Introduction
Mold design for liquid silicone rubber molding is very challenging, it is the most critical step in silicone parts production. We need to understand the basic information before start a LSR mold design. In most case, LSR tooling requires more precision and expertise than molds for thermoplastics. In reason of liquid silicone rubber (LSR) is processed differently from thermoplastics, it is difficult to translate certain strategies of tooling to LSR, even the most experienced thermoplastic tooling manufacturer.
Flash Prevention
Both LSR and thermoplastics molds are able to have high complex geometries. Furthermore, these molds must be designed to prevent flashing in the injection molding process. The main difference between LSR and thermoplastic molds are requirement of fitness and cavity inserts. As normal, thermoplastic molds can limit flashing by fitness of up to 0.05 mm insert. However, LSR molds with fitness of 0.0025 mm or less are also difficult to eliminate flash. This fitness requirement results in different level of machining and processing precision in mold manufacturing process.
Ejector System
Ejector system is the simple difference between LSR and thermoplastic injection molds. We commonly use straight-wall ejector pins in plastic molds, these pins can cross over the parting lines. Therefore, half of ejector pins are on parts and half on parting lines shutoff. This molding design in thermoplastics is typically no flash. However, ejector pins in liquid silicone molds have more strict demand, these pins must have tapered shutoff and never invade the parting-line shutoff area. In reason of precise controlling in LSR molding, any debris or rubber on ejector valve will give rise to failure.
Vacuum Demand
LSR molding has special requirement of vacuum, in order to guarantee final successful molding process. As we know, in thermoplastics molding process, the molds require enough vents and gaps will release out-gassing and contained air in cavity area. Furthermore, LSR molding has a chemical curing process, different compounds (A/B) will react in mixing process and create some gas. Therefore, it is critical to remove air in cavities before injection, and vent gases properly in curing process. Because very tight fit of mold components in LSR mold to prevent flashing, we must incorporate proper land venting in design. In order to allow air escape in LSR materials filling process. With tight flash controlling in liquid silicone parts, we need to keep venting tolerance reach to ±0.0002 mm.
Temperature Control
Plane features on mold opening direction require necessary drafts, in order to allow milling of molds. The mold design requires almost 1 degree draft in 25mm mold depth to prevent scraping. Because shear-thin of liquid silicone rubber, it can navigate easily into thin walls. This allows parts design with different thickness.
Tool & Part Design Differences
LSR materials are flexible, the LSR molds don’t require features intended to facilitate parts removal, such as draft angle. However, precise parting lines are required for good venting or shut-off areas once molding on substrates. This is the reason why our engineering team design and manufacture LSR molds in product development stage.
It is extremely important to involve mold design and manufacturing in the product design phase. There is a good example of 2-shot LSR molding project, normally mold LSR materials onto thermoplastic substrate. Such products require many little adjustments for mold design and molding process. In order to ensure proper shut-offs, venting and gate locations. Finally, guarantee proper pairing of substrate and over-molding LSR materials.
In addition, the thermoplastic substrates need to withstand high LSR mold temperature. Otherwise, the project will fail once thermoplastic substrates deform at or below LSR curing temperature. Furthermore, we also require considering the bonding methods of two materials, like: LSR self-bonding, mechanical bond, or both. These elements will impact the mold design concept.
Cold Deck and Runner
In LSR molding process, it requires a cool runner to prevent curing before mixed materials reach to the heated mold cavities. The barrel and deck on molding machines are kept cool by a water mixture, these components’ temperature range from 15℃ to 40℃. The most critical area is the contact point between the cold-deck nozzle and the mold, we typically use a needle-valve gate. Furthermore, as molds size become larger, the cavitation become higher. This will make LSR temperature sensitivity and out-gassing tendency more and more difficult. Normally, we will control cavities numbers under 64.
Runner-less LSR Molds
There are more economic motivations to apply runner-less LSR molds in mold design. The main reasons are:
- Firstly, cured LSR materials in runner scrap are unable to reuse.
- Secondly, LSR is premium-priced materials, its unit price is higher than normal thermoplastics.
- Thirdly, LSR materials generally require longer curing cycles than thermoplastics.
- Fourthly, In micro size LSR molding, the runner is thicker than LSR parts, therefore, we need to wait for LSR materials to cure in the runner.
- Finally, it is difficult to remove runner waste in automation process. It will increase overall cost in the project dramatically.
Conclusion
From the reason above, it is critical to evaluate the mold design in liquid silicone rubber molding process. We require precise and high-effective LSR molds to guarantee further silicone part production. If you have any question in mold design, why not contact our engineering team now?
