Forum Replies Created
December 14, 2012 at 9:11 am in reply to: Does anyone has experience overmolding a thermoplastic with silicone? #650
Hello! I would guess that this is occurring for one of two reasons:
(1) There could be a shrink differential between the two materials, since there is such a greater volume of silicone than thermoplastic, the silicone could be warping the plastic as it shrinks.
(2) Depending on the temperature at which the silicone overmolding is taking place (if it is higher than the glass transition temperature of the thermoplastic) it could be releasing any stresses that were molded into plastic components when they were molded/extruded.
I would try overmolding the silicone at a lower temperature, and if there is any postbaking or postcuring process being performed try doing so with the parts under compression.
Thanks for your question, let me know if we can help you further!
Hi Chuck- We (at Albright Technologies) do not work in material formulation or sililcone extrusion, we specialize in rapid silicone prototyping via means of injection, compression, and transfer molding. For transfer molding HCR materials, I would advise you to purchase the following equipment: heated shuttle presses (or hand crank presses), ovens (for part postbaking, mandatory with peroxide cure HCRs), metrology equipment for part verification and first article inspections, CNC machines for mold cutting, CAD and CAM software and a computer powerful enough to run it efficiently, and lastly packaging equipment to streamline your shipping process.
- This reply was modified 8 years ago by Silicone.Pro.
Low durometer parts do tend to be sticky, fortunately they also tend to be very elastic. For easier release of molded parts, we would recommend a nickle teflon coating on the tool. You can also try seasoning/conditioning the mold with a material that is easier to run. Basically, you just run numerous shots of a material that you are comfortable running to fill any micro structures in the tool. After 50 shots or so, you’ll start to notice the difference. You can try using a blast of compressed air to drive the part out of the cavity as well. Alcohol can act as a lubricant and causes silicone to become a little more pliable, on a hot mold it evaporates quckily so you’ll need to act fast. There are lots of commercially available mold releases, some of them are FDA approved, some are not, some work well, and others don’t, but one of the best mold releases (also cheap and easy to make) can be created by mixing equal parts dish soap, distilled water, and Isopropyl Alcohol.
Hope this answers your question, if not, feel free to ask any other questions you have!
When the mold temperature is high, liquid silicone will have a low hot tear strength. If mold temperature parameter is permitted to be adjusted, lowering the mold temperature would make increase the tear strength, which could ease in degating the part. The disadvantage of the lower temperature mold would increase the cycle time .
Degating can be influenced by gate designs: type, size, and location. Direct gating is preferred over other types of gating; the nozzle directly feeds the material into the part or by using the cold deck. Since the nozzle keeps the silicone uncured, the gate can be easily degated. Smaller gating is commonly used to solve degating problem. The gate in injection molding silicone is smaller than in thermal plastics injection molding. Nevertheless, it is very important to make sure that the gate size must be large enough for the silicone to fill the cavity before the gate completely solidifies. Just like real estate, “location, location, location” is important in degating process. For instance, it would be more difficult to degate when the gate locates at a thin section of the part vs. thick section.
The parameters that are discussed above are considered as tips; there isn’t a standard or best way to approach degating solution. It really depends on case by case situation.
I hope this answers your question. If you have any other questions, please email me directly at firstname.lastname@example.org