In the extrusion process, co-extrusion is a widely employed technique that cleverly combines two or more different plastics to form multi-layered plastic sheets or films. The primary purpose of using this technique is to combine the characteristics of various plastics, enhancing physical and mechanical properties, as well as meeting specific product requirements such as transparency, temperature resistance, or cost reduction.

There are two main methods of co-extrusion : using multi manifold dies and the well-known combination of feed block with extrusion die . The differences between these two methods are as below:

	Multi manifold extrusion die	Feed block with Extrusion dies
Number of Layers	2-5 layers	2-11 layers
Applicable Plastics	Temperature difference of 20 degrees or more	Plastics with similar temperatures
Operation Method	Different plastics directly enter separate mold cavities and combine in the mold forming area	Different plastics enter the layering device, combine within the layering device before entering the mold

Co-extrusion in two ways: (Left) Multi manifold extrusion dies, (Right) Feed block with extrusion dies

Learn more: Are you looking for the way to make perfect multi layers? Let GMA multi-manifold extrusion die help you to achieve it.

The feedblock plays a crucial role in co-extrusion and its operation can be summarized in three letters: DCA

D: Division

The feed block is designed based on the placement and quantity of the extruders, allowing for various inlet sizes and positions. The internal channels of the feed block are designed according to the structure and proportions of the product, ensuring that different plastics can enter and stack in the feed block based on the set ratios. For example, a two-layer structure implies two internal channels, a three-layer structure has three channels, and so on.

inlets and channels in the feed block designed based on the extruder and product structure.

C:Combination

Different plastics are guided to the channels through various inlets in the feed block. Before entering the extrusion dies, they combine within the feed block cavity. The short combining path makes it suitable for processing plastics with similar temperatures and properties. In cases where there is a significant temperature difference (20 degrees or more), multi manifold extrusion dies are recommended.

A: Adjustment

The feed block features two adjustment tools to facilitate achieving optimal layering effects. The most commonly used tool is the adjustment lever, used to finely tune layering proportions. It functions like a flow valve, altering the flow rate in the internal channels. For instance, it can adjust the material ratio from 15% to 10% for a single layer.

Another tool, the distribution pin, is used in specific situations to optimize interfaces in each layer. In cases where uneven layer interfaces occur despite uniform product thickness, the distribution pin allows for minor adjustments.

In addition, to accommodate various changes in product structure, the feed block can be used in conjunction with a selector to alter the direction of the internal flow paths, offering several different structural variations. This allows for product diversification in the simplest manner. For example, a two-extruder, two-layer structure (AB) can be transformed into a two-extruder, three-layer (ABA) structure by using a selector.

The feed block includes different adjustment tools for fine-tuning layering proportions.

The feed block, playing a crucial role in the co-extrusion process, allows for the optimization of layering quality through meticulous adjustments. Whether using a feed block with extrusion die or multi- manifold extrusion dies, adept use of co-extrusion technology provides a flexible and effective solution for the development and production of plastic products.