Precision glass moulding

Precision glass moulding is a replicative process that allows the production of high precision optical components from glass without grinding and polishing. The process is also known as ultra-precision glass pressing. It is used to manufacture precision glass lenses for consumer products such as digital cameras, and high-end products like medical systems. The main advantage over mechanical lens production is that complex lens geometries such as aspheres can be produced cost-efficiently.
Process.
The precision glass moulding process consists of six steps:
The process is executed on a specialized moulding machine, which precisely controls the temperature, travel, and force during the process. The tools used must withstand high temperatures and pressures, and need to be resistant to chemical interaction with the glass. The mold materials also have to be suitable for machining into the precise surface profiles.
Process chain.
In order to ensure the desired quality the parts are measured between each process step. Additionally, the parts are handled and transported carefully between the processing and metrology steps.
Tool and mould design.
Lens shapes.
Shape of optical element.
Precision glass moulding can be used to produce a large variety of optical form elements such as spheres, aspheres, free-form elements and array-structures.
Concerning the curvature of the lens elements, the following statements can be drawn: Acceptable lens shapes are most bi-convex, plano-convex and mild meniscus shapes. Not unacceptable but hard to mould are bi-concave lenses, steep meniscus lenses, and lenses with severe features (e.g. a bump on a convex surface).
In general, plano-curved lenses are easier to mould than lenses with both sides curved since matching of flat faces is easier.
Moulding concave forms with small centre thickness is difficult due to sticking of the moulded part to the mould occurring as a result of the different thermal expansion coefficients.
Furthermore it is recommended to avoid undercuts and sharp edges. For the lens design it should be considered that the lens has to be mountable in measurement systems.
Shape of preforms.
The shape of the preform or "blank" needs to be chosen according to the geometry of the finished optical element. Possible preforms are spherical (ball), near spherical (gob), plano-plano, plano-convex, plano-concave, biconvex and biconcave blanks. Ball and gob-blanks do not have to be premachined whereas other preforms require grinding and polishing.
The following section describes basic traits of preform choice:
“Used specifically for lenses with positive power: biconvex, plano-convex, and meniscus where the convex side is stronger than the concave side, this only works for a relatively small volume of material.”
“As a lens changes to negative in power biconcave, plano-concave, and meniscus
where the concave side is stronger, an alternative preform shape, plano-plano, is required
for the molding process. […] Relative to a formed preform an increase in cost is observed for the manufacturing of this type of preform.”
“When the geometry of a lens extends beyond the volume range of a formed ball
preform, a ground and polished ball preform is required. Used for lenses with positive
power: biconvex, plano-convex, and meniscus: where the convex side is stronger, this
geometry allows for molding of lenses with larger total volume. […] Relative to a formed
preform and a plano-plano preform, an increase in cost is observed for the manufacturing
of this type of preform.”
“The Lenslet preform is primarily for lenses with positive power, biconvex, planoconvex,
and meniscus: where the convex side is the strongest surface. The use of this
type of preform allows for molding of the largest volume of glass at any given time in the
molding machines. The Lenslet is traditionally ground and polished to a near net shape of the final lens, and then pressed. [...] The cost associated with the manufacturing of the lenslet preform is the highest of all preform types.”
Precision gobs can be used as preforms for the production of aspherical lenses in a precision molding process. They are manufactured from a continuous glass melting process. The resulting precision gobs exhibit a very smooth firepolished surface with an excellent surface roughness and high volume accuracy.
Dimensions.
The dimensions of the optical elements that can be moulded depend on the size of the moulding machine.
The precision glass moulding process is not limited to small optics. For the right element geometry, it can enable economical production of aspheric lenses up to 60 mm in diameter and more than 20 mm thick.
General design recommendations:
Size:
Radius:
Optical Surfaces:
Volume:
Tolerances.
Although the form, dimensional and positional tolerances that can be achieved in precision glass moulding are subject to a natural border, the values being achieved in practice strongly depend on the degree of control and experience in mould making and moulding. The table below gives an overview of achievable manufacturing tolerances in precision glass moulding at different companies.
For aspherical lenses, the design should be able to tolerate 0.010 mm of lateral shear between surfaces plus 5 micrometres Total Internal Reflection of wedge (across the part without considering the lateral shear) to be considered robust.
