7 Factors to Consider When Choosing the Right Plastic for Injection Molding

The most flexible method for mold-based manufacturing with plastic involves injecting material into a mold. A much-used means to mass produce parts and products, injection molding of plastics uses relatively basic processes. Yet, with injection molding, plastics of all types are used to make various items of different sizes, shapes, and complexities. Though versatile, there are limits to the materials that can undergo the injection molding process. Manufacturers must carefully consider plastics of different types for various factors that make certain types more useful for particular applications or products.  

7 Things to Consider with Injection Molding of Plastics 

Designers, engineers, and others involved in developing new parts or products could go on ad nauseam about the many factors that influence the injection molding process. Plastics of certain types have qualities that make them uniquely superior for this manufacturing method, while other types are left wanting. For example, how a particular plastic responds to heat can affect how it reacts during injection molding. Plastics must also present traits that allow liquified resins to flow into a mold easily, with certain materials more suitable to produce intricately shaped components than others.

Yet there is more to consider beyond a specific plastic’s material properties. Manufacturers must contemplate the costs involved, with specific engineered polymers considerably more expensive. Cost may also determine whether a material is suitable for larger-scale production via injection molding. Plastics used today must also be environmentally friendly, as the public is increasingly aware of the importance of sustainability in manufacturing. Considering all these factors regarding injection molding, plastics used in the process should be chosen partly according to the following considerations.

1. Material Properties 

For injection molding, a plastic’s material properties help determine the best one for a particular application or product. These characteristics also establish the manufacturability of a part or product. This step is generally the first in the design phase, which involves establishing the most important traits of the materials used. Chemical and physical characteristics will also define the complexity of shapes and tolerances that can be achieved. For a manufacturer to engage a vendor for their injection molding needs, seeking expert opinions on the best materials to select is essential.

With injection molding, plastics should consider properties that include: 

• Chemical resistance: Many plastic components must withstand harsh chemicals to prevent them from deteriorating. This often involves injection molding plastics like epoxies or PTFE (polytetrafluorethylene), commonly known as Teflon.
• Flexural modulus: Measured in psi (pounds per square inch), this is a crucial indicator of flexibility, with lower flexural modulus indicating greater flexibility and higher flexural modulus representing increased stiffness; injection molding plastics with lower flexural modulus ratings include silicone and LDPE (low-density polyethylene), while those with higher flexural modulus ratings include epoxies and PEEK (polyetheretherketone). 
• Impact resistance: This trait relates to the durability of injection-mold plastics when exposed to a sudden force, such as being dropped; often, materials like phenolic resins and ABS (acrylonitrile butadiene styrene) are used when impacts are a concern.
• Mechanical strength: This characteristic determines how well a material can withstand external forces without breaking or bending. Injection-molding plastics like BMI (bismaleimide resin) and PA (polyamide—aka nylon) are commonly used for this purpose.
• Shrinkage rate: After injection molding, plastics cool and shrink to a certain extent, so manufacturers need to take this into account to ensure dimensional accuracy after a component is removed from the mold; though mitigating shrinkage can be done by adjusting material density, epoxies and PP (polypropylene) are two plastics used in injection molding processes with lower shrinkage rates.
• Thermal stability: Related to a plastic’s melting point, this property determines the molding temperatures that can be withstood without deformation; plastics used in injection molding with high thermal stability include PEEK and PPS (polyphenylene sulfide).
• UV resistance: UV resistance is especially important for products that will be used outdoors and in direct sunlight. PU (polyurethane) and ABS (acrylonitrile butadiene styrene) are both resistant to degradation from ultraviolet light.

These properties affect other characteristics of plastics used in injection molding. For example, the lower the melting point, the less viscous and more liquid a polymer becomes. Lower viscosity will augment the precision of a component coming out of a mold, but such plastics will not work well in high-heat environments. This, in turn, relates to the thermal stability of the material used for injection molding. Plastics that can better handle thermal stress tend to maintain their dimensional integrity when exposed to high heat.

Chemical resistance can apply to various materials, which can cause other materials to deteriorate. When applied to injection molding plastics, it explains how a particular resin responds to gases, liquids, oils, solids, or solvents, along with other environmental dynamics. Exposure to chemicals can also be mitigated by providing a part with specific surface finishes, which can protect the underlying polymer. Industry standards and government regulations also stipulate material properties like corrosion resistance, electrical resistance, flammability, heat conductivity, and tensile strength for specific applications.

