Rotational Molding

Introduction

Rotational molding (RM), also called rotomolding or rotational casting, is a process for producing bellow, seamless items of all sizes and shapes.  The molded products range from domestic tanks to industrial containers, from small squeeze – bulbs to storage vessels for highly corrosive materials.   Commercial and industrial containers for packaging and materials handling may be covers and housings, water softening tanks and tote boxes; bay cribs, balls, doll parts; display figures; sporting equipment such as golf carts, surfboards, footballs, juggling pins, helmets; playground equipment and games; housings for vacuum cleaners, scrubbers, and lawnmowers; traffic barricades; display cases; boat hulls (Fig. 13.1); and so on.

Rotational moulding can produce quite uniform wall thickness even when the part has a deep draw of the parting line or small radii.  Ti is also useful if other techniques make it impossible to obtain a uniform wall thickness when molding a flat surface, especially if the surface has a large area.  The liquid or powdered  plastic used in this process flows freely into corners or other deep draws upon the mold being rotated and is fused/melted by heat passing through the mold’s wall.

This process is particularly cost-effective for small production runs and large part sizes; the molds are not subjected to pressure during molding, so they can be made relatively inexpensively out of thin sheet metal.  The molds may also be made from lightweight cast aluminum and electroformed nickel, both of them light in weight and low in cost.  Large rotational machines can be built economically because they use inexpensive gas-fired or hot-air ovens with the lightweight mold-rotating equipment.

Large parts range from up to 22000gal (83m³) in size, with a wall thickness of 1.5in. ((38mm). One tank this size used 2.4 tons (5300lb).

Processing

In most operations the mold cavities are filled with a certain amount of liquid or powder (charging the molds); the mold halves are clamped or bolted together; the charged and closed mold is then placed in a heating oven and the equipment biaxially rotates the mold during the heating cycle.  During heating, the plastic material melts, fuses, and densifies into the shapes of the internal cavities by directional centrifugal forces.  After heating is competed, the molds are moved into a cooling chamber where they continue to rotate and are slowly cooled by air from a high-veiocity fan and / or by fine spray of water.  After removal from the cooling chamber, the molds are opened and the solidified products are removed. 

Although RM machines can be built very inexpensively, they are lab-intensive.  Both rotations have to be programmed, either at the same rate or at different rates, depending on the product shape.  The temperature and duration of the heating cycle also need to be dontrolled.  Most machines that are being built have horizontal rotating arms with closed, recirculating, high-velocity, hot air ovens, with total automation of the compete process.  Many of these machines are also computer programmed to obtain consistent product quality.  The most common combine recirculating hot-air, gas-fires ovens with molds made of cast aluminum or fabricated sheet metal.  Fast operation is achieved by having four positions; load, heat, cool, and unload.  They use four arms, each holding a mold; thus, each positon is constantly in use.

More conventional RM uses three-position machines, where one arm is used for unloading and loading, followed by heat and cool (Fig. 13.2).  figure 13.3 shows the feeding inlet to form hollow products inside a closed mold while the mold is heated and rotated about two axes.  This system allows different plastics to be molded in multilayers; it is called corotational molding.

Different designs are used to meet different processing requirements: caroused, shuttle, clamshell, rock-and-roll machines, and so on.  Shuttle machines are principally used for rotational molding of large products such as tanks.  A frame for holding one mold is mounted on a movable table.  The table is on a tract which allows the mold and the table to incve into and out of the oven.  After the heating period is complete, the mold is moved into an open cooling station.  A duplicate table with a mold moves into the heating oven, usually from the opposite side of the oven.

As one mold is being cooled, the other mold is in the heating stage, and so on.

The clamshell machines have only one arm.  The same location provides mold loading, heating, cooling, and unloading.  It uses an enclosed oven that also serves as the cooling station.

The rock-and-roll technique is popular for molding long products such as canoes and kayaks.  It rotates on one axis and tilts to provide action in the other axial direction.  Most of the motion is in the long direction of the product, which relates to the main rotating motion.

Molds

Cost aluminum molds are most frequently used, especially for small to medium-size products.  Cast aluminum has better heat transfer than steel and is very cost-effective when several molds for the same part are required.  Sheet metal molds are normally used for larger parts.  They are easy to produce since sections can be welded together.  Since the molds are not subjected to pressure during molding, they are not built to take the high loads required in molds for injection, compression, and other pressure operating molds (Chapters 2,4,8).

Two –piece molds are usually used but molds in three or more pieces are sometimes required to remove the finished products.  Molds can be as simple as a sphere, and molds can be complex with undercuts, ribs, and /or tapers.  Design considerations include heat transfer, mounting techniques, parting lines, clamping mechanisms, mold releases, venting and material stability in storage and during the RM process.

Mold – release agents are usually required because the plastic melt may adhere to the surface of the mold cavity, particularly if the cavity has a very complex shape with contours, ribs, etc.  many molds must have very little or no draft, so they require a  mold-release agent.  There are mold-release agents that can be baked or applied to the cavity by wiping.  By coating with fluorocarbon, the need for mold release could be eliminated.  With conventional RM, after the initial mold-release agent is applied, several hundred parts can usually be molded before a stripdown of the mold cavity is required and another baked on coating is applied.

