good choice for you.

3) Space occupying

Gravimetric blender is quite big while the volumetric blender is quite small, for some factories with narrow space, the volumetric blender seems to be the only option for the dosing job.

4) Dosing accuracy

Gravimetric blender is with highest dosing accuracy of 0.5%, while the volumetric blender’s dosing accuracy can only reach up to 1%.

For products which require high accuracy, the gravimetric blender is the best choice. Otherwise, the volumetric blender is acceptable.

5) Secret recipes protection

Some factories have secret recipes that the factory owners don't want to disclose to anybody; then it's best to have gravimetric blender into which the factory owners can input the recipes by themselves and name them as recipe 1, recipe 2, ...without showing the details of your formulas. 

6) Dosing output capacity

The gravimetric blender can dose up to 2000kg per hour, while the maximum dosing capacity for volumetric doser is only 50kg per hour.

7) Lead time

The lead time of master batch feeder is pretty quick, normally 2-3 weeks, some models are even quicker because we keep stock for some hot-selling models.

While the gravimetric blender takes much longer time to make, usually 20-35 working days, as most of them are customized and the testing also needs 2-3 days to make sure the dosing accuracy can meet customer’s demand.

How to Select a Suitable Model of Gravimetric Blender

Before you choose a gravimetric blender, please take all the following elements into your consideration:

1) What dosing accuracy do you need?

2) How much dosing capacity is required?

3) How many materials need to be dosed and the ratio of each component?

4) How many kinds of recipes do you use in your production?  

5) Where would you like to place the blender, either on your molding machine or on the ground?

Operation of Gravimetric Blender

It’s very important to read through the manual book before you start the blender. And hereafter is some basic instructions for operation.

The gravimetric blender is controlled by PLC, and all settings can be edit through the touch screen. 

7.1 Power on

Please check the following items before you switch on the blender:

* The machine is wired correctly.

*the compressed air is properly connected, no air leakage.

*All material hoppers are filled with correct materials.

And the following requirements must be followed:

1) Do not operate the touch screen with sharp objects instead of hands and any violent bump is also prevented.

2) The touch screen may generate much static electricity in dry air. So before you touch it, use earthing metal to release the static electricity.

3) Use economic alcohol or benzine to scrub the touch screen. Other solvents may cause the touch screen to fade.

4) don't open it without permission or take out any circuit board for that may cause damage to the blender.

For more operation instructions, please download the blender manual on our websitehttps://www.plasticmach.com.

Maintenance Instructions


1.1 Safety

1.1.1 qualifications of personnel

The maintenance and repair work should only be carried out by qualified and authorized technicians.

If by insufficiently qualified personnel, the danger of injury or incalculable risks may happen.

1.2 Before starting work

Before starting maintenance, the following instructions must be followed.

1.2.1 power of the blender/disconnect from the compressed air supply

Danger of injury: mixing paddle inside the mixing chamber may bring body part injury.

1.3 Inspection

Checking emergency stop function and testing safety interlock switch.

For more maintenance tips, please download the blender manual on our website.



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