As industrial engineers seek to achieve ever-higher safety standards for machine guarding, many are turning to extraordinarily tough but flexible polycarbonates to do the job.
One such widely used product is Safeguard PC from engineering thermoplastics specialist Dotmar EPP, which has a shock load resistance 250 times that of glass and more than 30 times that of acrylic clear sheet.
An advantage of these tough but lightweight see-through machine guards is that they keep potentially hazardous machinery and processes in full view so operators can ensure their correct functioning rather than hiding potential hazards.
According to Dotmar, custom-fabricated from rod, sheet or tube, Safeguard PC is easy to handle and suitable for use in machine surrounds and as protective panels in applications traditionally served by metal and similar material.
However, as with many innovative materials, engineers need to understand the different characteristics of the new material to get good performance from it.
While Safeguard is practically indestructible and has sound deadening properties comparable with plate glass of equal thickness, it is necessary to understand the major design parameters affecting the material.
These include knowing how to fix Safeguard in the frame, bending, installation, cutting, bolt sizes, drilling and maintenance:
- Deflection: while Safeguard will not break easily, it may deflect when subjected to high load. To avoid the material slipping out of its sash in these circumstances, it is wise to reserve a wide edge engagement in a sash
- Expansion: because Safeguard is a material of large linear thermal expansion, installations should allow sufficient space for expansion and shrinkage in their design. Safeguard expands more than glass, expanding in the ratio 7.0 X 10(5) cm/cm/deg C
- Thickness and supporting sheet size: ensure suitable thickness is selected to minimise deflection of the sheet, and ensure appropriate span of supports to support this. Reputable suppliers, such as Dotmar EPP, will advise on this.
- Proper selection of auxiliaries: before actual installation, check the compatibility of auxiliaries such as sealants, packings, beads etc that come into contact with Safeguard.
- Cutting: This can be undertaken with a circular saw, band saw, zig saw or shearing machine. To achieve narrow tolerances and fine finish of cut sheets, a circular saw with hard alloy tipped blades is highly recommended.
- Note: Ensure that no burring or melting occur at cut edges. It is recommended that sheets be cut with protective plastic film in place to avoid damage to sheet surfaces.
- Bending: Unlike glass, Safeguard is flexible and may be subjected to forced bending. However, since this creates great internal stress, it should be noted that cracking may occur when load is applied during long-term use.
Allowable radii of curvature:
- Outdoor use, 180 times or over sheet thickness
- Indoor use, 100 times or over sheet thickness
- Centre of angle bending should be less than 90 degrees at maximum
- When exceeding 90 degrees, sectional reinforcement should be added so that each reinforced sections shall be a maximum of 90 degree
- Heat bending: When the radii of curvance exceeds the limit of forced bending (less than 180 times sheet thickness), the curvature should be made by heat bending. Heat bending entails no restriction on curvature radii, but there is a limit to the angle achievable. The centre angle of the curvature should be a maximum of 180 degrees.
- When fixing with nuts and bolts: Holes should be made with a fine sharpened drill. They can also be made using a metal working drilling machine.
Nuts should be tightened using a slightly larger size washer or flat bar. This is particularly important where a bolt and nut are directly fixed on the material, which could otherwise crack because of a concentration of stress created by high load and thermal stress.
Do not fasten the nuts and bolts too tightly because this will create a great deal of flexural stress. Ideally, tighten the fasteners to their minimum level then ease them around another 180-degree turn of the spanner.
Bolt diameter should be 5-12 mm, with bolting holes 2-4mm larger than the nominal diameter of the bolt, so as to allow for expansion and contraction because of temperature change. The centre of the bolt hole to sheet edges should be 2.5 times or over of hole diameter
- Painting: Sashes should be painted before Safeguard is fixed to them. When painting is required after the sheet is fixed, mask the material with adhesive tape or use the protective film is this is still attached.
- Outdoor installation: Because the material’s protective film is affected by sunlight and may become difficult to remove, it should be taken off as soon as the installation is completed.
- Removal of stains such as paint, oils, sealing materials etc: These should be taken off immediately with a soft cloth wetted with ethanol or isopropanol. Stains become quite difficult to remove once they dry on the material’s surface.
- Rinsing: Cleaning the surface should be done using a soft cloth or sponge dipped into warm water containing neutral detergent. Wash the surface and rinse using clean water.
Note that Safeguard surfaces that are under stress may craze or crack after contact with normal solvents that are not harmful to it under ordinary conditions.
With care and forethought during installation, followed by simple maintenance, materials such as Safeguard will reward users with high safety margins and superb mechanical properties, including:
- Perfect mechanical strength in the temperature range –100°C - +120°C
- Up to 90 per cent light transmission and optical clarity
- Good dimensional stability and electrical insulating properties
- Low moisture absorption and self extinguishing properties
- Non-toxicity, making it suitable in food and medical applications
- Electrical insulation properties
- Maintains high impact strength even after long periods outdoors
- High heat deflection temperature under load
According to Dotmar, where Safeguard is installed, the chances of accidents are dramatically reduced because panels are custom-cut to suit each requirement. In machining applications that cause stray objects or pressurised liquids to be thrown out, the polycarbonate provides a vital shield.
Safeguard PC comes in sheet sizes 2440mm x 1200 mm and 2440mm x 1850 mm; thickness’ of 3.0mm, 4.5mm, 6.0mm, 10.0mm; and in the grades of ECK 100 (general purpose clear, suitable for most guards) and ECK 100 XX hard coated PC (superior scratch-resistant quality grade).
Rods are available in diameters of 6mm – 200mm. The product can be cut, drilled, finished, punched and water-jet cut. It can even be solvent-cemented, adhesive-cemented, bent or vacuum formed.