This article will take an in-depth look at thermoplastic molding.
The article will bring more detail on topics such as:
This section will explore what thermoplastic molding is and how the molding process works.
Thermoplastic molding involves shaping plastic materials by injecting molten resin into a mold to produce functional components. Compared to thermosetting polymers, thermoplastics are preferred in injection molding due to their ability to be reheated and reformed multiple times. This characteristic makes thermoplastics an excellent choice for recycling and reuse.
Excess material from earlier molding cycles is reprocessed and mixed with fresh pellets before being added to the injection chamber. However, the amount of recycled material is capped at 30% to prevent compromising the plastic’s original quality and performance.
Thermoplastic molding has several sub-categories, such as rapid injection molding, which is best utilized in fine-tuning prototypes before a product is given the go-ahead for production. Another sub-category of thermoplastic injection molding is production injection molding; this is best utilized for full product runs. Developers get to use the thermoplastic injection molding process for many applications, as it can produce anything from cell phone cases to car door panels with good accuracy and surface finish.
Widely recognized as the standard method for manufacturing plastic components, this process ensures high-quality results when utilized in product development. The field of thermoplastic injection molding has evolved significantly over time. Initially focused on producing items like combs and buttons, it now supports a diverse range of applications across various sectors such as automotive, medical, aerospace, consumer goods, toys, plumbing, packaging, and construction. This molding technique leverages the properties of thermoplastics to inject molten plastic into molds, creating precise shapes. It is particularly well-suited for both large and extensive production runs.
The process of plastic injection molding revolves around heating thermoplastic (TP) pellets until they become pliable. These softened pellets are then mixed and pushed through a rotating screw under pressure, which forces the material into a mold. Once inside, the material conforms to the mold's shape and is allowed to cool, solidifying into the desired form. After cooling, the newly formed part is ejected from the mold, which can be used repeatedly for additional production runs. This technique is particularly advantageous for producing numerous high-quality identical items, making it ideal for both large and extensive production quantities due to the mold’s reusability and efficiency.
One key benefit is that parts produced using this method often need minimal to no additional machining. However, this process involves the expense of creating a steel mold, which is necessary for producing large quantities of parts. As a result, this approach is not ideal for small production runs. For smaller series, alternatives such as silicone molding are typically preferred.
Thermosets and thermoplastics are produced through different processes and require distinct approaches during injection molding. Below are some key differences between molding thermosets and thermoplastics.
| Thermosets | Thermoplastics |
|---|---|
| When producing parts, the cold material is injected into a hot mold | When producing parts, the plastic material is melted and injected into a mold |
| Can’t be remolded or reshaped | Can be remolded and recycled |
| Forms a permanent chemical bond | 100% reversible, as no chemical bonding takes place during the process |
| Comparatively difficult to surface finish | Thermoplastics result in accurate, flexible, and pleasing surface finishes |
| Does not require high heat and high pressure compared to thermoplastics molding | Requires high heat and high pressure |
| Thermosets are made by condensation polymerization | Thermoplastics are made by additional polymerization |
| The production process includes compression, transfer, and casting | The production process includes injection molding, extrusion, and blow molding |
| Some of the end products that come from thermosetting injection molding include: Handles of tools, billiard balls, insulation, computers parts, television parts, any electronic equipment, gardening equipment, tools, sprockets, and cooking utensils | Some of the end products that come from thermoplastic injection molding include: Vacuum cleaners, toys, machine screws, gear wheels, kettles, packaging film, sacks, power tool casings, toasters, gas pipes, and fittings |
| Disadvantages of thermosets are: unable to be recycled, and they release emissions referred to as volatile organic compounds (VOCs) | Disadvantages of thermoplastics are: they are expensive, easily melt when heated, and are hard to prototype |
The cycle of thermoplastic injection molding occurs within an injection molding machine, which primarily includes three key components: the injection unit, the clamping unit, and the mold.
