The approaches to noise control are designed to address noise problems in the rubber and plastics industries in a specific manner. It should be recognised, however, that machinery usage will vary from plant to plant.
While the general approaches to noise reduction presented here should be applicable to a wide variety of plants, careful engineering judgement should be made for each potential application to insure acoustical, production and safety constraints which are considered and dealt with.
General Approaches to Noise Control:
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Three approaches to noise control should be considered for any noise problem:
1. The noise source may be modified.
2. Noise may be blocked or reduced along the path from the source to the receiver.
3. Sound may be isolated from the receiver by means of barriers, operator location or hearing protection.
The optimum approach for any operation must be determined based on acoustical effectiveness, production compatibility and economics. It should be pointed out that OSHA recognises hearing protective devices as only a temporary solution to noise exposure and stipulates that other engineering methods must be employed as permanent compliance measures.
The first step in reducing noise is to define specifically how the acoustic energy is being generated.
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All noise sources generate sound by one of the following two mechanisms:
1. Acoustical radiation from a vibrating surface.
2. Aerodynamic turbulence.
Noise Control Systems:
Six types of noise control systems may be considered to solve any noise problem:
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1. Sound barriers.
2. Sound absorbers.
3. Vibration damping.
4. Vibrations isolation.
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5. Mufflers.
6. Machine redesign, process modification or noise source elimination.
Each of these six conceptual approaches is considered in the noise control solutions for specific items of machinery.
Overview of Noise Problems in the Rubber and Plastics Industry:
The following major types of specific noise problems are found in many rubber and plastics plants:
1. Granulators.
2. Parts ejection from moulds using air.
3. Rubber mills.
4. Injection moulding.
5. Container blow moulding.
6. Extruders.
7. Planers.
8. Vacuum loaders and air conveyor equipment.
In addition, various other common noise problems may be observed: air exhausts, fans, motors, etc.
Feasibility:
To establish that solution of a noise control problem is feasible, one must consider three areas:
1. Acoustical feasibility.
2. Production feasibility.
3. Economic feasibility.
To establish acoustical feasibility, it must be shown that designs exist which would provide adequate noise reduction.
Each proposed noise control design must be reviewed to insure suitability to the application for which it is intended and to establish production feasibility.
Non-acoustical considerations related to any design include:
1. Employee safety and hygiene.
2. Fire code compliance.
3. Operational integrity-
(a) Accessibility to equipment.
(b) Maintenance serviceability assurance.
(c) Product quality assurance.
4. Machine system compatibility-
(a) Mechanical (power, speed, etc.).
(b) Service life.
(c) Ventilation and cooling.
Figure 17.1 illustrates the matrix of decisions to be made in determining feasibility. In cases where doubt arises as to acoustical or production feasibility, a design prototype may be required.
The following design investigation procedure has been adopted to establish a basis for acoustical non-feasibility for several industrial operations:
1. A literature search is performed of all available publications in the noise control field and in the general field of the alleged violation.
2. The problem is discussed with colleagues within the professional community to identify where potential solutions to the problem may have been attempted.
3. Recognised authorities in the academic community are solicited for ideas.
4. The literature of all manufacturers of acoustical materials and systems is reviewed for solution approaches and many are contacted personally.
5. The manufacturer of the noise-producing equipment is contacted, as are several manufacturers of similar equipment.
6. Trade associations are contacted and the industry-wide state-of-the-art is sought.
7. Solution approaches are solicited from the OSHA personnel involved in the citation.
Granulators:
A-weighted sound levels due to plastic scrap grinders or granulators may be as high as 115 dBA.
Several approaches to noise reduction may be considered such as:
1. Reducing the structural vibrations by means of vibration damping.
2. Absorbing the sound before it propagates from the interior of the granulator through the inlet.
3. Closing the infeed as much as possible to reduce sound transmission.
4. Enclosing the entire granulator.
5. Use of a quiet cutter knife design.
The sound levels of the granulator are primarily due not to the grinding, but rather to vibrations induced by the grinding forces. Sound is radiated from the vibrating panels, with approximately 50 per cent of the acoustical energy radiated to the interior and radiating from the inlet and 50 per cent of the energy being radiated from the exterior surfaces.