Other properties may increase or limit a polymer’s manufacturability. For example, PEEK or POM (polyoxymethylene) are easily machinable, while the low melting points for PE (polyethylene) and PP make them excellent for welding parts. With good adhesive properties, epoxies, PC (polycarbonate), PU, and nylons work well for bonding components. Availability and cost are also factors in what materials manufacturers use to fabricate their products.

2. Thermal Aspects

Among the most essential material properties are the thermal aspects of a polymer, which directly affect the injection molding process. Plastics are distinctive, and each behaves differently when exposed to heat, a key part of molding. These facets include injection molding plastics’ melting points, heat deflection temperatures, thermal conductivity, thermal expansion, and glass transition temperatures. For example, melting points establish the heat at which a polymer is molded and the time it takes to cool.

With injection molding, plastics’ thermal qualities that matter include: 

• Melting point: Directly controlling viscosity during injection molding, a plastic’s melting point determines the temperature necessary during the process; UF (urea-formaldehyde) and LDPE are injection molding plastics with lower melting points at around 248°F to 320°F (120°C to 160°C) and 221°F to 239°F (105-115°C) respectively, whereas BMI and PEEK (polyetheretherketone) present higher melting points at about 572°F (300°C) and 649°F (343°C) respectively.
• Heat deflection temperature: This measures how well a material can maintain its shape when exposed to heat, with a higher temperature allowing a quicker molding process. Injection molding plastics with high heat deflection temperatures include BMI and PEEK, which can withstand temperatures of 572°F (300°C) and 482°F (250°C), respectively.
• Thermal conductivity: This trait affects how quickly material cools within a mold, with low thermal conductivity taking longer to cool and higher conductivity taking less time; plastics with lower thermal conductivity include phenolic and LDPE, whereas those with higher conductivity include epoxy composites reinforced with powders made from metal or carbon and PEEK.
• Thermal expansion: Temperature increases alter the density and size of a part, impacting its accuracy and stability as the material within the mold increases in area, length, or volume during injection molding; plastics with lower thermal expansion rates include epoxy composites reinforced with carbon fillers or fibers and PEEK.
• Glass transition temperature: Critical for amorphous thermoplastics to prevent them from deforming when heated, this transition point also impacts performance properties of thermosets.
• Thermal deformation temperature: Relates to the rate at which a material expands when exposed to rising temperatures and contracts to falling temperatures.
• Specific heat capacity determines how much energy is necessary to increase a material’s temperature. Plastics with low specific heat capacity increase in temperature more quickly than those with high specific heat capacity. 

Knowledge about these thermal aspects of the materials used for injection molding allows plastics to be chosen more logically. For example, low thermal conductivity in materials like ionic polymers makes them cool more slowly, so they must cool sufficiently to enhance the mold’s cooling effect. During injection molding, a plastic’s properties can be combined, affecting cooling rates, filling conditions, fluidity, and other aspects of fabrication.    

3. Flow Characteristics 

In terms of general material properties and thermal characteristics, the viscosity of a melted resin influences how well it flows when melted, affecting manufacturability. These factors then affect how complex a mold can be made, as flow characteristics affect how well-melted polymer will fill the mold’s cavity. Generally, this fluidity can be divided into three categories. For example, BMI, PEEK, and PVC typically have poor fluidity, so they are best for making simpler parts via injection molding. Plastics like PU, PC, and nylon can produce more complex components. Finally, when appropriately heated, phenolic, ABS, PP, and epoxies offer good fluidity for the most intricate parts.

Fluidity also changes based on a variety of molding factors. While a higher temperature increases fluidity during the injection molding process, plastics can differ greatly in their flow characteristics depending on the temperatures to which they’re exposed. The fluidity of plastics like PS (polystyrene), PP, PMMA (polymethyl methacrylate), PC, nylons, CA (cellulose acetate), or ABS will vary greatly depending on the temperature, for example. Conversely, plastics like PE or POM are largely unaffected by changes in temperature.

Pressure also affects fluidity and alters flow characteristics at the point of injection. Molding plastics like PE and POM are especially sensitive to pressure, which can be adjusted to control fluidity during production for these plastics. The layout, size, and shape of the molds’ structure also cause resistance to the flow of molten polymers and their fluidity within the mold. These, in turn, affect cavity shape, runner thickness, wall finish, and other structural aspects of a molded component. For these reasons, the mold design should consider the fluidity of the plastic to be used, along with other factors like injection pressure and speed, as well as material and mold temperatures during the injection molding process.

