Plastic injection molding is a common process used to make plastic components which are used by various industries.
While conventional injection molding is the most versatile of all plastics fabrication processes, it is not suited for very large parts.
However, as injection moulding technology has evolved and consumer demands have changed, alternative processes have been developed to meet them. There are several alternatives to injection and below are some common alternatives to Injection Molding
With the growing use of plastics, each of these technologies has sophisticated computer-controlled machines that, in conjunction with innovative tooling, are used to efficiently make multimaterial parts that can be decorated during the part forming process. The evolution of these technologies have made possible an unending number of everyday use items such as water bottles, food packaging, ketchup bottles with multifunctional lids, containers with handles, sophisticated toys, baby strollers, canoes and kayaks, many automotive components, and a host of medical devices.
Gas-assisted injection moulding
An extension of injection molding, gas-assist partially fills the mold with material in what is commonly referred to as a “short-shot.” Near the end of polymer fill, nitrogen is injected into the system. The gas bubble forces the polymer deep into mold cavities, hollowing out channels as it travels through the material. Nitrogen is either injected via the same nozzle as the polymer, into the mold cavity by way of runners, or directly into the part or rib sections. Multinozzle gas fill is called web molding. Molders must design both the part and nozzle’s location carefully to optimize the flow patterns of the gas. Otherwise gas bubbles will follow the path of least resistance and may bleed into thin areas.
The injection of gases can help to achieve features such as hollow cores – and reduced weight – in a plastic component and popular in the medical sector.
These parts are lighter than typical injection moulded components and yet cheap to manufacture. The components can also have enhanced strength and stiffness, even at lower weights. With the added benefit of reduced cooling times, the injection moulding cycle times can also be reduced.
Water injection moulding is said to be even more effective than gas-assisted injection moulding at reducing cooling, and thus cycle, times. However, both are equally effective at creating strong, hollow components
Constructed of foam core encased in a solid skin, structural-foam-molded parts have higher strength to- weight ratios than those made by conventional injection molding. Tooling costs may be lowered by 10 to 20%
Structural foam provides good dimensional control. Inserts such as brackets, threaded fasteners, or structural supports are easily molded-in and eliminate costly secondary assembly operations. The process also reduces sink marks on part surfaces even when designs incorporate large ribs, bosses, standoffs, or mounting pads.
The primary disadvantage of the process is a characteristic swirl-patterned surface. When surface cosmetics are a priority, structural- foam parts need secondary sanding and painting operations to get the same quality as injection molding. The process also requires longer cycle times because of thicker walls and foam’s insulating properties
Coinjection — Also called sandwich molding, co-injection uses a specially designed nozzle to inject two materials into a mold so that one completely encapsulates the other. Several constructions are possible. The most common are composites with either a solid-skin/solid-core or solid-skin/foam-core.
Coinjection molding provides an aesthetic surface in either case, even when recycled or EMI shielding material serves as the core. Solidskin/ solid-core gives a rigid part while maintaining a nice looking or flexible surface. Advantages of solid-skin/foam-core include thick walls with injection quality surfaces, no sink marks, and good rigidity at less cost.
Material selection for coinjection, however, is critical. Core and skin material must shrink and expand at the same rate and be compatible. Viscosity of the materials must also match up well. Typically, the skin is a lower viscosity material and lets higher viscosity core materials flow through its center. Use of recycled core material and a quality surface make this process attractive. However, there are some economic hurdles. For one thing, the capital equipment required costs 50 to 100% more than that of injection molding
Reaction-injection molding (RIM) — In-mold polymerization lets RIM form large parts with 10 to 15% less injection pressure and less than 5% of the clamping force used in conventional injection molding.