Vibrations are forced into the structure at the blade passage frequency, given by:
f = n x rpm/60
Where,
f = fundamental frequency, Hz.
n = number of blades.
rpm = blade rotational speed.
Some acoustical energy will be forced into the system above this frequency due to the non-sinusoidal nature of this frequency; however, the primary higher frequency vibrations are associated with structural resonances. The resonant vibration levels may be significantly reduced by vibrations damping treatment, although the forced vibrations will not be influenced by this treatment. To provide effective vibration damping, either the interior or exterior sides of all panels and wall surfaces (excluding heavy structural members) should be treated with a constrained layer vibration damping treatment.
The constrained layer treatment consists of a viscoelastic (rubber) material in contact with the surface being damped, with a sheet metal constraining layer. The constraining layer should be at least ½ the thickness of the structure being damped. The ‘sandwich’ construction may be assembled by means of bolts or adhesives such as hot melt, thermosetting epoxy or urethane.
Interior Absorption:
A portion of the sound generated within the granulator may be absorbed before it would propagate through the inlet.
The achieve a sound absorption coefficient of 0.9 at 125 Hertz, a sound absorptive material thickness of 4″ must be used. To protect the absorptive material, a perforated panel facing may be used. If the perforated panel is thin, the perforation diameter should not be less than 5/64” and a minimum of 20 per cent area must be exposed.
The sheet metal may be as thick as considered necessary to withstand operational requirements. This material should be available from local suppliers. The perforated sheet metal should be pressed very tightly against the glass fibre to provide a vibration damping of the perforated panel. The acoustical panels should be inspected periodically and replaced if damaged.
The infeed of granulators should be closed as much as possible to reduce sound transmission. This may be achieved by installing a curtain on the front of the granulator. The curtain should be constructed of two sheets of vinyl ⅛” thick, with staggered slits for passage of materials.
Any flexible plastic or rubber material may be used as an alternative, but must be of a minimum ⅛” thick. It is extremely important that the curtains provide a nearly perfect air-tight seal of the front opening. They should extend from side to side and come within 1/32″ of the bottom panel.
The classic approach to control granulator noise generally involves either a partial or total enclosure of the machine. Pelton and Storment described a solution to the granulator noise problem which involved either closing or shadowing the wall openings and acoustically ‘sealing’ off the noise sources.
Closing the openings was accomplished as follows:
1. Acoustical baffles are placed at conveyor feed openings and the opening physically closed down to the minimum allowable, using a piece of clear plexiglass.
2. The doors were gasketed and closures placed on them so they will not stand open.
3. The small granulor infeed was modified. A new infeed chute was fabricated, acoustically treated, with three leaded vinyl flaps placed in a series to reduce direct radiated noise. The infeed chute was made bigger and permanently placed in the opening and sealed around the outside of the chute.
4. The operator is placed on an elevated stand to remove his head (ears) from the direct noise path.
Another acoustical enclosure reduces plastic scrap grinding noise by a reported 42 dBA. The sound reducing panels were made of 16-gauge stainless steel with especially composed filler made-up of a visco-elastic damping compound, a high-density attenuator and a sound-absorbing material assembled in a multiple-layer construction.
While enclosures may be acoustically effective, there may be several potential disadvantages, including:
1. Operator access to the machine may be hindered.
2. Excessive heat can build up if ventilation is not provided. This heat rise may reduce efficiency in the cutting chamber, create motor problems, melt the plastic scrap being processed and create a potential fire hazard.
3. Maintenance of the grinder may become difficult and access to blades for sharpening may be restricted.
4. This solution may be costly in terms of materials, increased energy consumption and decreased productivity.
For many years, the use of helical or spiral blade design has been employed for the noise reduction of planers. This technique has now been applied to granulators by Foremost in the design of the ’21st Century Grinder’.
