Plastic injection molding is a highly repeatable manufacturing process for mass-producing identical plastic parts. It is ultimately a cost-effective way to produce the plastics we use every day, such as bottle caps, toys and electrical parts.
To start the process, thermoplastic polymer particles are melted. These polymers can be colored or filled with other additives and injected into molds under pressure. The liquid plastic cools and solidifies. Once solidified, the mold opens and pushes the part out. Next, the mold is closed and the process is repeated.
The process produces consistent results within tight tolerances and provides manufacturers with a high degree of design freedom. Although there is a high initial investment in mold creation, it pays for itself in high volume production.
Types of Injection Molding
Injection molding can be used to produce tiny parts like model airplane parts or larger items like body panels. This process is mainly used where thousands or millions of identical parts need to be produced.
Common injection molding types include:
l Blow Molding: Used to shape bottles and other plastic shapes, blow molding is done by blowing compressed air into a mold.
l Overmolding: Also known as co-molding or overmolding, overmolding involves forming a mold with one material and then partially or completely covering that mold with another material.
l Insert Molding: In insert molding, a non-plastic part is inserted into a mold and covered with plastic.
l Cube Forming: A forming process used to make round parts, cube forming is characterized by the vertical rotation of components around an axis.
Materials for Injection Molding
Various materials are used in plastic injection molding. Materials are selected based on their weight, strength and resistance to certain environments (heat, acid or moisture). Plastics can also be combined to produce different properties and effects.
l ABS (Acrylonitrile Butadiene Styrene)
l ABS/PC (Acrylonitrile Butadiene Styrene + Polycarbonate)
l Acetal
l Acetal Copolymer
l Acetal Homopolymer
l ETPU (Expanded Thermoplastic Polyurethane)
l HDPE (High Density Polyethylene Resin)
l LCP (Liquid Crystal Polymer)
l E (low density polyethylene resin)
l LLDPE (Linear Low Density Polyethylene Resin)
l Nylon 6
l Nylon 5/12
l PBT (polybutylene terephthalate)
l PC/PBT (polycarbonate/polybutylene terephthalate)
l PEEK (polyetheretherketone)
l PEI (polyetherimide)
l PET (polyethylene terephthalate)
l PETG (polyethylene terephthalate)
l PMMA (polymethylmethacrylate acrylic acid)
l Polycarbonate
l Polypropylene
l PPA (polyphthalamide)
l PPE/PS (polyphenylene ether + polystyrene)
l PS (polystyrene)
Injection Molding Design Process
For the best end result, there are several factors to consider when planning your injection molding design. Considering the following factors can reduce the risk of defective or substandard final components:
l Material: Each plastic material has different properties (elemental resistance, strength, cost) that must be considered. During molding, resins also react differently when heated and have different properties when cooled.
l Tolerances: Plastics will warp and shrink in the mold, so this needs to be taken into account in the design. The chemistry of each plastic also lends itself to different tolerances. For example, acrylic resins have low shrinkage and can hold tighter tolerances, but LDPE cannot withstand very high temperatures and warp easily.
l Mold Design: The mold itself needs to be carefully designed. Wall thickness should be uniform; elements such as ribbing can help. The mold also needs a certain amount of draft so that the part can be ejected correctly. This is especially important in injection molding, where high pressure allows the plastic to be injected into all the small nooks and crannies of the mold, which can make the part difficult to remove. Parts with intricate details also require additional sketches to maintain the integrity of the design.
l Shrinkage: Different plastics have different shrinkage rates due to differences in the resin structure, the mold itself, and the way the plastic flows into the mold. Injection pressure can increase shrinkage, but different resins require different tons per square inch.
l Finish: According to the Plastics Association, there are 12 grades of plastic finishes, ranging from matt to glossy. Each type of shine requires a different polishing service - sandblasting produces a dull finish, while diamond polishing produces a mirror finish. The plastic itself must also be considered. For example, you can't use powder coatings on low melting point materials, but plastics like PVC can withstand high temperatures.
The benefits of injection molding
The main benefit of plastic injection molding is that it can quickly manufacture large numbers of identical parts. After covering the initial mold making costs, the production cost of injection molded parts is low and continues to decline as production increases.
A variety of materials are available for injection molding, and the process is highly repeatable. It produces finished parts with tolerances of ±0.500 to ±0.125 with excellent visual appearance and little or no additional finishing.
Compared to other types of traditional manufacturing processes, plastic injection molding produces the least amount of waste because no material is removed from the cured product.
