Tiebar Set up and Monitoring using Ultrasonics

Injection molding presses rely on tiebars to apply clamping forces to molds during plastic injection and cooling cycles. Balancing the loads applied to individual tiebars ensures uniform clamping pressure of the mold improving overall process quality. Irrespective of process quality another very important reason to measure tiebar-clamping forces is to ensure that individual tiebars are not overloaded leading to eventual failure of the Injection molding press itself.

While numerous methods have been employed in an attempt to address this issue, the Ultrasonic Extensometer provides a simple, fast, and repeatable alternative.

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ULTRASONIC BASICS

Ultrasonic Extensometers are designed to provide precise length measurement of acoustically conductive materials. The instrument applies an electrical pulse to an ultrasonic transducer creating an acoustic wave. Piezo electric material within the transducer converts the electrical pulse to mechanical energy and vise versa.

The mechanical energy produced by the transducer is that of a sound wave with a specific center frequency typically in the range of 1.0 to 25.0 MHz. The sound wave generated by the transducer travels through the material being tested to the op-posite surface or back wall of the tiebar. The sound wave is then reflected off of the back wall and returns to the transducer where it is again converted to electrical energy. The ultrasonic instrument measures the time it takes the sound energy to travel from the transducer through the tiebar and return to the transducer referred to as time of flight. The instrument then calculates the length of the tiebar based on this time of flight and the velocity of sound for the tiebar material.

LENGTH, LOAD, AND STRESS

The Ultrasonic Extensometer compares the time of flight measurement in a tiebar under load to that of the same tiebar in a relaxed condition. This difference in time of flight appropriately factored for temperature and stress effects determines the elongation of the tiebar under load and is the basis for all other data provided by the Ultrasonic Extensometer.

What separates Ultrasonic Extensometers from traditional ultrasonic thickness gauges is their ability to adjust to variations in sound velocity caused by stress loading and temperature changes. As a tiebar is stretched the velocity of sound within it decreases making the tiebar appear longer than the physical change in length. Temperature has a similar effect. As the temperature of a tiebar increases, its ultrasonic length increases at a rate greater than its physical length. Not considering these effects will introduce unacceptable error in the measurement result.

Stress and load data provided by the Ultrasonic Extensometer is calculated within the instrument based on simple physical laws. When a mold closes, the hydraulic clamping pressure in the injection molding press stretches the tiebars the greater the clamping pressure the greater the elongation. In the elastic region of the tiebar material below the yield stress limit, the relationship between tiebar elongation and applied stress is linear and described by Hookes law. The modulus of elasticity of the material being tested is a constant and determines the ratio of stress to the change in length. Therefore by measuring the time of flight of the acoustic wave and knowing the sound velocity of the tiebar material, the Ultrasonic Extensometer calculates the change in length, the load and the stress applied to each tiebar.

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APPLICATION OF THE ULTRASONIC EXTENSOMETER

While the theory behind the operation of the Ultrasonic Extensometer requires some knowledge of physical laws, the ap-plication of these instruments is conversely simple. The user simply follows a specific setup procedure for each new test to be performed and the instrument takes care of all the complex measurements and calculations. Once created, individual setups are stored in the instrument and can be recalled at a later date to automatically set the instrument up for future tests.

A single Ultrasonic Extensometer will appropriately instrument an entire factory floor of injection molding presses. A trans-ducer can be permanently affixed to each tiebar on each press if desired and a single instrument can be circulated through-out the shop to perform a variety of set up and in process testing of multiple presses helping to ensure process quality and machine up time.

Modulus of Elasticity, E = =

Where: P = Applied Load or ForceL = Relaxed Length of tiebarA = Cross sectional area of tiebar? L = Elongation of tiebar under load

Unit Stress PLUnit Elongation A ?L

Contact InformationStressTel50 Industrial Park RoadLewistown, PA 17044

Tel: 1 866-243-2638Tel: 1 717-447-1562E-mail: geit-info@ge.com