Sound levels are 80 dBA or less and power consumption is reduced by 20 per cent. It should be pointed out that cutter design is an effort which must be undertaken in the design of the original machine and not on a retrofit basis by the machine user.
Parts Ejection from Moulds Using Air:
Noise is generated by the ejection of rubber or plastisol parts from moulds using pneumatic air. Sound levels of this operation typically exceed 100 dBA.
Analysis of the operation identified three separate components of air noise:
1. Noise of the nozzle in free air during the time not ejecting.
2. Noise generated by turbulence of air passing the edge of the mould as the part is broken loose from the mould.
3. Noise generated during the air-assisted pulling of the part from the mould. This noise is theorised to be common air nozzle noise which is amplified by virtue of the directional characteristics associated with placing the nozzle in the mould. Essentially, all of the air noise, which would normally be radiated over 360° and maximum at 180° from the operator, is reflected back toward the operator.
Three other mechanisms of noise generation may be present when air is blown inside the mould:
(a) The impingement of the air jet on the mould, as a rigid body will cause an increase of up to 7 dBA.
(b) The part shape may amplify noise at certain frequencies associated with the Helmholtz frequencies of the part.
(c) The part material may vibrate.
These three noise generating mechanisms are considered secondary with respect to the directional amplification.
In applications which require a jet of air to do useful work, effective thrust must be maintained to transfer force to the object.
1. Air ejector mufflers.
2. Directional control of air flow.
3. Regulation of air.
4. Barriers.
5. Mechanical ejectors.
Two substitute methods of part removal from moulds may be considered:
I. Vacuum pull.
II. Thongs.
These methods may not be considered feasible for some operations for various reasons, including the following:
1. The method is slow. Vacuum pulls have been used only on machines which operate at a very slow speed.
2. Each part pattern would require a separate vacuum head.
II. Thongs:
1. Thongs were used in one plant; however, the operations were so slow that their use was abandoned.
2. The use of thongs was applicable only where the parts were pulled horizontally rather than vertically.
3. Mould release with thongs is poor and an inferior product results.
Rubber Mills:
Sound levels in a typical rubber mill producing inner tubing are shown in Table 17.1.
It is seen that no noise problems exist for this type of operation in terms of OSHA compliance.
In mixing plants, however, the ‘popping’ noise associated with rubber mills is quite high and is inherent to the process.
The lack of engineering feasibility for some rubber mill operations has been recognised by industry.
‘Examples of operations and equipment that might not be amenable to sufficient noise control, either by engineering methods or administrative controls, include certain chemical milling operations, such as those involving rubber mills’.
While the noise of the mill itself may not be reduced, the noise may be shielded from other areas. Enclosure system can be installed to protect adjacent workers from noise levels created by the reduction unit of a rubber mill. Noise level readings of 98 dBA were recorded prior to installation. The after-enclosure reading was 90 dBA and 8 dBA attenuation.
A mylar covering was used to allow for easy cleaning and prevent oil splashes and other debris from clogging or destroying the sound-absorbing properties of the foam. The track-and-roller suspension method allowed for ready service and maintenance accessibility to the unit.
Injection Moulding:
The principal sources of noise associated with the moulding equipment are the hydraulic pumps and the pneumatic control blow-off nozzles. The noises from the hydraulic pumps basically are discrete frequency tones at the impeller or vane passing frequency and Fourier harmonics thereof.
Reduction of noise in this area has been most successfully achieved by the utilisation of a hydraulic muffler on the discharge of the pump. The selected muffler must naturally be tuned to the discrete frequency tones. Vibration isolation mounts must also be included to isolate the pump from the frame or other radiating surfaces.
Container Blow Moulding:
Containers are produced from a moulding process by injecting compressed air into a tube of hot plastic which has been clamped within the cavity of a shaped mould. The associated high frequency noise is related to the speed that the pneumatic system of the moulding machine can peak and then rapidly release air pressure.
