Plastic injection moulding transparent parts requires particular attention to detail during the tool design stage to ensure that the mould matches the requirements of your chosen material. In addition, because blemishes are more visible with translucent materials, more stringent control of the plastic injection moulding parameters throughout the production process is needed to prevent contamination and ensure the quality and precision of the final part.
There are different materials for injection moulding clear plastic components. Each polymer has particular properties and qualities that provide specific advantages. These properties, such as clarity, strength and resistance to UV, temperature and a range of chemicals, determine which is the best transparent or translucent plastic for your project.
A key consideration for choosing a particular transparent material over another is optical clarity and the level of light transmissivity. This directly influences the aesthetics and functionality of the component.
| Material | Light transmissivity (%) |
|---|---|
| Glass | 95 |
| Acrylic (PMMA) | 93 |
| COC | 91 |
| Polypropylene (PP) | 90 |
| Polycarbonate (PC) | 88-90 |
| Polyetherimide (PEI) | 82 |
| High Density Polyethylene (HDPE) | 80 |
Clear plastics for injection moulding
Here are the different types of polymer you can choose for your plastic parts when taking into account the different physical and chemical requirements of your project.
1. Acrylic (PMMA)
Acrylic, also known as Polymethyl Methacrylate (PMMA) is a thermoplastic with a higher percentage of optical light transmissivity than other clear polymers. It is in fact very close to that of glass and, because it is lightweight, UV-resistant, shatter-resistant and scratchproof, is often used as an alternative for applications such as lenses, light fixtures and windows.
PMMA is a popular material for plastic injection moulding using general purpose equipment. It is strong, has outstanding rigidity and clarity, is non-toxic, has good flow and can achieve tight tolerances. Colourants and other additives can also be easily mixed and evenly distributed throughout the moulded part.
Acrylic can however be inflexible and brittle, so it can’t be used in products that have to withstand a lot of pressure. It also absorbs moisture so must be dried which can slow down the manufacturing process. If highly polished surfaces are required, the mould surfaces need to be polished and high-quality steel must be used which can increase the cost of production.
OGM has produced components from PMMA for rear taillights in the automotive industry, PMMA was chosen because it weathers well, is scratch-resistant, doesn’t change colour over time and can be polished to maintain a crisp, clear finish.
2. High-density Polyethylene (HDPE)
HDPE is easy to process and has a low process temperature that delivers faster cycle times for thin-walled parts. It is also less brittle than acrylic, is resistant to stress cracks and can easily be moulded into different shapes. Because of this durable but flexible nature, and the fact that it is less expensive than acrylic, HDPE is suitable for products and parts that are mass produced such as tubes, containers and bottles. HDPE is also moisture, chemical and UV-resistant and non-toxic. HDPE is however not fully transparent, it has lower clarity with the lowest percentage of light transmissivity of the clear plastics used for plastic injection moulding and is therefore classed as translucent rather than transparent.
OGM has used HDPE for orthopaedic surgery applications such as flexible tubes that need to be resistant to chemicals used in bone cement.
3. Polycarbonate (PC)
Polycarbonate (PC) has good clarity and is dimensionally very stable when exposed to changes in temperature or moisture levels and post shrinkage. This delivers accurate and precise parts, even for complex designs for applications such as housings and lighting fixtures. PC is affected by some chemicals: methyl alcohol, acetone, ketones, ethers, aromatic and chlorinated hydrocarbons, aqueous or alcoholic alkaline solutions, or ammonia. It is as UV resistant as acrylic and has exceptionally high impact strength making it a great option for transparent safety products such as goggles, heat shields or visors on helmets.
As with acrylic, PC must also be dried prior to injection moulding and it is not easily ejected from the tool without good drafting or polishing.
Due to its high resistance to chemicals and exceptional impact strength, other applications include medical devices, automotive components and viewing windows for industrial fluids. However the use of PC in environments that use the above chemicals would be detrimental to the product and other more suitable polymers should be used.
