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How to choose the right injection molding material?
According to incomplete statistics, there are currently 45 polymer series in the material library, as many as 85,000 kinds of plastics, which can be roughly divided into two categories: thermosetting plastics and thermoplastics. Injection molding is one of the most common methods of manufacturing high-volume parts, and choosing a suitable material can seem like a daunting task. This requires a detailed understanding of the material and must refer to the purpose, efficiency and cost of making the part. Surely, certain materials may be more suitable, but there is no “one size fits all” when it comes to injection molding. By choosing the right material, you can improve the form, fit and function of your parts. In the final analysis, the material chosen is always directly related to the application of the part.
1. The difference between thermosetting plastics and thermoplastics
The key difference between thermosets and thermoplastics is that they react to heat with different results.
Thermosetting plastic. This material increases in strength when heated or exposed to high temperatures; for example, thermoset plastic products retain their overall strength and shape when exposed or exposed to higher temperatures. This characteristic decision is beneficial when manufacturing large permanent parts and assemblies. They stand up well to more uses and extreme conditions. Thermosets also have significant disadvantages. Once heated, their internal structure changes and cannot be reshaped, making reuse impossible. The high melting point of thermoset plastics is not suitable for injection molding process. Also, not all thermosets have the same melting point. Each material reacts differently to heat, so for a certain type of thermosetting plastic, a special machine may be required, which is generally not universal.
Thermoplastics.Thermoplastics are recyclable materials that can be reused without changing their chemical structure after repeated heating and cooling. For injection molding, thermoplastics have the advantage of having a relatively low melting point, making them more suitable for large-scale manufacturing of plastic parts.
* Thermoplastics generally cost more than thermosets.
2. Factors to be considered when selecting materials
To find the right material for your plastic part, it may be helpful to think backwards. Be ready to answer these questions: What will the part be used for? What kind of pressure will the part under? Are they working in a harsh environment? Is assembly complicated? Prioritizing the following factors can help in selecting the right material:
Installation position: The installation position of the parts is a factor that must be considered.
In the sun, outdoors, in a humid environment, or installed inside or outside the module?
Temperature: Does the part work in a cold refrigerator, or in a high temperature environment such as room temperature or under the hood?
Life cycle: Is the average working time of parts 5 years, 10 years, or longer?
Warranty: Especially in the automotive industry, it is necessary to consider that the parts may be damaged after a few years. How much will the repair cost?
Cost constraints: Commercial-grade plastics such as high-density polyethylene or polypropylene, usually have high density, low heat and relatively cheap. The other is engineering plastics, such as PEEK, PEI and other materials, which are high temperature resistant and very hard, but the cost is relatively high.
Appearance requirements: whether the part needs texture, how high the surface finish needs to be, whether colored parts are more suitable than transparent parts, etc.
After you figure out the initial question, you can exclude most of the material, but you still have to consider the next series of questions to further narrow down the material:
Design function: Considering the mechanical properties of the part, do you need functions such as flexibility, compressibility, and adhesion? Does it require tensile strength? What are the impact toughness or electrical insulation requirements of the part? Does the material require bonded inserts, such as multi-material overmolding or insert molding? Part weight is also an important consideration.
Environmental factors: What operating environment will the part be exposed to? Will it be exposed to chemicals, does it need to be flame retardant, what are the UV requirements?
Compliance: For some specific industries, parts require material access thresholds. Does your part need food certification, does it need to be FDA compliant? Or does it require medical grade, ISO, electrical compliance, etc.?
