May. 13, 2024
In manufacturing, molding refers to the process of shaping a type of pliable material or liquid by using a rigid matrix or mold. In plastic injection molding, raw polymer resin granules are melted into a liquid and injected into a mold, where the plastic takes the desired shape. Varying levels of heat and pressure may be applied to create the final profile for the end product.
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The basic plastic injection molding process consists of the following steps:
In today’s ever-evolving world of manufacturing, plastics are being used to make everything from automotive body parts to synthetic human body parts. To create critical components and ensure optimal performance, many manufacturers choose to make use of plastic injection molding.
To suit diverse industry needs, there are several other types of plastic molding available, each offering unique features and benefits. An application’s particular specifications and requirements will determine which type of molding process is best-suited for a specific part.
Below are a few of the most common different styles of plastic molding.
The blow molding process follows the same basic steps found in the art of glass blowing. To blow mold a part, the manufacturer inflates a parison — a heated plastic mass, usually in the shape of a tube — with air. The parison inflates until it fills the mold and conforms to its shape. In this way, the plastic is blown into its desired form. Once cooled, the newly formed plastic part is ejected from the mold.
Blow molding is especially useful for economically manufacturing one-piece, hollow objects in large volumes, as the process can quickly create uniform, thin-walled containers — perfect for small objects like bottles, as well as larger ones like storage containers and drums.
Depending on the specific application, manufacturers can use a variety of thermoplastics in blow molding to create a more customized product. Commonly worked materials include low-density polyethylene, high-density polyethylene, polyethylene terephthalate, polypropylene, and polyvinyl chloride.
Compression molding involves placing a heated plastic inside of a heated mold, then closing it to compress the plastic into the desired shape. Once cooled, the part is removed from the mold. The heating process, called curing, helps ensure that the final product will maintain its integrity and shape. Compression molding offers many unique advantages; it’s both cost-effective and highly efficient. The process is also quite versatile, allowing manufacturers to create parts that vary greatly in thickness, length, and intricacy.
Because compression molding often uses advanced composites for the plastic material, the process yields stronger, more durable parts, making it popular across a range of different industries. For example, compression molding often employs high-strength materials, such as thermosetting resins, fiberglass, and reinforced plastics, resulting in products that are sturdier and more resilient than those offered by other molding processes.
Allowing for the creation of high-strength parts, compression molding is used to produce components for a vast range of applications, including automotive parts, household appliances, clothing fasteners, and body armor.
While other forms of plastic molding use extrusion to insert the plastic resins into the mold, extrusion molding extrudes the melted plastic material directly into the die. This process is unique in that the shape of the die, not the mold, determines the shape of the final product.
Extrusion molding is ideal for manufacturing parts with continuous length and uniform cross-sections. Similar to a plastic injection molding machine, the extrusion molding machine has a screw that turns to feed the plastic resin into the feeder. The molten plastic then moves through a die, creating a long, tubular shape. The shape of the die determines the shape of the plastic tube. Once the extrusion is cooled, it is removed from the machine.
Extrusion molding is well-suited for long, hollow-formed applications, such as tubes, pipes, and straws. Plus, manufacturers can create these parts in many different shapes, including T-sections, U-sections, square sections, I-sections, L-sections, and circular sections.
Of the various molding processes available, plastic injection molding is considered to be the most versatile, as it can be used to create a variety of parts, ranging in both size and shape. Presses also come in different sizes, based on the pressure they exert and their tonnage.
Injection molding basic principles are fairly simple, but the actual process can be quite complex when it comes to maintaining part consistency. The process involves the injection of melted plastic into a mold, which is made of steel. The mold itself has cavities that will form the parts; once injected, the molten plastic fills the cavities and the rest of the mold. Once cooled, the parts are ejected by pins.
Thanks to its excellent versatility, injection molding can be used to create everything from large automotive parts to small, intricate parts used in surgical equipment. Injection molding also allows for a high level of customization, as various plastic resins and additives can be used, allowing designers and engineers to create unique parts to meet highly complex or unusual application needs. And there are some enhancements and techniques available — such as an array of resin and finish options — for manufacturers looking to create even more specialized parts.
Though it can be expensive to initially make the molds themselves, once built, the production costs become quite low. In fact, injection molding is best-suited for the creation of very high volumes of precise parts; once production begins, the cost per part drops significantly, making the process very economical for high-volume runs.
Plastic injection molding is a highly reliable solution for producing large numbers of precise, consistent components. It’s also more efficient and cost-effective than other molding styles, in that it produces much less waste. As a result, injection molding is most often used for the manufacture of high-quality parts in high volumes.
Rotational molding, also known as rotomolding, uses high temperatures and rotational movement to coat the inside of a mold to form a part’s desired shape. First, the mold is filled with a polymer powder. The heated mold then rotates on two perpendicular axes so that the powder adheres to the entire interior of the mold. As it continues to rotate, the mold eventually cools and is removed, ultimately forming an even-walled component.
Rotational molding is best suited for the creation of large, hollow, one-piece containers, such as tanks. Though cost-effective, it is not a fast-moving process. However, rotomolding wastes little material, and what excess material is produced can often be reused, making it an economical and environmentally friendly manufacturing process.
Other key advantages include the ability to produce parts with consistent wall thicknesses, enhanced design flexibility, and great strength
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Each of these plastic molding processes offers unique features and advantages, making each style well-suited for different types of applications.
At The Rodon Group, our designers and engineers are well-versed in custom, high volume plastic injection molding and work closely with all of our clients to ensure their unique needs are met. In most situations, there are several ways to approach a manufacturing challenge, and we can provide expert insight into the various techniques and materials best suited to a specific project.
