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Aug 06, 2023

Co-injection technology has been in existence for more than three decades, but recently transformed into a mainstream alternative for manufacturers in plastics processing. As consumer demands continue to change, and government sustainability compliance standards for plastics become increasingly difficult to achieve with standard mono-injection molding, the co-injection alternative has manufacturers around the globe paying closer attention.

Milacron pioneered co-injection technology starting in the 1990’s. Before co-injection was utilized in the food and beverage industry, those processors used a monolayer of special and expensive materials incorporating barriers that reduced the permeation of gas, moisture or light in order to extend the shelf life of products. With co-injection, these high-performance barriers are minimized, injected into the core layer, to make up less than 2 percent of total part weight. This not only creates significant savings but allows for the injection molded parts to re-enter the recycling stream.

Currently, the industrial packaging industry answers customer requests using this co-injection alternative in response to the impending US Plastic Pact that incorporates four targets addressing plastic waste slated for 2025. A wide range of packaging materials can now be co-injected to incorporate PCR or regrind materials into the core layer.

When the exterior appearance of a molded part is of critical importance, manufacturers utilize co-injection to fill the core layer of the part with PCR (or regrind) resin while maintaining the outer layer’s appeal utilizing virgin resin.

Co-injection is the ability to inject one material in between another as a fully encapsulated internal layer. It’s the simultaneous injection of two materials into a single three-layer melt stream. The easiest way to think about this process is “sandwich molding.” With co-injection, different polymers are injected into the same mold to produce a sandwich-like effect. A skin material at the top and bottom (typically virgin) melt stream, and a core material melt stream, are molded together. Simply put, co-injection produces a plastic part with an outer skin and an inner “core” molded into one.

In food or beverage packaging applications, the skin material (food contact layer) is typically a standard virgin resin, while the core is typically a resin with barrier properties that prevent the oxygen, carbon dioxide, or moisture from transferring through container walls.

Proper placement of that barrier wall within food and beverage packaging is critical, and that’s why co-injection is such a viable option for this application. “Commonly, the barrier material is intentionally a very small percentage of the overall container due to the high expense of the barrier material. Placement within the sidewall must be precisely controlled,” said Andy Stirn, Advanced Systems General Manager at Milacron, a tenured OEM in the manufacturing and distribution of plastics processing equipment and solutions.

An efficient co-injection barrier system can deliver with the same high-volume production efficiency and cycle times as a standard monolayer injection molded part through simultaneous injection. With advancements in resin and barrier layer materials, excellent clarity can still be achieved for brand owners that desire a clear package for their consumer-packaged goods application.

A co-injection machine cell could include a multi-material-capable injection molding machine with at least two barrels. Milacron offers this as an option for new machines, or a standard single-shot machine can be upgraded with a secondary E-Multi™ auxiliary injection unit from Mold-Masters.

Another component offered through Mold-Masters that can be critical in the co-injection process is the integration of the hot runner. An effective hot runner application not only applies precise simultaneous melt stream control but ensures proper injection sequencing – a process critical in food packaging, where the core material must be enclosed.

“Precise melt delivery is paramount to ensure adequate barrier layer execution,” says Igor Kim, Global Sales Director, PET & Co-Injection of Mold-Masters.

Cincinnati-based Milacron introduced a sustainable bucket encapsulation technology at NPE 2015. This type of encapsulation application is cutting-edge for producing a core of up to 50 percent total weight, instead of the standard 25-30 percent core most buckets were produced with at that time.

Stirn says process control is a critical factor to consider for manufacturers looking to make the transition to co-injection. “Material uniformity throughout a product with co-injection can be inconsistent, which can be a big problem when you’re producing an item like a bucket, for instance, where it’s vital to maintain structural strength,” he said. “The importance of process control when utilizing co-injection can’t be emphasized enough, especially for manufacturers who place an emphasis on enhanced production quality and scrap reduction.”

“It took the industry four years to realize that the technology we unveiled in 2015 was a necessity,” said Stirn, who collaborated with his fellow engineers at Milacron, as well as peers at fellow Hillenbrand operating company, Mold-Masters.

