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Material Distributors provide an ‘off-the-shelf’ solution, usually supplying branded products manufactured by a range of leading feedstock manufacturers. They can offer a wide range of polymer types, (e.g., Nylon 6,6, Acetal, ABS) but each of these will have a fairly standard formulation and be predominantly uncoloured (Natural). It is possible to buy some polymer grades with a limited range of added components such as flame retardants and glass fillers at a set percentage.
Material Compounders can formulate your material to an exact ‘recipe’. They take a standard feedstock and run it through an extrusion line along with any selected additives. Coloured pigments are the most common addition, but a huge range of additives are available that can improve the processing characteristics, appearance or properties of the final product.
The extrusion (compounding) line ensures that all components are thoroughly blended before being pelletised.
When designing a product, you should give as much consideration to the properties the product will need to have as you give to its physical form. Selecting the correct material can make a major contribution to meeting these requirements.
When selecting a material, considerations should include:
Thermoplastics are more widely used within the moulding sector than thermosets, being used to produce everything from washing up bowls to automotive components. They are characterised by how they are processed, meaning they are heated until molten and then formed and solidified within a mould through cooling.
There are many types of thermoplastic materials, each grade offering differing properties to a finished component. They can be roughly grouped into ‘commodity’ and ‘engineering’ grades.
Common commodity grades are polypropylene (PP), polystyrene (PS) and polyethylene (PE, also known as polythene). These grades are called commodity grades because they are generally used for mass-produced price-sensitive products (they are considerably cheaper than engineering grades), are easily processed and can be readily recycled.
Here are some examples of materials and some typical applications:
|Low-density polyethylene (LDPE)||Plastic film, squeezable bottles, heavy-duty sacks, plastic tubing.|
|High-density polyethylene (HDPE)||Heavy-duty drainage pipes, plastic cases, carrier bags, bottles for chemical products, wheelie bins.|
|Polypropylene (PP)||Storage boxes, car bumpers, bottle caps, washing up bowls.|
|Polystyrene (PS)||Food containers, CD cases, plastic cutlery (not to be confused with the expanded PS used for packaging).|
|High impact polystyrene (HIPS)||Plastic vending machine cups, linings for fridges and freezers, vacuum formed packaging.|
Engineering grades typically offer a particular characteristic, and as their name suggests, are often used where a component needs specific properties. For example, nylon grades (Polyamides) are readily machined and are resistant to impact or shock. Polycarbonate is ideal for the production of lenses due to its strength and clarity, while Acetal is used for its structural rigidity, making it ideal for metal replacement.
Some thermoplastic grades can substitute thermoset materials that are harder to process, such as rubber. Examples are TPE and TPU, which are processed on standard injection moulding machines but have the flexibility associated with an elastomer.
Here are some other examples of commonly used plastic grades and typical applications:
|Polyethene terephthalate (PET)||Drinks bottles (carbonated in particular), plastic jars, microwaveable food packaging.|
|Polyurethanes (PU)||Used for insulating and cushioning foams, as well as thickly walled engineering products.|
|Polyvinyl chloride (PVC)||Flexible grades are widely used for electrical wire insulation. Rigid grades for window profiles and plumbing pipes.|
|Acrylonitrile butadiene styrene (ABS)||Electronic housings, keyboards, computer casings, printers.|
Thermosetting polymers rely on a chemical reaction that permanently solidifies them within the mould tool. Once this reaction has taken place, they cannot be reprocessed, but offer unique characteristics such as resistance to high heat levels or chemical attack (saucepan handles are a typical application that benefits from heat resistance). Some grades are also excellent electrical insulators, so can are frequently used for manufacturing household switches and sockets.
Once the right material is identified, you can add further refinement by considering additives that can be introduced. For example, these can be used to enhance structural properties, provide an exact colour match (RAL), or add characteristics such as resistance to UV light, antistatic or antimicrobial properties, or even fluorescence.
Unlike in the case of compounded materials, these additives are directly dosed into the processing machines throat at a set percentage. The plasticising unit (screw and barrel) of the machine combine the base material and additives during the standard machine screw-back (recovery) phase.
To dose the masterbatch or additives at the required percentage, specialist additive feeding or blending equipment is used.
Purging compounds are designed to efficiently remove a polymer from a processing machine’s plasticising unit. For example, before a tool change on an injection moulding machine, or material change on an extrusion line, the machine is first run dry to remove as much molten material as possible.
Empty machine hopper > Mould or extrude as much material as possible from the plasticising unit > Use a purging compound to remove any ‘hung-up’ material left in the system > Move on to the next material to be processed.
The purging compound is designed to scrub away or dissolve residual material, shortening the time taken and amount of material that would otherwise need to run through the system before a new production run can start. Colour changes are a good example of how a change-over can be sped up by using a purging compound.
Both liquid and solid (granular) purging compounds are available, with grades designed to work best with particular material grades. In the case of compounds designed for injection moulding, some are also ‘mouldable’, meaning they can also be used to clean a tool’s hot runner system.