Differences Between Thermosets and Thermoplastics
| Thermosetting Polymers | Thermoplastics Polymers |
| Can be heated and molded once only | Can be heated and molded several times |
| They harden during their formation and do not soften when heated | They soften when heated (sometimes liquefy) |
| They do not melt. They can withstand high temperatures. | When heat is applied, they turn white, regaining their hardness upon cooling. |
| Up to 5 minutes to stabilize | Stabilization can be achieved in 10 seconds |
| High thermal and chemical resistance. | Good mechanical properties and easy processing. |
| Insoluble. | Insoluble in organic solvents. |
| Examples: silicones, some types of polyester and phenolic materials. | Examples: polyethylene, polypropylene, polystyrene, polyvinyl chloride. |
3. Advantages and applications of commonly used thermoplastic materials
A professional injection molding factory usually provides dozens of engineering-grade plastics in stock, and must also support more special materials requested by customers. The advantages and applications of commonly used thermoplastics are summarized based on the inventory of materials in stock by DDPROTOTYPE, an injection molding supplier in China.
① ABS (Acrylonitrile Butadiene Styrene).
Advantages: ABS is a tough, impact-resistant plastic with low shrinkage, stable dimensions, and excellent acid and alkali resistance, and is widely used in different fields. The price of this material is relatively cheap.
Application fields: Including but not limited to electronic products, remote controls, computers, telephones, cosmetics, handheld devices, casings, etc.
Precautions: Injection molded parts made from ABS will show weld lines and may have depressions and voids in some areas where it is thicker. Fortunately, ABS can be mixed with PC, and the improved material can greatly solve the defects.
②ABS/PC
Advantages: ABS/PC hybrid material has both the strength and heat resistance of polycarbonate and the flexibility and dimensional stability of ABS, which is a material with excellent mechanical properties. This material has higher heat resistance than ABS. At low temperatures, this material has a higher impact resistance than PC.
Applications: These hybrid materials are common in the automotive, electronics and telecommunications industries, among other industries.
Precautions: ABS/PC material maximizes the defects when molding with a single material, such as thick molding problems. In the case of choosing excellent mechanical properties and wanting to reduce costs, this hybrid material can be selected.
③PC (polycarbonate)
Advantages: PC is a transparent plastic with an optically clear grade, high strength, extremely impact resistance, low shrinkage, and good dimensional stability. In addition, PC has good heat resistance, and the surface finish of the machined parts is very high.
Applications: Including but not limited to lenses, lights, mobile phone casings, electronic components, medical equipment, bulletproof glass, etc.
Precautions: PC makes thicker parts, there may be voids, air bubbles or depressions. In addition, the chemical resistance of PC parts is relatively poor. ABS/PC blend material is a good substitute for PC and can solve some defects, but the manufactured parts are opaque.
④PA or PPA (Aliphatic Polyamide)
Advantages: PA is an engineering plastic with excellent performance. It has excellent mechanical properties, outstanding corrosion resistance, oil resistance, heat resistance, etc., and its reinforcement and flame-retardant modification can significantly improve its heat resistance. properties, stability and flame retardancy. Nylon comes in many varieties (4, 6/6, 6, 6/10, 6/12, 12, etc.). Each material has its own advantages. Nylon has high strength and high-temperature strength, excellent chemical resistance. For example, nylon 6/6 has high strength and hardness and is very wear-resistant. Nylon 6 is very stiff and tough at low temperatures. Nylon 6/12 has better impact resistance.
Applications: Including but not limited to parts with thin-walled features, shafts, gears and bearings, screws, pumps, guides, etc.
Precautions: Nylon is easy to deform, which is common knowledge. In some specific environments, such as inside a humid refrigerator, nylon parts are generally avoided. Because nylon is a water-absorbing material, it can cause changes in the size and structure of the part and damage it.
⑤POM (Polyoxymethylene)
Advantages: With toughness, stiffness, hardness and strength, it is very hard compared to other plastics. At the same time, it has good lubricity and resistance to organic solvents, and also has good elasticity. Therefore, this plastic is very suitable for the manufacture of bearing surfaces and gears.
Applications: Including but not limited to gears, pumps, impellers, blades, conveyor chains, fans, switch components, buttons and knobs, etc.