During the clamping stage, the mold halves are securely closed before molten plastic is introduced. Once the material is injected into the mold cavities, the clamping unit maintains this closure. The clamping unit is crucial for applying the necessary force to withstand the injection pressure and keep the mold halves together until the plastic has sufficiently cooled and solidified.
The clamping unit also handles several other key functions, including ejecting the molded part once the cooling phase is complete, managing the opening and closing of the mold plates between cycles, and ensuring the precise alignment of the mold plates throughout the process.
The clamping unit is composed of:
During the injection step, raw plastic pellets are melted and transferred into the mold through the injection unit. This unit is crucial for several functions, including delivering the molten plastic to fill the mold cavities. The amount of plastic injected in each cycle is known as a shot, and its volume is determined by the size of the part being produced.
The injection unit provides the necessary heat to melt and uniformly mix the plastic pellets before they are injected into the mold. Additionally, it delivers the required injection speed and pressure to ensure the molten plastic fills all the mold cavities effectively.
The injection unit consists of:
After the molten plastic is injected into the mold, it is allowed to fill the cavities and begin solidifying. During this phase, holding pressure replaces the injection pressure to help compact the plastic as it cools. The cooling process begins as the molten plastic contacts the mold's surface, with cooling facilitated by a built-in coolant system. To account for any shrinkage that may occur during cooling, additional molten plastic is injected to compensate. Once the plastic has cooled sufficiently, the mold halves are separated, and the finished part is ejected.
During the ejection phase, the cooled component is detached from the mold. This occurs within the clamping unit, where the ejection system plays a crucial role in extracting the molded component from the mold cavities. The ejection system features a mechanism with an ejector bar that operates to push an ejector plate fitted with ejector pins. These pins push the solidified component out of the mold as it opens at the end of the molding process. Sufficient force is necessary because the component tends to stick to the mold as it cools. To aid in this process, a mold release agent is applied, which may be reapplied before each new clamping operation or permanently integrated into the mold cavity surfaces.
Trimming is the final stage in the production of injection-molded plastics. During this step, any excess plastic—leftover from the flow of molten plastic into the mold—is removed from the finished part. Each molded unit is then separated from the remaining parts. Trimming is performed using specialized equipment. As the molten plastic is injected, it fills the mold channels, including the sprue, runners, and gates, and can solidify along with the material in the cavities. Additionally, flashes may form along the edges of the part. After cooling, these excess plastic materials must be trimmed away from the part.
The mold consists primarily of two plates attached to the clamping plates. The rear mold half is connected to a movable plate that enables the mold to open and close, and is positioned near the ejection system of the clamping unit. Before each molding cycle begins, it is essential that the two plates are clean and free from any contamination. The mold cavity, located within the plates, is the designated space where the molten plastic takes on the final shape of the part. As the molten plastic enters the cavity, it conforms to the hollow space, acquiring its intended volume and form.
The front mold half typically holds the majority of the mold's volume and may contain one or more cavities. The parting line, visible when the mold is closed, marks the separation between the two mold halves and can appear as a straight line or curve, depending on the complexity of the tooling design. This line allows for easier venting of air, which can cause the molten plastic to flow more readily in this area. Consequently, some finished parts may exhibit a visible line or curve, indicating that the part was formed from different plates.
Within the mold channels:
Other mold tool features include air vents that eliminate entrapped gasses inside the mold and the cooling channel, which facilitates the dissipation of heat to a coolant.
The injection molding parameters include:
Thermoplastic molding is a widely practiced industry in both the United States and Canada, with numerous companies specializing in this field. The industry is well-established across North America, serving a diverse range of sectors. Here are five leading companies renowned for their expertise in thermoplastic molding:
Proto Labs, headquartered in Minnesota, provides rapid prototyping and on-demand production services, including thermoplastic injection molding. They employ cutting-edge manufacturing technologies and automation to enhance efficiency, minimize lead times, and produce high-quality components. Proto Labs offers an online quoting system that allows customers to upload their CAD designs and receive prompt quotes for their molding projects. Their reputation for expertise, speed, and a customer-centric approach has made them a prominent player in the thermoplastic molding sector.
