4 Ways Hot Runner Systems Improve Injection Molding Efficiency

Injection molding is the plastic manufacturing process of choice for many of the plastic industry’s largest customers, including the automotive industry and the medical products industry. In fact, the U.S. has nearly 16,000 injection molding and plastic manufacturing facilities spread across all 50 states.

Like any mass produced product, however, injection molded parts are often profitable only when they are produced in bulk quantities. As a result, plastics manufacturers are often looking for ways to scale production and realize greater efficiencies. One way to address these needs is through hot runner systems. Hot runner systems use heated runners to direct melt into the molds. This is distinguished from cold runner systems in which the runners are often part of the molds and are unheated.

Here are four improved efficiencies that come from hot runner systems:

Faster Cycling Times

Since the runner is not part of the mold, the cooling time is calculated based on the piece itself rather than the piece plus the runner. Faster cooling times means faster cycling times. And faster cycling times, of course, means more pieces produced per time period.

Lower Pressures

One disadvantage of cold runner systems is that the melt can cool between injection cycles inside the nozzle. This means that higher pressures are required for cold runner systems so the cooled melt can be forced from the nozzle and into the mold when starting a new part.

Hot runner systems, on the other hand, have a heat source, either internally or externally, that maintains the temperature of the melt after the hot runner valve gate systems close. This means that hot runner systems will often use less energy to push melt into molds and require less maintenance to replace parts that wear under the high pressures needed for cold runner systems.

Fewer Under-filled Pieces

When cold runner systems are used, a risk exists that the melt will cool off too much as it travels through the cold runners and clog the runner. This will prevent plastic from flowing into the mold through that runner and can leave under-filled pieces. Even if these pieces are re-ground and recycled, the energy, time, and labor of forming a defective part is wasted.

Conversely, hot runner systems have a much lower risk of under-filled parts. Each nozzle of a hot runner system is heated so that the melt can be delivered to each fill point without clogging. This produces parts having a more consistent quality than cold runner systems.

Moreover, this allows the production of larger parts using hot runner systems than cold runner systems. For example, hot runner systems can be used to produce automotive parts because the melt can be distributed over a large mold area through multiple hot runner molding machine nozzles.

Less Finishing Required

This may be the consideration that distinguishes a hot runner system from a cold runner system. Since cold runner systems are unmolded with the runners and sprue attached, these must be removed. This can be performed automatically by a robot programmed to snap the runners from the piece or manually by an employee who snaps the runners from the pieces.

In either case, the removal of the runner can cause significant quality control issues due to the stresses introduced during runner removal. The part may become discolored or weakened when the runner is removed. Similarly, residual material may remain attached to the piece after snapping the runner from the piece which would require an additional step of grinding after removal.

Less Waste

The runners that are left over after molding represent waste. These runners can be reground and recycled. However, the energy used to melt that plastic and inject it into the mold, as well as the time and effort to remove and regrind the runners cannot be recovered.

One criticism of hot runner systems is that they are more expensive than cold runner systems. However, this criticism often ignores the labor and waste that come with cold runner systems. While there is a price difference, the efficiencies realized from hot runner injection molding systems can more than recoup the initial price difference.

Hot runner molding systems have significant advantages over cold runner molding systems. Aside from the manufacturing and quality benefits, hot runner injection molding provides substantial efficiencies not found in cold runner systems.