1.15 Polymer Processes Flashcards
(22 cards)
What are the forming methods for of polymers
- vacuum forming
- thermoforming
- calendaring
- line bending
- laminating (layup)
- injection moulding
- blow moulding
- rotational moulding
- extrusion
- compression moulding.
State two features of vacuum forming
- Moulds are relatively easy to make but need a draft angle, air holes, and rounded corners
- Not cost effective for one-off manufacture
- Small-scale batch and mass production
HIPS (High Impact Polystyrene): Tough, lightweight, excellent forming properties, easy to trim
What are the detailed steps involved in vacuum forming a polymer sheet into a product?
- Mould preparation:
A mould (also called a former) is created from MDF, urethane resin, or aluminium.
The mould must have draft angles (sloped sides) and vent holes to aid airflow and removal.
- Plastic sheet loading:
A sheet of thermoplastic (e.g. HIPS, PVC, or PETG) is clamped into the vacuum forming machine’s frame.
- Heating:
The plastic is evenly heated using a radiant heater until it reaches its forming temperature (usually between 140–180°C, depending on the polymer).
- Raising the mould:
Once the sheet is softened, the mould is raised into the softened plastic, positioned centrally.
- Applying the vacuum:
A vacuum pump is activated, sucking the air out from beneath the plastic sheet.
Atmospheric pressure forces the plastic to tightly conform to the shape of the mould.
- Cooling:
The plastic is allowed to cool while still in position. Fans or compressed air may speed up this process.
- Removing the formed part:
Once hardened, the vacuum is released, the mould lowers, and the plastic part is removed.
- Trimming:
Excess plastic (called the flashing) is cut away, and edges may be smoothed or drilled for finishing.
State two features of thermoforming
- Like vacuum forming, except an additional mould is pressed on the surface of the polymer sheet at the same time as the vacuum is applied
- Good for fine details such as moulding in logos
- Slow process, used in industrial production
What are the detailed steps involved in the thermoforming process?
- Polymer sheet loading:
A sheet of thermoplastic (e.g. HIPS, ABS, PETG) is clamped into place.
- Heating:
The sheet is heated until it reaches its plastic forming temperature (around 160–200°C, depending on the material).
- Moulding:
The mould closes around the softened plastic, using either a male mould (pushing up) or a female mould (pulling down), or both.
Compressed air or vacuum helps to force the sheet into every detail of the mould surface.
- Cooling:
The formed part is left in the mould to cool, sometimes with air jets or water-cooled tools to speed up the process.
- Ejection and trimming:
The mould opens and the formed part is removed and trimmed, often using a CNC router or guillotine.
State two features of calendering
- Heated rollers squash and stretch the polymer to make it thinner
- Industrial production method, carried out by specialist manufacturers
- Used for continuous production
- Calendaring is best suited for high-volume, continuous production due to its efficiency in producing large, consistent sheets or films of thermoplastics like PVC.
eg. Plastic films and sheets (e.g. PVC sheets) and Shower curtains, tablecloths, conveyor belts
What are the step-by-step stages in the calendering process?
- Polymer preparation:
Granules or pellets of thermoplastic (commonly PVC) are heated and softened using an extruder or internal mixer.
- Feeding into rollers:
The softened polymer is fed into the calendering unit — a series of heated, polished steel rollers, usually between 3 and 5 in total.
- Forming and rolling:
The material passes through the rollers, which flatten and stretch the polymer into a thin sheet.
The gap between the rollers can be adjusted to control the sheet’s thickness.
- Cooling:
After forming, the sheet passes through cooling rollers to solidify and set the plastic.
- Winding or cutting:
The finished sheet is either wound into rolls or cut to size depending on its application.
State two features of line bending
- Uses an electrically heated element that provides heat along a line
- Quite slow and labour intensive used in school or college workshops
- Suitable for one-off or limited batch production
State two features of lamination (lay-up)
- Requires a former to mould around
- Polyester resin used as the polymer
- A top gelcoat is applied which may contain pigment
- CFRP requires an autoclave for the product to ‘cure’
- One-off or low-volume batch production
Lay-up laminating is typically manual or semi-manual, making it best suited for custom, low-volume, or prototype production where high strength and tailored shapes are needed.
eg. Fiberglass or carbon fiber parts, Boat hulls, Car body panels and race car parts
What are the detailed steps in the laminating (lay-up) process?
- Mould preparation:
A mould or former is cleaned and coated with a release agent (e.g., wax or PVA film) to prevent the resin sticking.
- Gel coat application (optional):
A gel coat may be applied for a smooth outer finish (especially for products like boats or car panels).
- Layering fibre mat:
A layer of glass fibre matting (or carbon/aramid fibre) is laid over the mould.
- Resin application:
Thermoset resin (e.g., polyester or epoxy) is applied using a brush or roller, saturating the fibre matting.
- Repeating layers:
Multiple alternating layers of fibre mat and resin are built up to the required thickness.
- Consolidation:
A roller or brush is used to remove air bubbles and ensure even distribution of resin.
- Curing:
The lay-up is left to cure at room temperature or may be heat-cured depending on the resin.
- Finishing:
Once hardened, the part is removed from the mould, and any excess is trimmed or sanded.
State two features of injection moulding
- Complex 3D shapes can easily be made
- Fast industrial production method
- High initial tooling cost
- Used for large-scale mass or continuous production
What are the detailed stages of the injection moulding process?
- Polymer granules loaded:
Thermoplastic granules (e.g. ABS, polypropylene) are loaded into the hopper.