Nypro, now integrated into Jabil, is a leading global provider of contract manufacturing services, including thermoplastic molding. Based in Massachusetts, Nypro boasts extensive expertise in offering design, tooling, molding, and assembly services across multiple industries, including healthcare, consumer electronics, and automotive. Their advanced facilities and dedication to innovation have established them as a top choice for complex and large-scale thermoplastic molding projects.
Based in Indiana, Berry Global is a prominent manufacturer and supplier of plastic packaging and engineered materials, encompassing thermoplastic molding products. The company caters to various industries, including food and beverage, healthcare, and personal care, among others. Berry Global's success is driven by its broad product range, robust distribution network, and ongoing investment in cutting-edge molding technologies.
U.S. Farathane, based in Michigan, excels in thermoplastic injection molding and delivers engineered plastic solutions primarily for the automotive sector, among other industries. Renowned for their emphasis on innovation and tailored solutions, they offer comprehensive services that span from product design and engineering to manufacturing and assembly. Their capability to adhere to rigorous industry standards and meet complex supply chain requirements has established them as a significant player in the thermoplastic molding field.
Crescent Industries, located in Pennsylvania, offers comprehensive custom thermoplastic molding services. They serve a diverse range of sectors, including medical, aerospace, electronics, and consumer goods. Known for their precision in molding, rigorous quality control, and attentive customer service, Crescent Industries has established a strong market presence.
It's important to note that the industry landscape may have evolved since this update. For the most current information on these and other companies' thermoplastic molding capabilities, it is advisable to conduct additional research and consult recent sources.
This chapter will explore various types of injection molds and examine the common thermoplastics utilized in the injection molding process.
The choice of injection mold depends on factors such as part geometry, production volume, budget, and tool design. The type of mold selected can influence both the manufacturing costs and the quality of the finished components.
Single cavity molds are designed to produce one part per cycle. They are a cost-effective option for low production volumes due to their lower construction costs and shorter lead times. However, the downside is that they result in a higher cost per part during production.
Multi-cavity molds are designed to produce multiple parts per cycle. Advantages of these molds include increased production capacity and reduced cost per part, as more components are manufactured within the same cycle time.
A family mold features a single mold base with multiple cavities, each designed to produce different parts. This setup allows for the simultaneous manufacturing of several distinct components, or selective production using shut-offs to isolate specific cavities. To achieve optimal results, the parts should be similar in shape, material, size, and expected production volumes.
Ensuring that the parts are similar is crucial when running all parts simultaneously. Automation might be required to separate the pieces either during or after production. A family mold can offer cost savings and flexibility, particularly when mold costs are a significant consideration and production volumes are low.
Unscrewing molds are commonly used to create threaded holes within a plastic part. Unscrewing molds are automated with small drive systems, including rack & pinion, electric, or hydraulic motors. These drive systems are tied into the process and rotate threaded features to extract undercut features. Threads can either be internal or external, and the extraction is tied into the press cycle. Multi-shot or multi-component tooling allows a product designer to use two or more materials that are not similar on one part within the same cycle.
Different materials may be required in molding to achieve varying physical properties or aesthetic effects. Unscrewing molds frequently incorporate multiple manifolds within a single tool. Multi-shot tooling is particularly useful for complex products or for incorporating color changes within a product line. This approach necessitates the us
Hot runner systems use a temperature-controlled manifold to minimize or eliminate runner scrap during the molding cycle. Injection points can be positioned either outside or directly within the part. The presence of a sprue system, also known as a runner system, affects the overall cycle time of the mold. By eliminating the runner, hot runner systems help reduce material waste, leading to cost savings.
The controller must be appropriately sized to match the manifold in the mold. Although the initial investment may be higher, the long-term savings in material and cycle time can offset these costs. This is especially true for applications requiring costly engineering-grade resins or for high-volume production runs.
Cold runner molds are traditional tools that use sprues and runners to direct material into the part. While this method is simpler, it often results in greater material waste and longer cycle times. Depending on the application, some of the waste material can be re-ground and reused, although this may impact the physical properties of the resin. For more advanced or high-cost materials, such as those used in medical or engineering applications, a hot runner system may be more suitable. This is also the case when re-grinding of components is not feasible.
