Huge volumes of resin are used to form plastic bottle closures, and there remains huge disagreement about the best way to mold those closures. Injection mold? Compression mold? Tough choice. We asked experts at two leading machinery suppliers to make the case for their process, and they eagerly picked up the gauntlet. Let the clash begin.

Compression: Sacmi makes its case
In compression molding, everything depends on a lower plastic extrusion temperature, 30-40ºC less than injection, which gives these advantages:

• Shorter cycle time = higher productivity. The lower extrusion temperature allows the cap to be cooled in the mold more quickly. This has reduced cycle times, which have, in the case of lightweight caps, been cut to less than 2 seconds.
• Less energy consumption. Lower extrusion temperatures mean less energy is needed to bring the plastic to extrusion temperature and, since the plastic is colder, less energy to cool it. Overall energy savings per cap produced can be up to 45%.
• High specific density. Molding pressure being equal, lower temperatures allow attainment of a higher specific density; moreover, the injection point, which puts added strain on the molten plastic, is absent. This technological condition provides good mechanical properties and a consistency that would be unattainable with injection. 
• Maintenance-free molds that last decades. Unlike the injection mold, which is burdened by its own weight, the vertical and axial movement of the compression mold ensures the molds are protected from all kinds of wear.
• Quick color changeovers. These can be completed quickly without having to clean a hot chamber, which is absent in the compression process.
• Less waste = lower costs. In a scenario where the major retail multinationals demand both high standards and lower final product costs as necessary conditions for any increase in volumes, even the plastic cap is beginning to play a key role in decision-making.
• Online video inspection. Since compression operates with continuously rotating turrets, orientation and alignment of outgoing caps allows for the simple insertion of video cameras, feedback-connected to the machine itself at a practically negligible cost.

Injection: Netstal responds

Compression has enjoyed a perceived “king-of-the-hill” position with respect to productivity for a long time, particularly for certain types of closures. That picture has changed dramatically in recent years, as injection molding solutions have surpassed the capability of compression molding, particularly for single-piece closures. This is particularly evident in the lightweight water closure segment.

Many of the old arguments are no longer valid. The historical claims in favor of compression are related to higher productivity, lower capital investments, faster color changes, lower energy consumption, greater process consistency, and lower production costs. These are all compelling reasons to choose a technology, but make sure you look into all of the claims.

• Higher productivity. A modern injection system will be able to produce in excess of 2000 caps/min on a medium-platform machine and in excess of 2750 caps/min on a large-platform system. The smaller and lighter parts allow for reduced cycle times and so the output of one system will win in terms of output. If you combine the better efficiency with the small footprint, the output per square foot is staggering.

Measurement in terms of instantaneous output is also often misleading. It’s actually quite easy to have small buffers, even on an inline process of an injection system, without significant impact on floor space. Without buffers, an interruption anywhere in the process means a loss in efficiency. In both the output and efficiency, injection is the winner.

• Lower capital investment. A high-output injection molding system has become very efficient and cost competitive. The fact is that for the popular super-lightweight closures, the volume demands are high and so allow for the use of high-output systems. In a single-piece closure, if the demand is sufficiently high, the capital investment per million parts will favor the high-capacity injection molding system. Each application has its own requirements, but when you compare the cost of the total solution, injection is the winner.

• Faster color changes. Common claims are that a compression system can change colors in about 20 minutes. The photograph shows a blue to natural color change that was done on the fly in a matter of a few shots.

In addition to fast color changes, IM has a process advantage. There are a number of parameters that can be implemented in injection molding to compensate for the different shrinkage inherent with colors. More importantly, these parameters can normally be done without a negative impact on the cycle time.

• Lower energy costs. The real measure of energy efficiency must be done with respect to productivity. It is commonly accepted that the best measure is in kWh per kg of raw material throughput. Since the output (actual usable output) is higher, the consumption per kg would be lower. Be careful, though, since as the part weight goes down, the consumption per kg can go up. In terms of the energy consumption per kilogram, injection is the winner.

• Greater process consistency. The common claim is that the compression system has far fewer variables and therefore greater consistency. On the surface, it makes some sense. However, if you look into the practical side, you’ll find a different picture. If the machine is capable, you will have control parameters, and not variables. If the system is in control and you have the flexibility of using a number of process setpoints, you will be able to establish an optimum and, more importantly, a robust process.

Often in compression molding, only one parameter is available to compensate for shrinkage: process speed. Injection molding has the flexibility of process parameters such as backpressure, hold pressure, and fill time to achieve the same dimensions without taking on cycle time penalties. —[email protected]