EDGE-SWEETS COMPANY ELASTOMER PROCESSING ROOM TEMPERATURE CASTING HOT CASTING Presented by Dario Ramazzotti and Stephen Hoffman
EDGE-SWEETS COMPANY Typical processes and equipment to make elastomers with 60 to 95 shore hardness
Basically the machinery industry manufactures two distinct types of cast elastomer machinery ü Room Temperature Process Equipment ü Hot Cast Process Equipment
Room Temperature Systems R T systems are material systems, which can produce cast elastomers at room temperature. Generally these are a type of quasi prepolymers, which have component ratios close to 1:1.
Typical room temperature processing equipment
Hot Cast Systems Hot Cast Systems are material systems which are processed at elevated temperatures generally 180º to 230ºF. These MDI and TDI prepolymer systems typically have component ratios between 4:1 to 14:1.
Typical Hot Cast Schematic
List of differences Hot Cast R T Heat 130 c None- 20 c Pumps Internal External Degas Yes Yes & No Insulate Yes No Head Hot Yes No Cost Expensive Less expensive
Basic types of Cast Elastomer Processing Techniques Hand Mix Low Pressure Static Mixer Low Pressure Dynamic Mixing High Pressure RIM, including spray
Hand Mix Equipment Utilized Commonly referred to as bucket and stick Materials are pre-weighed and introduced into a common container and mixed by hand, generally using a spatula After mixing, material is hand poured into molds.
Hand Mix Advantages Minimum equipment investment Both Hot and R T systems can be processed. Versatility Ø Unlimited number of components can be processed Ø Very large parts can be produced
Hand Mix Disadvantages Ratio control: Residual in container causes ratio inaccuracies Limit on mixing and shear capabilities Part must be open poured Air entrapment Operator exposure to chemicals Limit working time Labor intensive
Static Mixer A static mixer consists of motionless mix elements used on reactive systems. They operate on the principle, that the main flow is broken up into smaller streams, divided, reoriented, brought back together, and the cycle is repeated again and again until the components are thoroughly mixed.
Equipment Required for Static Mix Materials are metered to the static mixers, generally by pneumatic driven pistons or mechanically driven gear pumps
Static Mixing Advantages Low equipment cost Hot cast and RT system can be processed Simple in design Good shot repeatability Closed mold pouring capabilities Limited air entrapment/bubble free
Static Mixing Disadvantages Shot size limit since most units are piston driven, shot size is limited to piston volume Non recirculation of materials to mixer Viscosity limited since mixers create pressure drop Difficult to process fast systems Solvent flushing or mixer disposal is required Limit shear/poor mixing/ratio limits
Low Pressure Dynamic Mixing A dynamic mixer consists of a mixing impeller that rotates at high speed. Speed of a dynamic mixer is generally between 1000 & 7000 RPM. Mixers come in all shapes and sizes. The mixer configuration combined with RPM determines the efficiency and shear.
Types of Mixers
Low Pressure DYNAMIC MIXER Plural component streams can be delivered to the mix head. In most cases, precision gear pumps driven by DC or Servo motors are utilized. On Hot Cast Systems, the metering pumps are submerged in the material tank for delivery of chemicals.
Submerged pump /continuous degassing
Dynamic Mixing Advantages ü Process both Hot and Room Temperature Systems ü Unlimited number of components can be introduced, including color and catalyst ü Processes the widest range of viscosities and ratio ü Open or closed pouring capabilities ü Continuous recirculation capabilities ü Continuous pour capabilities ü Viscosity ü Shear rates and pumping action can be manipulated by mixer design and/or RPM ü Most effective mixing efficiency ü Bubble free quality casting
Low Pressure Dynamic Mixing Disadvantages Requires prepolymer or solvent flushing Moving parts and mixer seal exposed to reactive materials Discharge pressure limited Bulky and heavy
Dynamic Mix Head
Other types of Dynamic Mixing NoFlush Low Pressure Dynamic Mixing ü Same features and benefits as the standard dynamic mixer, but need for flushing with solvent is eliminated ü Due to head design, it is not the best for absolute bubble free parts; however, some fillers can be directly introduced.
NoFlush Mix Head
Filled Systems It is often necessary to add fillers such as micro spheres, rubber particles, minerals and fibers. Typically these are preblended into the prepolymers or curatives
Progressive Pump
Filler Fed Head* FFH An alternate approach is to meter the dry filler into the mix head where they are blended with the chemicals. This eliminates the mess and inaccuracy of preblending, saturation of the fillers and additional tankage. *Edge-Sweets patent
FFH Head
FFH Cross-Section Show FFH schematic
High Pressure RIM Impingement Mixing High pressure impingement mixing sometimes called LIM (liquid injection molding). Mixing is accomplished by impinging the chemical streams under high pressure (1500 psi and above) in a very small chamber which creates turbulence causing mixing.
Basic types of High Pressure RIM Mixing Heads Spray Head used for lay up applications Straight Impingement Head -used to pressure fill closed molds L Type Mix Head reduced turbulence allows open mold filling
Straight Style Mixing Head
L Type Mixing Head
Spray Type RIM Head
Spray Systems Spray heads are typically short stroke impingement mixing heads with spray tip incorporated into the discharge.
RIM Heads High Pressure Mixing Heads-Straight heads are either short or long stroke. At the point of impingement there is a tremendous turbulence which, as the material flows down the mixing chamber, is energy is exhausted until the flow becomes laminar (a smooth non-turbulent flow). If too short a stroke is used, as the material exits the mixing chamber, it may not be in laminar flow causing the mixture to violently spray out of the mixing head.
RIM Metering Metering Systems that support the High Pressure RIM Mixing Ø Axial Piston Metering Ø Piston Metering Ø Lance Metering
Rotary Axial Piston Pump Several piston elements around the edge of a disc, which rotates inside a guide ring. Pistons have axial movement. Volumetric output is changed by adjusting the stroke length of the pistons.
Piston Metering Often modified to dispense material in both directions. Commonly utilized on spray equipment.
Lance Cylinder Metering Piston-less type metering cylinder. A piston rod (without a piston) is driven into a cylinder displacing material (positive displacement).
Advantages of RIM No solvent required Capable of spray applications at low cost with excellent performance Closed or open pour capability Only one moving part in head design Filled system processing is feasible Multi mixing head utilizing a single pumping unit
Disadvantages of RIM Basically Ratio restricted to Quasi system Investment cost is high Limited ratios and viscosities Utility cost high Limited materials streams Limited head life expectancy
Cast Elastomer Parts
Summary Material system and equipment must be integrated to insure proper operation for a particular process.
The End