Plastic injection molding is amongst the most widely used manufacturing processes when it comes to the production of plastic components. Through the use of injection molding, a wide variety of products can be manufactured that greatly differ in their size, complexity and application. The entire process of plastic injection molding makes use of an appropriate machine, raw plastic material and a mold. The process thereafter is quite simple – plastic being melted into the injection molding machine, injected into the mold and then cooled and solidified to achieve the final product.
Most commonly, injection molding is used as a manufacturing process where parts must be produced efficiently in large quantities.
What is Plastic Injection Molding?
Put in the simplest manner, plastic injection molding is a process that makes use of granular plastic resins to manufacture plastic parts ranging from automotive and airplane components to storage containers and even hair combs. Plastic injection molding is a widely used manufacturing process because it allows the producers to produce large amounts of the same item in a homogenous manner. The process of plastic injection molding makes use of a range of plastic composites like polycarbonate, polystyrene, polypropylene, acrylic, polyvinyl chloride and Teflon.
The melted plastic compounds immediately begin to cool and solidify when exposed to the mold surfaces. Due to rapid cooling techniques, some shrinkage in size could possibly occur. To avoid an impact on the final size of the product, extra material is usually permitted into the mold so that any shrinkage can be accounted for in advance.
As the product takes its time to solidify completely, the machine’s ejection system comes forth to open the mold and push out the hardened plastic object. Rigorous amounts of force paired with a mold releasing agent is used to properly eject the item without causing any losses. The process of opening molds and ejecting parts takes time and is calculated using dry cycle times and observing the amount of time it takes for the finished piece of plastic to fall into a final container. Molds are then shut and carried back to the area of the machine where the injection takes place, and the entire process is repeated until the required amount is produced.
Advantages and Disadvantages of using Plastic Injection Molding
The first and main advantage of plastic injection molding is its ability to produce at a very large scale. Once initial costs of the machine and molds are covered, the price per unit for any type of production is known to be extremely low. The price tends to further decrease as more parts are produced and the machine increases in efficiency levels.
Plastic injection molding also produces less waste in comparison with other traditional production processes such as CNC machining that cuts away substantial percentages of an original block of plastic. While latest technological advancements such as 3D printing is able to produce with even lower scrap rates, the process of plastic injection molding is known to be simple, quick and efficient. Waste material from plastic injection molding only has four significant sources which include the sprue, the runners, the gate locations and any additional overflow material that could leak out of the cavity itself.
A sprue is the channel that guides molten plastic compounds from the nozzle of the injection molding machine towards the entry point for the entire mold itself. A runner is another system of channels that connect with the sprue, typically as part of the mold tool, that further guides the molten plastic compounds into the correct part cavities within the mold tool. Finally, the gate locations are part of the channel after the runner that leads compounds directly into the part cavity. After one whole mold cycle, the molten plastic will cool and leave behind solid plastic in the sprue, runners, gates, part cavities and the areas where there might have been a little overflow.
Another important advantage of injection molding is the fact that it is easily repeatable. The entire process of creating identical products of plastic is made simple through the effective usage of plastic injection molding. Therefore, it greatly assists brand consistency and part reliability when producing homogenous products in high volumes.
However, there are also a few downsides to the attractive process of plastic injection molding. The first disadvantage is that initial costs tend to be very high due to the specific design, testing and tooling requirements of the machine. Since you have to produce the same product in large volumes, making sure that the design is 100% correct the very first time is extremely important. As simple as that may sound, the whole process is actually very complicated.
Creating the right design involves designing and then prototyping the part itself, down to the tiniest details. Initial prototypes can be developed using a 3D printer and different materials than what the final product will be constructed in. The second part is the designing of an injection mold tool and testing it with an initial production round to ensure high quality of the plastic products. Finally, the designing process includes the refining of all details in the injection mold tool before the mass production process begins.
The biggest potentially negative aspects of injection molding are the high tooling costs and large lead times required. As described above, tooling is almost an entire project in itself yet is only one part of the entire injection molding process. All of the iteration needed to make the correct tool prior to mass production requires both time and money. However, there are always certain ways to work around these. If a company is manufacturing something small and simple like a plastic bottle cap, they may choose to create a mold that produces all parts of the cap at once, instead of producing each small part separately. By using this technique, they will be saving both time and money as if there are any problems with the prototype, they can change it first without having to alter all cavities within the plastic injection molding machine.
Since some tools are typically made with steel or aluminum, it can be an extremely daunting task to make any changes. Attempting to bring forth any increase or decrease in the plastic means that you would either need to make the tool cavity larger by cutting away the steel or reduce the size of the cavity by adding metal to it. In most cases, any of these changes could call for the need to scrap the entire tool and start over from scratch. In a few rare cases, you might be able to strategically weld the metal into what is desired.