The brief, intermittent and hiss-like noise which results is an eighth power function of the velocity of the air which is released. Additional noise is heard when pellets of the virgin or reground plastic material are transported through ducts to the moulding machines. The associated pellet transfer noise is also intermittent.
The irregular timing cycle for grinding scrap into appropriate size pieces at the moulding machine produces yet another intermittent noise. After moulding, certain of the containers require a secondary treatment to bring the neck of the containers to specification.
Most often this is accomplished by hand feeding each container onto a multiple position, powered turntable. The reaming and clean put positions require air pressure for effect. This coring and trimming operation is yet another source for intermittent type noise.
The frequency of the offending intermittent noise from the moulding machines, pellet transfer and coring and trimming operations is mostly 8000 Hertz (Hz). This intermittent and high frequency energy is superimposed upon continuous sound energy from other plant sources which is most intense at the 250 and 500 Hz frequencies.
The offending high frequency hisses greater than 90 dBA are superimposed upon a low frequency steady state noise of 85 dBA. The duration of the noise skirt for the superimposed high frequency air hiss impulses was found to last less than half a second. The peak level for these impulses is 110-120 dB.
In addition, most often a significant background interval (longer than one second) occurs before the noise which follows is detected. A maximum of 27 of these impulses occurs per minute. Therefore, the duration of high frequency noise skirts last about 22 per cent of the work period.
The environmental noise data presented within Tables 17.2 and 17.3 shows the effectiveness of various noise reduction projects, including pre-designed engineering controls as well as cut and try experimentation at a number of moulding plants.
The single most effective control method found to reduce the noise exposure level for the greatest number of employees was to reduce the air pressures to the minimum level consistent with maximum product-productivity and quality. Not only is the noise level lowered by this action, but power is conserved and operating costs reduced.
Extruders:
The sound levels of an extruder operation which applied the jacket to copper wire was studied. It was found that the only major noise source on the extruders was the air wipes which are used to dry off the cooling water.
To provide effective noise reduction, the cover of the water collection box should be gasketed to insure a sealed box. The interior of the water collection box, including the cover, should be lined with one inch of the film faced glass fibre. The exit tube of the wir wipe should be converted into a muffler. This may be accomplished by replacing the exit tube with a perforated tube with at least 32 per cent open area. The perforated tube should be covered with one inch of film faced glass fibre.
Planers:
Plastic sheets may be finished by the use of planers which use a high-speed cutter heat. In one operation, sound levels measured 120 dBA when 24″ × 48″ sheets of Delrin were planed to a final thickness of 0.025″, with approximately 0.010″ being removed during each pass.
There are three major noise sources associated with planer operation:
1. The board, excited by cutter knife impacts.
2. The heavy structure under the cutter head, excited by vibration transmitted through the board.
3. Modulation of air flow by cutter knife chopping at the chip collector air stream.
One enclosure design approach is a localised enclosure which has an infeed and outfeed tunnel. The enclosure should be constructed of ½” minimum plywood and lined with a 2″ thick sound absorptive material such as glass fibre or open cell polyurethane foam with a protective facing (not more than 2 mils thick) to prevent dust accumulation. Provisions should be made for ventilation and accessibility.
In some cases planning the plant properly will allow one to enclose the planer without fabricating an additional structure. That is, by placing the planner in a room of its own and providing conveying equipment, it is possible to achieve isolation from planer noise for surrounding activities without the construction of a separate structure.
The problem with most planer enclosures is one of maintaining the smallest possible openings to accommodate the kind of sheet being planned. In many cases the feeding arrangement is such that the operator still has to be very close to the opening into the enclosure and so he receives minimal protection from the enclosure.
Infeed and outfeed openings should be as small as possible. A tunnel type opening provides room for multiple baffles of old conveyor belting or lead-filled vinyl to block the noise path. Slit the belt at intervals to accommodate various board widths, keeping the unused portion of the tunnel width blocked. The outfeed tunnel should be at least as long as the longest sheet fed through the planner so that noise caused by the vibrating sheet is confined inside.
