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Cascade Molding
Cascade molding (also cascade injection or cascade fill) is a form of sequential valve gating. Using a valve gate controller like the Rheo-Pro® CV08, optimum part fill is achieved by opening individual valve gate nozzles in a staggered sequence, creating a cascade progress of the flow front from gate to gate. This method is used primarily to create high quality plastic parts without weld lines. It can also help to reduce clamp tonnage for large surface parts. Rheo-Pro® pneumatic valve actuators have a response time with millisecond accuracy, allowing for complete control of your melt distribution.
Low Pressure Cascade Injection
The steady increase in material cost and energy prices continues to reduce the profit margin on large plastic parts such as containers, crates, automotive bumpers and similar products. However, new approaches in part design and better methods of filling the mold cavity are resulting in cost efficiency and increased part quality.
A simple reduction of the wall thickness is not the answer to restoring healthy margins of the part price even though it would save material cost and reduce the cooling time of the molding cycle. A thinner wall would require an increased fill pressure, which again would require more clamp force and a larger molding machine. Consequently, the increase of the machine hour rate would offset some or most of the gains in the anticipated savings of the cycle time and material cost. Wall reduction only pays back when the mold can stay in a machine that has the same or lesser clamp force. This means that the fill pressure of the mold cavity has to be reduced, even if the thinner wall presents a natural increase in flow resistance. These seemingly conflicting conditions can be overcome with low pressure cascade injection (LPCI). The principal of LPCI is easy to understand.
In order to understand how LPCI is different we must first look at traditional injection molding that uses high melt pressure of up to 20,000 or 30,000 psi to advance the plastic melt over a flow distance in a cavity as far as possible.
Historically, material processing properties are tailored to mold large parts with thin walls for ease of flow. They are characterized by a low molecular weight or a high melt flow index and they allow a high injection speed or melt velocity and accept high shear rates. The shear thinning effect at a high shear rate essentially drops the melt viscosity and the achievable flow distance increases. Consequently, high velocity injection developed and the flow distance increases for large thin wall parts molded in machines with increased clamp tonnage. A clamp force of 2.5 to 3.0 tons per square inch of the projected surface area of the part is common.
The low pressure cascade fill method (LPCI) follows a different line of thought than traditional high velocity or high pressure injection. LPCI reduces the injection melt pressure from 20,000 psi to 5,000-7,000 psi. This drops the clamp force to only 0.6 – 0.8 tons from 2.5 tons. Rather than pushing the melt with high pressure over a long flow distance, the LPCI method uses multiple injection points strategically placed along the flow path at shorter intervals.
Cascade front control avoids high melt pressure. Each time the melt pressure starts to peak, a new injection point is reached at the melt front and the next valve gate open is activated to advance the flow front further. It is like a relay in the Olympics; after a short distance, a new sprinter takes over. This cascading process is seamless and does not cause weld lines. The total flow length is divided into manageable flow segments, each being no longer than the flow ratio that does not exceed a melt pressure of 5,000 to 7,000 psi in order to overcome the distance to the next injection point.
For example, a large 600 x 1200 mm part with a wall of 1.0 mm (.040”) is molded using LPCI with a 600 ton clamp force. This part would have required a 2,000 ton pressure when molded with traditional high pressure injection. The savings in the machine hour rate is substantial. |
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Another benefit of LPCI is a part with reduced stress and warpage because the melt pressure difference in the cavity is lowered in comparison to a high pressure injection method. The flow analysis calculates a more uniform melt pressure distribution across the part surface. Advanced hot runner valve gate nozzles with quick response actuators that are precision controlled with a Rheo-Pro® CV08 controller achieve a melt front control in the cascading fill sequence of ± 5 mm accuracy over a flow distance of 200 to 400 mm at each gate along a wall of .040”. The fill sequence can cascade the melt front advancement in series within fractions of seconds. The CT05, a hand held touch screen unit connects to the mold mounted microprocessor/solenoid box so the machine operator can optimize the cascade sequence of the hot runner valve gates according to the processing characteristics of the plastic material and the part requirements. The CV08 has an internal memory which stores numerous process program setups with an automatic mold identification.
This new low pressure cascade injection technology is ideal for large surface parts that could have been molded only in large presses before. An important tool that helps to determine the placement and number of hot runner cascade valve gates is the computer simulated flow analysis. The flow analysis calculates the exact flow front and the melt pressure throughout the cavity and accurately provides the required machine clamp tonnage. Thus the theoretical clamp force mirrors, up front, the clamp force results in the actual molding machine.
Processing Support
As a complete solution provider, Mold Hotrunner Solutions® can approach each molding project with a detailed flow analysis to establish an accurate calculation of the part price. This feasibility study gives results for material weight savings, cycle time reduction, reduced machine size, floor space savings and energy cost reduction. The sum of these savings can add up to a substantial part cost reduction. The return of investment on LPCI hot runners with the cascade controller is usually only a few months.
Rheo-Pro® valve gate sequence controllers |
