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in most cases the two competing goals can be met. The moldmaker can machine vents to ± 0.00001 in. to ensure they work properly and generally hold vent depths to around 0.000020 in. (20 millionths). Some of the same defects—shorts, gas traps, and knit lines—can also result from mold heating problems. Inconsistent part dimen- sions could also be an indicator of improper heating. Common technical problems that cause uneven mold heating are improper sizing of the heaters relative to the size of the mold and improper heater spacing. Heaters must be spaced so that the cavity is heated uniformly, not just the backing plates. AUTOMATION Automation is another key concern for LSR molders. For example, the part must stay on the correct side of the machine every cycle so that automation can properly pull the part of. Since the parts are so small and the runs are often so large, consistency in automation is a key component. If parts do not stay in the same location every time, it makes the automa- tion more complex and expensive. Some reasons that parts don't stay on the same mold half from cycle to cycle include varia- tions in the material, which can contribute to parts sticking on the wrong side. Mold coatings, engineering changes to the mold or part, and processing technique are all keys to keeping parts on the correct side. Some LSR parts simply cannot be engi- neered in such a manner to ensure they will stay in the same place on every cycle, so more expensive automation is required. Overcome Typical Challenges in LSR Molding As the need for high-quality liquid silicone rubber (LSR) products has increased, so has the demand for machines and molds that support the production challenges faced each day. LSR molders often look for machines with all-electric drives, technology integration, and ability to run small parts consistently. Yet even with the repeatability aforded by high- quality machines and tooling, there are common day-to-day issues that every LSR run encounters. MOLD HEATING & VENTING Several mold issues can bring LSR production to a halt. LSR appli- cations often involve small parts that may weigh 0.017 g or less, which presents signifcant challenges. Molds for such small parts must vent air and gases efciently, as they can cause defects if not properly evacuated. In production, several defects that signal improper venting include short parts, knit lines, or fow lines. Short parts typically result from inadequate venting, improper process settings, or possibly an unbalanced cold deck. The size and style of vents are best determined at the prototype stage to prevent delays and rework later on. The moldmaker will design the vent depth and width to balance gas evacuation efciency with the need to control fashing of very low-viscosity silicones. Some part geome- tries require more aggressive venting and therefore a compromise with fash, but By Troy Smith, Roembke Mfg. & Andy Stirn, Milacron Here's a quick overview of basic mold and machine factors behind common LSR molding headaches and how to avoid them. Proper sizing and spacing of the heaters to provide uniform heating of the cavity— not just the plates—are essential. Venting, heating, automation, and controls are the key determinants of a successful mold and machine system for LSR. 56 DECEMBER 2014 Plastics Technology PTonline.com Troubleshooting