Injection Molding
with water injection technology (WIT)
Product |
BIC® Cristal® ballpoint pen |
Designer |
Marcel Bich |
Materials |
polystyrene (shaft); polypropylene (lid and plug) |
Manufacturer |
BIC |
Country |
France |
Date |
1950 |
Millions of BIC® Cristal® ballpoint pens are sold worldwide every day. All the elements of this iconic ballpoint pen are made using injection molding, except for the cartridge and nib.
Is injection molding the mother of all plastic-processing techniques? It is through this process that we are able to transform plastic into a mass of packaging, toys, and casings for electronics. It could well have been an injection mold that French philosopher Roland Barthes was referring to when he wrote, in his Mythologies (1957), of “. . . an ideally shaped machine, tabulated and oblong (a shape well suited to suggest the secret of an itinerary) effortlessly draws out a heap of greenish crystals, shiny and fluted dressing-room tidies. At one end, raw, telluric matter, at the other, the finished, human artifact, hardly watched over by an attendant in a cloth cap, half-god, half-robot.”
The process employs plastic pellets, which are fed from a hopper into a heated cylinder, which contains a screw. The screw carries the hot plastic, slowly melting it, and finally injecting it at high pressure into a series of gates and runners, which feed the polymer into a water-cooled steel mold. Once the part has solidified under pressure, pins eject the finished part from the mold.
Water injection technology (WIT), or water-assisted injection molding, is a relatively new technology that promises several advantages over conventional injection molding and gas-assisted injection molding (see p.201). It is based on several variations, which either employ the injection of water to ram the melt (polymer) into the mold, or use water injection as a means of forcing the polymer outward, to the walls of the mold, to create hollow parts. The use of water eliminates some of the problems that are associated with gas-assisted injection molding, such as migration of the gas into the plastics. In addition, due to the fact that water cannot be compressed, a greater degree of pressure is produced than can be provided by gas, which results in several advantages in terms of the complexity and finish of the final parts. Faster cycle times are also achievable due to the cooling effect of the water.
– Highly versatile in terms of molding different shapes.
– Highly automated production.
– Cost-effective parts.
– Involves considerable investment and high production runs.
– Can involve long lead times.
Volumes of production
Small injection-molding manufacturers can be found to produce simple components of 5,000 units or less. However, the minimum quantity is generally accepted to be 10,000 units.
Unit price vs. capital investment
Unit price is very low, but this must be set against the high tooling costs, which can run into tens of thousands of dollars.
Speed
Cycle times vary depending on the type of material, wall thickness, and the geometry of the part. As an example, simple bottle caps have the fastest cycle times of between 5 and10 seconds. A common speed for more complex parts is between 30 and 40 seconds.
Surface
This is determined by the steel mold, and can vary from spark-eroded to highly glossy. The points where ejector pins are located in the mold need to be considered when designing a part, as these leave small, indented circles. Parting lines, where the various parts of the mold come together, also need to be considered.
Types/complexity of shape
If the volumes of production are particularly great, injection molding can be used to form highly complex parts. However, features such as undercuts, variable wall thicknesses, inserts, and threads will add significantly to the cost of the tooling. Generally, injection molding is suited to thin-walled sections.
Scale
Micro-injection molding is a specialist area and there are certain manufacturers who specialize in parts that are often less than 1/25 inch in size. For large-scale products such as garden chairs, it is worth considering gas-assisted injection molding (p.201), and, where thick walls are required, try reaction injection molding (RIM) (p.199).
Tolerances
±1/250 inch.
Relevant materials
Predominantly used for thermoplastics, but thermosets and elastomers can also be used.
Typical products
It is impossible to state “typical” products produced by injection molding because its use is so widespread, from candy packaging (tic tacsTM boxes, for example) to medical implants.
Similar methods
The equivalent process for metals is metal injection molding (MIM) (p.216) or high-pressure die-casting (p.219).
Sustainability issues
This process offers a precise, controlled, and optimized use of material and energy. Water injection technology can help to improve the energy efficiency of injection molding through faster cycle times and by forming a closed-loop cycle in which the water is not disposed of but is put back into the process. However, with its ability to churn out plastic parts quickly and cheaply, injection molding can be accused of encouraging disposability as there is no cost incentive to reuse products. It can also come under fire for the release of toxins during heating and further emissions through high energy use.
Further information
www.bpf.co.uk
www.injection-molding-resource.org
Reaction Injection Molding (RIM)
with R-RIM and S-RIM
Reaction injection molding (RIM) is a process that is used for producing structural foam components. Unlike standard injection molding (see p.196), which uses pellets as the starting point, RIM involves feeding two reactive thermosetting liquid resins into a mixing chamber. They are then injected through a nozzle into the mold, where an exothermic chemical reaction produces a self-forming, smooth skin over a foam core. Depending on the formulation of the resin, parts produced using RIM can either be soft foams or solid, highly rigid components.
Composites can be produced by introducing short or long fibers into the mixture, to add reinforcement. This form of production can be broken down into two categories: reinforced-reaction injection molding (R-RIM) and structural-reaction injection molding (S-RIM).
– Allows for varying wall thicknesses within the same part.
– Because of the low pressures and temperatures required for this process, tooling costs can be low compared with other high-volume plastic methods.
– Produces parts with a high strength-to-weight ratio.
– Suitable for making large parts.
– A multiple-cavity mold is needed for small parts.