The choice of plastic material and additives directly determines whether medical device components are reliable, function properly or result in failed production runs. Additives and fillers enable plastic injection moulders to customise properties of the final product in terms of strength, flexibility, biocompatibility, sterilisation resistance and antimicrobial performance. However, such enhancements seldom come without compromises.
In this blog, we will explore how additives can affect the key injection moulding parameters to produce medical-grade plastic components and why the choice of additives is important for repeatable quality and productivity.
Why additives are important in medical injection moulding
The plastic used for medical moulding must withstand sterilisation, mechanical force, and long-term use. Manufacturers may require additives for one or more of the following reasons, depending on their application:
- Mechanical enhancement: Adding stiffness, impact resistance or flexibility to instruments and housings.
- Functional properties: Materials should feature better visibility during imaging along with antimicrobial properties and lower friction between moving parts.
- Compliance with regulatory requirements: Biocompatibility, sterilisation resistance.
- Aesthetic or identification reasons: Applications of colorants combined with transparent materials allow users to visually inspect and identify components.
Additives can be used to adjust the properties of a polymer so that these requirements are met. However, they also affect the behaviour of the material during the injection moulding process such as melt flow, cooling, shrinkage, warpage, process stability etc.
Key injection moulding parameters affected by additives
1. Melt flow and viscosity
The addition of fillers such as glass fibres, mineral fillers or high-molecular-weight modifiers results in increased melt viscosity which leads to reduced flowability. This sometimes necessitates a higher injection pressure or temperature to fill complex geometries, thereby affecting cycle time, and potentially, causing internal stress if not controlled.
2. Cooling time and rate
Thermal conductivity is dependent on the filler type. Glass or mineral fillers may enhance heat conduction; this may improve cooling rate but could lead to non-uniform cooling and localised stress. The use of plasticisers or elastomeric additives causes longer cooling times which results in extended cycle times and reduced production efficiency.
3. Shrinkage and dimensional stability
Additives affect crystallinity, thermal expansion, and stress development. Filled resins show reduced shrinkage compared to unfilled varieties which enhances dimensional stability, but they also demonstrate anisotropic shrinkage that might cause warpage or internal defects without proper adaptation of mould design and processing conditions.
4. Flow behaviour and fill balance
Additives can cause flow balance issues in multi-cavity tools or complex parts. Higher viscosity can result in non-uniform filling, cold-flow problems or weld lines which can all lead to inferior critical medical components. Adjustment to gate location, runner design and venting are often required.
5. Ejection and surface finish
The use of additives impacts part cooling and shrinkage which consequently modifies ejection forces and surface integrity. Relevant applications need smooth surfaces for sterility or aesthetics because lubricants and other additives like impact modifiers or fillers can change surface finish and gloss.
6. Cycle time and productivity
Cooling and solidification effects of additives can either increase or decrease cycle time. Injection pressure, temperature, or cooling channel design may need to be modified to ensure high production rates are kept to a minimum in order to keep high volume production cost effective.
Medical plastic injection moulding: It’s all about balance
Plastic injection moulding is a precise science, and the process of additive selection is a critical step that should never be overlooked. In general, the biggest challenge with additive selection is getting it to work for both the performance requirements while still achieving the manufacturability of the product. The performance of the material and regulatory requirements are balanced with the need to keep a mould part manufacturable. For the best possible balance, some key tactics should be put in place.
- Design and simulation: Flow simulations and design-for-manufacture (DfM) reviews allow for the visualisation of how resin flow is impacted by additive incorporation in injection moulding.
- Material selection: Select the lowest possible level of additive to satisfy the functional requirement with minimal impact to process behaviour. It should be noted that additives may migrate out of the polymer, which is a serious concern in applications such as medical devices where inappropriate additive type or excessive loading can compromise safety and regulatory compliance.
- Pilot runs and validation: Trial moulding cycles demonstrate how materials fill the mould and show shrinkage/warpage behaviour and surface finish quality.
- Process control: Continuous monitoring of melt temperature and injection pressure with holding pressure and cooling time allows for production reproducibility.
- Quality assurance: Dimensional verification and traceability of each batch is very important in the medical manufacturing process.
Why correct additive management matters to medical device moulders
Medical devices require consistency, precision and compliance. The consequences of poor additive management or process control can be:
- Distorted or dimensionally inaccurate parts.
- Surface defects that may affect sterility or aesthetics.
- Unreliable mechanical performance.
- Longer cycle times and higher production costs.
- Failure to meet regulatory requirements due to sterilisation or biocompatibility issues.
The performance of medical-grade plastics can show significant improvements when additives and fillers are used. These substances also affect essential injection moulding processes including melt flow characteristics and cooling dynamics as well as shrinkage and surface finish quality. It is essential to maintain a balance between material functionality and manufacturability in order to produce medical components that fulfil top-quality standards in precision and regulatory compliance.
OGM – your trusted partner for medical injection moulding
OGM specialises in precision plastic injection moulding for medical device production. Whether it’s product design and prototyping or large-scale manufacturing in a clean room environment, we have the experience and know-how to help you choose the best materials and additives for consistent, compliant, and superior quality components.
Get in touch today to find out how we can help you with your next medical device manufacturing and take advantage of our extensive expertise with medical grade plastics and injection moulding.