Understanding Mold Insulation As a Heat Management Tool
When insulation is necessary to build a mold, the moldmaker must consider the insulation's properties and what effects these properties will have on mold performance.
The Purpose of Thermal Insulation Boards
The rubber, plastics and wood panel industries work with heated tooling when material is pressed directly between hot platens. If a platen or heated tool is mounted to the press head, press table or beam without a layer of protective insulation, a considerable amount of heat is transferred to the press and the press heats up. In hydraulic presses, not only do the tables, beams and frames heat up, but so do the pistons and hydraulic fluid. This causes premature wear to packings in the entire system.
Overheated hydraulic fluid eventually damages packings and seals in the pumps, valves and filters. Extreme temperatures in the guide systems of any press will lead to a loss of lubricants, and subsequently, to excessive wear.
Insulation should be understood as an effective means to precisely control and manage heat in the molding process, improving the performance of the mold. Therefore, it is important to use great care when selecting the correct grades for every application.
When manufacturing thermoplastics parts and materials, thermal insulation offers great advantages. Experience has shown that flow characteristics are improved if cool and hot components are insulated effectively. It is advantageous to shield the heated mold from the press to reduce deflections. If work is done with a heated core, it is recommended that you separate the heated plates from the cooler tooling by means of insulation board.
Heat Conservation Pays Off Immediately
Installation of thermal insulation will result in energy savings between 50 and 75 percent, depending upon conditions. Thus, thermal insulation is economical. The cost of insulation is quickly amortized and paid off through lower production costs and improved quality.
How to Improve Press Tolerance
If accuracy and parallelism in a press need to be improved, compensation pads may help. They are placed between the hot platen and thermal insulation board, then cured in the press under heat and pressure. This compensates for unevenness and the press will show much better tolerances after treatment - eliminating shimming. Even presses that are decades old can be restored to present-day conditions and productivity requirements. New presses, too, can operate much more accurately with the help of compensation pads.
Characteristics of Thermal Insulation Boards
Thermal Load Stability
Oxidizing decay of organic components in thermal insulation boards increases with rising temperatures. Continuous thermal load is defined by the temperature at which the application of a particular grade can be technically and economically justified, so that the lifetime will extend to several years.
Marked with the symbol l this is one of the most important factors in thermal insulation boards - the spec sheet expresses it as W/mK. Low values in all grades make this insulation effective.
Dimensional Stability at Continuous Thermal Load
Most insulation grades are made of an organic base. Compared to inorganic materials, these have the advantage of being much tougher and suited to withstand thermal stress without the tendency for cracking. On the other hand, in organic materials thermal stability decreases with time, which means although there is no sudden softening, disintegration or melting, their lifetime diminishes with rising temperatures. The dimensional stability under continuous thermal load is a very important criterion for thermal insulation or machine components. This is defined as the temperature at which the material does not shrink more than one percent after 5,000 hours of exposure.
An organic material rated at 450xF temperature stability at room temperature will only be dimensionally stable up to 390xF while under continuous thermal load. An inorganic material rated 156xF also will dimensionally stable up to 1,560xF.
Compressive Strength at Maximum Operating Temperature
Compressive strength of materials is usually measured at room temperature, which is 20xC (68xF). It is generally known that this factor goes down with higher temperatures. In practice, compressive loads are usually much lower - a built-in safety factor.
The same organic material with a dimensional stability of 390xF will have a compressive strength of 300 N/mm² at room temperature, but only 100 N/mm² at 390xF.
The inorganic material of 1,560xF only has a compressive strength of 10N/mm² at room temperature, which remains the same under continuous thermal load.
Grade selection has to be based on specifications given for the equipment and production process - such as operating pressures and temperatures. Manufacturers of insulation furnish charts to aid with this selection.
Moisture absorption is a disadvantage in thermal insulation boards for two reasons. First of all, boards with high moisture content develop steam pressure at temperatures above 100xC (212xF), which may cause cracking, as often observed in boards with more than 10 percent moisture absorption. Secondly, the effectiveness of thermal insulation is significantly reduced, even though it is not noticeable while measuring the coefficient. A dry board, of course, has a more favorable coefficient than saturated material. If saturation occurs after every cooling cycle, additional heat is consumed during each heating cycle to evaporate the absorbed moisture.
Fiber-reinforced synthetics cannot be compared with metallic materials as far as tolerances and dimensional stability are concerned. For economic reasons and considering that an insulation panel cannot possibly retain its absolute dimension once in operation, demands regarding tolerances should be reasonable. It should be mentioned that thermal insulation panels are rarely ever used at room temperature. Change of dimension does not occur through thermal expansion, but through permanent changes such as drying, post-curing processes, long-term oxidative decay, etc. Nevertheless, most panel grades can be supplied with a parallelism of q0.05 mm = 1/500 inch.
Successful production of plastic components associated with the extrusion, injection and compression molding greatly depends on the mold material selection.
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