Injection molding has evolved into a family of specialized methods that let engineers balance speed, strength, cost, precision, and aesthetics. This guide explains the Top 10 Injection Molding Techniques in Plastics for learners who want practical knowledge with professional depth. You will see how each technique changes flow, cooling, and part performance, and how decisions about materials, tooling, and processing windows affect total cost. We focus on real manufacturing outcomes such as cycle time, tolerances, warpage control, weld line quality, and surface finish. Use these insights to match the right technique with product intent, from tiny medical parts to large structural housings.
#1 Conventional high pressure injection molding
This is the foundation of modern plastics manufacturing, delivering reliable, repeatable parts across consumer, industrial, and automotive markets. Pellets are plasticized, injected into a closed mold, packed to compensate shrink, and cooled before ejection. Tooling choices decide success, including gate location, runner balance, venting, and conformal or drilled cooling channels. Process capability hinges on clamp tonnage, screw design, residence time, and a robust window for melt temperature, injection speed, and holding pressure. Good part design removes thick sections, controls draft, and uses ribs for stiffness. With scientific molding, teams measure viscosity curves and optimize fill, pack, and cool phases.
#2 Two shot and multi material molding
Two shot molding produces parts with two resins in one press through sequential injection using a rotating or translating core. It enables hard soft combinations for grip, gaskets, living hinges, backlit icons, and complex color effects without secondary assembly. Adhesion depends on material compatibility, interface temperature, and surface energy. Tool design aligns two cavities precisely so the first shot becomes the substrate for the second. Typical pairs include PC with TPE, PP with TPE, and ABS with PC. Benefits include faster throughput, cleaner aesthetics, fewer leak paths, and stronger bonds than post assembly. Upfront tooling costs are higher.
#3 Overmolding and insert molding
Overmolding bonds a second material onto a preformed substrate, while insert molding encapsulates metal or other components directly in the mold. Both reduce assembly steps and improve ergonomics, electrical integration, and structural reinforcement. Success requires precise placement, anti shift features, and controlled clamping to avoid insert movement. Designers add undercuts, through holes, and knurling to create strong mechanical locks. Thermal matching and gate strategy prevent stress at interfaces. Typical applications include screwdriver handles, connectors, threaded bosses, and medical device grips. Process engineers validate adhesion, flash control, and knit lines through design of experiments and careful cavity balance.
#4 Gas assisted injection molding
Gas assisted molding injects pressurized inert gas into thick flow sections during or after polymer fill to create hollow channels that reduce sink and weight. The gas core pushes plastic against cavity walls, enhancing surface quality while limiting clamp tonnage and cycle time. It is ideal for handles, appliance frames, and thick ribs where conventional packing would cause warpage. Key variables include gas entry timing, pressure ramp, hold duration, and venting to prevent blowouts. Tooling requires gas pins, seal integrity, and controlled flow paths. The method reduces material usage, improves stiffness to weight ratio, and minimizes sink marks.
#5 Water assisted injection molding
Water assisted molding is related to gas assisted methods but uses liquid water to form smooth internal channels with excellent heat transfer. The water rapidly cools thick sections and can produce long hollow geometries with uniform walls, making it valuable for automotive handles, ducts, and tubular parts. Process control focuses on water timing, pressure, temperature, and evacuation to avoid residual moisture. Tooling needs corrosion resistant materials and dedicated inlet outlet circuits. Advantages include shorter cycles, reduced part weight, and superior interior surface finish. Designers must plan for consistent wall thickness and robust seals to prevent leakage and dimensional drift.
#6 Thin wall high speed molding
Thin wall molding targets parts with very low nominal thickness to achieve low material consumption and short cycle times for packaging and electronics. High injection speeds, elevated melt temperatures, and high clamp tonnage drive complete cavity fill before freeze off. Tools often have hot runners, balanced manifolds, and advanced venting to prevent burn marks. Materials like PP and PS with good flow indexes are common. Structural features such as ribs, gussets, and radii help stiffness without thickening walls. Gate design minimizes shear hotspots and visual defects. This technique demands stable dryers, high response controls, and robust preventive maintenance.
#7 Microcellular foam injection molding
Microcellular foam molding introduces a supercritical fluid into the melt to create fine closed cell structures that reduce density and clamp demand. The process, often known by proprietary systems, helps mitigate sink in ribs and bosses while improving dimensional stability. Proper dosing, nucleation control, and shot volume are crucial to achieve uniform cell size and avoid swirl marks. Designers can target weight reductions while maintaining bending stiffness due to foam core geometry. Cycle time can drop because less packing is needed. Surface quality improves with skin layers and optimized injection profiles. It suits housings, interior panels, and structural brackets.
#8 Micro injection molding
Micro injection molding produces miniature parts under a gram with micron level features for medical, optical, and micro actuator components. Machines use precise metering, short residence times, and high resolution screw or plunger systems to avoid material degradation. Tool steels, mirror finishes, and sub micron venting protect delicate geometries. Gate types include micro pin gates and edge gates placed to limit shear and jetting. Metrology with vision systems, CT scanning, and statistical process control verifies critical dimensions. Drying, filtration, and cleanroom practices are essential. The technique enables complex freeform features and micro textures that would be impossible by machining.
#9 Co injection and sandwich molding
Co injection uses two materials in a layered structure to combine properties and manage cost. A common approach forms a higher performance skin with a recycled or barrier core, improving sustainability and functional performance. Valve gating sequences the materials so the skin flows first, followed by core penetration, and then skin encapsulation. The process can enhance gas barrier, impact resistance, or aesthetics while hiding regrind inside. Tool balancing, residence time control, and rheology matching prevent core breakthrough at the surface. Typical products include packaging, appliance panels, and durable housings that demand premium surfaces with economical interiors.
#10 In mold labeling and in mold decoration
In mold labeling and in mold decoration integrate graphics, textures, and functional films inside the molded part during the molding cycle. A printed label or decorated foil is placed in the cavity, held with vacuum or electrostatics, and fused by the incoming melt to form a durable surface. The method eliminates secondary printing and improves scratch and chemical resistance. Tooling needs precise registration, optimized venting, and surfaces that allow quick pickup and release. Materials and inks must bond at processing temperatures without distortion. This technique enables premium branding, backlit icons, tactile grains, and even conductive or protective films.