Miwo provides liquid silicone rubber molding for various silicone parts productions. Although LSR molding has many similarities with conventional injection molding, there are still some notable difference. Normally, thermoplastic resin is melted before injection, while LSR is consisting of two thermeset compounds, it is chilled before injection, heated to solidify in mold and cured ultimately into final parts. LSR molded state is permanent in reason of its thermoset polymer property, it cannot be melted again as thermoplastics.
LSR has its inherent characteristics, it is a strong, elastic materials with excellent thermal, chemical, and electrical resistance. It also can maintain physical properties at extreme temperature, and withstand sterilization. In addition, LSR also has bio-compatibility, it is ideal for medical products with skin contacting. All these properties encourage LSR applied in medical, automotive, electronic, infant care, and food appliance industries, in the form of seals, valves, cables and gaskets.
Optimal Design Tips
Although parts designing for LSR and thermoplastics are similar, there are some specific guidelines to consider in LSR molding.
Dimensions
In Miwo, we are able to produce the maximum LSR parts in size of 300 mm*200 mm*100 mm, The maximum part surface can reach to 400 cm2 and the maximum volume can get 400 cm3. While features are less than 50 mm depth from any parting lines. We should notice that deeper features are limited to a small and simple outline.
Wall and Rib Thickness
LSR has shear thinning property, this encourages it to fill wall sections as thin as 0.3 mm with minimal challenges in large areas, and are possible over smaller areas down to 0.2 mm. Detailed wall thickness also depend on wall size and location of adjacent thicker sections. Rib thickness is normally 0.5 to 1.0 time of the adjoining wall thickness. LSR can accommodate to wall thickness variations without sink issues.
Shrink and Flash
The shrink rate of LSR is fairly high, however, its shrinkage is consistent throughout the whole part with tolerance of 0.25 mm/10 mm.
LSR is easily to flash during molding process, even mold gaps as thin as 0.005 mm. We need to incorporate additional features in mold design, in order to reduce flash possibility.
Parting Lines
In LSR parts design, simplify and minimize parting lines can accelerate LSR molding and solidification process, and assist final cleaner LSR parts removal as quickly as possible.
Undercuts
LSR is more easily molded to accommodate parts with undercuts comparing to thermoplastics, as these LSR parts can be removed manually by our press operators. We applied mechanical tooling to release undercut selectively. LSR molded pats are easily torn at shape edges, so it is critical to add fillets or radii to prevent tearing at general sharp edges. Especially for interior feature with final manual removal undercut.
LSR Over-molding
LSR materials are widely applied for over-molding, in order to add another resin layer on existing molded parts. This can provide a characteristic of multi-materials combination. One of the most popular applications is adding the soft, functional, and hand-friendly layer of liquid silicone rubber (or thermoplastic elastomer) over a hard substrate. Another application is enhancing cosmetic appearance by over-molding different colors or finishes on molded thermoplastic parts.
LSR Insert Molding
LSR insert molding is normally used to increase LSR parts strength and durability. The common LSR methods are bolstering certain pivotal points of LSR parts with metals or high temperature plastics. The metal items include brass bearing journal, stainless steel threaded inserts, stainless steel mobile-phone card tray. In reason of high cure-temperature in LSR molding, the recommended plastics include PEI, VALO or PEEK. In LSR insert molding process, the mechanical bonding features are required to maintain final parts durability. Because LSR generally don’t have chemical bond to other materials.
Part Ejection
Because of LSR natural flash properties, LSR parts are removed manually from molds, rather than application of ejector pins. Thus, in LSR parts design, we should make sure that the entire LSR parts will be retained on one mold half once mold opened. Then these LSR parts can be manual removal with air assistance.
Conclusion
Miwo has been in LSR molding industry for a long time, our high production capability can satisfy various volume requirement range from 1,000 to 10,000 per week.
If you are preparing a new LSR molding projects, contact us for further design analysis and interactive quotation.
