Volumes of production
High-volume production process.
Unit price vs. capital investment
Unit price for components is more than that for standard injection molding (see p.196). Tooling costs are higher due to the need to incorporate a second material.
Speed
Because it is a two-step process that involves forming a material over a pre-formed component, it is slower than some similar methods of multicomponent (that is, two-shot) molding.
Surface
The key feature here is that it allows for secondary “skins” to be applied over plastic moldings, so the surface is determined by the material you choose as the covering.
Types/complexity of shape
Because of the secondary material, over-mold decoration is best suited to flat surfaces and those with a deep draw.
Scale
The largest standard size is approximately 12 by 12 inches.
Tolerances
Depend on the shrinkage of the various materials.
Relevant materials
A variety of thin materials can be used to over-mold, such as aluminum sheet, leather, fabric, and thin wooden veneers.
Typical products
Over-molding has been used in a range of products that fall into the category of personal mobile technology, including cell phones, Personal Digital Assistants (PDAs), and laptop computers.
Similar methods
In-mold decoration (p.212) and insert molding (p.206).
Sustainability issues
Over-mold decoration requires an additional stage of processing to apply decoration to a molded product which, of course, increases energy consumption. However, the enhanced decorative surface could help to extend the product’s lifespan through increased value perception. As with any multimaterial component, recycling can be problematic because of the need to separate materials.
Further information
www.dow.com/inclosia
www.filminsertmoulding.com
Metal Injection Molding (MIM)
Product |
engineering components [pen nib for scale only] |
Materials |
low-alloy steel and stainless steel |
Manufacturer |
Metal Injection Mouldings Ltd, part of PI Castings |
Country |
UK |
Date |
first produced in the UK in 1989 |
This range of small-scale, complex engineering components is typical of the type of products that are made using MIM. It offers us the chance to create precise, solid metal products from metals with high melting points that cannot easily be formed by casting. The strength and hardness of these components offers several advantages over other forms of metal production.
A variation on standard injection molding (which uses plastics; see p.196), metal injection molding (MIM) is a relatively new way of producing complex shapes in large numbers from metals that have a high melting point, such as tool steel and stainless steel that would not be suitable for high-pressure die-casting (see p.219). The process is limited by the suitability of the metal powders that are used as the raw material, which need to be particularly fine.
MIM involves more processes than plastic injection molding, because of the necessity of adding binders to the metal. The various companies involved in using this technology each have their own unique binder systems but typically the binders, which can account for 50 percent of the compound, are made from a variety of materials, including wax and a range of plastics. They are mixed in with the metal powders to produce the molding compound. Once the shapes have been molded, the binder is no longer needed and is removed from the metal particles. What is left is then sintered (see p.168), which shrinks the component by about 20 percent.
– Can be used to form high-temperature alloys.
– Used to form complex shapes.
– Cost-effective for large numbers.
– No post finishing required.
– Parts have exhibited good strength.
– Low overall part size.
– Compared with standard injection molding in plastic (see p.196), only a limited number of manufacturers can offer MIM.