Our engineering team conclude the critical liquid silicone rubber (LSR) molding design guidelines. In order to improve silicone parts mold-ability, enhance cosmetic appearance, and save overall production time and cost. Liquid Silicone Rubber Molding is a cost-effect silicone parts manufacturing technology, it is applied widely in various industry.
Once design a component for LSR molding, we should meet several challenges. In order to create proper molds for high level accuracy and quality of final products. Liquid Silicone Rubber(LSR) Molding requires tighter tolerance than other traditional thermoplastic molding technologies in reason of flashing.
This LSR molding design guide will concentrate on tolerances, accuracy, shrinkage, gates, undercuts, drafts, finishes, and other features. Furthermore, these guidelines will explain the complexities in LSR designing, and ensure the manufacturability of design parts.
Liquid Silicone Rubber materials have unique advantages over traditional thermoplastics. Otherwise, we also need to consider the specific design rules.
LSR Molding Design Considerations
Liquid Silicon Rubber (LSR) is an elastic material, flashing occurs frequently in the removal process. Therefore, we don’t often use the ejector system as traditional thermoplastic molds. LSR molds normally comprise two or three plates. So we require undercuts and other design features for LSR molds to ensure consistency in manufacturing.
In addition, LSR elasticity will result ino hot tearing in force ejection process. It is important to consider LSR shrinks, the shrinkage rates vary in different LSR grades. This make it difficult to determine the precise rate, so prototypes are critical in design process for tiny details.
LSR Materials Specifications, Grade, and Durometer
LSR is bio-compatible, it is applicable for medical technologies with body implementation. Medical grade LSR can be used for healthcare components, it requires sterilization in medical standards.
The high elasticity of LSR can reach to almost 1000% elongation of the highest grade LSR. However, high elongation also give rise to difficulty in removal process. On the other hand, it will achieve greater undercuts in ejection process. In addition, LSR is also non-conductive, it provides unique electrical and thermal properties. Our standard LSR curing temperature is between 130℃ and 200℃.
Durometer refers to the hardness of cured LSR materials, it ranges from 0 to 80 Shore A. Normally, different silicone manufacturers provide variation in properties of LSR materials, which are normally specified by durometer and grade.
Miwo LSR Molding Materials
Miwo has various LSR molding materials as following:
1. Standard LSR (form 30 to 80 Shore A)
- KEG-2000 Series
- Elastosil 3003 Series
2. Medical grade LSR
- Dow Corning QP1-250
- KET-1001 Series
- KE-1950 Series
3. Optical grade LSR
- Dow Corning MS-1002
- KET-3008 Series
4. Fluorosilicone (fuel and oil resistance)
- Silastic FL60-9201
LSR Part Size
Miwo LSR molding maximum dimensions as following:
- Size: 300 mm×200 mm×100 mm
- Volume: 400 cm3
- Depth: No deeper than 50 mm from parting line
- Projected mold area: 400 cm2
- Tolerance: Miwo can produce silicone parts with a molding tolerance of 0.08 mm, and a liner tolerance of 0.01 mm.
LSR part manufacturability is influenced by part size and production volume. LSR component size and dimensions will impact injection molding and ejection process significantly. Furthermore, thin geometries are easy to blemishes or surface irregularities in manufacturing. In reason of LSR natural high liquidity, it can fill in thin walls easily. For well-design LSR parts, we should achieve tighter linear tolerances to avoid flashing.
Accuracy
LSR parts requires high dimensional accuracy, Miwo can achieve the tolerance of ± 0.08 mm in well-designed LSR parts. For specific tight tolerance, we can reach to ± 0.01 mm. Furthermore, we need specific prototyping to refine and optimize LSR component design before mass production. Such as, evaluate shrinkage rate by prototyping and testing of final LSR materials. This will achieve detail vales in guideline for LSR component design.
