Plastics Technology

AUG 2016

Plastics Technology - Dedicated to improving Plastics Processing.

Issue link: http://pty.epubxp.com/i/707262

Contents of this Issue

Navigation

Page 51 of 75

Bolt-on actuators allow the hot runner to be supplied to the mold maker as a preassem- bled and tested drop-in system, complete with hydraulic disconnects, so that no plumbing is necessary during installation. These systems also can be removed as a unit and tested outside of the tool, making troubleshooting and maintenance much easier. One of the disadvantages of bolt-on actua- tors is they need to be cooled to protect them from the heat of the manifold. The common solution to this challenge has been to run water-cooled isolation plates between the hot manifold and the actuator. The water-supply lines for these plates add complexity to the drop-in system and take up further space in the mold. These water-cooled isolation plates are only effective when the water is circulating. Frequently at the end of production, the cooling water to the mold is turned off at the same time as the hot-runner temperature controller. Large manifolds will take hours to cool down, thus transferring heat into the actuators, which are no longer receiving the benefit of the cooling water. This excessive heat can prema- turely degrade the seals in the actuator as well as the hydraulic fluid in the case of hydraulic systems. Impurities in the water can collect over time inside the cooling plates and at the connection of the water lines, restricting the water flow and making the cooling plates less effective. BEAT THE HEAT Bolt-on actuators are now available with passive cooling devices, which are designed to transfer excessive heat out of the actuator and into the top clamp plate. These devices prevent the actu- ator from overheating when the cooling water is turned off at the same time as the hot-runner heats (see graph on opposite page). In many cases, they can even eliminate the need for dedicated cooling lines to the actuator. This greatly simplifies the hot-runner system and can save a significant amount of space inside the tool, helping with tool support. Eliminating the water-cooled plates also reduces the water connections on the side of the tool, where space is often at a premium. Pin-height adjustment is another area where newer designs can greatly benefit the tool maker. In early actuators, adjusting the final forward pin position in the gate was done by adding and removing shims behind and in front of the valve-pin head. With that style of actuator, adjusting the pin position can be a time-consuming process. Many actuators today have the capability to adjust the pin height with a simple turn of an adjustment mechanism. In some designs this can even be done through the top clamp plate with the tool completely assem- bled, making pin-height adjustments a simple process requiring no disassembly of the mold or the actuator. Quick valve-pin decoupling is another feature that sets newer actuators apart from their simpler predecessors. In many of the older-design actuators, the pin is locked into the piston, requiring both the pin and the actuator to be removed as a unit. In other designs, it is necessary to disassemble the actuator to separate it from the valve pin. New designs are now available that incorporate quick- decouple features, allowing the pin and actuator to be separated easily without having to remove one to remove the other. This saves a great deal of time and preserves the pin adjustment when maintenance is required for either the tool or the hot-runner system. Today, actuators can be hydraulically, pneumatically, or electrically powered. Because of the availability of high pres- sures when running hydraulics, hydraulic actuators can be small yet powerful. Pneumatic actuators need to be larger because of the much lower pressure typically available with air, but are generally considered cleaner than hydraulics. Electric actuators offer a clean option and some even have the ability to adjust the QUESTIONS ABOUT HOT RUNNERS? Visit the Hot Runner Zone. Valve gates allow molders to eliminate gate vestige, decrease cycle times, and manipulate the filling of various gates through the injection cycle. Passive cooling devices (shown in blue) make contact with the cooled top mold plate, preventing the actuator from overheating. In many applications these passive cooling devices can eliminate the need for water-cooled isolation plates. 50 AUGUST 2016 Plastics Technology PTonline.com T ips & Technique s

Articles in this issue

Links on this page

Archives of this issue

view archives of Plastics Technology - AUG 2016