Precautions: Due to the shrinkage of POM, it is necessary to design a uniform wall thickness when manufacturing parts. Because of its lubricity, it is difficult to paint or coat it, and it is also difficult to achieve an aesthetic effect.
⑥PMMA (Polymethyl Methacrylate)
Advantages: Also known as acrylic, it is also a transparent plastic with good optical properties, surface finish, scratch resistance, and low shrinkage.
Application: Including but not limited to lens, light pipe, lens, lampshade, optical fiber, Logo, etc.
Precautions: PMMA is relatively brittle, easy to crack after applying force, and has poor chemical resistance.
⑦PP (polypropylene)
Advantages: PP has a good formability, good surface stiffness and scratch resistance. It is a low-cost plastic with good impact resistance, wear resistance, very toughness, good elongation, acid resistance alkali.
Applications: Including but not limited to hinges, fans, bottle caps, medical pipettes, etc.
Precautions: PP will become brittle at low temperature. Manufacturing thicker parts can create air pockets, and there is also a chance of shrinkage and warping.
⑧PBT (polybutylene terephthalate)
Advantages: PBT is an engineering material with excellent performance, which has excellent toughness and fatigue resistance, heat resistance, good weather resistance, good electrical properties, and low water absorption. Reinforcement and flame retardant modification can significantly improve its heat resistance, dimensional stability and flame retardancy. It is very suitable for automobiles and provides good electrical properties for electronic components. It is medium to high strength, tough, also has good resistance to fuels, oils, fats, and many solvents, and is non-odor absorbing.
Application: Not limited to sliding bearings, gears, grinders, vacuum cleaners; buttons, etc.
Precautions: PBT resin is easy to warp and difficult to process into thin-walled parts.
⑨PPSU (Polyphenylsulfone)
Advantages: PPSU has the characteristics of high toughness and heat resistance, and is a high temperature resistant and dimensionally stable material. It also has radiation resistance and certain acid and alkali resistance.
Applications: Not limited to medical device components, sterilization trays, hot water accessories, sockets and connectors, etc.
Precautions: For thicker parts, PPSU may cause voids, air bubbles. Organic solvents and hydrocarbons have certain corrosion on PPSU materials. Colorants generally cannot be added to PPSU resin.
⑩PEEK (Polyether ether ketone)
Advantages: PEEK is a high temperature resistant, good chemical resistance, flame retardant, excellent strength and dimensionally stable material, commonly used in medical, aerospace and automotive industries.
Application: Not limited to bearings, piston parts and pumps, insulated wires, etc.
Notes: PEEK is a high-performance material, so the cost is very high.
⓫PEI (Polyetherimide)
Advantages: Similar to PEEK, PEI is a material with heat resistance and flame retardancy, excellent strength and dimensional stability, and good chemical resistance. Commonly used in the medical, aerospace and automotive industries.
Applications: not limited to medical and chemical instruments; air conditioners, pipelines, etc.
Notes: PEI is also a material with high turnover, but it is cheaper than PEEK.
The above 11 materials are commonly used in injection molding. Other plastic options are also included in DDPROTOTYPE memory, such as PPS, TPE, TPU, LCP, HDPE, LDPE and PSU, and these resins can also improve performance by adding glass and carbon fiber.
4. Typical materials for medical injection molding
Plastics have long been considered to have advantages over metals for medical applications. Because, when in contact with the human body, the metal may react chemically with the saline solution in the human body. In the injection molding process, the medical industry has the highest level of requirements and is in great demand. The quality of medical injection molded parts is related to human health and even threatens human life. When working with medical injection molding manufacturers, it is very important that they fully understand the characteristics of the most commonly used thermoplastics in medical injection molding, which is one of the manifestations of whether the manufacturer adheres to strict manufacturing standards. Below we introduce the common materials and applications of medical injection molding parts. Usually these medical materials are not used as spare stock materials, but need to be used in production after strict testing before manufacture.
