Transferring Injection Compression Molding Technology to Thin Wall Packaging Production

Increased lightweighting, down-gauging and savings in both energy and materials compared to conventional injection molding can create interesting sustainable solutions for Thin Wall Packaging production

Bart van der Aar, Sr. Scientist Technology & Innovation Rigid Packaging, SABIC

Reto Gmür, Product Manager Packaging, NETSTAL Maschinen AG

The packaging industry is facing several challenges, including reducing weight and costs, improving sustainability and complying with more-stringent food safety and consumer safety regulations. One important solution is thin wall packaging, which is forecasted by Mordor Intelligence to grow by 6 percent between 2021 and 2026.

Thin wall packaging refers to plastic packaging parts with a ratio of flow length to wall thickness higher than 200:1 and a typical wall thickness of less than 0.02 in. (0.5mm). It is used in many different sectors such as dairy, beverages, frozen foods, fruit and vegetables and meats.

Despite strong demand for thin wall packaging, conventional injection molding is reaching the limit of its ability, from both a material and a machine perspective, to reduce the wall thicknesses of packaging applications. The adoption of injection compression molding (ICM) for packaging production offers new opportunities for further down-gauging and lightweighting at lower injection pressures and clamp forces, while ensuring good mechanical performance and aesthetics and reduced warpage.

Although ICM is a common technology in other industries, such as automotive and optical, its translation to thin wall packaging is challenging because of differences in part geometry, material type and machine setup.

In the packaging industry, polypropylene (PP) homopolymers, random copolymers and impact copolymers with a high melt flow rate (MFR) (50g/10 min. at 230°C and up) are commonly used. Typically, such high-flow materials have a drawback – lower mechanical performance (e.g., impact and stiffness) compared to materials with a lower MFR.

The ICM process avoids this issue by allowing the use of materials with lower MFRs (down to 20g/10 min. or lower) compared to the higher MFR materials used with conventional injection molding. Beyond materials, the injection molding machine and tool play a vital role, as they must ensure a fast, reliable and repeatable process with rapid setup.

ICM vs. Conventional Injection Molding

The thin wall packaging application space is dominated by very high flow-length-to-wall-thickness ratios. Efforts to increase this ratio to 350:1 and above are encountering roadblocks, as longer flow lengths and thinner walls exceed the limits of standard injection molding or reduce mechanical performance of the application. In contrast, ICM provides ways to further reduce wall thickness or accommodate longer flow paths by taking advantage of the compression stroke.

Figure 1: Steps in injection compression molding

As shown in Figure 1, in ICM a compression step is added to the injection molding process. The tool is equipped with a special compression frame that allows the injection sequence to start with the tool slightly open. This is called the compression gap (1). The melt then begins to fill the cavity (2), while in parallel – on an injection stroke-dependent basis – a compression stroke further drives the melt into its final shape (3). Importantly, this compression step does not extend cycle time compared to conventional injection molding.

Because the drop in cavity pressure from close of injection location to end of flow is lower with ICM than with conventional injection molding, the packing at end of flow can be improved. In turn, improved packing can reduce the tendency for sink marks for better aesthetics and can also increase the stability of the rim area of a molded product. In the case of lids, this lower cavity pressure drop with ICM can reduce warpage.

Trials have demonstrated significant benefits when this process is combined with proper materials and optimized mold and application designs. In one example – an airline cup made with PP random copolymer – ICM enabled the injection pressure to be decreased by as much as 50 percent vs. injection molding, resulting in a potential reduction of 20 percent in both the cup’s wall thickness and weight (Fig. 2).

Figure 2: Benefits of ICM in a thin wall packaging application

Looking at the differences in more detail, the wall thickness of the injection molded cup was limited to 0.35 mm, whereas the lower injection pressure in the ICM process provided further down-gauging of 0.07 mm. Thinner walls translated into a reduction in weight from 6.5 g for the standard injection molded cup to 5.2 g for the ICM molded cup. This difference of 1.3 g can add up to a savings of 40 metric tons of resin per year based on these parameters: a 3.3 second cycle time with in-mold-labeling and a production rate of approximately 32 million parts per year. Other potential improvements included lower MFR and higher top load and impact resistance.

Thanks to further developments in material chemistry and ICM process control, wall thickness can be reduced even further – by 30 percent vs. 20 percent – while lower clamp force offers the possibility of using a smaller machine with lower energy requirements.

Another ICM application example, a lid for a food container made with PP impact copolymer and manufactured in an 8+8 stack mold, illustrates additional benefits. Besides reducing the weight of the lid, the lower cavity pressure of ICM lowers molded-in stress, thus minimizing warpage. Less warpage results in a flatter lid, which is considered an important quality feature. Flatness improves the stackability of the lids on the filling line, as well as their ability to close completely after the container is filled.

Additional Process Controls

Maximizing the effectiveness of ICM in thin wall packaging calls for functions that are normally not available in the controls of conventional injection molding machines. Those machines are designed to apply and maintain uniform clamp and locking forces when the mold is closed. As described earlier, the ICM process has a slightly different sequence, which requires customized control mechanisms, e.g. control over compression speed, which is required to optimize the ICM molding cycle, depending on the geometry of the molded part.

Another example is a control feature that addresses process safety. Safety mechanisms incorporated in the standard controls of injection molding machines are insufficient if the machine is set to run an ICM process. This is because there is no function available to monitor the force at the compression gap or at the end of the compression step.

Force at the compression gap must be controlled to ensure the compression frame is properly actuated and has created a fully enclosed cavity before injecting material. Force control also allows the operator to determine whether the compression stroke of the machine is effectively compressing the hot polymer melt. Finally, by monitoring force throughout the complete compression cycle, the machine can detect parts that were not de-molded properly during the previous shot, thereby preventing damage to the tool.

Conclusion

Thin wall packaging is in high demand as a result of several market trends, including sustainability (e.g. reduced raw material usage and energy consumption during manufacturing and transport), cost pressures from e-commerce companies and consumer preferences for packaged items such as snacks and to-go foods.

As a replacement for conventional injection molding, injection compression molding provides significant potential for greater weight, material, energy and cost savings in thin wall applications. In turn, deriving the full value of ICM depends upon optimized materials, equipment and process controls.

In the case of materials, ICM technology provides a broader processing window than injection molding, allowing manufacturers to choose from a wider range of polymers for their thin wall packaging applications. In fact, this range is so extensive that materials that normally do not qualify for use in thin wall packaging applications can now be deployed – for instance, bio-based and recycled grades.

New thin wall packaging applications, materials and processing solutions are being developed, studied and tested through a collaboration between SABIC and NETSTAL Maschinen AG at the latter’s Thin-Wall Packaging Application Center in Näfels, Switzerland.