Introduction to Polymer Science and Technology

Polymer processing

charging the mould

heating & tumbling the mould

demoulding after cooling

Figure 3.44Illustration of rotational moulding

Figure 3.45Section of a mould covered with insulation fabric

The products often require secondary operations,such as trimming by a router (much safer than using a Stanley knife) and vacuum lifting of the trimmed swarf for recycling; glossing/sheening the surface of products with a short passing of naked flame over it (torching/flaming); decorating/labelling; joining together the sections of a product by hot-bar, hot-gas, or ultrasonic welding, as well as many other mechanical assembly work that may require drilling, etc.

The rotational moulding process enables the production of large hollow products, which are stress-free and contain no weld-lines, using comparatively low-cost moulds. Materials commonly used are polyethylenes (LDPE, LLDPE, MDPE, and HDPE), PP, ethylene vinyl acetate and PVC. For some applications cross-linked PE is also used. Typical products include agricultural products (e.g., feeders, drinking bowls/troughs, wheel barrows, calf hutches and chicken coops (Figure 3.46)); oil and water tanks; rainwater-harvesting tanks; diesel fuel and hydraulic tanks; toys and playground items; traffic cones, and marine products(e.g., canoes and kayaks, navigation buoys and mussel floats).

Introduction to Polymer Science and Technology

Polymer processing

Figure 3.46Rotational moulded chicken coops (courtesy of JFC Manufacturing Ltd.)

Rotational moulding machines are specified by their swing diameter (the envelope in which rotation should occur freely when operating in heating (oven) and cooling (cooler) chambers/stations. There are two types of rotational moulder:

1) Drop (offset) arm/spindle (Figure 3.47-a): suitable for medium to large mouldings

2) Straight arm/spindle (Figure 3.47-b): suitable for multiple small mouldings

Introduction to Polymer Science and Technology

Polymer processing

Figure 3.47Rotational moulding machines with (a) drop arm and (b) straight arm

Most modern rotational moulding machines involve complete biaxial rotations about two perpendicular axes and the machines are classified as shuttle machines or carousel machines, depending on their modes of operation and lay out of the stations. The shuttle machinesconsist of two mould carriages, each with a single arm moulder, operating on rectilinear tracks/rails. The system includes one oven and two cooling/service stations. During the operation, the carriage/mould in the oven is moved into its own cooling/service area on straight rails, and simultaneously the other mould enters into the oven from the other cooling/service area. The use of dual-carriages improves efficiency, since the oven is always occupied by a mould while the other mould is being cooled, de-moulded and re-charged.

The carousel machinesrotate in and out of the oven, the cooler and the service area. As long as there are extra stations available, the machine can be fitted by up to six arms of the same type or different types. Note that in a service area, the finished product is removed from the mould and then the mould is charged with fresh plastic powder ready for the next run.

The carousel machines come in two different models, fixed and independent. On a fixed machine three or four arms move in succession into and out of various stations. The moulds spend the same length of time in each operation chamber and, therefore, the fixed machines should work with the same moulds. The independent carousel machines can accommodate more arms that can be fitted with different types of moulds. Of course the arms cannot rotate past each other but they can move separately from each other and spend different lengths of times in different stations. This allows moulds of different shapes and sizes charged with different materials, with different heating and thickness requirements, to be processed. Detailed coverage of various rotational moulding machines/processes is given by Crawford & Kearns (2003).

Temperature measurement is critical for monitoring the process:one can establish various temperature-cycle time profiles (Crawford & Kearns 2003, p72) representing inside the oven, outer-surface of the mould, plastic and the air inside the mould with a data logger attached to the frame of the moulds and harnessed with a suitable number of the thermocouples

Introduction to Polymer Science and Technology

Polymer processing

with sufficient length of loose thermocouple wiring to accommodate rotation of the frame. Thermocouple wires for inside the mould cavity are inserted through a filter (akin to a cigarette filter) placed inside a PTFE sleeve. The filter stops the polymer powder from falling out but allows air venting out. In the oven when the tool begins to rotate initially the temperature fluctuates (± 40 °C) but it soon stabilises.

Processing with PE, the air temperature inside the mould reaches approximately 200 °C when the air temperature inside the oven stabilises at a set temperature of 310 °C. The tool stays rotating in the oven for approximately 10 min, then comes out to the cooling station, continues to rotate for a further 10 min for the temperature to fall down to below 100 °C. The temperature-time curve (Figure 3.48) for the air temperature inside the mould cavity shows that temperature climbs up to Point A as a function of the oven temperature, where the melting begins and the smallest particles begin to adhere to the tool/mould surface. The heat is needed for melting and, therefore, the internal-air temperature increases only very slowly between Points A and B. When all the powder has melted at Point B, the internal-air temperature begins to increase again rapidly.

