The Expert's Corner

BACKGROUND:

Thermoforming is the process of forming thermoplasticsheet by the use of heat and pressure (or vacuum).Plastics such as polystyrene, polyethylene, ABS andpolycarbonate are commonly formed by this method.

PROBLEM:

One method of Thermoforming utilizes a male plugto assist the vacuum or pressure in forming the plasticpart. These plugs have traditionally been machined fromblocks of aluminum which can be expensive. In addition,the aluminum plugs usually need to be temperaturecontrolled - plugs that are too hot can result in plasticsticking to the plug; plugs that are too cold can promote“freezing” or premature hardening of the plastic resultingin an improperly formed plastic part.

SOLUTION:

Syntactic Foam plugs have been used very successfullyas a substitute for aluminum plugs. Syntactic foamsare microsphere filled resins such as epoxies or polyurethanes.They are characterized as having low densitiesand high compressive strengths, but more importantlyfor Thermoforming applications, they are thermallyinsulating materials, neither chilling or heating the plasticbeing formed. The end result is a material that iseasily machined to any shape -and ultimately, a lesscostly Thermoforming plug.

PROCESSING:

Syntactic Foams require careful mixing techniquesto insure their low density, high strength characteristics.In addition, the shapes that are cast must be completelyvoid free. Any pores or voids in the finishedplug would transfer that defect to the molded plasticpart.

The Ross Vacuum Double Planetary Mixer is employedby companies worldwide for Syntactic Foamproduction. The gentle, yet thorough mixing action insurescomplete mixing of the hollow microspheres andresins while minimizing the number of fracturedmicrospheres.

In a typical procedure, liquid components are firstadded to the mix can of the Double Planetary Mixer.The fragile microspheres are then charged to the resinwhile the stirrers of the Double Planetary mixer rotateand revolve. When the microspheres have been wetted,a vacuum of at least 29.5" Hg is drawn on thebatch to remove air that can result in voids in the casting.

The finished material is usually pourable and canbe cast by tilting the mix can or by opening a flushmounted ball valve in the bottom of the mix can. Moreviscous formulations can be discharged using a RossFollower Plate Discharge System. The platen of thedischarge system forces the product through a flushmounted ball valve in the bottom of the mix can.

The product is usually cast directly from the mix caninto molds. These molds can form either net shapeparts, or blocks from which finished parts are machined.

Ross Follower Plate Discharge Systems, like this elevatedconfiguration, can be used to discharge viscous Syntacticsfrom the mix can of a Double Planetary Mixer.

MIXER SPECIFICATIONS:

Some of the recommended features for Double Planetary Mixers used for Syntactic Foams include:

• Vacuum processing capability - allows the removalof air from the product, resulting in a stronger, voidfree casting.
• Variable speed control - enables the user to finetune the charging sequence and the final mixing sequence.
• Jacketing of the mix can - lets the user control the temperature of the batch. Heating of the resinlow ers viscosity, allowing higher solids loadings;cooling the batch can control the exotherm ofroomtemperature curing resin systems, yieldinglonger pot life.
• Sight ports and charging ports - allow the operatorto view the batch and to add in gredients withoutopening the mixer
• Thermocouple - flush mounted to the mix can wallcan be used to monitor batch temperature and to giveearly warning of exotherms
• Flush mounted ball valve - in the bottom of the mixcan facilitates the discharge of product with minimaldead space between stirrer and bottom of mix can
• Caster wheels - on the mix can promote easy movementof the mix can in and out of the mixing position.Other options that may be considered, dependingon the specific application include:
• Dust shield - beneath the planetary gearbox reducesdusting in the upper gearbox area.
• Teflon scraper blades - to scrape the sides and bottomof the mix can are useful for ensuring batch homogeneityand promoting better thermal transfer froma jacketed mix can.
• Mechanical seals and gearbox caps - are importantwhen using aggressive solvents, to prevent the breakdownof gearbox lubricants. Solvent resistant lubricantsare also recommended for these applications

Important Note: Blade-to-bowl and blade-to-blade clearances on Double Planetary Mixers, for Syntactic Foam applications, should be liberal (1/8" for lab mixers and 1/4" - 3/8" for production units) to prevent excessive breakage of microspheres.