Extrusion This process can be compared to squeezing toothpaste from a tube. It is a continuous process used to produce both solid and hollow products that have a constant cross-section. E.g. window frames, hose pipe, curtain track, garden trellis. Process The photo below shows a typical thermoplastic extruder. Thermoplastic granules (below) are fed from a hopper by a rotating screw through a heated cylinder. The tapered shape of the screw (below) compacts the plastic as it becomes plasticized. This part of the process is similar to the heating and compacting stages in the injection moulding process. The difference being that the softened material is allowed to flow out through a die in a continuous stream rather than be pumped intermittently in measured amounts into a mould. The die (below) which is fitted to the end of the extruder barrel determines the cross-section of the extrusion.
This extrusion is part of a window seal made from thermoplastic elastomer (TPE). Extrusion rates typically vary between nine and sixty metres per minute. Thicker crosssections are extruded more slowly as more time is required for the initial heating and subsequent cooling of the larger quantities of material which are involved. As the extrusion leaves the die it is cooled by passing through a cooling trough (below) containing cold water. The photo below shows the full length of the cooling trough and the extrusion passing through it on its way to be cut to length.
The saw (below) cuts the extrusion automatically to the required length. The extrusions (below) can then be assembled into the finished product. The photo (below) shows a completed garden trellis.
Injection Moulding process moulds material Injection moulding is a highly automated production process for producing large quantities of identical items. Granulated or powdered thermoplastic material is heated, melted and then forced under pressure into a mould. Once in the mould the material cools, forming a component that takes on the shape of the mould cavity. The photo below shows a typical product manufactured by injection moulding. This toy saxophone has been made in high impact polystyrene (HIPS). Process Plastic powder or granules are fed from a hopper into a hollow steel barrel which usually contains a rotating screw. The barrel is surrounded by a jacket of heaters which melt the plastic material as it is carried along the barrel by the screw towards the mould. This part of the process is similar to the heating and compacting stages in the extrusion process. The screw is forced back as the melted plastic collects at the end of the barrel. Once a sufficient charge of melted plastic has accumulated a hydraulic ram forces the screw forward injecting the thermoplastic through a sprue into the mould cavity. The photo (below) shows a typical injection moulding machine. This one is capable of exerting forces of up to up to 250 tonnes. Pressure is kept on the mould until the plastic has cooled sufficiently for the mould to be opened and the component ejected. Moulds The two part mould shown in the photo below has been machined from high chromium steel (stainless). Moulds like this one cost several thousand pounds to manufacture. However this can be economic as many thousands of components can be produced from a single mould
thus resulting in a low cost per item. Different moulds have to be fitted to the machine whenever a new type of product is required. The following set of photographs show both halves of a mould used to produce an "electronics control box". The hydraulic ram has to exert a force of 100 tonnes to produce this moulding in fire retardant ABS. The first photo shows the completed control box ready to be ejected from the mould. The next photo shows the mould after the component has been ejected. The ejector pins can clearly be seen protruding from the mould. The other half of the mould is shown below. The thermoplastic material is injected through the sprue hole located in the centre of this part of the mould.
Material Normally thermoplastics are used in this process although a few thermosetting plastics can also be injection moulded. Material properties of the thermoplastic can be altered by adding plasticisers and other chemicals to improve such things as impact and ultra violet (UV) resistance. A wide range of colours are available and this is achieved by adding a 1% colour mix to the usually colourless raw material prior to melting.
