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Cascade Molding

Cascade molding (also called cascade injection or cascade fill) is a form of sequential valve gating. In certain hot runner injection molding applications, optimum part fill is achieved using a valve gate controller to open each individual valve gate nozzle in a staggered sequence, creating a cascade progression of the melt's flow front through the mold cavity, from one gate to the next. This method is used primarily to create high quality plastic parts without weld lines. It can also be used to reduce clamp tonnage for large surface parts. This type of automated injection molding technique requires accurate valve gate control to achieve reliabililty, repeatability, and processing optimization.

Sequential Valve Gating

A cascade injection sequence graphed to illustrate the advancing flow front inside the injection mold

Low Pressure Cascade Injection (LPCI)

The steady increase in material cost and energy prices continues to reduce the profit margin on large plastic parts such as containers, automotive components, contruction supplies and similar products. However, new approaches in part design and better methods of filling the mold cavity can offer improved 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. The principal of LPCI is easy to understand.

Cascade molded automotive bumper featuring weldline-free, high quality surface finish and new cooling-free Black Box™ valve gate actuators

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 was developed and the flow distance increased 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.

Sequential VG Hot Runner

Valve gate hot runner manifold system featuring gas assist injection and cascade fill for leightweight shopping cart. Rheo-Pro® pneumatic valve actuators feature fast, millisecond response times and unparalleled reliability for complete control of the mold.

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.

A cascade injection sequence modelled on a graph illustrates the relationship between melt pressure and the opening/closing of individual valve gate nozzles

Cascade flow 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.

New iVG™ internal valve gate nozzles feature cooling-free actuation directly inside the nozzle body for extremely compact and reliable valve gate solutions like this back-to-back stack



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Valve Gating
Flow Analysis and Optimization
Sequential Valve Gating
Rheo-Pro® Valve Gate Controllers
Cooling-free Valve Gating
Hot Runners for Automotive Parts

Case Studies


Sequential valve gate hot half with Rheo-Pro® hot runner and mold mounted controller for multi-component injection

Another benefit of LPCI is a part with reduced stress and warpage because the melt pressure difference in the cavity is lower compared 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 fast actuators, and a reliable valve gate controller can combine to deliver 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”.

Cascade Injection Molding
Illustration of a cascade injection sequence used to fill a large surface, thin-wall plastic part

The fill sequence can cascade the material so that the melt front can advance in series, and within fractions of seconds. A controller like the Rheo-Pro® CV allows the machine operator to optimize the cascade sequence of the hot runner valve gates according to the processing characteristics of the plastic material and the part requirements. These controllers have an internal memory which store numerous process program setups with automatic mold identification.

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 computer simulated analysis. The fill analysis calculates the exact flow front and the melt pressure throughout the cavity and accurately provides the required machine clamp tonnage. The theoretical clamp force can be accurately calculated up front, predicting the clamp force results in the actual molding machine during production.

Solenoid valve bank for air actuated Rheo-Pro® valve gate hot runner featuring clean, accurate control of the injection sequence

SoftFill™ offers high quality surfaces for large injection molded plastic parts. This feature reduces injection pressure peaks during the opening of the valve gate (cavity fill phase) by compensating for sudden melt velocity changes. The optional flow control provides variable speed pin actuation designed for sequential valve gating and cascade molding. The system offers a more gentle flow of melt into the cavity to eliminate surface blemishes and hesitation marks. Applications include large surface parts for appliances and automotive.

Valve Gate Hot Runner Controllers
Rheo-Pro® valve gate sequence controllers offer solutions for all injection molding applications with 4 to 300 control zones

Comprehensive Processing Support
As a complete solution provider, MHS approaches each molding project with detailed analysis to establish an accurate calculation of the part price. A feasibility study can provide results for material weight savings, cycle time reduction, reduced machine size, floor space savings, and energy cost reduction. The sum of these savings adds up to a substantial part cost reduction. The return of investment (ROI) on LPCI hot runners with a cascade controller is usually measured in only months.



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