Isothermould distributed power rods for mold parting line temperature control.

Properties of a molded rubber part depend on its state of cure, i.e., on cure temperature and time. Temperature variations between the different points in the mold can have significant effects on the uniformity of the molded part and imply setting a less than optimum cure time.

A uniform temperature offers numerous advantages. Increasing temperature uniformity makes it possible to meet tighter part characteristic tolerances, resulting in higher quality. It also makes it possible to reduce cure time, and in some cases, have more cavities on the same mold surface area (since the cavities can be placed nearer the edges), leading to increased productivity.

A global approach

While some manufacturers of injection presses focus on optimizing the thermal equilibrium of the heating platen, Rep has opted for standardizing the thermal mapping at the mold parting line where the mold cavities are located. Various systems were considered.

When heating by thermal fluid, the fluid can circulate in the heating platens, but only homogenizes them, so it would only be useful with very thin molds. Having the fluid circulate in the molds carries very considerable limitations from a technical point of view, and further costs with each new tooling.

Heating platens with heat pipes was also considered. Here again, heat pipes in the heating platens are only worthwhile with very thin molds. However, heat pipes can be installed in the molds, but the cost and advantages need to be considered on a case by case basis.

Adding power to the faces of the heating platens so as to compensate for the losses producing edging effects was considered. The limitations imposed by space requirements and the strength of existing engineering solutions make this somewhat unworkable.

With multiple heating zones, effective control is achieved with probes near the heat source so as to distinguish the effect of one area from another. This means using different temperature setting so as to ensure a temperature uniformity in the cavities.

This arrangement requires the user to work on a case by case basis. Experience and prototypes have shown that the results were insufficient and that the settings involved were very tricky.

The solution of a control area per platen with distributed power rods in the heating platens is simple and user-friendly. Its effectiveness is reproducible as it is not dependent on the settings made when the mold is being set up. Tests have shown that a target of [+ or -] 2.5 [degrees] C could be achieved at the mold parting line, where the cavities are located. A major part of the project was devoted to accurately defining distribution and developing a reliable industrial solution.

As a result, the distributed power rods were decided on, along with the methods introduced to optimize the system and test it in a variety of configurations. Another conclusion in the specifications is the requirement that this system should be compatible with the existing machines and should be able to be easily retrofitted.

An aid to prediction or comprehensive solution

A measurement project carried out enabled the development of a tool which makes it possible to predict the heat dispersion values obtained with every possible design of the heater plates.

Accordingly, a finite element model in a steady state integrating heat exchanges by conduction, radiation and convection at every interface was developed. This was thoroughly checked out by varying the test conditions, and a calculations/measurements correlation was established.

At the request of customers, the model decided on involves a non-insulated mold, as mold insulation is seen as very restrictive with regard to production (mold cleaning), and very costly.

Perfecting the system

Using this tool, the company was able to study optimum heat dispersion with molds of medium thickness. Calculations showed that this compromise was equally valid for both thin and thick molds. With this platen setup, the company was able to check its effectiveness on different thicknesses.

The excellent results on its static measuring plate confirmed that the system was highly effective and that the system is achieving its dispersion target of [+ or -] 2.5[degrees]C in the cavity area.

Lastly, the company worked in close collaboration with its power rods supplier for the industrial application and installation of these rods, which resulted in the assurance that the reliability of the system was fully in line with customer requirements.

Molding confirmation

The question is: Is equilibrium maintained when the system is disturbed? The fact is that when rubber is injected into the cavities, heat exchanges occur inside the mold. Similarly, when the mold is opened and closed, the conditions governing exchanges with its environment continuously change.

Two molds fitted with instruments for a 400 ton press (platen dimensions 630 x 800 mm) were tested. The first mold produced massive (anti-vibration) parts that cause major disturbances; the second produced thin parts typical of another configuration (short curing time involving frequent mold openings). With Isothermould, both molds, whether static or in production, gave temperature differences never achieved before, whether with systems using one or more areas. Thus, the company has exhibited one of these molds working with a maximum difference of [+ or -]1.5 degrees]C in the cavities.

Industrially applicable results

This system has already been installed at a number of customer locations on V48, V58 and V68 models, and is included in Rep's standard range. All customers agree: Some cure times were reduced by up to 20%, and a decrease of up to 40% was achieved in the dispersion of the properties of the molded parts.

In some reported cases, this system was the only one, on a large injection press with a large number of cavities, to meet the very rigorous specifications governing the finished part.

Advantages of the solution include the following: The use of insulation around molds is very restrictive and costly, thus design has been optimized by doing away with it; the new system consists of a single area per platen with the probe located on the mold as near as possible to the cavities, and this arrangement has the advantage of optimum control of the usable areas, as opposed to having the probe in the heating platen, as well as a very simple control; the system can be retrofitted very easily and at low cost onto the company's existing presses; and the distributed power rods are compatible with Thermotrac 2 and 3, and strengthen all the well-proven advantages.

Thermotrac 3 is an automatic calculation program for mold PIDT control coefficients and is standard on the company's injection presses.