The Milacron machine, utilizing Mold-Masters co-injection technology, can inject and control a higher percentage of PCR content (up to 50 percent) than any other competitive systems that tend to average only around 35 percent of less-controlled PCR. Mold-Masters patented co-injection technology, in combination with the Milacron machine, provides a unique advantage - the capability to achieve the highest PCR percentage, ensuring uniform distribution and complete coverage of PCR resins (in turn, preventing contact of PCR with product).

According to industry Process Engineer Tyler Boss, who has been consulting in the plastic processing space for nearly two decades, the Milacron machine, which incorporates Mold-Masters co-injection technology, is a “game changer” in the industry.

“In fluid dynamics, laminar flow keeps fluid particles following smooth paths in layers, with each layer moving with ease past the adjacent layers with little to no mixing. At low velocities, the fluid tends to flow without lateral mixing. In other words, the core layer stays in its own lane without mixing with the outer layers. The amount of core can be increased or decreased depending on the application, and with the precise machine control,” he explains.

With a fully encapsulated core layer through this co-injection technology, the PCR inconsistency from lot to lot is managed. The odor and contamination risk are contained to the core, maintaining a reliable food contact layer. In addition, the color remains consistent with that of the desired skin material.

This sustainable solution of injecting PCR as the filler core layer has the potential to generate significant savings for the molder. It reduces the use of virgin material, can maintain cycle times, utilizes components of existing tooling, and sometimes even has a lower cost per part (when regrind is less expensive than virgin material, the result is significant savings and favorable ROI for the manufacturer that offsets the initial investment).

Prior to launching this sustainable PCR molding solution more than five years ago, Milacron already had technology to produce co-injection parts, like toilet seats, refrigerator handles, tractor hoods and chemical containers using their proprietary co-injection technology.

The only downfall of maximizing PCR content may be a visible difference. In the case of dark PCR an ‘odd shadow’ may be seen on the thin wall buckets as the outside virgin layer cannot fully mask the dark (or sometimes black) PCR layer.

“Significant effort, engineering and development is being done in our industry to make sustainability improvements,” says Stirn. “We need brand owners and consumers to become more aware of the challenges and compromises that come with these new technologies or techniques.”

Stirn hopes that one day plastics processing leaders like Milacron and Mold-Masters can responsibly claim that they worked cohesively to pave the way for consumer awareness of recycled plastics. Doing so would transform improvements in circular recycling, allowing for greater PCR integration into everyday use.

Cutaway revealing outer and inner layer consistency at the leading edge.

Co-injection machines and retrofit solutions are available, but molders need to first understand what’s best for their application. Oftentimes a retrofit or rebuild of existing machines can solve for multiple issues. Mold-Masters co-injection CONNECT™ package is available as a retrofit to almost any existing single-shot injection machine. It's best to consult with an OEM that has proven co-injection experience and knowledge.

For more information on Milacron co-injection machine technologies, please contact Andy Stirn of Milacron. He’ll be present in Orlando for NPE 2024 next May at the Milacron booth. And, plastics processing subject matter expert, Tyler Boss of DME and Milacron, can field a wide range of questions pertaining to co-injection technology, technical molding, as well as other plastics processing / engineering concerns. Both can also be found on LinkedIn. In addition, Milacron and Mold-Masters are on LinkedIn, Facebook and Twitter with valuable information pertaining to plastics processing solutions.

Milacron ( is a global industry and technology leader in the manufacturing of injection molding and extrusion machines. Milacron also produces and provides machinery, auxiliaries and components that complement its plastics processing machines and systems. The company’s 1,900 associates worldwide collaborate to provide a diverse range of products across various plastics applications. Milacron holds longstanding relationships with more than 27,000 global customers across North America and India, as well as coverage in Europe and in many emerging markets, including South/Central America.

Mold-Masters ( is a supplier of hot runners, controllers, auxiliary injection, and co-injection systems. Credited for patenting the first commercially viable hot runner system in 1965, today Mold-Masters conducts business in more than 100 countries and employs a diverse workforce.

Milacron and Mold-Masters are operating companies of Hillenbrand (NYSE: HI), a global pure-play industrial company that provides highly engineered, mission-critical processing equipment and solutions to customers in over 100 countries around the world.

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