Now more than ever, manufacturers have more choice of plastics. Likewise, the possibilities of products made with plastics are endless. Consumer goods, vehicle parts machines, cosmetics food, healthcare, and more. Plastics seem to have found their way into all areas of our lives. Many of us are aware of the 7 recycle signs. Those are only a small fraction of the number of plastics that exist. The plastic number 7 which indicates “other” contains more types of plastics than 1 to 6 put together. Nylons, polymethyl methacrylates, polyurethanes, acetyl butyl styrene, polyvinyl acetate, polyacrylonitrile. These are some examples of other plastics. For any product, you can have more than one plastics. Those that are suitable candidates for making a product. To maintain quality and efficiency in manufacturing there need to be static processes. This includes everything from how you sort orders from suppliers. To how you make choices of materials. Here, we look at how to choose the right plastic for injection molding.
Is the Product Injection Moldable?
Say your company gets approached by a startup that has a new product idea. They want the product made out of plastic and they want to use injection molding. How do you begin with the client?. Well, first you want to be sure the product is injection molding compatible in the first place. Or does it even need to be injection molded? If the product has a uniform cross-section extrusion might work. This can be extrusion with a post-processing stage like laser cutting. One sign is the product has a noncontinuous complex solid shape another is if it can fit in a mold. Then injection molding is ideal.
Although your concern in injection molding is how the product gets made. You also need to consider how the product gets used. A material might be good for creating a part but does not serve the function of the part. Similarly, plastic might be suitable for a product function. But the plastic choice makes it very difficult to injection mold. You can avoid either case by taking the right approach to select the best material for your part. You might then come down to a plastic that suits your product. But you then have to find a way to manage issues like corrosion or heat sensitivity.
Product Function
Whenever someone tells you they have a product idea. The first thing that comes to mind is what does it do?. Knowing what the product does gives you an idea of a few important things. Like will it get exposed to high temperature, mechanical stress, or harsh chemicals? This helps you decide what class of plastics you want. First, are you going to need a thermoplastic or a thermoset?. In some cases, you might need a combination. Commodity, engineering, or high performance/specialty plastics.
Thermoplastics are the preferred choice for injection molding. For many reasons such as recyclability and ease of processing. So where a product can get injection molded using a thermoplastic, go for that. High flexible products for long have necessitated the need for thermoset elastomers. Today you have the option of thermoplastic elastomers. So that your part needs to be very flexible does not remove the option of using thermoplastics. There are also different grades of TPEs from food grade to high-performance TPEs.
Commodity plastics get used in everyday consumer products. Examples are polystyrene coffee cups, polypropylene takeaway bowls, and high-density polyethylene bottle caps. They are cheaper and more available. Engineering plastics get used in, as the name implies, engineering applications. You’ll find them in greenhouses, roofing sheets, and equipment. Examples are polyamides (Nylon), polycarbonate (PC), and acrylonitrile butadiene styrene (ABS). They can withstand more harsh environmental conditions. They’ll withstand load and temperatures well above room temperatures. High-performance plastics perform well under conditions where commodities and engineering plastics fail. Examples of high-performance plastics are polyethylene ether ketone, polytetrafluoroethylene, and polyphenylene sulfide. Also known as PEEK, PTFE, and PPS. They find use in high-end applications such as aerospace, medical devices, and gears. High performance is more expensive than a commodity or engineering plastics. The properties of the plastics help you decide which suits a particular application. For example, some applications demand strong but lightweight materials. For this, you compare their density and tensile strength.
Comparing the properties of example commodity, engineering, and high-performance plastic
| Plastic | Density | Ultimate Tensile stress | Category |
| Polypropylene (PP | 0.92 g/cm3 | 40 MPa | Commodity plastic |
| Polycarbonate (PC) | 1.22 g/cm3 | 75 MPa | Engineering plastic |
| Nylon 66 (PA66) | 1.31 g/cm3 | 93 MPa | High-Performance Plastic |
So once you know the function of the product you can decide which class of plastics you’re looking at. This narrows your choices down by a bit. You then have to choose from amongst the range of plastics within this group. Some plastic products can use both engineering and commodity plastics. For example, if you are designing a particular type of plastic furniture. Polypropylene gets used in plastic furniture so do nylon and polycarbonate. All these plastics can be injection molded. This is where you then look at other details of the product.
