It is better to prevent noise generation in machinery during the design stage than to try to reduce it later. There are Injection Molding and auxiliary equipment machines built with exceptionally low noise levels. However, at times noise reduction by external means is preferred. Design changes to reduce noise sometimes decrease efficiency. Although this is relatively unimportant in small, fractional-horsepower  equipment, it becomes costly and wasteful in large, high-power machinery that has been designed for maximum performance and efficiency.

           One of the best ways to reduce machinery noise by external means is to place it in an acoustic enclosure. Such enclosures provide more dB reduction per dollar than any other form of industrial noise control. For this reason many are in use today, and they are very efficient when designed  and installed  correctly.  A good acoustic enclouse can easily reduce noise by 20 to 30 dB and more; a very simple design, by 10 dB.

            The performance of an acoustic material can be described in terms of its transmission coefficient T, which is defined as the fraction of incident sound power transmitted through the material. Materials with low transmission coefficients isolate noise better than materials with higher coefficients. If  the material has, say, a transmission coefficient of  0.01, when airborne sound strikes one side of a wall, only 1% of  the sound comes out the other side. Of course, the sound does not “go
through” the wall; it makes the wall vibrate, and this radiates the sound again. Sound coefficients vary with frequency.

             The sound transmission loss TL of a wall or barrier measures its sound isolating ability. It is the ratio of the airborne sound transmitted by the wall to the airborne sound striking the wall. It is expressed in decibels (dB).

             TLis related to the transmission coefficient by the equation TL = 10 log(l/ T) 
For example, a wall having a TL of  30 dB transmits only 1/1,000 of the energy incident on it. The transmission loss, like the transmission coefficient, varies with frequency. To make a correct design, it is necessary to know the frequency, or frequency band, of the noise to be isolated. Approximate  TL values for several different materials, at 1,000 Hz, are given in Table 2.6.

            To obtain the best processing melts for any plastic, one starts with the Plastic Manufacturer’s recommended heat profile and/or one’s own experience (see the section on Processing Different Plastics in Chap. 6). There are different starting points for the various types of plastics, which have to be interfaced with the different capabilities of IMMs to be used. The time and effort expended on startup make it possible to achieve maximum efficiency of  performance vs. cost for the processed plastics. By the application of common sense with  available control systems, the information gained can be stored and applied to future setups (Chap. 7). As explained above (see the sub-subsection on the Hunkar test in the subsection “Injection Molding: A Technology in Transition to Electrical Power”), electric IMMs can provide higher process capabilities with quick startup and setup without the oil heating required in hydraulic IMMs. Specialty IMMs have their own procedures, as reviewed in the subsection on Structural Foam Molding the section on Startup for Molding in Chap. 15.