Install funnel-shaped metal facing inside to guide the stock into the tunnel opening. In recent years, helical knife cutters have been developed for wood planers. Results achieved by a helical knife cutter head are shown in Fig. 17.10 reduction from 106 dBA to 93 dBA. It is reported, however, that the helical blades cannot achieve a suitable surface finish for plastic sheets.
Vacuum Loaders and Air Conveyor Equipment:
The principal source of noise from the vacuum loaders and other conveying equipment is the rotary positive blowers. The noise levels here are basically discrete frequency (pure tones) at the impeller or hear passing frequency. The successful approach to noise reduction is the inclusion of a reactive muffler or silencer on the intake or discharge side of the blower.
The muffler naturally must be tuned to the impeller passing discrete frequency tones to be effective. In some cases, switch-back type silencers have been included as part of the frame. A carefully selected muffler has been shown to bring the noise levels from the noisier blowers below 80 dBA. The lower limit here is the noise radiated from the case of the blower itself. Reduction beyond this point would require an enclosure.
Air Exhausts:
Air discharges are observed throughout most plants and are considered a primary source of excessive sound levels. Most air discharges result from pneumatic exhaust of control systems and these air exhausts are either continuous or cyclic in nature.
Sound levels generated by air exhaust, where concentrated air flow is not required, may be silenced by the installation of pneumatic mufflers which diffuse the air stream.
The specification of an optimum pneumatic silencer for any application should be based upon:
1. Sound level reduction.
2. Low pressure drop.
3. Durability.
4. Non-clogging features for the air contaminants present.
5. Economy.
Mufflers should be periodically inspected for wear and effectiveness. If necessary an in-line filter system may be installed to prevent muffler clogging.
The following information should be provided to pneumatic muffler suppliers to assist in proper muffler selection:
1. Pipe thread size (N.P.T.).
2. Estimated or measured air flow (cfm).
3. Presence of airline contaminants (oil, excess moisture, etc.).
It should be pointed out that most air discharges have threaded outlets which would easily accept silencers. To insure that the silencers are not removed, they may be secured by means of welding or a set screw. Signs reminding employees that noise control devices are for their benefit may also help.
Man Cooler Fans:
The movement of high velocity air across the body is the most effective method of reducing employee heat stress in the working environment. Creating a high air flow by means of fans is very critical in maintaining employee health and comfort. In an inventory of over 100 man cooler fans, sound levels ranging from 85 to 103 dBA were measured at 6′, with the sound levels of 20 per cent of the units exceeding 90 dBA.
Sound levels of the fans are influenced by the following variables:
1. Speed or rpm of the motor.
2. Velocity output of the fan.
3. Number of blades.
4. Blade shape and pitch.
5. Horsepower.
6. Diameter.
Blade shape is an important factor in fan noise generation; a 10 dBA reduction may be achieved by blade shape modification.
Where excessive sound levels are observed from man coolers, the following approaches may be taken:
1. Replace man coolers with new quiet models.
2. Install new quiet blades on the man coolers.
If new fans are purchased, a specification similar to the following should be used:
The vendor shall guarantee that the sound level of the fan does not exceed 85 dBA, measured at 6 feet.
A specification requiring that the fan ‘comply with OSHA’ may not provide adequate results, as the manufacturer may measure his fan noise at a distance considerable greater than 6 feet.
Ventilation Fans:
The sound levels due to the roof ventilation fans often exceed 85 dBA and occasionally exceed 90 dBA.
Silencing of these fans is frequently required to achieve satisfactory ambient levels in a plant.
Three approaches may be considered:
The installation of silencers is considered the most effective abatement method. The ‘straight-through’ type should be used so as to minimise air flow impedance.
There are commercially available replacement blades of a multitude of configurations for noise reduction purposes.
Suspending a barrier directly below the fan, obstructing the line-of-sight to the workfloor may be considered. If a sound absorptive barrier of twice the fan diameter were hung 1-½ times the diameter below the unit, a noise reduction of 1-4 dBA may be expected.