Vacuum Forming process moulds heater systems forming pressure materials trimming This process is used to manufacture a variety of products in thermoplastic materials. These products range in size from garden pond liners to food trays used in supermarkets. A typical industrial size vacuum forming machine like the one seen below is capable of producing vacuum formings up to 1.8m x 1.5m in size. Process A mould, like the one below, which is used for producing the interior panels of lorry cabs, is attached to a platen (support plate). The platen and mould are then lowered and rigid thermoplastic sheet material is clamped onto an air tight gasket and usually heated from above. Once the thermoplastic sheet is softened enough (i.e. reaches a plastic state) then air is blown in to raise the sheet in a slight bubble before the platen is raised bringing the mould into contact with the plastic. Any trapped air remaining between the platen and the heated plastic sheet is then evacuated by a vacuum pump. Atmospheric pressure acting over the top surface completes the forming process by pressing the plastic sheet onto the mould. Once the plastic sheet has cooled down to below it's freeze point the air flow is reversed to lift the forming off the mould. If this is not done quickly the forming tends to grip onto the mould and attempts to prise them apart often result in damage to the forming. The photo (below) shows completed vacuum formings made in ABS ready for the waste material around the edges to be trimmed off.
Moulds Moulds can be made from a variety of materials such as wood, medium density fibreboard (MDF), plaster of paris and clay. MDF is probably the most suitable for "one-off" vacuum formings or short production runs in school workshops Having no grain it is relatively quick and easy to shape compared with solid wood and is also stable whilst being heated in the vacuum forming machine. For higher volumes of production it is necessary to manufacture moulds from harder wearing materials such as aluminium or cast epoxy resin. The photo (below) shows an aluminium mould positioned on the platen of a vacuum forming machine. This pattern is used to produce the base part of a shop display board. It is necessary to have a 5-10 taper or draft angle on the sides of the mould to enable the forming to be separated easily from the mould. This can be seen clearly in the finished moulding (below) which has sides that taper in towards the top. It is important to avoid undercuts as these will also prevent the forming from releasing. Moulds which contain cavities should have vent holes (see below) drilled in positions where there is any possibility of areas of the mould being sealed by the hot thermoplastic sheet as the air is being evacuated.
Moulds should have radiused corners and the depth of the draw in straight sided cavities should generally be kept below a width/depth ratio of 2:1 to avoid thinning of the thermoplastic sheet whilst vacuum forming. Heater Systems Electrically powered ceramic heating elements are usually mounted under a heater hood which is attached to the forming machine. Heat from these elements is directed down onto the thermoplastic sheet in order to soften it in readiness for vacuum forming. Groups or zones of heaters are controlled separately by a power regulator so that fine adjustment of output is possible within each zone. This is to compensate for variations in heat loss such as around the heater edges. Some industrial machines can heat the thermoplastic sheet from both sides which can reduce heating times by up to 60%. Forming Pressure The vacuum pump can be connected directly to the platen in small and medium size machines. Larger machines may operate the vacuum pump during the heating cycle to evacuate a reservoir prior to removing air from the space between the mould and the softened thermoplastic sheet. The extra evacuated volume in the reservoir ensures that the air between the mould and the softened plastic sheet is evacuated quickly. The limiting factor of the vacuum forming process is that the forming pressure cannot be greater than atmospheric pressure (14.7psi or 1 bar). This can cause problems when high definition mouldings are required in thermoplastic materials which have poor plasticity properties. This problem is overcome by using a process called Vacuum/Pressure Forming. In this process a pressure chamber is sealed over the top of the hot plastic sheet and compressed air is pumped in to increase the forming pressure above atmospheric pressure. Materials Many types of thermoplastics are suitable for vacuum forming. The most popular is Polystyrene (HIPS). It is relatively cheap, comes in a wide range of colours and is easy to form. Typical thicknesses of sheet material used range from less than 1mm up to 6mm. Material Forming Temp. C Definition ABS 159 Good Extruded Acrylic 160 Good
Polycarbonate 200 Good Polyethylene 120 Fair Polypropylene 180 High Polystyrene (HIPS) 120 Good PVC 130 Fair Some thermoplastics (e.g. ABS) require pre-drying in an oven before vacuum forming as moisture blisters may appear on the surface when the material is heated if this is not carried out. Trimming Several methods can be used to remove waste material from the final product. For low volume quantities of vacuum formings a slitting saw can be used. For high volume quantities a clipping press can be used with thin sheet material and for thicker mouldings hand held or CNC (computer numerically controlled) routers are used. The photo (below) shows an ABS vacuum forming of a display sign which ready to be trimmed prior to assembly.