Most rotational molds require a venting system to remove the gas that develops during the heat cycle.  Venting of mould is also used to maintain atmospheric pressure inside the closed mold during the entire cycle of heating and cooling.  The vent ill reduce flash and prevent mold distortion as well as lowering the clamping pressure needed to keep the mold closed.  It will prevent blowouts caused by pressure and permit use to thinner molds.  The vent can be a thin-walled tube with an internal liner of PTFE.  The opening where enters the mold is located where it will not harm the performance or appearance of the molded product. 

Materials

Most RM resins are in powder form with a particle size of 35 mech (74-2000µm).  The other form is liquid.  Some high-flow resins, such as nylon, have been used in small pellet form.  About 85 wt% of molding applications use polyethylenes, particularly LDPE, LLDPE, HDPE and cross linked grades of PE (XLPE and other grades).  Ethylene vinyl acetate and adhesive PEs are also used in specialized applications as are PVC, PC, TP polyester, nylon, and PP.

Costing

Advantages exist with rotational molding since costs for molds and equipment are lower than those of most other processing methods.  Channels for cooling water and resistance to clamping force are not needed.  Different products and colors may be molded on the same machine and in the same cycle.  Quick mold changes are possible when several short production runs are required.  Large, hollow parts are conveniently molded.  Trimming can be eliminated because very little flash is produced.  The molded parts are relatively stress-free.  Corner sections are thicker than with other processes, processes, which provides additional strength when required.  Undercuts, molded-in inserts, intricate contours, and double-wall construction are routinely included.

The essential characteristics of thermoplastic sheet material namely that when they are heated to just below molting point they become rubbery or plastic in nature to an extent which enables them to be attachment out rather like a balloon.  They thus display a good hot melt strength and it is this feature which enables various types of sheet materials to be successfully formed.

Plastics sheet is manufactured by the main processes like extrusion, calendaring and casting.  In general, the greatest proportion of sheet is produced by the continuous extrusion or calendaring process with thickness ranging from 0.15mm to 12.5mm.

Thermoforming Materials

The materials most generally used for forming may be listed as follows:

Polystyrene (PS), Acrylo-nitrile-butadiene –styrene (ABS), Polyvinyl Chloride (PVC ), Acrylic (PMMA), Cellulose-acetate-butyrate (CAB) Polycarbonate (PC) and Polypropylene (PP).

Thermoforming molds

Wood, plaster, cement, metal epoxy or polyesters.   For forming prototypes and small batch quantities,  wood or plaster moulds may be used.  Hard wood is frequently employed but for long production runs epoxy resin or polyester resin coated are to be preferred.  For long runs cast and machined aluminium and steel moulds are preferred when durability, rapid cooling is required.  The moulds  must be drilled with a number of small holes for evacuating or supplying air.

Heating arrangements

Electric resistance heaters are used from one or both side of the sheet. 

Process

The mould is places over the air outlet.  The plastics sheet to be formed is placed over the open top of machine (box) and clamped down by the frame, thus sealing –off the box and making it an airtight  compartment.  The heater panel is them placed over the plastics sheet at a distance of 5-6 in .  (127 – 156mm) in order to heat the sheet as uniformly as possible.  When the sheet has been raised to a temperature below its melting point, the heater is withdrawn and air is evacuated by means of the evacuated by means of the vacuum pump.  This causes the plastic sheet to be sucked down into or over the mould and to form an accurate reproduction of the mould contours.  The mould can be either male or female or a combination of both, when the sheet has cooled and hardened the clamping frame is then released, the formed sheet removed from the mould and the surplus material trimmed-off.

The cycles of a vacuum forming sheet is :

-         Clamping of the sheet, heating of the sheet, forming of the sheet, cooling of the sheet, and removal of the sheet.

Thermoforming or vacuum forming machine consists of a vacuum pump, clamping  frame, a heating panel, air compressor and mould.

Different type of thermoforming or vacuum forming methods are :

-         Straight vacuum forming in a female mould

-         Drape forming on a male mould

-         Pre-stretched by the bubble or air assist method

-         Plug assist vacuum forming

-         Matched mould or die forming

Process variables in thermoforming

-         Materials variations :

1.   Sheet thickness :- Thickness variations should not be over 4-8% for high quality production it can be caused dye to

i)             Uneven heating      ii)       Uneven drawing          iii) or both.

2.   Sheet viscosity can melt index variations mainly applicable to olefins

3.   Sheet orientation : - For maximum uniformity, the sheet used must be un oriented.

4.   Sheet density :-  Variations in sheet density  (especially of olefins) can cause variations in tensile strength, shrinkage, hardness, stiffness and softening temperature.

-         Process variables:

1.   Heating temperature and final sheet temperature

2.   Prestreching

3.   Air or plug temperatures :- Air and / or plugs used to prestrech or form the port should be kept at controlled temperature to insure maximum uniformity of the part.

-         Mold variables

1.   Vacuum holes :- These should be as small as possible, so that no marks will be on the final part.  Slots are better than holes, but are more expensive.

2.   Speed of evacuation : In general, the faster the evacuation, the better the part will form.

3.   Mold temperatures : Mold temperature can affect the forming of part and final size by affecting shrinkage.  If size variations cannot be tolerated, optimum temperature has to be maintained.

4.   Mold Surface :- If varies depending on polymer processed.  For PE, a lightly sandblasted surface will suffice.