- Heating and plasticising:
Granules are fed into a heated barrel by a rotating screw.
Friction and heaters melt the plastic into a viscous fluid.
- Injection into mould:
The screw moves forward like a plunger, forcing the molten polymer into the closed steel mould at high pressure.
- Cooling and solidifying:
The mould is kept water-cooled, so the plastic solidifies rapidly into the desired shape.
- Mould opening and ejection:
The mould opens, and ejector pins push out the finished product.
- Cycle repeats:
The mould closes again and the process repeats automatically.
Cycle times can be as quick as 15–60 seconds, depending on product size and material.
State two features of blow moulding
- Produces hollow thin-walled components
- High initial tooling cost
- Used for continuous production
What are the detailed steps in the extrusion blow moulding process?
- Plasticising:
Thermoplastic granules (e.g. HDPE, PET) are fed into a heated barrel and melted by a rotating screw.
- Extruding the parison:
A hollow tube of molten plastic (parison) is extruded vertically from a die head.
- Mould closing:
The split mould closes around the parison, sealing the bottom and creating the product’s final shape.
- Blowing:
Compressed air is injected into the parison, forcing the plastic to expand and fill the mould cavity.
- Cooling:
The plastic is allowed to cool and solidify while still inside the mould, often with cooling channels.
- Mould opening and ejection:
The mould opens, and the finished product is ejected. Excess plastic (flash) is trimmed.
- Quality check and finishing:
Products may undergo trimming, inspection, and packaging.
State two features of rotational moulding
- Seamless hollow components with a large wall thickness
- Mould continuously rotated through heating and cooling
- Large-scale batch or mass production
What are the detailed steps in the rotational moulding process?
- Loading the mould:
A specific amount of thermoplastic powder (usually polyethylene) is loaded into a hollow metal mould.
- Sealing and rotating:
The mould is sealed and placed inside an oven while rotating on two axes.
- Heating and coating:
As the mould rotates, the powder melts and evenly coats the inside surface of the mould by centrifugal force and gravity.
- Cooling:
The mould is removed from the oven and cooled slowly, often using air or water spray, while still rotating to prevent sagging or deformation.
- Demoulding:
Once solidified, the mould is opened and the finished hollow part is removed.
- Trimming and finishing:
Excess plastic at seams or openings is trimmed and any post-processing like drilling or assembling is completed.
State two features of extrusion
- Like injection moulding expect polymer is forced through a die
- Industrial production method
- Continuous position
- Mass production / Continuous production
Extrusion is highly efficient for high-volume, continuous manufacturing, producing long lengths of uniform cross-section plastic products.
eg. Pipes and tubing (PVC, polyethylene)
Window and door frames
Wire and cable insulation
What are the detailed steps in the polymer extrusion process?
- Feeding:
Plastic granules or pellets are fed into a hopper, which funnels them into a heated barrel.
- Melting:
Inside the barrel, a rotating screw moves the thermoplastic forwards while heaters melt the material.
- Pressurising:
The screw builds pressure, forcing the molten plastic to move towards the die.
- Shaping:
The plastic is pushed through a custom-shaped die, forming a continuous profile (e.g. a rod, tube, or sheet).
- Cooling:
The extruded plastic is cooled using air or water baths, solidifying its shape.
- Cutting:
The material is cut to length or wound onto spools depending on the final product requirements.
State two features of compression moulding
- Uses a pre-weighted ‘slug’ of polymer
- Low to medium volume batch production
Compression moulding is best suited for medium-scale production, especially for thermosetting plastics and composite materials. It’s ideal when high strength and durability are required.
eg. Automotive parts (e.g. bumpers, panels, gaskets) and Cookware handles and knobs
What are the detailed steps in the compression moulding process?
- Preparing the charge:
A pre-weighed amount of thermosetting plastic (e.g. phenol formaldehyde) is prepared in the form of a powder, pellet, or preform (disc or sheet).
- Heating the mould:
The steel mould is preheated to the required temperature to initiate curing once pressure is applied.
- Placing the charge:
The plastic charge is placed into the bottom half of the open mould.
- Closing the mould:
The top half of the mould is closed, and hydraulic pressure is applied to force the material to flow into the shape of the cavity.
- Curing the polymer:
Heat and pressure are maintained as the material cures (cross-links and hardens) into its final shape.
- Opening and removing:
Once curing is complete, the mould is opened, and the solidified product is ejected.
- Trimming and finishing:
Any excess flash (material squeezed out of the mould) is trimmed off.
State two features of line bending
Signage: acrylic signs, illuminated sign bases
Lighting: light covers, diffusers
Production Scale:
Ideal for one-off and batch production
Common in custom work, prototypes, and small retail runs
Not suitable for mass production due to manual or semi-automated process
What are the detailed steps involved in the line bending process?
- Marking the bend line:
The sheet polymer (e.g. acrylic) is marked where the bend is required.
- Positioning:
The plastic is placed on a strip heater, aligning the marked line with the heated element.
- Heating:
The plastic is heated along the line only, softening the material just in that zone.
This usually takes 30 seconds to 2 minutes, depending on thickness.
- Bending:
Once soft, the sheet is manually bent along the heated line using a jig, former or edge guide to ensure a clean, accurate angle (often 90°).
- Cooling and setting:
The bent section is held in position (with a jig or cooling block) until it solidifies and holds the new shape.
- Finishing:
Any markings are removed and edges may be polished if necessary.
