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Thermoformed Trays: How Plug Assists Play a Crucial Role in Package Design, Material Distribution, and Product Safety

Wed, 01/22/2014 - 1:09pm
Noel Tessier, Senior Materials Engineer, CMT Materials

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When it comes to medical plastic packaging, device manufacturers rarely have the option to rank safety, compliance, performance, cost and quality in order of importance. For most device manufacturers and their suppliers, each of these elements is equally important. Whether you are working with rigid thermoformed trays, form/fill/seal trays or flexible pouches, the same rules apply. In this article, we will focus on thermoformed trays and how plug assists play a crucial role in package design, material distribution and product safety.

Plug assist technology allows plastics processors to reduce starting gauge, reduce cycle times and improve material distribution. There are different types of plug assist material, each with their own benefits (Table 1).

Table 1: Categories of plug assist materials with comparative benefits

Syntactic foam plug assists are purpose-engineered for thermoforming and provide unique properties that result in superior packaging.

There are several key pre-requisites for any thermoformed tray in medical device packaging including rigidity and clarity. Features such as impact resistance and ease of de-nesting (the ability to stack, or ‘nest’, one tray on top of another without sticking) are also considered. Material selection therefore is of primary importance. Certified, medical grade films made from HDPE, PP, PS and PETG are the most commonly used materials in thermoformed medical packaging. Each material type has its own characteristics and sheet suppliers generally provide the appropriate documentation outlining specific properties including specific gravity, tensile strength and, perhaps most importantly, thermoforming temperature.

Heating and Cooling the Sheet
Best practice suggests that the temperature of the plastic sheet should be measured to ensure the optimal forming window. New data-driven technologies are being integrated with thermoforming systems that allow processors to dial in very specific measurements. Generally speaking, the goal is to keep the heat in the sheet right up to the point where the plastic enters the tooling cavity. This is where the plug assist becomes part of the equation.

The plug is designed to pre-stretch the material into the right place at the right time. Doing so adds a level of predictability to a process that is notoriously filled with variables. Let’s look at two such variables as they relate to the plug: thermal conductivity and the coefficient of thermal expansion.

Thermal Conductivity
Thermal conductivity refers to the quantity of heat that passes in a unit of time through a unit of area when its opposite faces differ by a unit of temperature (BTU/hr-ft-°F). In this case, we are talking about BTUs as the quantity of heat that passes in an hour in a foot for every degree of temperature difference between the plug and the sheet. A high number either means the plug will quickly freeze the sheet or that it must be heated/cooled with an outside means to match the sheet temperature. This is logical, but it adds cost and complexity while increasing the cycle time.

It is also the case that air flow and ambient conditions can vary, which reduces consistency. Most processors choose a plug with very low thermal conductivity simply because it does not remove heat or chill the sheet under any conditions. The lower the conductivity number, the less impact it has on the sheet.

Coefficient of Thermal Expansion
The coefficient of thermal expansion (CTE) is the amount of expansion (or contraction) per unit length of a material resulting from a one degree change in temperature. In simplest and most practical terms for the thermoformer, it can be thought of as how much a material will grow when its temperature increases. In English units it is typically expressed in length/length/per degree Fahrenheit (in/in/°F). In metric, it is m/m/°C.

Syntactic foams are filled with hollow glass spheres, so even though they are polymeric in nature, the stable fillers mean that the CTE is relatively low, around 20-30 x 10-6 in/in/°F. When you couple this with the fact that syntactics are excellent insulators and therefore take less heat from the sheet and run at much lower operating temperatures, you have a material that maintains a much higher dimensional stability than other plug material types. By way of comparison, Delrin has a CTE of 59 x 10-6 in/in/°F. Nylon can range from 40-60 x 10-6 in/in/°F. This means they grow and change greatly during the process as they absorb heat from the sheet during contact. With more stable materials, the thermoforming process itself becomes much more stable, providing for a greater degree of consistency and repeatability.

Plug Geometry, Plug Material and Package Design
When it comes to package design, form and function must be balanced. Design engineers use state-of-the-art software to create innovative and eye-catching packages while still maintaining the fundamental goal of protecting and displaying the product inside. Syntactic foam plug assists aid in the design process by pre-stretching the sheet into position without removing heat or affecting its formability. This is critical because the design work is based on material specifications which are based on specific, optimal sheet forming temperatures. Overheating a sheet is often the cause of loss of plastic orientation, lower strength, loss of clarity, sheet stick to the plug and a wide range of uncontrolled issues, all due to compensation for a plug that chilled the sheet on contact.

It is important to understand the interplay of the plug material, plug geometry, tool design and sheet temperature, and not just to look at each element in isolation. Surface friction, roughness and temperature are all in play. To control the interaction between plug and sheet requires the ability to modulate release. Doing so reduces variability and increases repeatability.

Testing and Validation of Specific Plug Materials
CMT Materials of Attleboro, Massachusetts and RPC Cobelplast of Lokeren, Belgium performed tests to develop optimal plug materials for multilayer barrier films that would be used in both rigid thermoformed trays and form/fill/seal (FFS) applications.

RPC Cobelplast is a leader in coextruded, multilayer, high barrier plastic films for the European and international thermoforming market. RPC has a custom-made laboratory thermoforming machine to aid in the development of improved multilayer films for thermoforming. This machine was used to evaluate a range of plug materials in forming multilayer packages from a PE/EVOH/PS coextruded sheet.

Two series of different plug materials were evaluated: PTFE, POM, HYTAC-W, HYTAC-B1X, HYTAC-WFT and HYTAC-FLX. Initial trials showed HYTAC-WFT and -FLX to have the best potential for medical thermoforming applications. Based on these results, CMT went back to the laboratory and developed HYTAC-FLXT to combine the best performance properties of the two materials. A third set of trials showed HYTAC-FLXT to have the best forming and release characteristics of all the plug materials in multilayer applications with EVOH.

A final series of trials compared RPC’s standard PTFE plug material to HYTAC-FLXT using a starting sheet of 1.4 mm thickness of PE/EVOH/PS. The team looked at optimised plug geometry for PTFE as well as several process variables developed for the PTFE plugs and compared them to three different plug geometries for HYTAC-FLXT and process changes to determine the best conditions for HYTAC-FLXT.

Figure 1 compares the thicknesses for the multilayer with EVOH container formed with the standard PTFE with HYTAC-FLXT. Material distribution was much more consistent with HYTAC-FLXT versus the PTFE. Additionally, the minimum wall thickness with the standard plug material was 128 microns versus a minimum of 188 microns for the HYTAC-FLXT plug. This increase in minimum wall thickness allowed RPC to down gage the starting multilayer thickness by approximately 10%.

Figure 1: Comparative sheet thickness with two different types of plug assist material, HYTAC FLXT and PTFE.

Sustainability and the Bottom Line(s)
Reducing the starting gauge of the plastic sheet is a well-known reason to use plug assists, but the benefits extend beyond the package. Using less material through down-gauging (or light-weighting) has important environmental benefits. When considered in the context of millions of packages produced each year, the numbers can drive change at the top levels of major device manufacturers, especially those public companies with prominent commitments to sustainability. Whether it’s due to ESG reporting (environmental, social and governance) or CSR requirements (corporate social responsibility), business are seeing how innovations in packaging not only result in a lighter environmental footprint, but also in tangible cost savings.

The judicious use of plug materials and of plug assist techniques have proven to reduce material thickness without compromising the quality and integrity of the package. That’s a double-bottom line worthy of notice from the design lab to the C-suite.

Acknowledgements: RPC/CMT study (2009)

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