LSR Materials Property
LSR is a thermoset material, it has unique design characteristics different from thermoplastic injection molding. Therefore, LSR design considerations are less strict. Unlike thermoplastics are softening once heated, thermosets like LSR are cured in high heat temperature with various forms. LSR can withstand high temperature easily, while thermoplastics melt. So it is most suitable for high-heat automotive and industrial applications, or medical products with high temperature sterilization.
Liquid silicone rubber also can maintain its flexibility in low temperature of nearly -30℃. LSR materials have high flexibility degrees and varies with different compound. Such as: KE-2090/50 has elongation at break of 420 %, while KE-2090/40 has elongation of 630 %, and KE-2090/70 has elongation of 230 %.
In addition, LSR compound have different available hardness, we can select different LSR materials to match different application requirements. Such as: KEG-2000/50 has hardness of 50 Shore A, while KEG-2000/30 has hardness of 31 Shore A, and KEG-2000/75 has hardness of 75 Shore A.
LSR materials have excellent thermal, chemical and electrical resistance properties. However, it is easily corroded by solvent like gasoline or mineral spirits. It is an ideal material for elastomer in reason of its low compression set. LSR is also fire-retardant and never emit toxins or halogens in burning. This encourages it to be suitable for various safety applications.
Medical grade LSR is perfect for medical products with skin contact. Its stability can prevent affection between skin and medical parts surface. Specific grade LSR is capable for implantable applications in strict clean manufacturing environment. In addition, its hydrophobic property make it ideal for water-proof applications.
Reviewing LSR material specifications, consulting LSR molds for more detail information.
Thermoplastics vs Thermosets
Thermoplastics and thermosets are two main materials in injection molding process, as the natural properties of these two kinds of materials, the molding methods are opposite in some aspects. Thermoplastics like polyethylene, polypropylene, polycarbonate and ABS, are solid at room temperature, soften once heated, then re-solidify as cool down. This makes thermoplastics an excellent recycling materials. On the contrary, thermosets are typically in gel form, and solidify permanently once heated with catalysts. This guarantee the superior performance at high temperature, but not suitable for recycling.
The natural property differences of thermoplastic and thermosets determine the physical response to heat, and also define final respective molding processes. Thermoplastic pellets will be heated to liquefy before injection and then cool down in molds before ejection. Thermosets resin will be chilled before injection and then heated for curing in molds.
The cross-linking in thermosets materials determine the characteristics of strength, stability and heat resistance. However, thermoplastics don’t have these cross-linking in molecular structures.
Shear Thinning
Liquid silicone rubber is a shear shinning fluid, this is also as a pseudo-plastic. Shear thinning is the reduction of fluid viscosity once shear stress increase, it impacts LSR behaves in the mold significantly. Furthermore, it has both positive and negative effects on LSR molding. This will improve LSR flow through thin wall areas, and reduce the requirement of maintaining uniform wall thickness. Otherwise, LSR flows easily in molds, it has high tendency to flash and create unwanted vestiges. This must be removed by secondary operations. We should prevent and minimize flash by careful mold design, but it is still a critical factor in part design. In some case, LSR materials can flash into gaps as thin as 0.005 mm.
Molded LSR parts will continue shrinkage once removed from molds in the cooling down process. This is in reason of high mold temperature. Normally, molded LSR parts tend to stick on the mold half with the greatest surface area, rather than core area.
Due to LSR materials flexibility, different parts of molded piece may stick on different molds surface, and cause parts deformation once molds open. In this case, we need to redesign the part to prevent this issue.
Design Considerations for LSR Parts
Design guidelines for LSR are similar to that for thermoplastic. However, because LSR is a more flexible and forgiving material than thermoplastics. LSR design considerations are more relaxed in certain areas.
Drafts are required on the planes, which are parallel to mold opening direction. In order to allow milling process in mold tooling. Normally, one degree per 25 mm of mold depth is sufficient. Scarping prevention during ejection will never need to consider.
Uniform wall thickness is not critical in LSR design, in reason of shear-thinning LSR can flow easily into mold cavities, and traverse thin walls without fill problems.
Sink is not a concern in LSR parts design, because of LSR material is solidified by heat and cured before it cools down. We can design LSR parts features thicker than thermoplastics material as required.
Undercut is the biggest difference between thermoplastics and thermosets LSR mold design. LSR is flexible, molded parts are easily peeled out of molds. While the features molded in thermoplastics, would be trapped behind undercuts. The only constraint of undercuts in LSR is the milling capability in molds tooling.
LSR Molding Design Tips
Miwo current turnaround time for LSR production is 3 weeks, our automated aluminum molding milling can apply LSR materials for cost-effective prototyping in earlier industrial processes, which result in easier redesign with less time and cost.
In order to expand our LSR molding capability, our engineering team list the current requirement and design consideration as following:
Maximum Size
Our maximum LSR parts size is 300 mm×200 mm×100 mm. This size will fit in the injection molds, normally, the part cannot occupy full mold cavities space.
Maximum Volume
As micro molding produces very small parts, the shot weight is very low. This gives rise to long time duration of polymer material staying in barrels. Furthermore, this long time expose will cause degradation of polymers. However, because of cool temperature in LSR barrels, LSR materials do not degrade or cure.
Wall Thickness
Our thinnest wall thickness can reach to 0.25 mm. However, the issues are how to mill thinner wall features and remove thinner wall parts from molds without defects. Variation of wall thickness is not a general problem, in reason of LSR shear thinning properties.
Rib Thickness
Normally, the rib thickness is in the range of 0.5 to 1.0 time of the adjoining wall thickness.
Inside Fillets Radii
Inside fillets radii should be equal to wall thickness, in order to ensure milling accessibility in mold manufacturing.
Sink
Although LSR molding has no issue of sinking issues. Eliminate unnecessary thick area to reduce material cost.
Parting Lines
Parting lines should be simple and short as necessary to avoid flash.
Manual Removal
As in LSR molding, the removal process is typically manual rather than ejector. LSR parts design should retain entire in one half mold, parts features should be above the parting line surface for further hand removal.
Undercuts
Simple undercuts can be removed easily from molds without any mechanical assistance. Otherwise, We should review each LSR parts design with undercuts on case-by-case basis.
Draft
LSR parts have general draft angle similar to thermoplastics. However, zero draft angle is possible on shallow LSR parts. LSR molding has less draft angle than thermoplastics, but require reviews on details cases.
Gates
In reason of LSR high flow rate, it only requires small injection gates. Normally, the gate location is on the thickest or broadest cross-section. Otherwise, LSR edge gates should place on surface without dimensional or cosmetic demand, in order to avoid vestige or blemish.
Vents, Overflows, or Tear Strips
Our engineering team will add vents, overflows, or tear strips in molds to improve final LSR parts quality. Although these will leave vestige on parts surface.
Tolerance
Our typical tolerances are liner tolerance of ± 0.08 mm, specific tolerances of higher than 0.01 mm per 1 mm.
Surface Finish
The actual finish of LSR molds will determine the final LSR product’s mechanical and visual outcomes. Our normal finishes range from smooth, high polished uniform to machined marks, special texture.
Miwo LSR Molding
Silicone micro molding is a complex technology, it consists of precision molds, fin-turned cells and advanced experience. Miwo is one of the best LSR molding manufacturer in China, we are popular for our precision and accuracy LSR parts manufacturing. Our engineering team has extensive experience in a wide of industries, we are able to satisfy any requirement of intricate geometries and mass production. No matter micro molding in LSR injection molding, LSR 2-shot molding, or LSR over-molding.
