A precision injection mold has dimensional tolerances controlled below 1/3 of the dimensional tolerance of products. The precision of the mold depends on the precision of the cavity size and number of cavities in the design, positioning of the cavity, precision of the parting surface, material selection, and dimensional tolerance. The thickness of the bottom plate, supporting plate, and cavity wall as well as the runner size are all important factors of the precision mold. Design engineering is equally important. The mold is usually made of alloy steel, which has high mechanical strength.
Mold Making
The mold inserts (or masters) can be fabricated by a variety of techniques. For large features(> 50um) with tolerances and repeatability in the range of about 10um, traditional computer numerically controlled (CNC)-machining and wire electrodischarge machining(EDM) of materials like tool steel and stainless steel molding are often accurate enough. The advantage of this technique is that the tool materials used are the same as those in conventional polymer mould, so their design,strength, and service life are well established. Complicated 3-Dstructures can also be machined easily. The main drawbacks are that it is difficult to make sharp corners or right angles, and the surface quality is usually poor (surface roughness around several um). Diamond-based micromilling/microdrilling, micro-EDM, and excimer or femtosecond laser-based direct removal processes can reduce the surface roughness to 1 um or less. While diamond-based methods can also make features smaller than10 um, they are only applicable to “soft” metals such as nickel, aluminum, and copper. For prototyping, most of these methods can be directly used on polymeric materials to fabricate.
microfluidic devices. For smaller feature sizes (down to one micron or less), photo lithographic methods, e-beam lithography(EBL), or scanning probe lithography(SPL, such as AFM dip pen lithography have to be employed (i.e., surface machining).Here, a liquid photoresistor self-assembled monolayer(SAM) is placed on a galvanic starting layer bye ither spin-coating, thin film deposition or self-assembly. The microfeatures are formed after either radiation exposure through a photomask and development or direct e-beam or scanning probe writing. For prototyping, this photoresist structure can serve as a microdevice itself or be used as a mould (called photoresist mold) in low temperature and low-pressure mould processes. More generally, this structure is either used directly for electroplating or for wet/dry etching of silicon, which is subsequently electroplated. Both technologies yield a metal tool, usually nickel or nickel-cobalt. For features with a low aspect ratio (defined as the ratio of feature depth to width) or for rapid prototyping where the lifetime of mold inserts is not crucial, a glass or silicon wafer etched by wet or reactive-ion etching (RIE) can be utilized directly as a mold insert. For very small features (< 1 um) with high aspect ratios(up to 100 or higher), technologies like LIGA in thick resists (like EPON SU-8) or Deep RIE (DRIE) are needed to obtain the mold insert.