Common problems and solutions for plastic injection moulded transparent parts

Plastic is lightweight, versatile, durable and flexible. It is therefore not surprising that it is an ideal and cost-effective substitute for other materials such as metal or glass. Transparent plastics have benefits over glass when manufacturing clear, plastic parts such as optical lenses, medical equipment, automotive components and various packaging products. As well as being lighter weight, they have qualities such as impact, shatter and scratch resistance and can withstand extreme temperatures and pressures.

Despite these obvious benefits, clear plastics have to have an exceptional surface finish and it is not possible to hide flaws and blemishes. Transparent plastics therefore present manufacturers with a number of unique challenges to achieve outstanding aesthetics and ensure clarity of the material and quality of the finished part is maintained. There are obvious benefits to choosing a plastic injection moulder that is experienced in manufacturing transparent plastic parts and has excellent technical expertise, these are:

  • An injection moulder that has produced different transparent plastic parts will know what the best materials are and will have access to a good supplier.
  • The materials specialist will know how the transparent materials have to be handled and stored.
  • The materials specialist and engineers will know what the different properties of each transparent material are and the impact this will have on how they behave during the injection moulding process, how the finished component will perform during volume manufacturing and the functional and aesthetic qualities of the final part.
  • The plastic injection moulder will have worked on a range of different jobs, potentially across different industries, so will already have experience of design for manufacture (DFM) prototyping, tooling, the injection moulding process, assembly and validation of plastic parts. Knowing what will work and what challenges have to be overcome will save time and money.
  • The engineering team and machine operatives will have the necessary expertise to recognise and troubleshoot any problems quickly and effectively.
  • They will be open and honest with you and, by working with them, you will receive excellent advice and learn from them.

Key challenges for plastic injection moulding clear parts

Achieving a flawless finish is much more difficult when injection moulding transparent or translucent plastics compared with opaque materials. The following are some challenges that can occur during the plastic injection moulding of transparent parts and components, as well as solutions to make your next project a success in form, function and aesthetics.  

1. Level of transparency or clarity

Transparency or clarity of clear plastic is measured by the level of light transmission, for example glass transmits 95%. Some clear plastics such as PE and PP are classed as translucent rather than transparent because they have a crystalline structure which interrupts and slows the flow of light resulting in a lower level of light transmittance.

Factors that affect transparency of polymers

  • Contaminants – particulates in the polymer will disrupt and scatter light. Your plastic injection moulder should ensure all equipment is clean and dry and that materials are stored and processed properly to minimise the chance of contaminants affecting the clarity of your part.
  • Surface smoothness – any defects or surface roughness from abrasion or scratches will affect optical clarity. Extra care must be taken when handling, moulding or machining transparent plastic components to avoid damaging the surface.
  • Crystallinity – if your project requires optimum clarity, you will need to choose an amorphous polymer such as acrylic, polycarbonate (PC) or polyetherimide (PEI) instead of a crystalline polymer like high density polyethylene HDPE or polypropylene (PP).
  • Additives such as colourants and stabilisers will affect the level of light transmission which can reduce clarity. There are however some additives such as clarifiers or nucleators that can enhance the clarity of crystalline polymers.

There are many other factors which can influence transparency such as wall thickness in the design, mechanical processing, heat treatment, finishing etc. These factors should be taken into considerations by your plastic injection moulding company.  

2. Prevention of impurities and flaws

Transparent materials require much more care and attention throughout the entire injection moulding process. They must be stored in a contaminant and moisture free storage area as contaminants will cause visible flaws and if the polymer is not dry it will start to deteriorate once it is heated.

More preparation and planning is required to ensure the tooling and other equipment and processes are as clean, dry and precise as possible.

3. Difficulty ejecting transparent parts from the mould

Plastic selection, precision tooling and good design will help prevent problems with ejecting your transparent components from the mould. Precise draft angles will be required and some polymers will need steel tooling that has a highly polished mould surface.

4. Temperature resistant materials

Temperature resistant transparent materials are often used for safety equipment such as goggles, masks, heat shields and components for fire-fighting equipment. These materials will need a high-quality mould tool made from steel that can withstand very high temperatures. They may also require additional equipment such as high-density heater bands and an efficient cooling system to avoid long cycle times. This will add additional but necessary cost to your project to ensure that you get a high-quality finished product that will meet safety requirements.

5. Yellowing of non-UV resistant thermoplastics

Non-UV resistant thermoplastics will weather and turn yellow over time. If your product is for use outside or is likely to be exposed to UV light in a lab, you will need to choose a UV resistant material or add additives to prevent this from happening. Your plastic injection moulder will be able to advise you on the best material for your purpose without negatively impacting the transparency or function of your part.

6. Uneven cooling

Uneven cooling can cause multiple problems such as surface defects, deterioration, warping and dents which will all affect the aesthetics of your transparent part. The temperature of the molten material must be carefully controlled so that there is a steady flow and even distribution throughout the mould cavity. Thicker sections will obviously take longer to cool which can cause areas to contract at different rates.

A key cause of poor distribution that leads to sink marks is uneven wall thickness which is a result of poor design It is therefore important to partner with an experienced injection moulder who can avoid these problems, one that has worked on a range of projects using different transparent materials and understands how they behave and under what conditions.

To prevent uneven cooling the following factors should be addressed at the mould design phase: 

  • Wall thickness should be as uniform as possible but not too thin
  • Gate runners should be as wide and short as possible
  • The surface of the mould should be polished, clean and smooth
  • Injection temperature and pressure should be controlled to suit the properties of the polymer. Transparent plastics tend to have a high melting point and poor liquidity.

Clear planning and preparation are required at the design stage when working on plastic injection moulding projects, even more so when using transparent polymers. Because the finished components will be see-through, everything will be visible so having outstanding aesthetics is critical. The design phase therefore needs to be meticulous taking into account the material options, product design, mould design and equipment to ensure that all parameters and their associated challenges have been considered and solutions sought before starting the project. It is also important to validate your material choice to avoid costly delays during the manufacturing of your plastic components.

If you have a project that requires transparent plastic parts, contact one of our plastic injection moulding experts.

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