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Overcoming the Challenges in Using LSR Molding

by Nice Rapid | Oct 19,2022 | Liquid Silicone Rubber Molding

In recent years, liquid silicone rubber (LSR) injection molding has become increasingly popular in several industries. LSR is a two-part platinum-cured silicone material with a wide range of properties, making it an ideal choice for many applications.

Despite its many benefits, LSR injection molding can be challenging. It requires precise temperature and pressure control, making the material difficult to process. This blog post will discuss some of the challenges in using LSR molding and how to overcome them.

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Mold Heating and Venting

Several mold problems have the potential to stop LSR manufacturing. LSR applications sometimes contain tiny components that might weigh 0.017 g or less, which poses difficult problems.

Molds for these tiny components must effectively vent air and gasses, since they can lead to flaws if not done. Poor venting is indicated by short portions, knit lines, and flow lines. Short parts may arise from poor process parameters, insufficient venting, or even an uneven cold deck.

To avoid delays and rework later, it is preferable to decide on the size and design of vents at the prototype stage. The mold maker will design the vent depth and width to balance gas evacuation effectiveness with the requirement t prevent the flashing of very low-viscosity silicones.

Although some component geometries necessitate more aggressive venting and hence a compromise with flash, the two opposing objectives are typically reconcilable. Vents in the mold may be machined to 0.00001 inches to ensure they function correctly and typically limit vent depths to around 0.000020 inches.

Mold heating issues may also cause flaws, such as shorts, gas traps, and knit lines. A sign of poor heating might also be inconsistent part dimensions. Common technical issues that result in uneven mold heating include incorrect heater spacing and incorrect heater sizing concerning the mold size. In addition, heaters must be placed at appropriate intervals to heat the cavity rather than simply the backing plates evenly.

Automation

Another major problem for LSR molders is automation. For instance, the component must remain on the right side of the machine during each cycle for the automation to remove the part correctly. Consistency in automation is essential because of the tiny parts and frequently huge runs.

When parts are moved around often, automation becomes more difficult and costly. In addition, variations in the material might cause pieces to stick on the wrong side, which is one reason components don't stay on the same mold from cycle to cycle. To retain components on the right side, it is important to use mold coatings, engineering modifications to the mold or part, and processing techniques.

More costly automation is necessary because some LSR pieces cannot be designed in a way that ensures they will remain in the same location on every cycle. Therefore, creating a prototype is so important. Adjustments are simpler on a single cavity than on a mold with sixty-four cavities. The automation plan for a repeatable process and short cycle time may be developed using the prototype. Working with mold and machine makers with expertise in various LSR molding systems and who can deliver dependable, high-quality automation is also beneficial.

Machine Issues

The press needs a vacuum pump that is big enough and rapid enough to evacuate all the mold cavities to perform LSR efficiently and swiftly. While waiting for the cavities to be emptied, cycle time will lengthen if the vacuum pump is too small.

A valve-gated cold deck may be operated on the press thanks to provisions made within the machine controls. Although using a separate valve-gate controller is an option, this complicates the procedure by adding yet another piece of machinery. In addition, the right number of valve-gate controls must be installed on the new machines to accommodate the range of tasks they may be employed for. Starting with no less than eight valve-gate control solenoids is a decent rule of thumb.

Most cold decks are designed to be divided evenly by eight. Four of the eight solenoids can be used for a mold with four cavities. One solenoid may control one gate in an eight-cavity mold. The best level of control is provided by doing this, while a lesser amount of control may be obtained by driving several valve gates with a single solenoid. For instance, to exert some control over a 16-cavity mold, a bank of eight solenoids can be used to activate eight pairs of valve gates simultaneously. Operating 32 and 64 drops with eight solenoids follows a similar process.

The molder should understand the components and dimensions of the molds used in the press so that the machine is built with the ideal number of controls at the outset.

To debug the process within the constraints of the cycle, the press must also give appropriate feedback. The feedback systems must be able to read mold temperatures and provide a visual representation of all zones and their current operational status.

The machine's ability to vacuum the mold, as well as the injection rates, hold durations, and injection pressures from shot to shot, is an additional crucial piece of feedback. Using mold-cavity sensors, complete control of the molding cycle is facilitated.

In Summary

The LSR process is used to produce a variety of component parts with a wide range of applications, including everything from medical injection molding to high-strength, lightweight aircraft components. The process begins with a short curing period when the two parts (a liquid and a solid) are mixed. This is followed by a second period during which the mixture cures completely. During the second period, LSR performs the same as other thermal silicone molding processes. It is only after the mixture cures that it is used to produce parts.

During the entire process, the mold must maintain a temperature high enough to ensure the final component is cured. If the temperature falls below the required level, the mixture will not cure completely and may result in defects like air bubbles, voids, or broken molds. Even if the temperature rises high enough for complete curing, if the mold remains hot for too long, the surface may crack from thermal expansion.

For more details about LSR molding, contact Nice Rapid. They accept outsourced production of LSR products at affordable rates without compromising quality.

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