Insulated runner molds are similar to traditional cold runner molds but incorporate cartridge heaters or other heating methods to maintain a layer of molten resin around the runner system. This creates an insulating effect that mimics the performance of a hot runner system. Insulated runners are more cost-effective than hot runners, which require a temperature controller, and they facilitate quicker changes in color and material. However, they are not suitable for all types of materials and may not be effective with more demanding engineering-grade resins.
Three-plate molds are categorized as cold runner tools. The addition of a third plate to the runner system allows for flexibility in placing the injection point anywhere on the mold. This setup is generally more cost-effective than incorporating a hot runner system. However, three-plate molds can be more challenging to automate due to their large and cumbersome runners.
Some common types of thermoplastics used in injection molding include:
Acrylonitrile Butadiene Styrene (ABS) is an opaque, amorphous thermoplastic polymer. ABS is a terpolymer, composed of three different monomers: acrylonitrile, butadiene, and styrene. This combination results in a material that is flexible, lightweight, and easily moldable, making it suitable for a wide range of everyday products.
One of the advantages of ABS is its versatility in modifying properties to enhance impact resistance, toughness, and heat resistance. Molding ABS at higher temperatures can improve both the gloss and the heat resistance of the final product.Molding ABS at lower temperatures yields the highest impact resistance and strength. Beyond its use in molded plastics, acrylonitrile butadiene styrene is also employed in applications such as drain pipe systems, golf club heads, and auto
Polyethylene is a thermoplastic polymer characterized by its variable crystalline structure. It is among the most versatile and widely used plastics, with applications spanning a broad range of uses depending on the specific type of polyethylene.
Polyethylene is available in two common forms: high-density polyethylene (HDPE) and low-density polyethylene (LDPE). It is known for its high ductility, tensile strength, impact resistance, moisture resistance, and recyclability.
A polyethylene material with a higher density produces plastic that is stronger, more rigid, and more heat-resistant. The primary applications of this material are plastic bags, plastic films, containers, and geomembranes.
Polycarbonate plastics are naturally transparent, amorphous thermoplastics. They are used in applications that require both impact resistance and clarity, such as in the production of bulletproof glass and other durable, see-through materials.
Polycarbonate can experience large plastic deformations without cracking or breaking. As a result, polycarbonate is commonly used for greenhouses, eyewear lenses, medical devices, automotive components, and cellular phones.
Thermoplastic molding provides numerous applications and benefits, including:
Thermoplastic molding is versatile and used for producing items like bottle caps, wire spools, packaging materials, automotive dashboards, and pocket combs. This technology supports the manufacture of both small and large series with the appropriate material. Its applications are diverse within the plastics industry, spanning automotive, packaging, medical, and electronics sectors. Particularly in fields with stringent regulations, like the medical industry, thermoplastic molding meets rigorous testing and certification requirements. Additionally, it accommodates the production of both tiny components, such as electronic parts, and large items, like car panels.
To ensure high quality in the final product, selecting the appropriate material for injection molding enhances the durability and mechanical properties of thermoplastic components, making them suitable for testing and small-scale production. Molds used in this process deliver superior quality, with thermoplastic injection molding accommodating both tiny and large molds as required, while maintaining precise accuracy. This versatility makes it a popular choice for rapid prototyping, particularly in the medical and automotive industries.
Thermoplastic molding is a manufacturing process that works to create fully functional parts by injecting plastic resin into a pre-made mold. Thermoplastic polymers are more widely used than thermosetting polymers in injection molding. The main reason is that thermoplastics can be repeatedly softened by heating and solidified by cooling, making them highly recyclable materials. Materials left over from a previous molding process cycle are re-grinded and added back to the injection chamber along with virgin pellets. An injection mold is selected depending on the part geometry, production volumes, budget, and tool design. The type of injection mold has the ability to affect the manufacturing cost and quality of the components.
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