Processing Chapter 3

Compression Molding

Introduction

• In 1907 Dr. Leo Baekeland achieved a reaction between Phenol, a caustic, crystalline acidic compound and Formaldehyde, a colorless, pungent gas, by heating these ingredients under pressure in a reaction kettle.

• The resultant, hot liquid when cooled, became a hard brittle solid. It lacked physical strength, but possessed excellent thermal, chemical and electrical insulating properties

Introduction

• The hard brittle resin in a pulverized from could be mixed with fillers, reinforcement, colorants, lubricants and catalyst. The end product was a molding compound capable of further processing on heated mills and calendar mills.

• Thus the first thermosetting molding compound was created.

Introduction

• In the following years additional thermoset resin systems were developed to meet the requirements of diverse applications in automotive, communications, construction, transportation, household appliances, business machines, aircraft and aerospace fields.

Thermoset Definition:

• A polymeric material which can be formed by the application of heat and pressure, but as a result of chemical reaction permanently crosslinks and cannot be reformed upon further application of heat

– Chemical crosslinking must occur for the resultant product to be called a thermoset.

– Monomeric precursors may or may not be polymeric

Thermoset Families

– Allyls– Aminos– Epoxies– Phenolics– Polyesters– Polyurethanes– Silicones– Crossed linked thermoplastics

Classifications

• General Purpose – Average mechanical properties, lower resistance to heat, higher coefficient of expansion, low cost and commodity production.(Phenolics, Aminos and polyesters)

• Engineering – Higher mechanical properties and temperature resistance and more durable. More expensive with moderate production rates. (Epoxies and Polyurethanes)

Classifications

• Specialty – one or more highly specific and unusual properties. Very expensive and produced in small quantities. (Silicones, Allyls, Crossedlinked Thermoplastics)

General Properties

– Thermal and dimensional stability– Electrical insulation– Chemical resistance– High flexural modulus– Toughness– Surface hardness– Moldablilty– Cost effectiveness

Phenolics

Introduction

• First thermosetting plastic, first synthetic commercially available plastic resin

• Produced by chemical reaction between phenol (resin) and formaldehyde (curing agent)

• First made by Dr. Leo Baekeland in 1907

• By far the most widely used of all thermosets for molding applications

Chemistry

– Phenol – Formaldehyde resins are general purpose thermosets formed mainly by the polycondensation reaction between phenol and formaldehyde

– Resole process, other main process is Novalac

– Raw Materials• Phenol

– Show phenol structure

• Formaldehyde– Show formaldehyde structure

Phenol

• Cumene process for making phenol

Formaldehyde

• Formaldehyde is produced by the controlled catalytic oxidation of methanol

Phenolic Reaction

• Phenolic resin reaction process

Phenolic curing process

• The curing process to the final thermoset material can be initiated by just heating the resin in a mold above its gel temperature

• When heated, the resin forms larger molecules without the use of additional catalyst

• The reaction is a typical polycondensation, since water is given off as a by product

Phenolic curing process

Processes that use Phenolics

• Compression Molding

• Transfer molding

• Injection Molding

Properties of Phenolics

• Physical• Excellent dimensional stability• Low water absorption• High surface hardness• High creep resistance

• Mechanical• High compressive strength• High modulus, stiffness

Properties of Phenolics

• Thermal• Phenolics retain a high percentage of properties

at elevated temperatures• Low coefficient of thermal expansion• Good heat resistance

Properties of Phenolics

• Electrical• Good electrical resistance

• Chemical• Good chemical resistance

• Other• Good weather resistance• Easily molded• Good machining properties

Applications for Phenolics

• Appliances 17%

• Closures 7%

• Electrical 38%

• Housewares 23%

• Industrial 4%

• Transportation 11%

Molding and Molding Techniques

Introduction

• There are three fundamental molding processes for thermoset compounds.

– Compression molding– Transfer molding– Injection molding

• The process used will be determined by which process can produce the part to appropriate specifications at the most economical rate

Production Methods and Equipment

• The choice of methods and equipment for use in the production of thermosetting molding compounds is usually determined by the type of reinforcement being used

• Reinforcements are added to thermosets to improve properties. Impact, flexural strengths, shrinkage, etc.

Compression Molding

• Introduction– Uses a hydraulically operated press– Press consists of two heated mold platens, one

stationary and one movable– Platens have either “T” slots or tapped holes to fix

the mold halves to the platens– Tie Rods, usually 4, provide for precise alignment

as the press is opened and closed– The press open and shut height and the platen

temperature are controlled.

Compression Molding

• Compression presses are rated by their closing force capacities

• Can be manual, semiautomatic or fully automatic

• Preform temperature, molding temperature, molding pressure, molding time and cooling time are the most important design parameters

– Cavity depth is very important to achieve the proper molded density

Transfer Molding

• Similar to compression molding

• Four basic types– Pot type transfer molding – old technology– Plunger transfer molding– Automatic Transfer molding– Screw transfer molding

Pot Type Transfer Molding

• Mold design provides a “pot” into which the compound, heated or unheated, is placed and the press closed

• The clamping pressure exerted by the press also forces the compound from the pot into a runner system, then into the cavity

• Was innovative, providing an improved means for certain parts and producing less flash at the parting line

Pot Type Transfer Molding

• Also made it possible to feed the molding compound for a single location

• Produced a lot of waste due to pot volume being 15% higher than the runner and cavities volume, to overcome the pressure needed to force material to the runners and cavities

Plunger Transfer Molding

• Developed in early 1940’s to overcome difficulties/expenses of the pot type

• A hydraulic cylinder is mounted on the top platen of a standard compression press. It’s the plunger

• The clamping pressure is used to keep the mold totally closed while the top cylinder provides the pressure required to force the compound into the runner system and cavity

Automatic Transfer Molding

• An attempt to integrate perform and preheat as a function of press operations

• An overhead performer capable of feeding a precise weight of perform feeds a dielectric preheater directly below

• The preheater feeds the compound to the plunger

Automatic Transfer Molding

• The plunger feeds the mold

• Completely automatic, but confined to the use of free flowing granular materials

• They have been slightly redesigned for other materials

Screw Transfer Molding

• Uses an extruder to supply a precise weight, temperature and density, shot, that feeds into the mold on a completely automatic basis

Screw Transfer Molding