Debunking a Myth: Co-injection Molding is Too Complex and Too Expensive

Wed, 08/26/2020 - 14:27

When choosing a co-injection solution, there are a few different options available. You can buy a complete, turnkey co-injection system or you can retrofit an existing injection molding machine with a co-injection hot runner and auxiliary injection unit. Regardless of the chosen path, the end result is a co-injection molding cell.

Co-injection takes two materials, splits them into three layers and injects them into the mold through the same injection point at the same time, so there is no cycle loss with respect to monolayer injection. This statement alone makes potential co-injection users think that such a technique is too specialized (i.e. must be dedicated) and hard to justify.

In fact, co-injection can be very flexible. The key to this is looking at the whole range of applications a potential co-injection user has or may have; this even includes monolayer applications.

The first thing to look at is the material combinations. For example, PP/EVOH, PP/PP, HDPE/HDPE, LDPE/LDPE (the last three as PCR and/or for cosmetic effects), HDPE /EVOH, LDPE/EVOH can be processed with the same hot runner.

The second thing to look at is the plastic part geometry to configure the hot runner nozzle pitch so it can accommodate the largest part of the group being considered.

The next items to consider are the product demands and production cycles to determine the number of cavities. The flexibility of co-injection will allow for volumes to be consolidated into one system.

As an example, consider the following scenario involving 4 different applications:


The difference between part A and part B, apart from the diameter, is that the latter is twice as heavy but the EVOH requirements are the same in terms of percentages. Part C does not require EVOH but recycled PP (PCR) to save on material costs. Part D is a monolayer application. In this particular situation, it is assumed that the machine does not have enough throughput capacity to achieve the 12-second cycle required. In this case, the auxiliary injection unit is used to inject the same PP being injected with the machine’s main injection unit to achieve the cycle.

All of the above applications can be molded with a properly-configured, common injection molding machine and mold base for the different sets of mold inserts, a common co-injection hot runner and a common auxiliary injection unit with an easily adaptable second injection screw and barrel set. The hot runner can be built with an X*Y pitch of 250*250 mm pitch in mind as shown below.


Due to co-injection’s flexibility, all production requirements (Total: 7448 hrs/yr of good parts) can be achieved during the year in the same injection cell using one co-injection hot runner and one auxiliary injection unit. Therefore, the justification for the co-injection equipment gets easier as it can be made taking all applications into account.

Milacron has over 30 years of co-injection experience and over 100 systems in the field, 7,000 drops across a variety of market landscapes producing 12 billion parts in 18 countries. No other co-injection technology offering has the same experience.

The KORTEC CONNECT Co-Injection Solution includes everything you need to easily and economically convert your existing single shot injection molding equipment over to produce co-injected parts. KORTEC co-injection technology is fully customizable to create a moisture, gas or light barrier for containers of all shapes and sizes for applications across any industry.

For more information on our co-injection offering, visit our website:

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