Here are some common plastics and their features in the table below.
| Plastic | Desirable features | Short falls | Example applications |
| Polycarbonate (PC) | High impact strength, low shrinkage, good optical properties, heat resistance and dimensionally stable | Low resistance to chemicals, prone to formation of bubbles and sinks | High impact glass, lenses, lightings, mobile phone cases, carports medical devices, babycare products |
| Polyether ether ketone (PEEK) | High performance, high chemical resistance, heat and flame resistance, | Expensive and not stocked by as many suppliers | Gears, bearings, automotive parts, high vacuum and insulations |
| Polystyrene | Low cost commodity plastic with good optical clarity, good resistant to gamma radiation. | Brittle, low UV resistance and prone to degradation by organic solvents. | Coffee cups, egg crates, packaging, containers amongst many other consumer products |
| Acrylonitrile Butadiene Styrene (ABS | High resilient and high impact plastic with low shrinkage. Good chemical resistance and low cost. | Not very high temperature resistance. Parts with high thickness are prone to voids and knit lines. | Lego pieces, electricals, computer and telephone components. Often blended with PC. |
| Polyphenylsulfone (PPSU) | Exceptionally resistant plastics. Resistant to both acids and alkali. Low shrinkage, heat and radiation resistant. | Thick parts are likely to have bubbles and voids in injection molding. Less resistant to organic solvents and organic chemicals. Some grades are difficult to color. | Medical appliances, hot water pipe fittings, automotive and aircraft parts |
| Polyamides (Nylons or PA) | High strength, chemical and temperature resistance. Resistant to abrasion and good surface gloss. | Degrades in at low and high pH. Sharp glass transition temperature. | Gears, screws, bearings, pipes, automotive parts, combs and furnitures. |
| Polyoxymethylene (POM) | Low cost commodity lastic. Flexible, resistant to high and low pH. Good wear and impact resistance. | High shrinkage and tendency to warp. Prone to bubbles in parts with high thickness. | Bottle screw caps, food bowls, shampoo bottles and utensils. |
| Polyetherimide (PEI) | High performance, heat and temperature resistant, low shrinkage and good dimensional stability. | High cost (although costs less PEEK) | Electrical appliance parts, medical instruments, tableware and lightings |
| Polybutylene terephthalate | Good strength and flexibility. Resistants so a wide range of fluids like solvents, fuels and oils. Non reactive and non absorbent | Low resistance to acids and alkali. Likely to show warpage when glass filled. Not great for parts with thin walls or low thickness. | Casings and nozzles for heating appliances, coffee machines, gears and bearings, handles for electric cooking appliances. |
| High density polyethylene | Flexible and lightweight. Low melting point makes it easy to injection mold. Low cost | Low temperature and impact resistance. | Bottle caps, packaging sheets, toys and handles |
Target Niche
You have now narrowed down to the plastics that suit the function of your product. You also know that all these plastics are injection moldable. You now look at who you are producing for. For example in molding a plastic toy cart. You want the cart to be big enough to fit in whatever a toddler packs in a toy cart. You also want it to be light enough so the toddler can move it. The plastic must also be strong enough to take falls and abrasions which are likely. Or if it’s a plastic chair you’re looking at what age group the chair meant for. This relates to the load the chair would be born.
Availability
Chances are that the product will get made all year round. So you want to use a plastic that you can source. Changing the plastic at any point will need changes to your process settings. It might also affect product function and appearance. When the choice comes down to it, choose a plastic that has a reliable supply. Making a product out of a material that is not very available can lead to downtime. When the machine is not producing you risk not meeting demands. You might also have full-time workers who you are paying to wait.