RIM starts with two low viscosity components, that when mixed and injected into the closed mold, quickly react and polymerize. The low pressures needed to mold the low-viscosity components produce complex geometries with no molded-in stresses and little shrinkage. It accepts inserts or stiffeners and readily duplicates mold surfaces. The low-pressure, slow-cavity fill also handles walls whose thickness varies. Ribs and bosses can be molded without the threat of sink marks, and the process can accommodate in-mold coating which reduces finishing costs. Polyurethane remains the dominant RIM material
RRIM combines short-fiber or flake reinforcement directly into the reaction process, while SRIM uses molds containing structural preforms. Preforms are three-dimensional precursors of the part and can be plastic or metallic inserts, fibrous reinforcements, or core materials. The most common preforms are fiberglass and can be mats of either thermoformed continuous strands or chopped fibers sprayed onto part-shaped screens
Technical blow molding — Blow molding produces lightweight parts with the highest stiffness-to-weight ratio of any thermoplastic process. It starts with a hollow tube of material (parison) extruded between open mold halves. As the mold closes, the parison is grasped and pinched off at each end. Air injects through a blow pin, expanding the parison into the mold cavity, where it solidifies and takes shape.
Blow-molded parts have lower molded-in stresses than injectionmolded components. Consequently, there is less warpage and fewer failures from stress. This single-operation process easily molds complex parts or flat panels. The technique employs foam filling, molded-in stiffeners or compression welds — tackoffs — between opposite sides of the parison to boost stiffness and strength in flat panels.
It is also possible to form parts with varying compositions or multiple layers through sequential and coextrusion blow molding. Sequential extrusion mixes various materials as the parison forms so that selected locations have different properties. Coextrusion builds multilayer parts and can use recycled material for inner layers. Coextrusion also gives inner and outer surfaces different performance qualities
Blow molding cannot produce the same surface finish or complex surface detail as injection molding. And because the parison fills the cavity by stretching, thinner walls result in areas with radii or deep-draw. Sharp corners are not practical and parts will have “witness” lines where cavity sections meet. Blow-molded parts also need secondary operations of trimming and deburring to remove pinchline flash. Holes and cutouts must be machined as well.
Rotational molding — Large, thick walled hollow or open-side parts are good candidates for rotational or rotomolding. Using mainly thermoplastic liquids or powders, rotomolding forms parts by simultaneously rotating molds around two right-angle axes. As the molds heat up, the resins fuse together forming uniform layers on mold surfaces. The amount of resin placed in the mold controls wall thickness.
Rotomolding tools cost less and are easier to make than those for injection molding. The process can handle a large array of part sizes from doll heads to boat hulls with no mold lines, sprue, or ejection marks. As a low-pressure process, it makes parts having no molded-in stress. Structurally, rotomolded parts have good load-bearing properties and stiffeners can be molded in if additional stiffness is needed.
However, relatively few materials are suitable for rotational molding. The materials are also more expensive because they must be ground to a fine, uniform particle size to mold properly
SHAPED Thermoforming — Thermoplastic sheets can be formed in a number of ways using vacuum, low pressure, or a combination of both. The basic idea is to lay a heated sheet across a mold cavity. A pressure box makes contact with the sheet surface, forming two sealed areas above and below the sheet. Vacuum draws the sheet toward the mold, evacuating the cavity at the bottom, while compressed air pushes down on the sheet’s top surface.
Similarly, hollow parts are made using a twin-sheet forming process. Two opposing molds come together with the heated sheets between them. Vacuum draws each sheet to the corresponding mold half, while compressed air is introduced between them. Ballooning out, the sheets take the shape of the mold and fuse together where the molds meet. Vacuum can also be used alone to draw thermoplastic sheets into female molds or over male ones.
At low volumes, thermoforming produces near-injection molded quality. Thermoforming tooling is also less costly, less complicated, and easier to make than injection-molding tools.
Materials for thermoforming are more expensive because they have already been processed into sheet form. Finished parts also require secondary trimming or routing for holes, louvers, or grills. Robotic trimmers, however, have reduced the cost of these operations so thermoforming prices, especially at low volumes, are comparable to injection molding.
Any the alternatives to injection molding has the same main purpose that improving the end products and save the production costs, materials additives are also common choice of many plastic companies. To know properly about these, you can try US Masterbatch materials for reference