Here at The Merriott Group, we process many different types of Thermoplastic materials for a wide and diverse range of products and customers, ranging from commodity types to engineering and higher performance types.

To reset, Thermoplastic is the material most widely used worldwide by injection moulding. This process is where the material is fed through a heated barrel and screw arrangement, thus turning the original pellet form into a molten form to be injected under force into the closed mould tool. Once cooled and solidified the mould part is then ejected from the open mould tool, and the process is repeated. Cycle times can vary from a few seconds to a few minutes, in some extreme cases, but on average around 36-60 seconds is the norm.

Commodity Plastics

Commodity Plastics are the most widely used types of materials, namely, Polypropylene (PP), Polyethylene (PE) and Polystyrene (PS). Having a specific gravity of 1 or fewer means that they are all lightweight and hence have the best weight to cost ratio. Being less than 1 enables the moulded product to float on water, and can be easily identified by this simple test.

Polystyrene (PS)

The origination of this material can be traced back to the 1800s and from the resin of a tree!! However today the material is based on Benzene, Carbon, and Hydrogen.

In its original form, it is water clear and is thus used for display products and packaging, which enable the consumer to see the article contained clearly. Additionally, it can be used for many of the disposable items that we use, such as beakers and cutlery, etc, but of course, as we now see at an environmental cost. Here at Merriott Plastics, we use this material to produce a range of DVD Trays. Read our informative blog on How DVD Trays Are Made if you would like to know more.

Polystyrene (PS) is not a strong material and can be brittle. This can be improved with the use of additives to become High Impact Polystyrene (HIPS), which is then specified for more demanding products such as housings, frames, and toys. Polystyrene is a difficult material to re-cycle and is not widely accepted into the commercial waste stream, so please check for either the letters PS or HIPS or look out for the recycle triangle

Polyethylene

Polyethylene (or polythene) is a UK success story, in that it was commercially developed by the leading chemicals giant ICI before the second world war. It was a unique material, and at the time was considered to be a critical material for the war efforts as it was used to shield radar cables.

Polyethylene is a base polymer that has many uses for the base block of other Poly types of materials and is used as a low-cost carrier for colouring master-batches since the material has little or no impact on the main material itself.

There are several variants of PE, by changing the manufacturing process or adding other compounds. These are HDPE (High Density) which is much tougher than the standard and used for heavy impact applications like crates, pallets, agricultural machinery covers and more. LDPE (Low Density) is used for parts welded to film of LLDPE (Linear Low Density). It is this material that is widely used as milk bottles, which are blow moulded in a separate in-house facility at the major milk processing and bottling facilities.

Polyethylene type is the most widely used material for many household containers for everyday use of chemicals and food, etc, as well as film products, such as bags and stretch film. Being such is its downfall, as it is most likely these products that we see as litter.

Recycling rigid containers and other solid products is an easy process, but the films are much more difficult to re-process. Always check the engraved detail on the products and look for the letters HDPE or LDPE and number 02 or 04

Polypropylene (PP)

Polypropylene is specified in a wide and diverse range of applications. First developed in the early 1950s, and is now the second most used commodity thermoplastic material worldwide. In its natural state, it is white and can easily be either compounded or master-batch coloured in the full spectrum of colours.

In addition, fillers can be compounded into polypropylene, such as:

  • Talc – To improve stiffness.
  • Glass Fibres – To improve strength.
  • Minerals – To improve impact and strength.

Whilst at the same time, maintaining a high chemical resistance, zero water absorption, low weight, and product appearance since it offers excellent surface finish subject to the choice of filler. Cost is a key driver in specifying PP, due to its relative cost to manufacture, along with product weight.

Processing PP requires heating the material in the press barrel to temperature ranges of 220 °C to 260 °C and is an easy material to process. Generally, the tool temperatures are low, as the material has good flow properties and can be cooled quickly in the mould tool, thus leading to acceptable yield cycle times.

One of the key aspects of PP is that it is used for ‘living hinges’ in designed products, think of snap bottle tops and other enclosures with lids. Having a ‘living hinge’ in the product design requires the moulded item to have the hinge detail activated immediately on the mould tool opening.

Other examples of parts produced in PP are appearance interior car trims, as the material can be modified to reduce marking/scratching, but has a good appearance. Equipment covers and trays for chemical resistance, instrument casings where both water and chemical resistance are necessary, and many more. The chemical resistance is key for medical and food applications, along with the excellent surface finish of PP. Another reason for its use is in young children’s toys, as it is easy to clean and has a soft-touch feel.

Its ease of recycling is a key consideration at the design stage so that the moulded product can be converted back into pellet form, and be re-used. Products in PP are identified either with the letters PP or the recycling triangle

We will revisit Thermoplastic materials, and in particular, Engineering types in a future blog post. In the meantime, if you require any further information or have a design or a product that you are considering, then please do not hesitate to contact us and we will endeavour to answer any questions.