The sintering process, where the plastic particles on the mould surface fuse together to produce a smooth homogeneous layer, begins at Point B, reaches its optimum state at Point С and continues through into the cooling cycle until the start of crystallisation at Point D . Note that, once the tool comes out of the oven into the sintering/cooling chamber, there is still a temperature overshoot within the mould and then the temperature inside the cavity begins to decrease. The rate of fall in temperature slows down at Point D because of the contribution of the heat associated with the exothermic solidification/crystallisation process. Following complete solidification, Point E, the internal air temperature continues to fall at its normal rate. At Point F, the plastic shrinks and detaches itself from the tool face, which is accentuated by the release agent (usually silicone solution wiped on the mould), leaving a gap between the part and the tool surface. This layer of air (space) between the tool inner face and the outer face of the moulding acts as insulation and decelerates the cooling inside the tool cavity.

Introduction to Polymer Science and Technology Polymer processing

Developments in rotational moulding seem to concentrate on multilayer structures with different types of polymers for high barrier applications, e.g., for fuel tanks, or with layers that contain fibres/fillers/nano material (e.g., nano clay), or with biodegradable and biosustainable (e.g., polylactic acid polymer) materials, skin-foam layers, conductive materials such as Cu or Al powder and the use of micropellets.

A multi layer rotational mouldingmay consist of three layers: e.g., a layer of polyethylene, with a second layer of fibre-reinforced polyethylene or foamed polyethylene and another layer of polyethylene. The multiwall or foamed mouldings are normally produced by a multi-charge process:with the first charge of the plastic powder, which forms the outer layer of the part, being charged into the mould in the usual way. The subsequent charges of powder/micropellets are added during the heating cycle when the temperature reaches approximately the point shown by the asterisk on the graph (Figure 3.48), and the full heating/cooling cycle is completed after the addition of the final charge.

The single-charge methodinvolves placement of a plastic blend of lower and higher temperature melting point materials, or a blend of smaller and larger powder/pellet particle sizes in the mould. The principle is that the lower melting point material or smaller particles melt first and adhere/sinter onto the inner mould surface forming the outer skin/layer of the product.

The foamed rotational moulded products could be in the form of containers with walls consisting of a solid outer skin and an inner foam layer or solid inner and outer skins with foam core, or tubs and box sections (Figure 3.49) with solid outer walls completely filled with foam. I-FOAM (Insulation Foam) Ltd. produce PE pellets that contain expansion foaming agent with, apparently, up to of 40 times expansion ratios. The moulding process is a one shot method, allowing loading of all raw materials at the same time and therefore producing wholly integrated foamed products with excellent bonding between the skin and the foam.

Introduction to Polymer Science and Technology

Polymer processing

Figure 3.49PE box section insulated with double-layer PE foam pellets (source: I-FOAM)

Micropellets are a useful alternative to powder plastic for some applications: in rotational moulding powder covers flat uncomplicated surfaces well but for complex parts micropellets give better coverage. Micropellets wash out and do not remain put on flat surfaces.

Micropellets/microgranules are obtained by fast underwater granulation: the process involves extrusion of hot strands into water (submerged) and cutting at the die face produces instantly spherical microgranules (spheres form because of the energetic tendency of the material to minimise the surface area and because of the presence of hydrostatic pressure).

One of the obvious concerns in incorporating engineering fibres such as glass fibre, natural plant fibres or nanoparticles into polymers for rotational moulding is the possibility of distortion of the finished parts, for example the walls of a box-shaped container bowing inwards. Such distortions to the mouldings are a consequence of the differences in the coefficient of thermal expansion (CTE) values for the polymer and the additives, which may result in the generation of un-balanced internal residual stresses.

When selecting polymers for rotational moulding, it is important to select grades that process easily and yield the required mechanical and physical/chemical properties, e.g., when selecting PE for oil-tank production, the important properties would be MFI (approximately a value of 4), flexural modulus and environmental-stress-cracking resistance (ESCR).

Rotational moulded products with mottled look or shot-blast or shot-peened finishes can be achieved by shot blasting the tool/mould surface with grit or very tiny billiards, often shot-blasted moulds are Teflon coated to facilitate mould release. Teflon coating involves applying/spraying a PTFE primer first. The primer contains colour so that the area covered becomes visible, and also once the PTFE coating is completed, the colour helps in spotting any subsequent damage to the coating. The primer is baked on at 110 °C, then a second ingredient is applied, then the clear PTFE solution is applied and baked at 380 °C.

