Editor's note: Mold insulation is an often overlooked but important part of the molding process. Kurt H. Hartwig, marketing manager of Dielectric Corp., has prepared these strategies and guidelines for the selection of thermal insulation.
Wherever two adjacent areas are at different temperatures, heat will transfer from the warmer area to the cooler area. In compression molding and injection molding, this heat transfer takes place in three primary ways. First, it occurs by heat conduction through the mold and into the press platen. Also, heat moves by air currents within the environment, a process known as convection. In addition, heat transfers from the mold into the final product.
Insulating materials slow down the heat transfer rate in and around the mold. Thermal conductivity (K factor) is a measure of a material's ability to resist the flow of heat. The lower the K factor, the higher the material's insulating power, and thus, the lower the overall heat transfer and operating costs.
Figure 1 shows a mold with no insulation. Figure 2 shows mold insulation inserted between mold and press platen to obtain some energy and cost savings. Figure 3 shows mold insulation installed between press and platen as well as around the mold to achieve the maximum level of process efficiency and cost savings.
Cost Savings Strategies
Installing a sheet of mold insulation in and around molds that are heated by steam or electric power can conserve energy and correspondingly reduce energy costs.
Thermal insulation will also protect machinery components. When the mold begins to heat toward operating temperature, increased temperatures accelerate the breakdown of hydraulic oil, which reduces the life of oil seals and rings as well as the pump and valves.
In addition to energy conservation and machinery protection, thermal insulation has the ability to reduce the potential for cold spots. Generally, most molds distribute heat unevenly; a sheet of mold insulation can help provide a more consistent heat profile and better maintain mold temperatures.
Thermal insulation can decrease startup times and shorten cycle times. In both injection and compression molding, heat will transfer from the mold into the final product, and from the mold into the press. Mold temperature, therefore, can decrease as part count increases during production.
Selecting Thermal Insulation
There are many insulation materials available. Deciding which is best requires an understanding of the key properties shown in Table 1. The first property is compressive strength, which is the maximum force required to deform a material prior to reaching its yield point. The importance of this property is for maintaining mold and press alignments. Typically, the compressive strength of most insulation materials decreases as temperature increases.
The second key property for selecting a mold insulation is service temperature, which is the highest temperature at which a material can perform reliably in a long-term application (long-term being inconsistently defined by the manufacturers). Depending on the product, most presses operate between 275 and 450F. It is recommended to select insulation with a service temperature 25 percent higher than the operating temperature of the mold.
The third and most important key property is thermal conductivity, which is defined as the quantity of heat that flows through a unit area in a unit time under a unit temperature. Thermal conductivity is a useful measure for three purposes. First, it's used as a benchmark of material's performance during operation. Second, it is used to determine utility savings (e.g., electric or steam). Last, it's used to measure the return on investment.
The fourth property to consider in selecting mold insulation is water absorption, which is defined as the amount of water absorbed by a material when immersed in water for a period of time. The common measure is the percent swell. The disadvantage of water absorption to mold insulation is that swelling can cause mold misalignment and cracking. The lower the value the better a material is at resisting the absorption of water.
The fifth key property is thickness tolerance, which is the material's ability to maintain parallelism across flat areas. On most press applications, thickness tolerance is extremely important for achieving mold alignments and product quality. The influence of thermal expansion at operating temperatures is so low that operations are unaffected.
Figures 1, 2, and 3 illustrate the relative value of partial and complete mold insulations vs. no mold insulation at all. In addition to placement, the content of the insulation is a key consideration.
The last property to be considered is a material's resistance to lubricants and oils. Stray hydraulic fluid leaking from components can swell and crack mold insulation.
To calculate cost savings and return on investment (ROI), a customized energy savings worksheet using Fourier's law of heat transfer is used. For example: An insulating board has an area that measures 3 by 3 ft, a thickness of 1/2 inch, a thermal conductivity of 1.8 Btu/hr, and is mounted to a steel mold that is electrically heated at 325F. What are the energy savings and ROI per year if operating three shifts, 45 weeks in a year, five days a week?
Using Glastherm Grade S, the Btu savings per hour is 71,699 for an energy savings of approximately $3942 per year. The cost of the 3-by-3-ft sheet was approximately $190. ROI is about $3752 for the first year and $3942 for the years to follow.
Heat loss from bare molds varies with the difference between temperature inside the mold, that of the surrounding air, and the heat transfer to the final product. For maximum savings, thermal insulation is attached to mold sides as well.