Building a Better Machine Guard With Plastic

A machine guard is a safety feature designed to protect operators from hazardous moving parts, flying projectiles or sparks, or other dangers that may arise from use of a machine. Because it often stands as an operator’s last line of defense against injury on the job, it is an important investment for any industrial business. Add to that the attention paid by the U.S. Occupational Safety and Health Administration (OSHA), which maintains machine guarding requirements, and optimizing machine guard safety becomes a critical concern for plant owners, safety managers, industrial designers, and engineers.

But building machine guarding that can withstand the varied rigors and repeated punishment of industrial use is far from easy. To complicate matters further, design and performance requirements can vary dramatically depending on the industry using the machine. A vehicle manufacturer, for example, will have different needs than a food processing plant or a tool and die shop.

Enter plastics. This versatile family of materials, used for many purposes in machinery and equipment, can help a machine guard last longer and may resolve many application failure challenges commonly faced by designers and operators.

The key, of course, is selecting and applying plastics appropriately to match the work environment and the particular dangers posed by a piece of equipment. Here, we’ll take a high-level look at how the properties of some commonly used plastics — polycarbonate, acrylic, PETG, and clear PVC — make them advantageous for machine guarding.

What Makes a Machine Guard Successful?

Impact Resistance

In most cases, a machine guard needs to be tough, or impact resistant. While not all machine guards house moving parts, the most common purpose for a guard is to stop projectiles from harming operators. It’s no surprise, then, that OSHA specifically points to impact resistance when outlining materials from which safety guarding may be constructed: OSHA allows plastic, sheet metal, screen, wire cloth, bars, or any material “substantial enough to withstand whatever impact it may receive and to endure prolonged use.”

Impact resistance can be measured in different ways, and the test method varies depending on the material being evaluated. In the United States, the notched Izod impact resistance test, as outlined in ASTM D256, is a common method of measuring the toughness of a plastic material.

The test consists of cutting a small notch into a thin sample and hitting that sample with a hammer to record the break energy. The more energy required to cause a break, the tougher the material.

Understanding the relative toughness of materials allows designers to better determine what is necessary for a guard to be successful. A guard made of tougher material can be down-gauged to be thinner — and, therefore, more cost-effective — than one made from a more brittle material while still offering the same or superior level of protection for operators.

Stiffness

Stiffness is another important factor when designing a machine guard. Stiffness, or modulus, is a measure of how resistant a material is to deformation from an applied load.

Stiffness can be measured in different ways depending on the type of loading the measurer is interested in. For plastics, it is very common to measure the stiffness in flexure in order to compare the resistance to bending. This property is called the flexural modulus; the higher the value, the stiffer the plastic. Plastics are particularly sensitive to heat and will lose modulus rapidly as the temperature rises. It is beneficial to use a stiff material for machine guards in order to avoid sagging and to maintain structural integrity, but also to avoid creating optical distortions.

Source: The Effect of Temperature and Other Factors on Plastics and Elastomers, Second Edition, by Laurence W. McKeen, 2008, Norwich, N.Y.: William Andrew. Copyright 2008 by William Andrew Inc.

Light Transmittance and Optical Clarity

Light transmittance and optical clarity are essential in a machine guard when operators are tasked with monitoring the inside of a machine. While perforating an otherwise opaque material like sheet metal is sometimes sufficient, more often than not users need an unobstructed view.

Maintaining optical clarity over the life of a machine guard can be challenging, particularly in high-traffic areas or with machines that are regularly sterilized.

 

Clear Machine Guards: Common Issues Solved With Plastics

For operations that require visual monitoring, a loss of clarity in the machine guard is considered a failure. Two reasons clear machine guards lose clarity are abrasive wear and chemical attack. While plastics are significantly lighter than glass or metal, they are also much softer and more susceptible to scratching. Even mild abrasives applied with little pressure can create microdefects on the surface of a plastic that add up over time and reduce clarity.

