Servoelectric unscrewing device demolds threads with high precision

  • Part features five female threads with diameters of ⅛”, ¼” and 36 mm
  • Servo motors with resolver-type position encoders protect mold from damage
  • Servoelectric unscrewing vs hydraulically operated counterparts

The company Servomold has developed servoelectric unscrewing devices which provide precision-controlled and gentle demolding of threads in injection-molded parts. Five of such systems were combined by the Krallmann Group in a mould used by Festo Polymer GmbH to make the complex-shaped housing of the DB Mini pneumatic maintenance unit. The unscrewing device is distributed by Servomold’s sister company I-Mold.

Servoelectric unscrewing vs hydraulically operated counterparts

Hendrik Sanio, project manager at Krallmann, quotes the reasons why servoelectric unscrewing devices were preferred in this application over their conventional, hydraulically operated counterparts.

“They enable us to control the individual motions of each core with high precision. The respective unscrewing torques can be individually monitored and controlled, and the threads, varying in depth, can be demolded at the optimum speed in each case. The servoelectric motor unit requires less mounting space and yields shorter cycle times than a hydraulic solution. Moreover, the risk of soiling due to oil leaks is eliminated.”

Part features five female threads with diameters of ⅛”, ¼” and 36 mm

Festo Polymer produces the cube-shaped housing having an edge length of approx. 40 mm from a high-modulus, high-strength glass fiber reinforced polyamide (PA66-GF50). The molding comprises five female threads having diameters of ⅛”, ¼” and 36 mm, respectively, and measuring between 12 and 22 mm in length. In line with the positions of these threads, four of the associated unscrewing spindles are arranged at 90-deg. angles to each other, in parallel to the parting surface, on the ejector side of the single-cavity mould. A fifth spindle moves parallel to the ejector set in the central machine axis.

Electromotive unscrewing device for molds in injection molding

Five internal threads and multiple undercuts make demolding a distinct challenge in the case of this housing for a Festo pneumatic maintenance unit (above). Using servoelectric unscrewing devices from Servomold (below), the Krallmann Group developed an efficient solution to this task (Source: Servomold)

Servomold selected and sized the five unscrewing devices to match the appropriate torques (T), which were found to range from 6 to 33 Nm in a series of in-house trials. Accordingly, the mold is now equipped with four units of the smallest available standard type SAEW040-050-12-0055 device (T up to 23 Nm) for the ⅛” and ¼” threads plus one SWZ070-16-0207 type (T up to 75 Nm) angular (“cogwheel” type) servo drive for the large metric thread. Control is provided by a mobile SRS-8.6 rack-type servo controller capable of managing up to 6 servo motors. This controller adapts the speed of each spindle individually, so that, as the five liquid-cooled thread cores are screwed in, all will reach their respective limit positions simultaneously. The SRS also performs continuous torque monitoring of each servo unit and outputs appropriate signals when preset threshold values are reached.

Servo motors with resolver-type position encoders protect mold from damage

The servo motors are equipped with resolver-type position encoders whose zero positions are individually referenced on the clamped and preheated mold in each case.

Stefan Bernhard, managing director of Servomold, explains: “We like to use resolvers to protect the mold from damage during closing, for in everyday operation the molding machine controller will issue a start clearance only when the SRS unit reports that these reference points are reached.” As a further safety aspect, Mr. Bernhard mentions the fact that servoelectric systems support continuous force monitoring, which allows incipient mold damage to be detected at an early stage when low-cost troubleshooting options are often still available.

Dr. Jan Oliver Hauch, head of Project Transfer of Polymer Innovation at Festo Polymer, confirms the decision taken jointly with Krallmann. “Given the low space demand of these systems, the mould itself is of fairly small dimensions. We can therefore use it on a very energy-efficient Arburg Allrounder 570 S e² 2000-400 with a 570 x 570 mm tie-bar spacing. In fact, it has been in day-to-day operation on this machine since early 2015. There have been no problems whatsoever with the servoelectric unscrewing devices from Servomold to this day. And since this manufacturer relies mainly on standardized components, we anticipate that a reliable spare part supply will be ensured in the future.”

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