A Guide to Thermoplastic Commodity Materials

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

To reset, Thermoplastic is the material most widely used world-wide by injection moulding. This 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 repeated. Cycle times can vary from a few seconds to in some extreme cases a few minutes, but on average 36-60 seconds is the norm.

Commodity Plastics

These are the most widely used types of materials, namely Polypropylene (PP), Polyethylene (PE) and Polystyrene (PS). Having a specific gravity of 1 or less, means that they are all light weight 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 1800’s, 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, packaging, that enable the consumer to see the article contained clearly. Additionally, it can be used for many of the disposable items we use, such as beakers, cutlery, etc, but of course as we now see at an environmental cost. Here at Merriott Plastics we use the material to produce a range of DVD Trays. 

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) and 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 the recycle triangle.  


A UK success story in that it was commercially developed by the leading chemicals giant ICI prior to the second world war. It was a unique material and at the time was considered a critical material for the war effort, 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.

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. LDPE (Low density) used for parts welded to film of LLDPE (Liner 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 types is the most widely used material for many household containers for everyday use of chemicals, food, etc and for film products such as bags, stretch film. Being such is its downfall, as it is most likely these products that we see as litter.

Recycling of ridged 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)

Is specified in a wide and diverse range of applications. First developed in the early 1950’s and is now the second most used commodity thermoplastic material world-wide. In its natural state it is white in colour and can easily in either compounded or master-batched coloured in the full spectrum of colour.

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 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 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 in young children’s toys, as it is easy to clean and 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 by re-used. Products in PP are identified either with the letters PP or the recycling triangle.   

We will come back to Thermoplastic materials and in particular Engineering types in our next blog. In the meantime, if you require any further information on the above, or have a design/product that you are considering, then please contact us through this link, https://merriott.com/merriott-group-factories.html