Thin-wall applications with long flow length-to wall ratios of 150 to 400 have been used in the packaging industry for years. Now thin-wall technology is finding increasing usage in other industries (such as telecommunications, household appliances, auto parts and medical technology). Compared with packaging, these areas demand more in terms of precision and materials. The injection and plasticizing performance requirements are far greater than those of conventional injection molding. These requirements include:
- High injection speeds and pressure
- High dynamics for the injection movement
- Excellent response of injection movements
- High plasticizing rates
- Gentle melt preparation
- Excellent thermal and mechanical melt homogeneity
- Preferably lower melt temperatures
- High repeatability for all processes.
Implementing parallel functions with the mold movement, such as the injection unit- or core movement, plasticizing (e.g. an electric screw) requires very high cycle times.
Minimum Fill-Time with the Highest Injection Performance
A quick fill-time (between 0.1 and 0.3 sec.) minimizes the inner stress in the molding (less distortion from a shorter holding pressure) and also reduces the necessary clamping force by up to 40%. Fast filling has a positive impact on more than just part quality and reduced clamping force requirements: the cycle times benefit as well. To do this, a machine with a faster injection rate capacity and higher injection pressure capabilities is required. In borderline cases melt pre-compression can be used.
Using High-performance Screws for Optimum Melts
Since the cooling time for thin-walled articles is short, the plasticizing process must occur as quickly as possible. Although there is a weight savings with thin-walled parts, actual shot weights may be quite high if multi-cavity molds are employed. A homogenous melt can be achieved using high-performance screws (e.g. specially designed three-zone screws or barrier screws) with a mixing head. The HOP screw has been successfully employed with Polyolefins. Wear to the barrel and screw is increased when filled or reinforcing material is used. For this reason the design of the plasticizing unit has to be adapted so that it can withstand the effects of abrasion, corrosion and adhesion caused by the material.
Intelligent Technology – The Pack Pressure Device
An important innovation in thin-walled injection molding technology is the functional decoupling of the injection from the holding pressure phase through the use of a pack pressure device. Using a double piston, the pack pressure device provides holding pressure at the same time as shot recovery. This means that more than 80% of the cycle time is available for processing the melt. The result is a thermally and mechanically homogenous melt (no unmelted material, and good color dispersion). Choosing the right screw diameter ensures optimum injection strokes and short residence times.
A Solid Foundation
Our high-performance machines in the MAXIMA topline and K-TEC series are built to meet the demands of thin-walled injection molding technology. When it comes to choosing the most suitable configuration of machines and the mold design our applications team can provide technical advice and part cost information.