Draft Angle
In LSR molding design, we recommend adding draft angle in LSR parts design. Draft angle will facilitate parts ejection from the molds, and improve the overall mold-ability. Our engineering team summarize normal LSR features with recommended draft angles:
| Normal features | Recommended draft angels |
| Vertical faces | 0.5° |
| Most situations | 2° |
| Shut off | 3° |
| Light texture (PM-T1) | 3° |
| Light texture (PM-T2) | 5° |
In reason of LSR natural elongation, it allows more stretched draft rules than thermoplastics in mold construction permits. Although common LSR parts draft angle is 2 degrees, there will be zero draft angle occasional on shallow LSR parts. LSR natural mechanical properties and well deformation make these zero draft angle possible. However, we should pay attention to draft location, in order to reduce limitation of the parting line and improve LSR parts manufacturability.
Wall Thickness
Our engineering team recommended wall thickness considerations in LSR molding:
- The minimum wall thickness can reach to 0.2 mm. However, this is depending on wall size and location of adjacent sections.
- Ribs thickness ranges from 0.5 to 1.0 time of the adjoining wall thickness, in order to ensure structure stability.
- The radii of inside fillets should be almost equal to the wall thickness typically, in order to make LSR materials achieve corners area smoothly. Otherwise, too much lager or smaller radii will give rise to porosity on LSR parts surface.
- Different features with various wall thickness should be adjusted to eliminate effects in injection molding process. Proper wall thickness will guarantee features of function and reduce risk of cosmetic defects
Because of LSR materials shear shinning, variation of wall thickness is permitted in LSR designing and molding process.
Undercuts
One of the greatest advantage of LSR molding is producing parts with undercuts. In most case, simple undercuts are removed easily by press operators without mechanical assistance. However, undercut will complicate parts geometries, and prevent part ejection process in some case. So each LSR part design should be reviewed for feasibility on a case by case basis, sometimes to eliminate undercut once possible. In addition, LSR elongation will limit the undercut size, complex and sharp edge tooling will increase damage risk of molded parts.
Radii
As LSR molds are manufactured by automated CNC machining technology, normally part corners will be a radius rather than a sharp edge in LSR molds. We don’t need to change in parts model, as final radii will be identified by CNC machines before the mold is milled. Otherwise, with specific sharp edges demands, our engineering team will find a perfect solution for your LSR molding project.
Shrinkage
The shrinkage rate of LSR will impact overall quality of molded silicone parts. Because of difficult prediction of shrinkage, we require a lot of adjustment before production. There are many factors to affect shrinkage rate, including: tools temperatures in molding and ejection, pressure in mold cavity, direction of material flow from injection point.
As the normal shrinkage rate of LSR is 2% to 3%, it is crucial to adjust LSR design for shrinkage. Molding similar shape and size in existing mold will provide better LSR materials behaviors, so materials testing and proper curing method will eliminate design flaws, in order to guarantee final dimensional accuracy.
Parting Lines
Parting lines connect different mold pieces together and create a closed cavity for molding. It aligns with the parting surface to prevent lines or flashing in the molding process. In some specific cases, more than 2 parting lines should be considered to avoid irregularities on part edges.
Injection Gates
LSR molding has smaller gates than traditional plastic molding, in reason of LSR natural high liquidity. These gates are normally placed on surface without critical dimensional and aesthetic requirement. In order to avoid blemishes on finished surface. The vent or overflow area also can be designed in LSR parts to improve final quality by air exiting in molding.
Aesthetics
Different design principles for LSR molding will create higher quality and aesthetic products. To consider different material types and behaviors, we can incorporate undercuts and gates placement to reduce common blemishes or surface irregularities.
Except better technique design, mold finishes and tooling are also used to improve product aesthetic. Machined finish within mold cavities can provide smoother surfaces, special texture are also achieved by enhanced surface features in mold cavity.
Conclusion
Miwo is the top LSR molding company in China, we provide LSR products with excellent quality control and unsurpassed manufacturing capability. Contact us now for more LSR molding design information.
