What Is Dosing in Injection Molding?

Dosing is a critical part of the injection molding process. It involves delivering a precise amount of plastic material, along with any necessary additives like colorants or reinforcements, into the injection molding machine.

Getting the dosing step right is essential for producing consistent, high-quality plastic parts. So in this post, as a professional plastic fabricante de moldeo por inyección, I’ll explain everything you need to know about dosing in injection molding.

What Is Dosing in Injection Molding?

Dosing in injection molding is a critical step that ensures the precise and controlled delivery of plastic material, along with any necessary additives, into the injection molding machine. This precise mechanism that controls the mixture and volume of plastic materials and additives, ensuring each injection into the mold is consistent and of the highest quality.

Why Dosing Matters in Injection Molding

The whole point of injection molding is to produce plastic parts with consistent physical properties.

For example, let’s say you’re manufacturing LEGO bricks. You need each brick to have the exact same color, strength, and texture. Any variation between bricks means you have a quality problem.

And that’s why dosing matters.

If the amounts of plastic material and additives aren’t precisely controlled during dosing, you’ll end up with inconsistent parts. Some bricks might be stronger and darker than others.

Proper dosing ensures that the plastic shot that gets injected into the mold has exactly the same composition shot after shot. This consistency leads to consistent properties across all your molded parts.

So in a nutshell, that’s why dosing is such a vital step.

Now let’s look at how dosing actually works within the injection molding process.

How Dosing Works in Injection Molding

The dosing system in injection molding has to integrate with the molding machine itself. That way, dosing can be automatically triggered and controlled as the machine runs through its molding cycles.

Here’s an overview of how this works:

1. Plastic Pellets Enter the Barrel

The process starts with plastic pellets entering the injection barrel from a hopper. This raw material is usually in the form of small cylinders or spheres.

At this stage, colorants, fillers, or other additives haven’t been added yet. It’s just the base plastic resin.

2. Plasticiser Melts the Pellets

Next, a giant screw inside the barrel—called a plasticiser—works to melt the pellets.

The screw rotates and mixes the plastic, while barrel heaters help to melt the material. By the nozzle end, the plastic becomes a homogeneous molten liquid ready for injection.

3. Dosing System Adds Additives

Now the dosing action takes place.

The dosing device adds extra ingredients like colorants to the base molten plastic already inside the barrel. So crucial elements that enhance the properties of the final plastic parts get mixed in.

A machine mounted hopper feeds the additives into the dosing unit. Then an auger or pump transfers a precise dose into the barrel after measuring the proper amount.

4. Shot Gets Injected Into Mold

With the plastic mixture now uniform, the next step is injection.

A hydraulic ram pushes the screw forward, injecting a precise shot of plastic through the nozzle into the mold. The mold’s shape gives form to the plastic.

High pressure packs the material tightly into every crevice of the mold. This occurs in just a fraction of a second.

5. Parts Cool Down and Eject

After getting injected while molten hot, the plastic shot cools back into a solid part.

Cooling channels circulate water or oil to extract heat from the mold surfaces. This helps harden the plastic.

As soon as the part gets rigid enough, the mold opens and robotically controlled ejector pins pop out the finished molded product.

Then the free mold closes again, and a fresh shot enters to restart the whole molding sequence.

As you can see, dosing represents just one phase within the injection molding process. But it’s an absolutely vital phase for ensuring consistency.

Now let’s explore some key types of dosing systems.

Types of Dosing Systems for Injection Molding

Several designs exist when it comes to dosing units for injection molding machines. Let’s compare the two main categories:

• Volumetric dosing
• Gravimetric dosing

Volumetric Dosing Systems

In volumetric dosing, the feed rate gets controlled based on physical volume. Typically an auger rotates inside a fixed chamber to deliver a certain volume per revolution.

Since additives like colorants are free flowing powders, you can easily correlate the auger displacement with volume. These systems feature simplicity and low cost.

However, volumetric dosing lacks precision. As temperature or humidity fluctuate, powder can pack to differing densities.

Plus any wear on the auger or chamber can throw calibrations off. So maintaining accuracy over long runs becomes difficult. Frequent calibration is essential.

Overall volumetric systems work where precision tolerances aren’t too strict. But for color matching aesthetic parts within tight specs, gravimetric offers better consistency.

Gravimetric Dosing Systems

In gravimetric dosing, the feed rate gets directly controlled by continuous weight measurements.

An electronic scale integrates with a hopper containing powder additives like pigments. A controller tracks the precise loss-in-weight over time as powder dispenses.

So rather than relying on volume displacement, which varies, the system electronically adjusts the feed rate to maintain the exact weight target. This closed loop control guarantees consistency.

And because the system self-corrects for any changes in material packing density, temperature, humidity etc, it stays accurate over longer runs. Recalibration generally only gets performed during material changeovers or scheduled maintenance.

For high-precision color matching of injection molded products, gravimetric offers much tighter process control. And advanced software allows storing hundreds of recipes.

Overall gravimetric represents the gold standard technology for dosing in injection molding these days.

Now let’s answer some common questions about dosing.

FAQ About Dosing in Injection Molding

Here are answers to some frequently asked questions regarding dosing technology:

Why does dosing matter for part quality?

Controlling the precise additive content during dosing directly impacts the consistency of molded part properties like color, strength, wear resistance and more. Poor dosing causes unacceptable variations.

Does dosing help productivity?

Advanced dosing allows faster molding cycles and less downtime. Pre-mixed recipes can change automatically without needing operator input. And self-calibrating operation reduces maintenance needs.

What gets dosed into the plastic?

Typical additives introduced during dosing include colorants, reinforcements like glass fibers or flame retardants, blowing agents for foam molding, and materials like regrind. Anything affecting properties or appearance.

How to choose dosing technology?

For basic applications with looser tolerances, a simple volumetric auger system often suffices. But high precision aesthetic or functional projects with tighter color and performance specs demand gravimetric dosing control for best consistency.

Can other ingredients get dosed?

Some applications even involve directly injecting liquid additives like lubricants or adhesives with specialized dosing pumps synchronized to the molding sequence. But the main goal remains metering everything precisely shot to shot.

So in summary, whether color masterbatch for a phone case or catalyst for a chemical reaction, maintaining strict dosing accuracy is what enables injection molding processes to produce extremely repeatable results over millions of cycles.

Key Takeaways on Dosing Systems

Let’s recap the core concepts around dosing:

• Dosing in injection molding refers to the controlled volumetric metering of plasticators and additives into the heating barrel
• Tight regulation of ingredient quantities shot-to-shot maintains uniformity in the final molded plastic parts
• While basic volumetric feeders work for looser specs, advanced gravimetric dosing technology delivers maximum precision for color matching and performance consistency
• Automated coupling with the molding press allows dosing to occur in perfect synchronization with the machine injection cycles
• Precise ingredient dosing prevents fluctuations and defects, helping plastics factories achieve high quality, efficiency and output

I hope this gives you a solid overview of why dosing plays such a pivotal role within injection moling! Let me know in the comments if you have any other questions.