Micro injection molding

Micro injection molding is a molding process for the manufacture of plastics components for shot weights of 1 to 0.1 grams with tolerances in the range of 10 to 100 microns. This molding process permits the manufacture of complicated small geometries with maximum possible accuracy and precision.

Basic concept

The basic concept of the micro injection molding process is quite similar to the regular injection molding process. In this process, a micro injection unit is integrated in the injection molding machine. When it comes to the production of micro components the machine and process technology mainly depend on the below points:

Short dwell time
Low shear stress on the polymer melt
Homogeneous material preparation before molding
Precision injection and ejection
Accuracy of dimensions
Critical factors

Parting line issue

A parting line is the line of separation on the part where the two halves of the mold meet. The parting line matching for micro parts is a big issue. The interlocking features of mold cavity and core for precise mating are used to reduce such issues.

Degating issue

Another major critical factor of micro injection technology is that the smaller part size causes problems with degating.

Sprue and runner size

Runner and sprue diameters are another concern. The total volume of the feed system can exceed the volume of the parts by a factor of 100 or more.

Materials and applications for micro injection molding

The most common polymers used in micro injection molding are reported in the table below:

Polymer family	Application
Polyoxymethylene	Micro gears and filters
Liquid-crystal polymer	Connectors, ferrules and microelectronic devices
Polyamide	Micro gear wheels
Polysulfone	Housing for microfluidic devices
Polycarbonate	Lens and sensors for optical applications devices
Polyether ether ketone	Micro bearings & pistons
Poly	Optical fiber connector
Polyethylene	Child parts of micro actuators
Polylactic acid	Biodegradable implants

Machine used for micro injection molding

In the 1980s, micro injection molding techniques utilized traditional injection molding, but no dedicated machines were available until the mid-1990s. Currently, commercial micro molding systems are produced from Milacron, Arburg, and Sumitomo Demag as micro injection units for regular machines. At the same time, Wittmann Battenfeld, Babyplast and Desma are manufacturers of dedicated micro injection molding machines.
Milacron developed two types of micro injection units:

A two-stage and all-electric injection unit accomplished by an extruder and injection plunger
An all-electric injection unit with 14 mm screw to inject the polymer melt into the mold

Arburg developed a micro injection molding machine with an 8 mm injection to ensure high degree of dosing precision. This type of machine is combined with a second screw, which is responsible for melting and homogenous mixing of the material.
Sumitomo Demag developed a customized micro molding injection unit suitable for micro parts weighing of 5 g to 0.1 g.

Uses

Micro injection molding is widely used for parts and devices in the medical, pharmaceutical, electronics, automotive, optical and other industries. In general, the medical micro injection molding market is the leading one, due to an increase in the usage of sophisticated micro components for endoscopic surgery, minimally invasive treatments, point-of-care testing and other advanced technology developments.
Applications in other fields include parts for electric motors, micron-tolerance door components, thing-wall containers, etc.