Introduction to Polymer Science and Technology Polymer processing

3.3 Processing and forming thermosetting polymers

Thermosetting polymers/thermosets(TS) consist of a resin (linear polymer with pendant functional groupssuch as hydroxyl (OH), carboxyl (COOH), amino (NH₂) groups or containing double bonds) + cross-linking agent+ catalyst + heat (or cold curing with a catalyst activator). Typical TS polymers include polyesters, alkyds, amino resins, epoxy resins and polyurethanes. TSs contain cross-links in their structure and in general offer greater resistance to temperature and creep cf. TPs, however, they suffer from low impact resistance.

Most thermosets are available in two forms: resinsor as moulding compounds.

The resins maybe used in neat form, e.g., in encapsulation processes, or as matrix in polymer-matrix composites. Moulding compounds in the forms of dough moulding compounds (DMC) and sheet moulding compounds (SMC) are a mixture of TS resins with both fillers and fibre reinforcements. In SMC longer glass strands are used. Low viscosity formulations, known as low pressure moulding compound (LPMC), are suitable for low-cost tools/moulds.

Polyester resins (unsaturated)are produced by condensation polymerisation of a polyfunctional acid with a polyfunctional alcohol.

Maleic acid (HOOC - HC = CH - COOH) contains 4 functional groups. The carboxyl groups undergo esterification (condensation) reaction with a di-alcohol (e.g., ethylene glycol), and the double bonds can enable addition reactions.

HOOC- CH= CH- COO- C₂H4- О(- CO- CH= CH- COO- C₂H4-O-)_n

Ester link

Unsaturated (i.e., containing unreacted double bonds) polyester resins are usually supplied as a solution in styrene, which acts as a solvent and also as the cross-linking agent (by reaction through the double bonds).

Therefore, unsaturated linear polyester + styrene + an activated catalyst -> a cross-linked polymer.

Polyester resins are usually used with woven glass-fibre cloth or mats of chopped glass-fibre strands to produce strong laminates. DMC and SMC containing unsaturated polyester resins are widely used. Polyester DMC is of a putty-like consistency and has a very low viscosity at moulding temperatures and, therefore, can be compression moulded using low pressures. Their relatively high temperature resistance and desirable electrical properties make it attractive for the electrical industry.

Introduction to Polymer Science and Technology Polymer processing

Amino resins such as urea formaldehyde (UF)

H₂N - CO - NH₂ + CH₂O -> UF

Urea formaldehyde

and melamine formaldehyde(MF), produced from melamine (Figure 3.50) and formaldehyde, which are available as neat resins, and also as moulding powders.

NH₂

A

N N

С. С

Н₂]чГ v_N/ \ш₂

Figure 3.50Melamine (a cyclic amino compound)

UF resins are used as adhesives for plywood and chipboard, to impart crease resistance to fabrics (it is a clear resin), and to improve the wet strength of paper. The use of melamine-formaldehyde polymers include kitchen and picnic ware (Figure 3.51).

Figure 3.51Le Cadeaux melamine fruit-motif assorted plates

Phenol-formaldehyde(PF) resins were the first man-made plastics to be commercially used. The PF prepolymers are novolaks and resols. The novolaks are prepared using excess phenol (molar ratio of 5:4) under acidic conditions. A resol is formed by using excess formaldehyde (phenol to formaldehyde molar ratio up to 1: 2.5), normally under basic (alkaline) conditions.

Early applications were in the electrical industry. PF impregnated paper and cotton fabric laminates find application as very durable gear wheels (e.g., Tufnol products). PF and MF exhibit good flame resistance. The PF (an inherently fire resistant material with low smoke emission, like PVC) now replaces polyester in normally polyester-based products. London Transport specifies phenolic glass-fibre reinforced plastic (GRP) for use in underground rolling stock and as cladding in station escalator wells. Serious concerns were raised in the London Underground after the 1987 Kings Cross Station fire regarding health and safety. Similarly, the Dusseldorf airport fire in 1999, which claimed 16 lives, focused attention on the type of materials used in public places. The victims died mainly as a result of suffocation and poisoning from the thick smoke emitted by the burning material. Other uses of PF include the weapons and equipment storage boxes on British naval ships, where, following the Falklands conflict, fire retardancy and low smoke emission were found to be essential.

Processing methods for thermosets involve the reactive processing of prepolymer/monomers with a catalyst or a curing agent as part of the shaping operation, and include:

compression/transfer moulding

injection moulding

reaction injection moulding (RIM)

- vacuum infusion
resin/foam dispensing

autoclave and resin transfer moulding pultrusion

- SMC/DMC moulding
filament winding.

3.3.1 Compression moulding

The process is suitable for moulding powders or DMC/SMC. The mould is placed in a hydraulic press and is heated (160-200 °C), being thermally insulated from the platens (Figure 3.52). The required amount of prepolymer (mixed with catalyst

Date: 2015-12-11; view: 787