Many product designers think of material selection as an exercise to be engaged after most aspects of the design have been finalized. However, for any structural and display product, material selection is the decision upon which all others are contingent – and choosing incorrectly at the prototyping stage not only wastes time, but subsequently misleads every engineering decision to follow.
The form-first trap that undermines prototyping
When seeking to develop a scale model to verify proportions or present the concept to a client, a designer’s first impulse may be to send the request to the 3D printing department to produce it with PLA or ABS filament. However, while useful in validating approximate proportions, using a readily accessible prototyping method may lead to decisions that are inappropriate for the finalized product. A part printed on an FDM printer will flex and scratch in different ways than cast acrylic that has been CNC routed for the commercial product. By the time a designer gets feedback on loading performance or bonded joint performance, iteration has already been delayed needlessly by inappropriate assumptions about material performance.
Beyond appearance, optical clarity can be a requirement
In applications calling for retail cases, museum display cabinets, or exhibition stands, optical clarity is not merely a bonus, but an essential requirement. While standard silica glass offers 90% light transmission with a greenish tint that changes the appearance of objects at a distance, higher-end polymethyl methacrylate acrylic offers better light output (92%) and no color shift
Acrylic’s 1.49 refractive index compared to 1.50 for silica glass means the two materials bend light in nearly the same way, making acrylic appear optically “clear” in much the same way as glass. Consulting a fabricator such as Productive Plasticsearly on can help highlight these sorts of considerations, as acrylic’s 17x impact resistance (vs. equivalent thickness silica glass) and 50% lower density make it the preferred material for custom acrylic cases in high-traffic environments or for the safe display of valuable items.
These considerations are not apparent when working with a 3D-printed prototype. The choice to use optically clear acrylic over some lower-cost option or polycarbonate will require upfront consideration, as will the choice of bonding method and edge-finishing technique. Similarly, the choice between acrylic and polycarbonate is not obvious to someone familiar only with prototyping techniques – and yet has enormous implications on downstream processing.
Acrylic vs. polycarbonate: choosing the right material
Designers that recognize the need for a transparent thermoplastic in their design may gravitate towards polycarbonate, since they are often sold as inherently tougher alternatives to acrylics. And yet, few polycarbonate applications actually require the material’s unique “impact-resistant” qualities – and those that do must be prepared to make concessions elsewhere.
The primary benefit of polycarbonate is its extreme resistance to impact without sacrificing light transmission. A polycarbonate sheet or rod will resist breaking under impact in a way that acrylic cannot – making it a natural choice for safety glass and similar applications. However, polycarbonate is far easier to scratch than acrylic, and will yellow drastically under prolonged UV exposure – a problem in any display under glass or with LED lighting. Polycarbonate also presents challenges in achieving a consistently good edge finish during machining, and is more challenging to bond with solvents.
By contrast, acrylic is generally far easier to machine – cast acrylic, in particular, can be machined to excellent finishes with standard CNC routing and laser cutting equipment. Proper tooling geometry keeps heat – and therefore deformation – to a minimum, while flame polishing and, for the most detail-oriented applications, diamond polishing take the sharp edges off to create a display-quality edge. Similarly, acrylic is relatively resistant to UV-induced yellowing compared to most other plastics.
There is a direct trade between the two plastics, such that the choice between acrylic and polycarbonate is generally simple: choose polycarbonate for applications calling for impact resistance and acrylic for applications calling for long-term optical quality. For protective applications, polycarbonate has few viable competitors. By contrast, acrylic is generally the best choice for display fabrication, signage, and similar applications. For general-purpose protective displays, polycarbonate is generally preferable; similarly, for shelving displays even outdoors, acrylic can be used with confidence. If in doubt, there are two simple tests that can narrow things down.
Crazing, chemical exposure, and the bonding challenge
Crazing represents a severe point of failure for acrylic fabricators and designers attempting to use solvents and bonding methods unsuitable to the material. In most cases, it is the result of using an incompatible material in an application where a solvent is used to assemble or finish the acrylic display.
Crazing takes place when a chemical solvent or adhesive attacks the acrylic polymer, causing stresses within the sheet leading to a network of fine cracks on the surface of the material. Practical ways to prevent crazing can be demonstrated using common bonding and cleaning practices. A fabricator uses a solvent cement to bond two acrylic panels together with mild tension. The resulting joint appears fine, and is ready for installation – until six weeks later, when a network of cracks appears at the joint. Or, a retail display is cleaned with an ammonia-based glass cleaner, causing the surfaces to turn cloudy and crack – rendering the display unusable.
Most displays fabricated with 3D-printed prototyping materials would never encounter these issues. In most cases, materials compatibility is easy to test ahead of production if the materials selection stage takes place concurrently with production planning. Cast acrylic and extruded acrylic (see below) react differently to solvents, which is another reason to specify which type of acrylic to use ahead of production. Extruded acrylic sheet tends to suffer more from crazing than cast acrylic sheet due to internal stresses from its manufacturing process.