Polyethylene (DPE)
Polyethylene, by far the most widely used plastic in the world, is a cost-effective medical-grade material that is non-absorbent, non-biodegradable, and color-resistant, making it ideal for sensitive medical devices and components. Polyethylene does not easily retain dangerous bacteria and can withstand harsh cleaning agents. It is commonly used in containers, bottles and pipes, etc., but is susceptible to UV radiation and is flammable. It has a tensile strength of 4,000 psi.
Polypropylene – a commonly used thermoplastic for injection molding of medical devices.
Polypropylene is a thermoplastic material with excellent mechanical properties and chemical resistance. Polypropylene is relatively strong and durable, with a very high tensile strength of 4,800 psi, and is used in applications ranging from automotive bumpers to medical tools. Polypropylene is commonly used in the manufacture of disposable syringes, connectors, knuckle prostheses, non-absorbable sutures, containers, vials and clear bags, among others.
Polystyrene
Polystyrene is one of the most widely used plastics. It is a glassy, transparent, hard plastic that is relatively inexpensive but has a poor barrier to oxygen and water vapor and a relatively low melting point. Polystyrene is commonly used in the manufacture of test tubes, Petri dishes, trays, disposable plastic tableware, etc.
Acrylic
PMMA has near-perfect transmission of visible light and has the unusual property of keeping light beams reflected within its surface, so it is often used to make optical fibers. It is often used in medical devices to make artificial teeth, dental implants, denture materials, dental fillings, intraocular lenses and membranes for dialysis.
Polyvinyl Chloride
Polyvinyl chloride (PVC) is one of the most commonly used thermoplastic polymers in the world. It is most commonly used in construction industries such as flooring, piping and siding in hospital sterile laboratories. It is also used as a substitute for rubber in some cases and is also commonly used in the manufacture of hemodialysis or hemoperfusion devices, blood tubing, blood bags and prosthetic materials.
Polycarbonate – a commonly used thermoplastic for injection molding of medical devices
Polycarbonates are a group of thermoplastic polymers that are naturally transparent to visible light and resistant to UV rays. They are commonly used in eyeglass lenses and are considered a good substitute for glass. Polycarbonate is a very strong material that is not brittle and is commonly used in medical devices. Parts manufactured from polycarbonate can be sterilized using steam at 120°C, gamma radiation, or ethylene oxide (Eto) methods.
5. Find a trustworthy injection molding manufacturer
When you entrust your injection molding project to a manufacturer, you expect them to consistently meet your expectations and add value to your project. So, you need to know how to look for in an injection molding manufacturer.
Appropriate injection molding certification.
The injection molding industry has strict guidelines, especially for special industries such as medical.
Design and Fabrication Qualification:
It is very important that the manufacturer’s equipment meets the quality standards for IQ/OQ/PQ process validation. In addition, using design software such as SolidWorks CAD is an important performance in determining their prototyping ability.
Quality Control and Assessment Certification
For manufacturers, ISO 9001:2015 is an important certification as it implies a suitable quality management system.
Material safety and procurement certification
Especially in the medical field, traceability is very important. Ensure that the manufacturer’s production records are in full compliance with relevant legal and ethical constraints.
Manufacturer’s level of quality control
Injection molding requires extremely high stability, so make sure your supplier implements quality control and safety measures throughout the entire production process. Check out their factory for a hands-on solution for manufacturing a range of thermoplastic parts. It is wise to hear them describe specific examples of practice.
In-house manufacturing equipment and professional mechanics
In-house advanced manufacturing equipment and machinists are an important basis for delivering high quality and meeting production expectations. A 5-axis CNC machine tool, a three-coordinate measuring instrument, etc. are a necessary condition. Of course, experienced mechanics will have to follow the entire development stage.
If you have any doubts in choosing injection molding materials, please do not hesitate to contact DDPROTOTYPE immediately, they will advise you free of charge, based on their over 20 years’ experience.