To learn more about plastic injection molding and how it can help with your next project, download our free eBook, “Plastic Injection Molding 101.” Contact us or request a quote to start your solution.
We often get asked what the best plastic injection molding resin is to use for certain projects. And honestly there is no one answer for that. The best resin is the one that works for you and your unique circumstances. The good news is that there are many thousands of plastic injection molding resins to choose from with similar – but not identical – properties, so you have many potential solutions available.
When choosing a plastic, the first thing you have to consider is the part’s function and the physical environment that it’s going to be exposed to. So clearly you’ll need to consider what type of injection molding material is going to stand up to various environmental challenges such as temperature fluctuations, moisture, chemical exposure, UV light, and many more. Then the material’s strength, durability, flexibility, color, and cost come into play. Therefore, we’d like to help you refine your search by explaining the properties of the ten most common plastic injection molding resins that we use for the majority of parts that we make here at Star Rapid.
Nylon is often used to produce strong mechanical parts like bushings, gears, and bearings. It’s very common in automotive applications because not only is it tough but it helps to reduce weight and lower production costs compared to a metal analogue. You should be aware that, although it’s a strong plastic, it tends to absorb water. It’s not the ideal choice for marine applications. Nylon is also known by its chemical designation PA (Polyamide).
We use acrylic to produce transparent parts such as windows, view screens, and various lighting equipment. It’s often used as an alternative to glass due to its high tensile strength and weather and scratch resistant nature. It takes dyes and colorants very well so you can produce many aesthetic effects. On top of its optical and transparent properties, acrylic is odorless and tasteless and doesn’t contain Bisphenol A (BPA). BPA is a harmful organic compound, so plastic injection molding resins like acrylic are considered safe for food storage.
Polycarbonate is another clear injection molding resin that has excellent optical properties and is extremely durable. When molding with this amorphous thermoplastic material, precise dimensional control can be maintained as it has predictable and uniform mold shrinkage. We use polycarbonate when we need something substantially stronger than acrylic. However, be aware that if you’re making optically clear plastic parts the mold tool must be highly polished, which in turn implies the use of a higher grade of stainless steel that costs more. Now, you can see that your choice of plastic resin can very much influence the appropriate mold tool material as well.
Polyoxymethylene (POM) is a type of acetal resin used to make mechanical and automotive parts that would usually be made with metal. This engineering thermoplastic material is very strong, tough, and rigid. It’s often used to produce gears, fasteners, knife handles, and ball bearings. Although POM has high resistance towards solvents such as alcohols, gasoline, detergents and motor oils, it shouldn’t be exposed to hydrochloric acid and nitric acid.
When it comes to injection molding resins, there are two types of polystyrene that are commonly used: High Impact Polystyrene (HIPS) and General Purpose Polystyrene (GPPS). GPPS is transparent, while HIPS is opaque. Hard cases for toolboxes and bodies of power tools are also made using High Impact Polystyrene. As with so many things there is a tradeoff to be aware of. On the one hand, PS is tough and durable. It can take a lot of abuse in the field. But that also means it’s not very environmentally friendly.
ABS resin is an opaque thermoplastic polymer and an engineering grade plastic. There are many advantages to using ABS. It’s tough, has good dimensional stability, it resists impacts and scratching, and is hard to break. Also, the low melting temperature makes it easy to mold. It’s commonly used to produce electronic parts such as phone adaptors, keyboard keys, and wall socket plastic guards. Why is this? Because ABS is a good insulator and won’t conduct electricity or give off fumes if it’s exposed to fire. These are important considerations for product developers working on electrical devices.
This thermoplastic injection molding material is widely used in the food storage and packaging industry because it doesn’t let chemicals mix with food products. Polypropylene (PP) can be washed in hot water without degrading, and it has high chemical and moisture resistance. PP has incredible impact strength, elasticity, and toughness. Designers should also note that PP is easy to recycle, and because of its flexibility, it can be used to make live hinges that can be bent many times without tearing.
Polyethylene (PE) is a lightweight thermoplastic molding material that has high chemical resistance, elasticity, and electrical insulating properties. It’s not especially strong or hard, but it’s inexpensive. You’ll find it everywhere in consumer plastic parts, milk bottles, medicine and detergent bottles, plastic bags, and trash cans. PE is also the most common injection molding resin for making toys because it’s non-toxic and can take a beating without complaint.
Thermoplastic Polyurethane (TPU) is soft and elastic, with great tensile and tear strength. That’s why it’s often used to make parts that demand a rubber – like elasticity. You should know that TPU is more expensive than other resins but for many applications, like protective wire and cable sheaths, there really is no substitute. Another advantage is that TPU improves the grip for products that need to be held securely in the hand.
Thermoplastic Rubber (TPR) resin is actually a mixture of plastic and rubber, and it’s easy to use in the injection molding process. It has outstanding chemical and weather resistance and high impact strength. Because of this, TPR is used in many types of fluid dispensers, flexible hoses, catheters, and other places containing different liquids, including acid.
You can find this recyclable material in medical catheters, suspension bushings and headphone cables. Thermoplastic rubber is also known as thermoplastic elastomer (TPE).
We hope this brief overview helped you to understand some of the most popular kinds of injection molding resins as well as their properties and applications. Please note that we can work with you to modify all of these materials with lubricants, glass fibers and UV stabilizers to suit your unique project needs.
You can also explore the most common injection molding defects and how to handle them. Are you ready to collaborate with us on your next manufacturing project? Request a quote today!
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