Res Solutions: Selecting The Right Materials For Your Parts

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Figure 1: Protomold offers a number of different materials that possess varying degrees of strength, flexibility, resistance, lubricity and other benefits.

There are more than 85,000 commercial options for plastic materials listed in materials databases, and within those, there are 45 polymer families which can be broadly classified into two categories: thermosets and thermoplastics. We primarily focus on the latter category, offering hundreds of stocked engineering-grade thermoplastic resins through our Protomold Injection Molding service (see Figure 1). In addition to our stocked supply of resins, we have the capabilities to support many more customer-supplied resins as well.
Narrowing down that extensive list of materials can sometimes seem like a formidable task, so we’ve prepared a reference guide that outlines the benefits and applications of some of our most commonly molded materials. By choosing the right material, you can improve the form, fit and function of your parts — a resolution we can all strive for in the new year.

ABS (Acrylonitrile Butadiene Styrene)

Benefits: ABS is a tough, impact-resistant plastic that is widely used across many different industries. It has low shrink and high dimensional stability, and it has good resistance to acids and bases. ABS is a good choice for handheld consumer devices. It’s also relatively inexpensive.
Applications: cosmetic parts, handheld devices, housings and moldings for electrical tools, remote controls, computers, telephone components.
Considerations: ABS-molded parts will show knit lines and can have sink and voids in thick areas. Reduction of sink can possibly be had by switching to an ABS/PC-blended material. ABS also mates well to PC parts.

PC (Polycarbonate)

Benefits: PC is strong and extremely impact resistant with low shrink and good dimensional stability. It’s a transparent plastic that is available in optically clear grades. PC has good heat resistance and accepts high cosmetic finishes well.
Applications: lenses, indoor and outdoor lighting, cell phone housings, electrical components, medical devices, bulletproof glass.
Considerations: There can be sensitivities in thick sections of parts that may cause voids, bubbles and sink, and PC parts possess poor chemical resistance. An ABS/PC blend is a good alternative for opaque parts with these issues, and Acrylic (outlined below) is another option for parts with thick geometries.

PPA (Aliphatic Polyamides)

Benefits: There are many types of PPAs, or Nylons (4, 6/6, 6, 6/10, 6/12, 12, etc.), each with its own advantages. Generally, Nylon parts provide high strength and high-temperature strength, especially when reinforced, and are chemically resistant, except to strong acids and bases. Some, like Nylon 6/6, offer great hardness and stiffness, and are abrasion resistant. Nylon 6 is extremely hard and tough at low temperatures, but Nylon 6/12 has the better impact strength.
Applications: thin-walled features, combs, spools, gears and bearings, screws, structural parts (with glass), pump parts, under-hood components, cameras.
Considerations: Nylon can be more susceptible to warp due to non-linear shrink, so keep that in mind when choosing a specific type of Nylon. If your part will be exposed to moisture, you should consider avoiding Nylon as it is a hygroscopic material that will absorb moisture, causing dimensional and potentially structural issues.

POM (Polyoxymethylene)

Benefits: Commonly called Acetal, it is characterized by high toughness, stiffness, hardness and strength. Acetal has good lubricity and resistance to hydrocarbons and organic solvents. It also has good elasticity and is slippery, so it works well for bearing surfaces and gears.
Applications: gears, pumps and pump impellers, conveyor links, soap dispensers, fan and blower blades, automotive switches, electrical switch components, buttons and knobs.
Considerations: Due to shrink, customers must specifically design for Acetal with uniform wall thickness. Painting and coating Acetal parts is difficult because of their lubricity. It’s also difficult to achieve a high-cosmetic finish using Acetal. If a limited number of prototype parts are needed, it can easily be machined through Firstcut.

PMMA (Polymethyl Methacrylate)

Benefits: PMMA, aka Acrylic, has good optical properties, high gloss and is scratch resistant. Acrylic also has low shrink and has less sink in geometries with thin and think sections.
Applications: light pipes, lenses, light shades, optical fibers, signs.
Considerations: Acrylic can be brittle and thus more susceptible to stress cracking. PC is a good alternative, if needed. Draft is always required in Acrylic parts due to its brittleness, often times twice the draft of other materials. Acrylic also has poor chemical resistance.

PP (Polypropylene)

Benefits: PP is an inexpensive resin option with higher impact resistance in some grades; propylene homopolymer can be brittle in cold temperatures, with copolymers more resistant to impact. PP is wear-resistant, flexible and can have very high elongation. It’s also resistant to acids and bases.
Applications: integral hinges or living hinges, fans, snapover lids (e.g. shampoo bottle tops), medical pipette tubing.
Considerations: Thick sections in part geometry can void and show bubbles. Shrink and warp may also occur in PP parts. If the part has living hinges that require higher stiffness, K-Resin is a good alternative.

PBT (Polybutylene Terephthalate)

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