Mold insulation to support cost reduction strategies.

Wherever you find two areas of different temperature, you will find heat transfer from hot to cold. In the case of compression 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. Second, it moves by convection by currents of air within the environment. And finally, heat leaves the mold by transferring into the final product.

Responsibility of mold insulation

Insulating materials control the heat by slowing down the transfer rate in and around the mold. Thermal conductivity (K factor) is important in determining 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 (figures 1, 2 and 3).

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Figure 1 shows a mold at its lowest level of process efficiency, which results in zero energy and cost savings when no insulation is used. Figure 2 demonstrates some energy and cost savings when mold insulation is inserted between mold and press platen. Figure 3 highlights the maximum level of process efficiency and cost savings when mold insulation is installed between press and platen, as well as around the mold.

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Cost saving strategies

Historically, total quality programs have been the focus for improved process efficiency. Energy conservation, however, is a strategy that has little, if any, exposure for cost reduction initiatives. The purpose of a program is to minimize overhead costs from the consumption of utilities such as water, gas, oil, air, steam and electric. Installing a sheet of mold insulation in and around press molds that are heated by steam or electric will dramatically conserve energy, and thus reduce costs.

Other than conserve energy, thermal insulation will also protect machinery components. When the press mold begins to heat toward operating temperatures, other major parts and components also increase in temperature and can cause major complications to machinery. First, it breaks down the hydraulic oil sooner, which reduces the life of oil seals and rings, as well as to the pump and valves. Second, it causes thermal expansion on moving parts, which invites increased drag and wear. With time, system components will fail prematurely.

In addition to energy conservation and machinery protection, thermal insulation has the ability to reduce the potential for cold spots. All molds distribute heat differently, and to a certain point, unevenly. Installing a sheet of mold insulation in and around press molds dramatically improves the ability to control and maintain mold temperatures, which provides an overall tighter machine tolerance, as well as an increase in uniformity and stability into the final product.

The last cost saving strategy that is realized from thermal insulation is its ability to lessen cycle and shorten start-up times. When it is injection or compression molding, heat will transfer from (1) the mold and into the final product, and (2) from the mold into the press. Mold temperature, therefore, decreases as units increase during production. Installing a sheet of mold insulation between press and platen allows the mold to recover quicker, without losing time in mold reheat. Mold temperature will also transfer into the press upon initial setup, which results in longer start-up times. Again, installing a sheet of insulation will minimize the time required to reach full operating temperature.

Six key considerations in selecting thermal insulation

There are many insulation materials on the market claiming to be the most efficient. How to decide which material is best requires an understanding of five key properties (table 1). The first property is compression strength (unit: psi), 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, compression strength of most insulation materials decreases as temperature increases. In fact, a small increase in temperature can result in a significant decrease in compression strength in some insulating grades. Manufacturers can provide this data through their technical bulletin.

Table 1 - comparative properties

                                         Glastherm     Sheet
                                           Grade       Grade
Property                   Units             S          HT

Compressive
strength
@  75 [degrees] F          Psi            45,000      49,000
@ 302 [degrees] F          Psi            16,000      27,000
@ 392 [degrees] F          Psi            11,000      18,000
@ 425 [degrees] F          Psi            10,000      17,500
Maximum service
  temperature              [degrees] F   425         550
Thermal conductivity
  Btu/hr./[ft..sup.2]/
  in./[degrees] F                        1.8         1.9
Water
  absorption               %             0.4         0.2
Thickness tolerance        In.           [+ or -]    [+ or -]
                                         .002        .002
Resistance to lubricants
  and hydraulic fluids                   No effect   No effect

                           Asbestos   Calcium
                           Concrete   Silicate
Property

Compressive
strength
@  75 [degrees] F           14,000      2,350
@ 302 [degrees] F              ---        ---
@ 392 [degrees] F              ---        ---
@ 425 [degrees] F              ---        ---
Maximum service
  temperature
Thermal conductivity
  Btu/hr./[ft..sup.2]/
  in./[degrees] F          4.5        0.88
Water
  absorption               22         89.0
Thickness tolerance

Resistance to lubricants
  and hydraulic fluids

The second key property for selecting mold insulation is service temperature (unit: [degrees] F), which is the highest temperature at which a material can perform reliably in long term application (long term being inconsistently defined by the manufacturers). Depending on the product, most presses operate between 275 [degrees] F and 450 [degrees] F. It is recommended to select insulation 25% above the operating temperature of the mold.

The third and most important key property is thermal conductivity (unit: Btu/hr./[ft..sup.2]/in./[degrees] F), which is defined as the quantity of heat that flows through a unit area in a unit time under a unit temperature. The value of thermal conductivity is used for three purposes. First, it is used as a benchmark of a material's performance during operation. Second, it is used to determine a utility's savings (e.g., electric or steam). Last, it is used to measure the return on investment. These three purposes for understanding thermal conducivity aid in making a good buying decision of a mold insulation material.

The fourth property for selecting mold insulation is water absorption (unit: %), 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. Therefore, the lower the value, the better a material is at resisting the absorption of water.

The fifth key property is thickness tolerance (unit: inch), which is the material's ability to maintain parallelism across flats. On most press applications, tolerance is extremely important for achieving mold alignments and product quality. The value of thermal expansion at operating temperatures is so low that operations are unaffected. Parallelism, therefore, is important only during initial purchase of mold insulation. Most insulating boards claim to hold a thickness tolerance of [+ or -] .005 inch. Glastherm thermal mold insulation is the only material to claim to hold [+ or -] .002 on all thickness grades.

The last, yet less talked about, property is a material's resistance to lubricants and hydraulic oils. When hydraulic fluid leaks through components, the oil runs down the machine and contacts the mold insulation. Again, this gives rise to the potential of swelling and cracking. This condition is seen more on vertical presses such as compression molding where a hydraulic cylinder and components are mounted above the press.

Calculating utility savings and return on investment (ROI)

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 18" by 18", a thickness of 1/2", has a thermal conductivity of 1.8 Btu per hour, and is mounted to a steel mold that is electrically heated at 325 [degrees] F.

What is the energy saving 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 24,012 for an energy savings of approximately $2,268 per year. The cost of the 18" by 18" x 1/2" sheet was approximately $150. ROI is approximately $2,118 for the first year.

Note: 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. To achieve maximum savings, thermal insulation is attached to the sides of a mold as well.

Conclusion

These five key properties are important because of the life expectancy differences in thermal mold insulation materials. The life of any insulation, however, is dependent upon three factors, including time in actual operation, operating temperature and actual compression. As mentioned, the most important property is thermal conductivity. The second is both temperature and compression. The third is thickness tolerance. And the fourth is water and oil absorption.

In the future, compression and injection mold manufacturers will continue to experience pressures to increase quality and efficiency by developing and implementing cost-saving strategies to their processes in order to remain competitive.

Thermal mold insulation is the answer for reducing costs that are associated with poor product quality and excessive utility usage. In the world of economics, rational individuals and institutions should do the most cost-effective activities first and then continue to seek better and more effective cost-reduction strategies to gain competitive advantages.