Another significant part of plastic injection molding is how it requires uniform wall thickness throughout the production process. Preventing these walls from becoming too thick is necessary in order to avoid inconsistencies in the cooling process and avoiding major defects like sink marks in the plastic. A general rule of thumb is to keep walls less than or equal to a thickness of 4mm. Thicker walls represent more material being used, which in turn means that each cycle will be longer and cost more per unit produced. On the other hand, if wall thickness is kept thinner than 1mm, there will be lots of trouble faced while filling the mold tool properly. Therefore, another disadvantage of plastic injection molding is the complexity of the process and the need to pay attention to even minor details.
Additionally, large undercuts of plastic call for experienced designers and tested prototypes to avoid added costs to the project.
Plastic Injection Molding – Industrial Applications
Generally speaking, plastic compounds are part of almost every product that consumers come in contact with. Therefore, the demand for plastic is high all over the world and there are multiple manufacturing processes being used to effectively produce small parts of plastic.
Plastic injection molding can be used to create a wide variety of products ranging from daily use consumer goods to higher level producer goods. Common products such as wire spools, packaging tools and bottle caps are mass produced using plastic injection molding, Other products such as automotive parts and components as well as important parts of airplanes or cars can also be produced using plastic injection molding. Additionally, toys, pocket combs, musical instruments, small chairs and plastic tables, storage containers and even some mechanical parts such as gears are produced using plastic injection molding.
Materials Used in the Process of Plastic Injection Molding
During the plastic injection molding, a wide range of materials can be effectively used.
Thermoplastics are a type of polymers that can be continuously molten by heating and solidified by cooling. During the production of thermoset materials and products, a physical change takes place in the plastic compound instead of a chemical one.
Acrylonitrile Butadiene Styrene (ABS) is another opaque thermoplastic polymer consisting of three distinct monomers. Joined together as one, they create products that are flexible, lightweight and can be molded into many items that we use in our daily lives. The main advantage of using ABS is it’s improved resistance, toughness and resistance to heat. These properties during the manufacturing process will also influence the properties of the final product.
Polyethylene is also a thermoplastic polymer with a distinctive and changing crystalline compound and a large range of applications depending on its specific type. Developed by the Germans and Italians in the 1950’s, it continues to be one of the most commonly consumed plastics in the world to date. The main advantages of it is the high levels of ductility, strength, resistance to impact and moisture, and recyclability. Popular products produced with polyethylene are plastic bags, films, storage containers such as bottles and geomembranes.
Polycarbonate (PC) plastics are an essentially see-through kind of thermoplastic. These plastics are commonly used to manufacture a wide range of materials and is specifically useful when impact resistance and transparency are both required. The most widely known use of this plastic is for the production of bullet-proof glass as PC can resist great levels of impact without cracking or breaking. Additionally, PC is also used for greenhouses, digital disks, eyesight lenses, medical tools, important automobile parts and even cellular mobile phones.
Nylon material is also used in a wide variety of applications due to its significant conductor properties, high strength, wear-and-tear resistance and even chemical resistance. Nylon plastics also have greater levels of firmness and is known to be resistant to multiple external factors such as abrasion, impact and chemicals. It is commonly used to produce plastics in the medical industry, automotive products, athletic equipment, clothing and footwear, and many industrial-use components.
Polypropylene (PP) is a widely used kind of plastic which is popular for its flexible nature. This dynamic plastic has a high melting point, greater resistance to cracking and breaks, impact resistance and does not react when in contact with water or harmful acids. PP is generally safe to use for food containers as it does not transfer any chemicals into the food products. PP is also commonly found in kitchen utensils, athletic clothing, rugs and even car batteries!
During the plastic injection molding process, these materials are usually in their raw form of a fine powder. The selection of the correct material for each specific injection mold is not inly based on the desired characteristics of the final part but also the strength and function derived from the final product. Each material requires a distinct set of processing parameters including temperature, pressure, ejection and cycle time.
Why Should You Choose Essai As Your Plastic Injection Molding Service Provider?
Initially created in 2006, Essai has worked long and hard to provide high quality plastic and metal parts using injection molding and tooling, dye casting and tooling, and sheet metal stamping. Essai specializes in the field of plastic injection molding and provides remarkable and unmatched service to all our customers. With extensive prototyping services producing a wide variety of metal and plastic parts, Essai uses only the highest quality materials, machines and molds to ensure the manufacture of outstanding final products of every kind.