Injection compression technology is a low-pressure process employed in the production of low-stress moldings. It is also used for back-molding of textiles or films (in-mold-decoration) where it serves to protect decorative elements from high injection pressures. In these cases the melt is injected into a mold which is not fully closed. During or immediately after completion of the injection process the mold is fully closed. With thick-walled parts (such as optical lenses) the compression process exerts pressure evenly over the whole parting line of the mold during cool time thus evening out shrinkage. For thin-walled parts, injection compression offers better material orientation and reduced stresses. This is especially true for thin-walled molds with microstructures, such as optical data media.
The following chart presents an overview of the parts that are usually produced using injection-compression technology.
| Parts | Requirements | Clamping Force Range [kN] |
| Optical components, lenses, prisms | Low double refraction, optical quality | 400 to 1,500 |
| Optical data storage (CDs, DVDs) | Low double refraction, high store, good molds for small structures | 350 to 650 |
| Medical devices, cuvettes | Low double refraction, optical homogeneity | 1,500 to 3,000 |
| Thin-walled parts for technology applications (plugs, housings for mobile telephones, laptops, etc.) | warp-free, form consistency for higher temperatures, low residual stress | 800 to 3,000 |
| Auto part panes (mirrors, windows etc.) | Optical homogeneity, distortion-free | 1,500 to 40,000 |
| Passenger vehicle interior surfaces with textile or back molding | Protection for decorative elements, warp-free components | 2,500 to 13,000 |
Chart: Examples of injection compression applications
Machine- or a Mold-Induced Processes
The compression process can be effected by the machine or via hydraulic pistons inside the mold. Machine-induced injection-compression uses molds with shut-offs on the parting line or the outer contour of the mold can be realized with the placement of springs. Mold-induced injection-compression requires moveable mold inserts in the cavity which can be activated by the core pull program. The injection-compression process can be controlled from the compression path (clamping force or piston pressure) or from the internal mold pressure. Often part microstructures can be optimized by gradually building up (or reducing) clamping force during molding.
Expansion injection-compression for foam injection molding
Expansion injection-compression for foam injection molding presents a special case. Here, after the melt and a blowing agent are injected into the completely closed mold, the cavity is opened by a pre-determined amount. The result is a well-defined, integrated foam structure with a uniform cell distribution in the core and an only slightly foamed skin.
A Solid Foundation
Injection-compression technology is available in the entire machine series (MAXIMA, ELEKTRA and K-TEC). For the production of optical molds the ELEKTRA series, in combination with a fully electric injection-molding machine, is the best there is when it comes to precision. The ELEKTRA disco is an injection molding system specially designed for the production of optical data storage units including CDs and DVDs. Our applications team is there to help you find the most suitable machine configuration and the best process technologies for your needs.
