Opening molds for rubber molding, especially liquid silicone (LSR) injection, can be very challenging. Many people believe that rubber molding is less technically challenging. The opposite is true. In many cases, LSR molds require more precision and expertise than thermoplastic molds.
Liquid Silicone Products
Preventing flying edges
Both thermoplastic and LSR molds have very complex geometries. The main difference between them is the requirement for fit and the relationship between cavity inserts. Many thermoplastic materials can use a 0.002″ gap fit without creating a flying edge problem. However, the same gap will allow many LSR materials to produce flutter, even if the fit is as tight as 0.0001 inch or less. This fit requires varying degrees of engineering, machining, and process precision.
Using ejector pins is a simple example of the difference in fit and association between LSR and thermoplastic injection molds. Commonly used in plastic molds, ejector pins can pass through the parting line on certain parts so that half of the ejector bar is on the part and half is on the parting line closing device. In the thermoplastic molding of this design, no flying edges are usually produced. Ejector pins can also be used in LSR molds, but they must have a tapered closure design that does not intrude into the parting line closure area. Trace amounts of debris or rubber left on the valve seat can cause serious production problems.
Another characteristic of LSR processing is the requirement for vacuum. Even though the thermoplastic material can be vented during the molding process, the mold will usually have sufficient clearance (vents) in the mating and associated areas to release these gases and the air in the cavity area.
When these two components are mixed, a chemical reaction produces some gas. More importantly, removing residual air from the mold cavity before the injection is important to ensure that the mold can properly vent the gases generated during the curing process. Since LSR molded parts will fit tightly to prevent flutter, proper vents must be incorporated into the mold design to allow air to escape from the cavity during the filling process. If the rubber part requires flutter control within tight specifications, the venting should be controlled within a tolerance of one-millionth of an inch in depth.
Another difference between thermoplastic and LSR mold design is temperature control. Since thermoplastic molding requires cooling of the material in the mold, temperature control is usually achieved by circulating oil or water. The temperature of thermoplastic molds is usually below 80 degrees Celsius. For LSR, the mold must be heated to 160 to 220 degrees Celsius to cure the material, and the most common way to heat the mold is with an electric cartridge heater.
Mold and Product Design Differences
There are many differences in mold and product design for LSR materials compared to thermoplastic materials. Many of the guidelines for thermoplastic materials do not apply to LSR doors.
This is especially important in the case of LSR secondary molding applications. Typically secondary molding involves molding LSR to a thermoplastic substrate or molding one grade of LSR to another LSR. These products often require many small adjustments to allow for a more user-friendly mold design and molding process. The design process needs to ensure proper shut-off valves, venting, gate locations, and most importantly, properly paired substrates and secondary molding materials.
Thermoplastic substrate resins need to withstand mold temperatures that are half of LSR. The project will fail if the thermoplastic begins to deform at temperatures below the LSR cure temperature. Another issue that needs to be discussed early on is the method of bonding the two materials. Are we using self-bonding LSR or mechanical bonding, or both? These items will affect the design of the mold. Once the product has passed all approvals, it is very difficult to change, so these factors must be discussed early on.
Cold Deck vs. Hot Runner
Both thermoplastics and LSRs can use runner-less mold designs, but there are significant differences in design. Thermoplastics require a hot runner to maintain the temperature and viscosity of the material between shots. LSR, in contrast, requires a cooling runner to prevent the material from curing before it reaches the heated cavity. (LSR can begin curing at room temperature once the two reactive components are mixed.) The engineering concepts are similar to thermally keeping the material delivery system from the mold body.
When using LSR, the molding machine’s barrel and cold table (runner system) are kept cool using a water mixture. The most critical area is the contact point between the cold plate nozzle and the mold, where a needle valve gate is typically used.
If this area is not designed properly, it can be susceptible to molding defects such as a “wet” gate appearance due to the area being too cold or an extended gate due to the area being too hot. As mold sizes get larger and cavities increase, the temperature sensitivity and venting characteristics of LSRs make the challenge even more difficult. This is why most LSR mold makers and mold builders do not prefer more than 64 cavities.
For several reasons, the economic incentive to use fewer LSR molds is even greater than thermoplastics because cured LSR scrap cannot be reused. Second, because LSR is often a high-priced material. Third, the curing cycle of LSR is usually longer than the cooling cycle of thermoplastics. When molding small LSR parts, the runners may be thicker than the part, so the molder has to wait for the runners to cure after the part is ready to be demolded. In addition, hot runner LSR systems are more difficult to automate because they require the removal of molded parts and runner scrap. This scrap must also be collected and eventually removed from the molding area.