There are few applications in which materials quality is more important than in the production of polycarbonate ophthalmic lenses. The slightest variation from perfection can alter the prescription and render the end product useless.
Critical Production Parameters
Traditionally, optical lenses are manufactured at high pressure using injection moulding. High melt pressure and high mould temperatures are required in order to achieve the necessary quality when the part thickness is more than 8 mm. Both the filling phase (when the material is injected through an injection gate into the cavity) and the holding pressure phase (during which the part is formed) are long, and large cold runners and gates are necessary, all of which has a significant effect on the productivity of the process. In addition, compensating for material shrinkage during the holding phase by screw movement automatically gives rise to high stress levels in the part.
Expansion Compression Moulding
An innovative development from Krauss Maffei, showcased at last year’s K show in Germany, could be set to revolutionise the production of optical lenses in terms of quality and productivity. This claim is based on a new highly-automated process, called expansion compression moulding (ECM). Centred around the company’s C Series C2+ machines the process overcomes many of the traditional drawbacks of injection moulding.
Advantages of Expansion Compression Moulding over Traditional Injection Moulding
In conventional injection moulding, the plastic melt is injected into a closed mould cavity, pressure is applied and the part cooled before the mould is opened and the solid component removed.
Traditionally, to get good part surface quality, high temperatures are required. The thicker the part, the higher the necessary mould temperature. In contrast, in the expansion compression moulding process, a thinner part is injected, so lower temperatures can be employed. Expansion compression moulding is a two-stage process. In the first stage a thin part is formed in a downsized cavity under low mould temperatures (80°C, as opposed to 110°C) using small cold runners in stage two, pressure is constantly applied to the melt, which gradually causes the mould to open. This expands the lens to the required thickness without applying excessive stress to the material, and produces semi-finished corrective lenses with high dimensional precision.
Other Features of the Expansion Compression Moulding Technique
Using lamps on robotic arms to heat up the mould cavity brings a further temperature advantage of about 40°C. The reduced size of the cavity helps avoid ‘jetting’, a turbulent flow of melt. A material drying and de-dusting system is used and the mould clamp area meets the required specifications for cleanroom production.
Expansion Compression Moulding Moulds – Key Components
The key to the expansion compression moulding process is the unique linear platten design of the machines. The plattens, onto which the mould tools are fitted, enable the mould to be opened in a controlled, continuous manner, applying uniform pressure levels at all stages via screw movement. This controlled opening is not possible with most injection moulding machines, which use a toggle system.
Improved Quality of Expansion Compression Moulded Optical Lenses
Expansion compression moulding achieves the highest optical quality, enabling the production of lenses that comply fully with ISO 10322-1/2. Owing to the uniform pressure on the cavity surface, sink marks are eliminated and internal stresses on the material greatly reduced. Reject rates are also much lower and material waste is greatly reduced, with sprue weight savings of up to 70% achievable.
Productivity Improvements of the Expansion Compression Process
Productivity improvements compared to the conventional injection moulding process are a key benefit of expansion compression moulding - the previous long cycle times can be reduced by as much as 50% thanks to a lower mould pressure and the efficient temperature management system (surface heating combined with water cooling). Another benefit is process flexibility - because of the nature of the expansion process, different wall thicknesses can be produced from the same mould.
Versatility of the Expansion Compression Moulding Machine
In a further boost to profitability the C2+ machine can, when not being used for ECM processing, be put to work in conventional injection moulding with a simple tool change.
Other Features of Expansion Compression Moulding Machines
In addition to possessing a number of specific adaptations for expansion compression moulding, C2+ machines are supplied ‘cleanroorn ready’. The machine’s clamping unit is cantilevered, enabling the cleanroom to be planned so that only the clamping unit protrudes into the room, so reducing the space needed and the associated costs. Machines are rail-mounted, allowing them to be moved in and out of the cleanroom quickly and easily for tool changes and maintenance tasks. The flexibility of the machines means that future production expansion can be planned in at the initial stages, with a progression from clean area to complete cleanroom being achieved cost-effectively and with minimum disruption to production.
Expansion compression moulding offers significant advantages compared to existing manufacturing solutions for thick wall components such as ophthalmic lenses. it enables an increase in productivity, significant improvements in quality and reduction in reject rates, and flexibility in production from the same machine and the same process. Ophthalmic applications include convex and concave lenses, spherical and aspherical lenses and progressive lenses. The process can also be used for magnifier lenses.