Regulations
Regulatory bodies and standard organizations like the FDA and ISO have a say here too. Certain products get approved for use in medical devices while others are not. The different reasons range from being sterilizable to dangers of toxins. For example, you can have polypropylene from the same company. But one is food grade and one is for general purpose. This has to do with the manufacturing conditions of the pellets and the additives used. Both materials would have the same chemistry and physical properties. The regulatory body you follow depends on your region. The respective agencies publish the regulations for different products.
Additives, Blends, and Composites
Where you desire to use a particular type of plastics and the properties do not meet the need. In such a case you have the option of including additives. Many of the plastic products we use have additives in them. These can be colors, plasticizers, antioxidants, antibacterial, stabilizers, and others. PVC is one of the plastics where additives make a lot of difference in how it gets used. PVC can go from the rigid plastic used in roofing sheets to the flexible PVC sheets used in file binders.
Other than the use of additives, different plastics also get blended. The blends combine the properties of the two plastics. This results in a superior material that performs better than either on its own. PVC and ABS are a common type of blends used in electric socket casings. You can also have blends of biodegradable plastics. For example polylactic acid (PLA) with synthetic polymers like polyethylene glycol. This gives a more flexible PLA that is more useful.
Composites are also another option. These are a combination of plastics and other materials. The plastic serves as the matrix. The fibers, particles, or powders are the dispersed phases or fillers. The fibers can be long or short. The powders and particles can have varying sizes from nanometers to millimeters. Examples are nylon filled with glass fibers. These filled plastics have superior properties to the unfilled plastics. So when you get limited to a particular type of plastic, you can tune the properties in many ways.
Design Features
The design features of a part go a long way in determining what type of material gets used. The texture, color, and size are determinants. Other features such as undercuts and threads also affect the materials used. For example, if the part needs to get made by overmolding. You then need to determine what material to use as the overmold and which as the substrate. The level of precisions also determines the material that gets chosen. For example, if you are making plastic gears you don’t want a material with high shrinkage. A material that has a high shrinkage makes mold design to a precision more challenging. If the material needs to be clear, you want plastics with good optical properties. Those that are not likely to yellow during processing or get hazy. The texture of the product is also an important design feature. Plastics like nylon can achieve a smooth silky surface feel. For more matte finish you might go for materials like TPE. Different textures can also get achieved by post-processing. The mold design can also impact texture on the part by design. The selection of plastic for a particular texture is thus based on your facility.
Expected Usage Period and Disposal/Recycle
How long the product is going to get used for is an important factor. This determines factors like how strong it needs to be. Also for how long it will get exposed to a certain environment. This of course relates to the function. For plastic which is getting used for a long time consider wearing resistant plastics like nylon. This way the part retains gloss and integrity over a long period.
Another important consideration is how the plastic is going to get disposed of. Today there is more demand for more materials that are better for the environment. Plastics like PVC get replaced by easier to recycle options like polypropylene. Products that once got made using thermosets are now made with thermoplastic elastomers. Manufacturers in the modern age now have to take into consideration the end of life of the product. One approach taken by one drinks company is to make its bottles clear. This simple choice increases the value of plastic when recycled. The color is an added component and affects the property of the plastic when reheated. The clear plastic is also easier to recolor in its next life.
The recyclability of the plastic is also important during its processing. Where you get scraps or rejected parts. The percentage of the scraps allowed in every product depends on the regulatory body. Plastics like polyethylene and polypropylene are easier to recycle than others. PVC is not easy to recycle due to the risk of chlorine release. There are also heat-sensitive plastics that lose a lot of their properties when recycled. Nylon for example does not withstand prolonged heating. So when choosing material for a plastic part also think about how the product will get disposed of. Or even meter how it gets recycled.
Here’s a checklist of a few items to consider when deciding on a material for injection molded parts.
Conclusion
There are several factors to consider when choosing the right material for a part. The material has to be fit for function and also fit for production. Material selection requires experience and in-depth knowledge of plastics and injection molding. At Cavity Mold, we have a team ready to help you decide the right plastic for your product.