4. Polyetherimide (PEI)
Similar to other transparent polymers, Polyetherimide (PEI) is resistant to UV. Where this advanced engineering plastic stands head and shoulders above other transparent plastic materials is with its ability to withstand extreme temperatures (up to 200°C), repeated pressure and exposure to acids. The material specification is therefore generally much higher than other materials.
This clear plastic which has an amber coloured hue, is often used for applications in the automotive, aerospace, electrical and medical industries such as components for temperature sensors, heat shields or aerospace engine components.
PEI has excellent dimensional stability, good resistance to a broad range of chemicals such as automotive fluids, fully halogenated hydrocarbons and alcohols. With its ability to withstand various sterilisation methods due to resistance against most lipids, detergents and disinfectants, PEI is ideal for medical applications that require stringent sterilisation methods.
In the automotive industry, PEI is a cost-effective alternative to metal as it is sufficiently strong enough to replace steel in certain applications and light enough to replace aluminium.
PEI has inherent flame retardancy with low smoke and no need for additives. It has very good retention of its mechanical and chemical properties at very high temperatures or when in contact with acids.
Moulding components from PEI is more expensive than with other clear plastics because the machine cylinder has to be equipped with high density heater bands and a steel mould tool is required for the plastic injection moulding process due to the high tool temperature needed.
An example of an application that OGMs technical engineers have produced using PEI are components for fire-fighting equipment.
5. Polypropylene (PP)
One of the lightest clear plastics, Polypropylene (PP) is a rigid but flexible commodity plastic that has excellent chemical and electrical resistance. It is often used for components in the electronics industry and, because it is also nonreactive to acids and bases, it is ideal for making containers for solvents and other corrosive materials. This outstanding toughness means that PP can be steam sterilised or autoclaved. As it is so light and flexible, it is also ideal for non-loadbearing parts such as living hinges.
As with HDPE, PP is classed as translucent rather than transparent because it is a crystalline polymer so light scatters rather than travels easily through it. It is also vulnerable to UV radiation.
6. COC
Cyclic Olefin Copolymer (COC) is a high-performance thermoplastic material known for its unique combination of properties: high transparency, extremely low water absorption, high rigidity, strength and hardness, excellent biocompatibility and high chemical resistance to both acids and alkalis. COC is ideal for transparent components because of its outstanding optical qualities including high clarity and low birefringence.
COC is a low-density polymer with a heat deflection temperature of up to 170°C. Despite being resistant to hydrolysis, acids and alkalis it is vulnerable to attack from non-polar organic solvents such as toluene and naphtha. It can be a challenging plastic to injection mould because process conditions have to be particularly precise to avoid moulded-in stresses.
Applications include medical devices and optical lenses. OGM’s technical engineers have used COC for projects such as orthopaedic surgical applications that required high transparency, excellent biocompatibility and very good blood compatibility.
It is advisable to partner with an experienced plastic injection moulding company early in your project. As you can see there are lots of different options for transparent materials with many and varied properties to suit your needs depending on what the key functional performance requirements and aesthetic criteria are for your product. In addition to choosing your perfect transparent material, the materials specialist at your plastic injection moulding company will also know how to improve specific qualities of the different transparent plastics with certain additives such as UV stabilisers, nucleators or clarifiers.
OGM: Your trusted partner for high-performance transparent components
An expert plastic injection moulding partner like OGM will be able to advise which is the best material for you. OGM has years of experience working with challenging materials to produce very technical and complex transparent components for customers in the medical, electronics and automotive industries as well as general industry. OGMs expert design and engineering team has developed solutions for orthopaedic surgical equipment that required high transparency and excellent blood and biocompatibility, fire-fighting equipment that needed a high weld line strength under exceptionally high temperatures and viewing windows for industrial equipment where clarity, dimensional stability and high impact strength were a must for viewing industrial fluids in harsh environments.
OGM’s material experts know the benefits and the unique combination of properties of each material, how they will perform under duress, what the clarity, surface finish and aesthetic will be. Working hand in hand with OGM’s engineers, they will find the best solution for your project and will help you design for manufacture taking into consideration the limitations of each material to avoid potential costly and time-consuming mistakes.