4. Component Design

The complexity, surface finish, wall thickness, and other elements of part design often affect the choice of materials used in injection molding. When molten, plastic contacts the inner wall of the mold’s cavity, cooling and forming a low-density shell around the still-molten material within. As plastic generally has poor thermal conductivity, the interior of a component cools more slowly, creating a higher-density solid layer with considerable shrinkage. As such, thick-walled plastic components will cool slowly and shrink more substantially.  

The properties of the polymers used to make components also affect how much and where they shrink, along with how material flows into the mold. Within the mold, the number and layout of inserts also direct where material flows, affecting the density of different sections of the part. The design features determine the best material, colors, size, textures, and other aspects of a plastic component. Threading and undercuts are other features that influence what type of material to use, especially when using over-molding processes in plastic injection molding. Overmolding is often used for toothbrushes, where a rubber handle is overmolded onto a plastic substrate.

The precision required often also influences what materials to use when making components via injection molding. Plastics with low shrinkage rates are preferred, for example, when fabricating gears from polymers, whereas for products that require parts with greater transparency, good optical qualities are most desirable. Nylons are often used by manufacturers who want smooth and silky surfaces, while thermoplastic polymers are best for matte finishes. Textured finishes can also be achieved after injection molding of plastics—the design of the component and the type of material used often impact texture.   

5. Cost-Efficiency

Cost is always a consideration in the manufacturing of parts. While specific polymers have desirable properties that make them more sought after, component designers sometimes need to compromise between cost and the required characteristics. Yet it’s also essential that the component designs and the polymers used don’t result in the need for additional reworking after injection molding. A plastic’s expense factors significantly into the choice of material for a project.

Manufacturers must also consider production volume, as raw materials bought in bulk are usually considerably less expensive. While mechanical and thermal properties must provide sufficient durability and other characteristics to the finished part or product, the cost of the material employed is a crucial consideration. Yet, though a cheaper option may be more desirable on the surface, if it results in substandard parts that fail prematurely, then using a pricier material may result in greater cost-efficiency. When making parts via injection molding, the plastics used must maintain a balance between cost and performance.

6. Volume of Production

The production volume also affects what materials manufacturers use when making a part or product by the process of injection molding. Plastics for smaller production runs may not be suitable for larger ones and vice versa. Additionally, manufacturability plays a role in the volume of parts that can be produced. A material that makes a component easier to process, with good flow characteristics, and presents quicker cooling times, will scale better than one that presents greater difficulties to production.

When it comes to injection molding plastics, it often depends more on the vendor providing services. In injection molding, plastics for longer production runs or larger volumes should be carefully chosen based on material availability to ensure production won’t be interrupted. When selecting a molding company as a vendor, choosing one sufficiently experienced for the manufacturer’s production goals is best.

While many plastic injection molding companies can provide production runs under ten thousand, it usually takes a more experienced partner to increase volume beyond this. While smaller producers may work well for prototypes or products produced in smaller quantities, bulk orders of parts in the hundreds of thousands or even millions must have sufficient capabilities. Additionally, it’s important to choose a partner that can provide the capabilities to handle injection molding.

7. Compliance 

Compliance is a key part of any business today, driven by local and state governments, along with federal agencies like the EPA (Environmental Protection Agency) and FDA (Food and Drug Administration). International standards organizations like the ISO (International Organization for Standardization) also play a part. Safety and sustainability are key elements in modern manufacturing regulations and standards that can negatively affect a manufacturer’s reputation, and result in financial penalties.

Regulatory requirements and industry standards also come into play when choosing materials for injection molding. Plastics like polypropylene, for example, can be made for either general industrial applications or specifically for food processing. Though both have the same properties and chemical makeup, their purity differs. Food-grade propylene must meet rigorous FDA standards to ensure it contains no dangerous contaminants, whereas industrial-grade propylene needn’t follow similar requirements.  

Environmental regulations also promote materials that are suitable for recycling and reuse. For example, due to their reduced impact on the environment, plastics like high-impact polystyrene (HIPS) or PP are often used for injection molding. Plastics manufacturers are increasingly seeking to lower energy consumption, minimize emissions, and reduce waste throughout the lifecycle of injected molded parts. To achieve greater efficiency, decrease waste, and maintain compliance, plastic makers continue to innovate.

To learn more about injection molding plastics and our capabilities, contact the experts at Spaulding Composites today!