To address the issue of abrasion resistance in clear plastics, there are hard, transparent coatings commercially available. These coatings are often formulated from siloxane, which provides a unique suite of benefits for machine guard applications. After curing, the coating becomes exceptionally hard and abrasion resistant. The hard coat also provides UV resistance, which can ward off long-term yellowing in machines that are exposed to ultraviolet light. Siloxane exhibits excellent chemical resistance and can provide long-term protection from cleaners that can cause uncoated plastic to become cloudy or yellow.

Chemical attack is a reaction between a substance and the polymer chain of a plastic material. The polymer chains of clear plastics are considered amorphous in that they show very little order — imagine them like strands of spaghetti in a bowl, with no consistent pattern. Highly ordered polymer structures like polyethylene are considered chemically resistant in part because of how this order protects polymer chains from chemical attack. When the chains are attacked, they break apart and light begins to bounce off them at different angles. This light scattering is what causes plastics to cloud up and lose clarity after being exposed to aggressive chemicals.

As with abrasion resistance, a siloxane hard coat can boost chemical resistance, but there is one important caveat: While a coating protects the surface of a sheet, it leaves the edges vulnerable. It is highly recommended that the edges of a hard-coated machine guard be sealed in a watertight manner in order to avoid chemical absorption into the bulk of the plastic.

Chemical Resistance of Transparent Plastics

Corrosion on metal guards can be problematic for clean environments and can reduce visibility in perforated sheet metal. While plastics provide superior corrosion resistance compared to metals, there are limitations. Certain classes of chemicals are particularly aggressive toward plastics and should be avoided in all cases. Organic solvents like MEK (methyl ethyl ketone), THF (tetrahydrofuran), and methylene chloride will attack both uncoated plastic and siloxane hard coats even with minimal exposure and in low concentrations. Concentrated acids and dilute sodium hydroxide will also attack both the coating and the plastic, particularly when exposed for extended periods of time or at elevated temperatures.

Stress, like temperature, has a dramatic effect on the chemical resistance of plastic. Residual stress from processing or machining will increase the likelihood of chemical attack. To prolong the life of a plastic machine guard, avoid stress concentrators like countersunk screws, sharp angles, and fasteners very near the edge of a sheet.

Siloxane coatings resist common cleaners that would otherwise attack uncoated clear plastics. Oxidizing chemicals like hydrogen peroxide and bleach (sodium hypochlorite) are better managed by siloxane — the same is true of most acids and bases. Alcohols, greases, and disinfectants are all generally less reactive with siloxane compared to uncoated plastics.

PETG (polyethylene terephthalate glycol), a clear polymer, is considered more chemically resistant than acrylic or polycarbonate. PETG stands up to most oils and greases better than other clear plastics and will show less color shift over time. Clear PVC offers certain advantages over PETG in that it holds up very well to phosphoric acid and weakly basic solutions, but is a poor choice for strong chlorine-based cleaners.

Adapted from: Plastics in Medical Devices: Properties, Requirements, and Applications, Second Edition (p. 124), by Vinny R. Sastri, 2014, Oxford, England: Elsevier. Copyright 2014 by Elsevier Inc.

Key Takeaways

• In many cases, plastic can be an advantageous substitute. Plastic machine guards are lightweight and provide superior corrosion resistance compared to metal guards.
• Plastics are more impact resistant than glass and will provide persistent, long-term clarity when chemical attack and abrasion are addressed with a siloxane hard coat.
• Coatings protect the surface of a machine guard, but leave the sides vulnerable, so it is advised that a coated plastic be sealed with a watertight frame to avoid absorption through the edge.
• Stress in a plastic sheet greatly increases the likelihood for chemical attack.
• High heat will dramatically soften the material and reduce the stiffness. High temperatures will also reduce the chemical resistance of plastics and accelerate the loss of clarity from both chemical attack and abrasion.

Choosing the right plastic to use in a machine guard depends on the specific requirements and circumstances of the application.

This article provides general guidelines and is intended for informational purposes only. Because every situation is unique, many factors must be considered when selecting a material for something as important as a machine guard. It is the reader’s responsibility to conduct his or her own research and make his or her own determination regarding the suitability of specific products for any given application.