The LSR tooling is different from plastic toolings

The LSR tooling is different from plastic toolings and it is very challenging to make such tooling. Intertech has good knowledge and expericne to make good quality silicone rubber tooling. Here we share one good LSR tooling introduction article to you !

Getting Into LSR--Part IV: How LSR Tooling Is Different           

Building molds for rubber molding, and specifically liquid silicone (LSR ) injection, can be very challenging. Many think of rubber molding as a low-tech process. It’s actually quite the opposite. In many—if not most—cases, molds for LSR require much more precision and expertise than molds for thermoplastics. (Part I of this series, in June 2010, covered LSR materials. Part II, in Aug. 2010, covered the injection machine. Part III, in Sept. 2011, covered mixing/metering systems.)

PREVENTING FLASH

Both thermoplastic and LSR molds can have very complex geometries. The major difference between them is the requirements for fit and relate between cavity inserts. Many thermoplastic materials can be molded with limited flash using fits between inserts of up to 0.002 in. But many LSRs will flash even with fits as tight as 0.0001 in. or less. This fit requirement necessitates a different level of engineering, machining, and processing precision.

A simple example of fit and relate differences between LSR and thermoplastic injection molds is the use of ejector pins. Straight-wall ejector pins are commonly used in plastic molds. The pins can cross over the parting line on some parts, so that half the ejector pin is on the part and half is on the parting line shutoff. In thermoplastic molding with this design, there is typically no flash. Ejector pins can be used in LSR molds but they must have a tapered shutoff and can’t invade the parting-line shutoff area. Any minute amount of debris or rubber that builds up on the ejector valve seat will cause process failure.

Another special characteristic of LSR processing is the requirement for vacuum. Even though thermoplastic materials can create outgassing during the molding process, the mold will normally have enough gaps (vents) in the fit and relate areas to release these gases along with the air contained in the cavity area. What’s more, LSR moldingrequires a chemical curing process. Liquid silicone typically comes in two parts. One part has a catalyst and the other has the crosslinker. As these two components mix a chemical reaction takes place that can create some gas. It is important to remove any air from the cavity before injection to ensure that the mold can properly vent the gases evolved during curing. With the very close fit of mold components to prevent flashing, proper land venting has to be incorporated into the design to allow air to escape from the cavity during filling. This venting should be controlled to millionths-of-an-inch depth tolerance if the rubber parts need flash to be controlled within tight specs.

One other difference between thermoplastic and LSR mold designs is in temperature control. Since thermoplastic molding requires cooling the material in the mold, temperature control is typically achieved with circulating oil or water. Thermoplastic mold temperatures are commonly less than 170 F. For LSR, the mold must be heated to 320 to 420 F to cure the material, most commonly with electrical cartridge heaters.