Specifications for aspheres:
Index drop.
Due to the fast cooling after moulding, the part retains a small amount of residual stress. Consequently, the glass exhibits a small change in the refractive index which has to be considered in the optical design. A higher cooling rate corresponds to a larger decrease of the refractive index. A lower cooling rate could circumvent the index drop, but would be less cost-efficient
Glass material.
Many glasses can be used with PGM. However, there are some limitations:
So-called "low-Tg-glasses" with a maximum transition temperature of less than 550 °C have been developed in order to enable new manufacturing routes for the moulds. Mould materials such as steel can be used for moulding low-Tg-glasses whereas high-Tg–glasses require a high-temperature mould material, such as tungsten carbide.
Substrate materials.
The mould material must have sufficient strength, hardness and accuracy at high temperature and pressure. Good oxidation resistance, low thermal expansion and high thermal conductivity are also required.
The material of the mould has to be suitable to withstand the process temperatures without undergoing deforming processes. Therefore, the mould material choice depends critically on the transition temperature of the glass material. For low-Tg-glasses, steel moulds with a nickel alloy coating can be used. Since they cannot withstand the high temperatures required for regular optical glasses, heat-resistant materials such as carbide alloys have to be used instead in this case. In addition, mould materials include aluminium alloys, glasslike or vitreous carbon, silicon carbide, silicon nitride and a mixture of silicon carbide and carbon.
A commonly used material in mould making is tungsten carbide. The mould inserts are produced by means of powder metallurgy, i.e. a sintering process followed by post-machining processes and sophisticated grinding operations. Most commonly a metallic binder (usually cobalt) is added in liquid phase sintering. In this process, the metallic binder improves the toughness of the mould as well as the sintering quality in the liquid phase to fully dense material.
Moulds made of hard materials have a typical lifetime of thousands of parts (size dependent) and are cost-effective for volumes of 200-1000+ (depending upon the size of the part).
Mould manufacturing.
This article describes how mould inserts are manufactured for precision glass moulding.
In order to ensure high quality standards metrology steps are implemented between each process step.
In order to save the quality and enable an early warning in case of any problems between every single step there has to be a step of measurement and referencing. Besides that the time for transport and handling has to be taken into account in the planning of the process.
Metrology and quality assurance.
Once process and tool have been developed, precision glass moulding has a great advantage over conventional
production techniques. The majority of the lens quality characteristics are tool-bound. This means that lenses, which
are pressed with the same tool and process, usually have only insignificantly small deviations. For example, an important characteristic of a lens is the form of the optical surface. In the case of aspherical lenses the measurement of optical surfaces is very difficult and connected to high efforts. Additionally, when working with tactile measurement systems there is always a risk that the optical surface might be scratched. For precision moulded lenses such measurements are only necessary for a small amount of sample lenses in order to qualify the tool. The series production can then be executed without further need for measurements. In this case, only the cleanliness of the optical surface has to be monitored. Another advantage is that the lens' center thickness can be estimated from the easily measurable edge thickness or by applying a contactless measurement system.
Protective coatings.
In order to enhance the mould insert's lifetime, protective coatings can be applied. “The materials that have been selected for the antistick coatings can be divided into 5 groups including: (1) single layer carbides, nitrides, oxides and borides such as TiN, BN,
TiAlN, NiAlN, TiBC, TiBCN, NiCrSiB and Al2O3, (2) nitrides or carbides based gradient and multilayers, (3) nitrides based superlattice films, (4) amorphous carbon or diamond-like carbon and (5) precious metal based alloys”
Experiments carried out by Ma et al. yield the following results:
“The higher the temperature, the smaller the wetting angle between glass gob and substrate could be observed. This indicates that severe interface chemical reaction occurred and resulted in the loss of transparency in glass appearance. The wetting experiment in nitrogen ambient improved the sticking situation. The combination of chemically stable substrates and coatings, such as Sapphire (substrate) / GaN (film) and Glass (substrate) / Al2O3 (film) can achieve the best antistick propose. The precious metal films such as PtIr(Platinum, Iridium) coated on the ceramic substrates can effectively reduce the interface reaction between the glass and substrates.”
Although PtIr is used as a standard coating material, it has the disadvantage of being expensive. Therefore, research activities aim at substituting PtIr with cheaper materials.