A more restrictive barrier may provide noise reductions up to 10 dBA; however, such units are generally impractical, because a 10 to 15 per cent impedance of the fan’s air flow would result.
When purchasing new fan units, the engineer should always include noise level specifications.
Reverberation:
Whenever machinery is operated within enclosed spaces, sound levels will be increased to some extent due to reverberation. When this reverberant sound level increase becomes significant, it is appropriate to install sound absorptive materials on the ceiling above offending machinery. A simplified procedure can be used to estimate the increase in noise due to the reflected component.
Ceiling treatment is also required wherever acoustical barriers are employed to prevent sound from being reflected off the ceiling and over the barrier.
The most convenient method of employing sound absorption is the installation of acoustical baffles.
Where fire sprinkler systems or air ventilation requirements preclude the use of baffles, 1″ thick conventional acoustical tile may be installed directly on the ceiling.
Where large surface areas are to be treated, a spray-on treatment may be most economical. Attention should also be given to machine location within a room.
Motors:
It is evident that high speed (3500 rpm and above) and TEFC motors are noisier than their slower speed and DRPR counter parts.
Solutions to motor noise problems include muting or muffling the inlet and outlet air and isolating or dampening the vibrations. Motors with horsepower ratings of 75 or more often require abatement. Motors below this rating do not normally generate noise levels that are above 90 dBA; however, these smaller motors may contribute to other equipment noise and the additive levels may exceed 90 dBA.
Several motor manufacturers have developed new lines of quiet motors with higher horsepower ratings. This has been accomplished through quiet fan designs and minor internal modifications. To insure minimum motor noise levels, specifications that sound pressure levels are not to exceed 85 dBA at 3 feet and that sound power levels are not to exceed 95 dBA, should be made to manufacturers for any new motors purchases.
OSHA:
The source of the greatest controversy and speculation surrounding OSHA relates to the revisions to the noise standard.
The proposed standard would require:
1. Employee exposure to continuous noise is limited to 90 decibels A-scale, time-weighted average over an 8-hour workday. The standard would set a doubling rate of 5 dB.
2. Daily exposure to impulse noise is limited to 100 impulses at 140 dB, 1000 impulses at 130 dB and 10,000 at 120 dB.
3. Workplace noise levels are monitored annually to determine if any employee is exposed to levels of 85 dBA of higher.
4. Development and implementation of engineering controls except where such controls are not feasible.
5. Baseline and annual audiograms for all employees exposed to noise levels of 85 dBA or higher.
6. Recordkeeping of monitoring, measuring and calibration of instruments.
Written notification is given to all employees who have been exposed to excessive noise levels, including a statement of what noise abatement action the employer is taking. Employers would also be required to allow employees to observe monitoring of workplace noise levels and allow employees and former employee’s access to their noise exposure records.
National Emphasis Programme (NEP) as a new effort to reduce the rates of accidents and occupational diseases in high hazard industries.
The programme includes the following items:
1. Inspections for every plant within and industry classification identified for high hazards, including with issuance of citation for hazards, proposed penalties and abatement deadlines.
2. Enlisting the involvement of the industries to be inspected, their trade associations, state safety and health agencies, organised labour, insurance companies and professional safety and health groups.
3. Publishing safety and health training courses for employers and employees and simplified guides to OSHA standards.
4. Training the officers and OSHA area directors in the processes and hazards of the emphasis industries.
5. Providing on-site consultations, on employer’s request, before the inspection teams arrive.
6. Making follow-up compliance inspections.
7. Collecting data during initial and follow-up inspections so that OSHA can measure the effectiveness of the programme.
8. Educate management and workers more fully about OSHA.
9. Simplify the language of OSHA safety standards.
10. Mend OSHA’s image by steering away from excessive attention to detail in OSHA inspections.
11. Staff OSHA with more industrial hygienists and physicians well versed in occupational health.
12. Step up the development of occupational health standards.