To translate optical technologies in synthetic material requires a high degree of precision in terms of mechanical engineering, tool or process engineering, in combination with a high degree of reproducibility and cleanliness of the production environment. Even smallest component defects or deficiencies such as fisheyes, silver streaks or flow marks will mean outright reject. Hence, the processing contractor has inevitably to be in possession of the required product, process or production know-how at highest levels. Determining process-relevant parameters or their tolerance limits and to establish permanent quality control for the products to be produced are very complex and costly challenges. In a word, the optical sector makes highest demands.
Group of companies project Optical Technologies 1&2
Collaborative project under the direction of Kunststoffinstitut Lüdenscheid, of ISK Iserlohner Kunststoff-Technologie GmbH and of the Fachhochschule Südwestfalen (South Westphalia university of applied sciences)
It is the purpose and objective of this collaborative project to resolve and clarify any open issues or questions in relation to the subject of "complex optimal elements made of plastic" with a clear reference to practical experience. Sumitomo (SHI) Demag makes use of latest machine technology to advance and develop further the area of competence of all-electric injection moulding machines.
Comparison of processes for injection compression moulding – an approach to a solution for complex optical elements
Through injection compression moulding, an additional influence can be exerted on the conditions of production and thus the properties of optical elements. In addition to the capacity of massively influencing the filling and holding pressure cycles, compression moulding offers the benefit of influencing a whole range of qualitative benefits for optical injection moulded parts. Which benefits this process offers in terms of quality and reproducibility, has been explored in comparative studies for various optical materials performed on the IntElect model all-electric injection moulding machine. For more information, read the specialist periodical Kunststoffe international 06/2010, "Playing with Light".
Material handling for optical pellets
Ideally, the processing of optical materials starts upon the inspection of incoming goods, viz. the plastics pellets. Once the material has been released for production, the material must be subjected to proper predrying in terms of time and temperature. Permanent control of the residual moisture is recommended. In addition, the feeding and drying systems should be dustproof and should not have an abrasive effect on the material or vice versa. Additional dust collection systems may be interposed just before the material enters the filling zone of the injection moulding machine. The feed characteristics, the dosing consistency and venting for precision applications can be enhanced through maintaining a uniform hopper filling level. As a basic principle, material travels should be short and manageable, and the pellet feeding rate should not be excessively high.
Long-term transparent plasticization
The plasticizing system as such, viz. the plasticizing cylinder as well as the screw and the back-flow lock including the respective protection against adhesion or abrasion, must be purposefully designed. Here, the required energy absorption and feed characteristics of the material to be processed are of particular importance. In many cases, the filling, compression or metering zones will have to be optimized by customizing the length, flight depth, pitch or compression ratio respectively. As optical materials strongly tend to adhesion or blooming, adhesion or wear protection are equally important. Here again, current systems offer various options to permanently eliminate material residues and thus what is called "black spots". Still, the back-flow lock should close very rapidly and reproducibly even if the screw return is very minimal or if there is no screw return at all. In many cases, the geometries, flow cross-sections or approaching surfaces are adjusted accordingly to match the application. Sharp edges, deflections or dead zones must be positively avoided.
The right cylinder size will avoid the need for later compromises
The right design of the cylinder size is a decisive measurement for later quality. Here, specifically parameters such as the shot weight, stroke utilization, residence time or the required injection moulding pressure are of special importance. The shot weight is determined by the design or material specifications. These should be used to select the right screw size such that the stroke utilization will ideally be between twenty percent (20%) and sixty percent (60%), or, in an extreme case, no more than seventy-five percent (75%). The required injection moulding pressure must be absolutely considered very closely in the design and configuration of the system as transparent materials are mostly strongly viscous. In this context, also the holding pressure should be assessed accordingly.
Proper mould environment for visible cleanliness
The area of the mould or its environment determine the consistent and lasting cleanliness of all components. Unnecessary objects or areas must be avoided. A high degree of accuracy and care must be guaranteed whenever entering this sensitive area at all times. In some cases, encapsulated or metal-clad clamping or unloading systems composed of a laminar flow cabinet and a completely self-contained periphery may be used to keep the production area lastingly clean. It is worthwhile to consider antistatic paintwork for the machine. Hydraulic components should be kept away from the area where the mouldings are produced. Pneumatic valves must be provided with adequate fine mesh filter systems. Temperature control hoses should be as short as possible, and should provide adequate safety and flexibility even at very high temperatures.
Selection of the appropriate machine platform: all-hydraulic, all-electric, or hybrid?
The decision in favor of an all-hydraulic, an all-electric or rather a hybrid drive concept will depend on several factors. What will be the resulting shot weight? What component geometry and thus what holding pressure times will be required? What precision is required for what level of cleanliness? Generally, all-electric machine axes will move more precisely by one decimal power, and will be in a position to implement parallel machine movements in a more straightforward way. This is an essential requirement especially for simultaneous compression moulding processes via the clamping unit of the machine. Owing to the mostly mechanical power transmission, the approached positions are maintained in a very precise and stable manner. Also, there will be more energy saving in the passive stages of the process such as residual cooling time. However, electric machines will also have a clearly higher installed power as compared to similar hydraulic concepts. The major benefit of oil-driven injection moulding machines undoubtedly is their long holding pressure capacity at a high level. This is supplemented by a higher injection capacity such as it can only be provided by hydraulic accumulators. In conclusion, it makes sense to make a product-related comparison of potential machine concepts with a view to analyzing which solution is best for your specific product. Just contact us, we will be happy to help.
Basic considerations regarding the processing of transparent plastic materials
When processing transparent materials in the injection moulding process, machine downtimes can often be minimized by merely following some basic rules.
Here is our advice when it comes to the processing of transparent materials:
- Amply heat the system before first-time dosing
- Allow the mould to preheat for a sufficient time (homogeneous temperature distribution)
- Amply flush the plasticising cylinder with the material to be processed
- Visually inspect the melt quality before switching to Fully Automatic mode, continue flushing as and if required
- Use go-parts once a thermal equilibrium will have adjusted only
- Do not discontinue or interrupt the process, if possible -> avoid idling time as much as possible
- Keep interruptions as short as possible, make provisions
- For extended interruptions, lower the cylinder temperature to the value specified by the manufacturer
- When cooling to room temperature, amply flush the plasticizing cylinder and drain the unit last
- For change of materials, flush with appropriate cleaning pellets or tracing material
- For very long periods of production, mechanically clean the cylinder, the screw and the back-flow lock at regular intervals
- Optical or technical lenses (such as converging lenses, divergent lenses, magnifying glasses)
- Light transmission and lighting design elements (such as signal panels, signs)
- Headlights and lamp lenses (such as for front headlights or backing lamps, indicators)
- Glazings or optical orifices (such as side windows, speedometer shades)
- Displays or optical sensors (such as cell phone displays, rain light sensors)
- Light structural components or optical surface structures (such as Fresnel surfaces, Lotus sheeting surfaces)
- General optical functional components (such as medical products, packagings)