Central heating for moulds

Dramatic improvements in efficiency have been achieved in manufacturing car interiors by a simple design innovation. Design Engineering investigates.

Mannings Thermal & Environmental Engineers of Southport have been able to speed up production and reduce energy consumption by modifications to the Thermoforming Tool Presses which are used to produce car trims, such as door panels, roof liners, parcel shelves and other moulded parts. The presses operate by bringing together two preheated metal tools which compress the raw material between them to create contoured shapes.

The improvements were achieved by embedding electric heating elements inside the moulding tools so that the heat could be directed at the point where it is required. As a result, power savings of over 75% have been recorded and heating time has been reduced to half.

The Thermoform Tool Heating System was developed in conjunction with the Lear Corporation of Colne in Lancashire, which manufactures interiors for many major car companies such as Jaguar, Rover, Toyota and Honda, using environmentally friendly materials. A prototype unit was produced on which heaters were installed to heat the front working faces of the top and bottom tools. Each ‘pocket’ in the rear of the tool castings was then fitted with individual heating elements. Measuring thermocouples were fixed to various points on the tool and temperatures, as well as power consumption, were carefully monitored and recorded.

HEATING ELEMENTS

The design is based on the use of Mannings flexible ceramic pad heaters. These heaters were originally developed for use on heat treatment applications, particularly on steel pressure vessels and pipework where heat treatment is used to relieve internal stresses, reduce brittleness and improve machinability. The pad heaters consist of multi-strand nickel chrome element wire which is electrically insulated by the use of interlocking sintered alumna beads to produce a flexible mat with high heat transfer capacity. Electrical joints are made by the use of special ‘camlock’ connectors. These heating elements have been used in the petrochemical, aerospace, chemical processing and power generation industries.

For stress relieving applications, the mats are simply wrapped around the part to be heat treated and covered with insulation to limit heat loss.

Electric heaters are manufactured in a variety of forms such as rope heaters for use in awkward situations, braided heaters for non-uniform shapes, shoelace heaters for small components and special purpose insulated pre-heaters. Pipe Butt Clamp On Heaters have also been produced.

These are pre-formed heater assemblies which are manufactured in a variety of sizes to suit specific pipe diameters and are indispensable for a pipe work production workshop where a fast throughput is required.

To control the heat output from these elements, self-contained Heat Treatment Centres have been designed to provide the power source as well as programmed temperature control and recording.

On production units of the Thermoform Tool Heating System for Lear, the heaters are arranged vertically and horizontally in the moulds to create separately controlled temperature zones. This enables different amounts of heat to be applied at different parts to cater for the complex shapes being formed by the press. Mannings had to work closely with the specialist tool designers so that the heating system for each new tool could be individually assessed and then custom built to suit the component to be manufactured.

TOOL CHANGING

So that tools can be changed after each production batch has been completed the electrical supply can be disconnected from the control system by means of flexible, quick release power and control cables. These cables connect the tool to a control panel which contains the necessary controllers, contactors, chart recorders and power unit. Digital readouts on the panel show the temperature allocated to each zone and this can be checked against the actual measured temperature.

Since each press is used to produce many different shapes, the control panel is pre-programmed with individual heating cycles for every tool. By selecting the tool number on the panel, the correct cycle is automatically initiated. Numerous programmes can be retained in a single programmer.

Graeme McFarlane, who is the Technical Engineering Manager for the Lear Corporation at Colne said, `before installing The Thermoform Tool Heating System we had to wait eight hours for the tool to reach the correct moulding temperature and this new development has made significant improvements to our production process as well as helping to reduce our running costs’.

As an added bonus it has been found that downtime between mould changes can be almost eliminated. This is made possible by the use of a standby heating system which is used to preheat moulds so that they can be made ready to be transferred to the moulding press when required. They are then simply plugged in and production of the next moulded shape can begin immediately.

Mannings Thermal and Environmental Engineers Tel: 01704 538202 www.mannings.uk.com