【正文】 獲得材料間好的結(jié)合也經(jīng)常取決于當(dāng)?shù)诙牧献⑸鋾r(shí)第一材料的溫度 Secret of successful thinwall molding Demands to create smaller, lighter parts have made thinwall molding one of the most sought after capabilities for an injection molder. These days ,”thin wall” is generally defined by portable electronics parts 32 having a wall thickness less than 1mm . for large automotive parts , “thin” may mean 2 mm . In any case, thinner wall sections bring changes in processing requirements: higher pressure and speeds, faster cooling times, and modification to partejection and gating arrangements .These process changes have in turn prompted new considerations in mold ,machinery ,and part design Machinery considerations Standard molding machinery can be used for many thinwall applications. Capabilities built into newer standard machines go well beyond those of 10 years ago. Advances in materials, gating technology and design further expand the capabilities of a standard machine to fill thinner parts . But as wall thicknesses continue to shrink, a more specialized press with higher speed and pressure capabilities may be required. For example, with a portable electronics part less than 1 mm thick, fill times of less than sec and injection pressures greater than 30,000psi are not unmon. Hydraulic machines designed for thinwall molding frequently have accumulators driving both injection and clamping cycles. Allelectric and hybrid electric/hydraulic models with high speed and pressure capabilities are starting to appear as well. To stand up to the high pressures involved, clamp force should be a minimum of 57tons/sq in. of projected area. In addition,extraheavy platens help to reduce flexure as wall thicknesses drop and injection pressures rise. Thinwall machines monly have a 2:1 or lower ratio of tiebar distance to platen thickness. Also, with thinner walls, closedloop control of 33 injection speed, transfer pressure,and other process variables can help to control filling and packing at high speeds and pressures. When it es to shot capacity, large barrels tend to be too large. We suggest you aim for a shot size of 40% to 70%of barrels capacity . The greatly reduces total cycle time seen in thinwall applications may make it possible to reduce the minimum shot size to 20%30% of barrel capacity, but only if the parts are thoroughly tested for property loss possible material degradation. Users must be careful, as small shot sizes can mean longer barrel residence times for the material ,resulting in property degradation . Molds: make ‘em rugged Speed is one of the key attributes of successful thinwall molding. Faster filling and higher are required to drive molten thermoplastic material into thinner cavities at a sufficient rate to prevent freeze off. If a standard part is filled in 2 sec, then a reduction in thickness of 25%potentially can require a drop in fill time of 50%to just 1 sec. One benefit of thinwall molding is that as wall sections drop, there is less material to cool. Cycle times can drop by 50%with aggressive wallthickness reduction. Careful management of the meltdelivery system can keep runners and sprues from diminishing that cycletime advantage. Hot runners and heated sprue bushings are often used in thinwall molding to help minimize cycle time. Mold material should be reviewed too. P20 steel is used extensively in conventional applications, but due to the higher pressures of thinwall molding, molds must be built more robustly. H13 and other tough steels add an extra degree of safety for thinwall tools.[If possible, 34 you will also want to select a molding material that doesn’t accelerate mold wear when injected into the cavity at high speeds.] However, robust tools cost moneypossibly even 30% to 40%more than a standard mold. Yet the cost is often offset by increased productivity. In fact, the thinwall approach is frequently used to save money on tooling. A 100% increase in productivity can mean that fewer molds to be built, thereby saving money over the life of a program. Here are some more tips on tool design for thin walls: For aggressive thinwall applications, use steel harder than P20,especially when high wear and erosion are expected. H13 and D2 steels have been successful in gate inserts. Mold interlocks sometimes can stave off flexing and misalignment. Cores that telescope into the cavity can help reduce core shifting and breakage. Use heavier support plates[often 2 to 3 in thick]with support pillars [typically preloaded in]under the cavities and sprue. Use more and large ejector pins than with conventional molds to reduce pin pushing. Consider strategic placement of sleeve and blade knockouts. Injection Molding Troubleshooter Avoid Pitfalls in MultiMaterial Molding Injection molding with two or more materials requires either a twoshot molding approach or a simultaneous coinjection technique. Regardless of the process used, molders face the same challenges in achieving high part quality. Three mon problems with any multimaterial process are insufficient chemical or mechanical bonding of the polymers, inplete 35 filling of one or more ponents, and flashing of one or more ponents. These conditions can occur whether the materials binat