Thermal expansion and environmental tolerances
Thermoplastics, including acrylic and polycarbonate, typically exhibit linear expansion coefficients an order of magnitude higher than metals. Put simply, thermoplastics are far more likely to expand and contract with changes in ambient temperature than steel or aluminum. This is generally less of a concern for static displays in a temperature-controlled environment, but becomes relevant for an outdoor display or a point-of-purchase case near an HVAC duct. If the display uses bonded joints and no mechanical fasteners to keep its panels together, the panels will either warp as they expand, or the bonds will fail entirely as the materials react differently to the changing temperature.
It is generally advisable to include expansion gaps in the design phase for any acrylic display case that is subject to moderate thermal variation. These gaps help prevent the warping or cracking of acrylic sheet panels, and their inclusion depends on the linear expansion coefficient of the material selected. This, in turn, may dictate the choice of assembly method. Mechanical fasteners that keep panels at a fixed distance relative to one another tend to resist thermal expansion forces by transferring the stress to the fastener itself. By contrast, adhesives typically allow panels to move slightly relative to one another, reducing the risk of delamination. Few fabrication shops are aware of these nuances until they get their prototype in production-grade material, which is yet another reason why FDM prototyping can waste time and money.
Compatibility with manufacturing methods
Both extruded and cast acrylic products are made from the same plastics material – PMMA – but the manufacturing process imparts different characteristics to the final sheet. Cast acrylic is created by curing liquid monomer in a casting frame, while extruded acrylic is formed by pushing molten plastic through a die. The differences in manufacturing are important to consider when selecting a material. Casting acrylic has a more uniform polymer structure with greater tensile strength than extruded acrylic, a property that becomes critical in thermoforming applications. Meanwhile, extruded acrylic tends to yellow sooner under UV light than its cast counterpart.
The differences also affect machining characteristics, particularly for laser cutting. Extruded acrylic is generally easier to cut using laser methods, but the material is more likely to deform under heat. CNC routing can also be affected by the differences between the two materials if the cutting parameters are not adjusted, with extruded acrylic presenting additional challenges during cutting due to internal stresses. A prototype made from extruded acrylic would have visibly different edge quality from a production part made from cast acrylic sheet. The grade of material specified also affects the final appearance of polished edges; flame polishing and diamond polishing respond differently to extruded acrylic, and cast acrylic offers a better degree of optical quality.
For shelving applications or other applications requiring CNC routing or laser cutting, the choice between extruded and cast acrylic is important to specify. Both materials offer reasonable performance, but cast acrylic’s superior tensile strength can be valuable in applications such as thermoforming.
Structural load capacity and shelf deflection
Using shelves within a display case or cabinet introduces the risk of shelf deflection – the risk that the shelf will bend under the force of the products placed on it. The magnitude of this risk is governed by a combination of factors, including the shelf material, thickness and length, and the mass of the objects on the shelf. Acrylic shelves will deflect more than glass shelves for the same thickness, and a given shelf material will deflect more with greater length and mass, and less with greater thickness. Adding support channels to the back of the shelf or increasing the thickness can reduce the likelihood of shelf bending.
Estimating the risk of shelf deflection is a matter of applying basic principles from beam bending physics. By inputting the mass of the objects on the shelf, the length of the unsupported span, and the flexural modulus of a given material, it is possible to estimate the expected deflection of a shelf. For display cases carrying particularly heavy products, artifacts, or samples, these calculations may need to be performed ahead of design finalization. The risk of shelf sag is rarely apparent in a 3D-printed prototype, as such prototypes rarely incorporate mass simulation or use a material with similar stiffness characteristics to production-grade acrylic. A shelf that appears fine in a prototype can prove completely inadequate in a production unit, undermining the visual quality of the display case.
The cost of under-specification
One way for material selection to go wrong is for it to underestimate a material’s performance requirements, leading to unanticipated failures downstream. A less obvious but potentially far more expensive error is to over-specify a material, choosing a higher grade engineering plastic when the situation does not require it. Choosing polycarbonate with aerospace-grade impact resistance for a retail display is unlikely to provide much value in terms of performance. Likewise, a UL94-rated fire-retardant plastic may represent an unneeded expense for a display in a low-risk environment. Flame retardancy is a common area of materials mis-specification in the display industry, particularly among contractors working in commercial environments.
UL ratings for flame retardancy exist to help specify materials in commercial environments where strict safety standards are in place. If one is working in a jurisdiction with commercial building codes, there may be need to use materials rated for commercial use – even for display cases that do not directly involve a risk of fire. If the worksite does not have commercial building codes, ordinary optical-grade acrylic may be entirely sufficient.
The wrong materials specification can increase costs, complicate supply chains, and in extreme cases, dictate the choice of manufacturing method – thereby increasing tooling and die costs for any given production run. It is important to make sensible decisions when it comes to materials selection, avoiding both conservatism and unneeded risk.
Getting these decisions right at the prototyping stage, taking into account production-grade materials, structural mechanics, and manufacturing tolerances, is what makes for displays that endure – rather than those that get replaced. The earlier one engages these considerations, the lower the cost of iteration and correction. Once fabrication begins, errors in materials specification can be exceptionally costly.
