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Although there is much training centered around the in-mold rheology curve, the basics are as follows:

• Fill the mold as fast as you can with a short shot during 1st stage velocity controlled injection
• Record the fill time and injection pressure at transfer
• Repeat this process using incrementally slower speeds
• Use 1/(fill time) for apparent Shear Rate
• Use (Pressure)*(Fill Time) for Relative Viscosity
• Graph Relative Viscosity (X axis) vs. Shear Rate (Y axis)

For more posts about rheology, just type rheology in the search box in the upper right hand corner of this page.

-Andy

I was recently asked this question…

lash is a common issue that must be dealt with in silicone injection molding. The low viscosity of the silicone prior to curing allows it to get out of the smallest spaces of the mold including the parting line, mold vents, and ejector pins. In order to prevent flash in silicone injection molding a few different things can be done to counteract the problem.

First, ensure your shot size is not to large. An excessively large shot size will contribute to flash because the silicone expands as it cures. Unlike traditional injection molding the mold does not usually need to be 95-98% full during first stage fill. Second, check your mold tonnage and ensure both platens are evenly applying the appropriate force. This can be verified with spotting fluid. Also, try increasing the mold temperature or preheat the silicone more prior to injection. This should reduce your curing and cycle times and also increase the silicones viscosity in the mold.

-Andy

Personally, I would ensure there is some protection to the machine. On most of these machines, there are very few moving parts on this machine, so you basically need to protect the heater bands and clamping unit (if it is not already protected).

Since these machines are very small, I have also seen companies just place a small Plexiglas enclosure around the whole machine.

-Andy

So, if you have a machine which has much older and inadequate guarding, remember that safety is not grandfathered in. Even though the machine was built when lower standards were in place, there is no reason to allow the machine to continue being unsafe. Always keep this in mind when you purchase a machine used machinery, or from a country which has a significantly lower safety standard.

-Andy