Retention Feature Design for Medical Blister Trays: Snaps, Posts & Undercuts

If you’ve ever watched a production line operator pack a surgical instrument into a blister tray, you’ve seen retention features do their job — or fail at it. A well-designed snap holds the instrument securely through 1,000km of truck vibration, 50kGy of gamma sterilization, and 3 years of shelf aging, then releases cleanly with a 5N pull when a nurse needs it. A poorly designed snap either lets the instrument rattle around and puncture the lid, or it’s so tight that the operator has to pry it out with forceps, generating plastic particulate right in the sterile field.

Retention features — snaps, posts, and undercuts — are the most under-engineered part of most medical blister tray designs. Most designers add a generic “clip” feature without calculating retention force, without considering demolding, and without thinking about what happens when the user actually removes the device. The result is almost always one of two failures: either the feature doesn’t hold, or it generates particulate when released. In this guide, we’ll break down the three most common retention feature types, their design specifications, demolding considerations, and how to design them to be clean-release and particulate-free.

What Are Retention Features, and Why Do They Matter?

Retention features are the raised or recessed plastic geometries in a blister tray that hold the device in position during shipping, handling, and sterilization. They’re not just “something to hold the device” — they’re engineered components that have to satisfy four competing requirements simultaneously:

1. Hold securely: The device must not move when the tray is dropped, vibrated, or turned upside down. A device that shifts in transit can damage itself, puncture the lid, or lose its sterile barrier.
2. Release cleanly: The user must be able to remove the device with one hand, with reasonable force (typically 3–10N), without excessive effort or sudden jerking motion.
3. No particulate generation: When the device is removed, the retention feature must not shed plastic fibers, chips, or dust. This is critical for sterile field integrity — a single plastic particle in a surgical wound can cause a serious infection.
4. Demoldable: The feature must be removable from the thermoforming mold without tearing, stretching, or permanently deforming the plastic. A feature that works perfectly on paper but can’t be demolded is useless in production.

Getting all four right is harder than it sounds, and it’s why retention feature design is as much art as science. Below, we’ll go through each of the three most common feature types in detail.

Three Common Retention Feature Types: An Overview

1. Snap (Clip) Design: The Most Common Retention Feature

Snaps (also called clips or C-clips) are the most widely used retention feature in medical blister trays. They consist of two opposing raised plastic arms that wrap partially around a cylindrical or rounded feature of the device, holding it in place with a spring-like clamping force. When the user pulls the device straight up, the arms flex outward and release.

Design Specifications and Reference Dimensions

Below are our standard design parameters for snap features in medical grade PETG and APET trays. These are starting points — always tune based on actual device diameter, material thickness, and required retention force.

Applicable Scenarios

Snaps are the best choice when:

• The device has a cylindrical or rounded feature (handle, shaft, tube, catheter hub) with a consistent diameter
• You need moderate retention force (3–8N) that’s predictable and consistent across production runs
• The device will be removed by pulling straight up (vertical removal), not sliding out sideways
• You need a feature that’s easy to inspect visually — it’s obvious if a snap is broken or malformed
• The tray material is PETG or APET, which have good flexibility and memory for snap action

Snaps are not ideal for:

• Very small diameter devices (<3mm) — the snap arms become too small to form reliably
• Flat or irregularly shaped devices — the snap can’t wrap around a consistent profile
• Devices that need to slide out laterally — snaps are designed for vertical removal only

Demolding Considerations

Snaps are relatively easy to demold, but there are a few common pitfalls:

• Undercut on the inner snap surface: The inner curve of the snap creates an undercut that can catch on the mold core during ejection. For shallow snaps (height <4mm), the plastic can flex enough to release without side-action. For deeper snaps, you may need a split mold or a collapsible core.
• Insufficient draft on outer arm surfaces: If the outer surface of the snap arm has less than 5° draft, the plastic will stick to the mold cavity and stretch during ejection, causing whitening (stress marks) or even tearing. Always use 5–10° draft on all vertical outer surfaces.
• Thin arm bases: If the nominal sheet thickness is 0.5mm and the snap is in a deep draw area, the actual formed thickness at the arm base may be 0.25mm or less. This causes the arm to fold over during demolding instead of flexing. Always calculate draw ratio and verify actual thickness at the snap base with prototype parts.
• Ejection timing: Snaps need to be ejected while the plastic is still warm enough to flex, but cool enough to hold its shape. If you eject too hot, the snap arms deform permanently. If you eject too cold, they crack. This is a process parameter that needs tuning for each material and tray design.

How to Prevent Particulate Generation

Snap-related particulate usually comes from one of three sources: the arm tip scraping against the device during removal, the arm surface cracking due to fatigue, or the lead-in edge being too sharp and shaving plastic off the device. To prevent this:

• Round all snap arm tips with a minimum 0.3mm radius: A sharp tip edge will scrape the device surface every time it’s inserted or removed, generating micro-particulate. A rounded tip glides over the device surface without cutting.
• Keep the snap opening width at 70–80% of device diameter, not narrower: If the opening is less than 70%, the arm has to flex too far to release, and the tip drags hard against the device surface, potentially shaving plastic. Above 80%, retention is too weak.
• Use a smooth lead-in angle of 30–45°, not steeper: A steep lead-in (60°+) creates a sharp shoulder that catches on the device and can chip plastic during insertion. A gradual 30–45° ramp lets the device slide in smoothly.
• Ensure arm base thickness ≥0.4mm after forming: Thin arms develop micro-cracks at the base after repeated flexing (even just the insertion during packaging). These cracks shed particulate over time. Thicker arms distribute stress more evenly and resist fatigue cracking.
• Polish the mold surface at all snap features to SPI A-2 finish or better: A rough mold surface creates a rough plastic surface, which sheds fibers when flexed. A highly polished mold produces a smooth, low-friction surface that releases cleanly.

2. Post (Locating Pin) Design: The Cleanest Retention Feature

Posts (also called locating pins, alignment posts, or mandrels) are the simplest and cleanest type of retention feature. They’re cylindrical or tapered raised pins that insert into an existing hole, lumen, or bore in the device, holding it in position by friction and geometric constraint. Posts don’t clamp onto the device — they locate it and prevent lateral movement, while the cavity walls prevent vertical escape.

Design Specifications and Reference Dimensions

Applicable Scenarios

Posts are the best choice when:

• The device has an existing through-hole, lumen, bore, or hollow hub (syringe barrels, needle hubs, catheter connectors, instrument handles with holes)
• You need the lowest possible particulate risk — posts have no clamping or scraping action, just smooth insertion and removal
• The device needs precise rotational alignment — a non-round post (D-shaped, square, or keyed) can lock the device in a specific orientation
• You want the easiest possible demolding — posts with proper draft release cleanly from the mold with no side-action required
• The device is removed by lifting straight up, and the cavity walls are deep enough to prevent the device from lifting off the post accidentally during shipping

Posts are not ideal for:

• Devices without an existing hole or lumen — you can’t add a hole to a device just for retention
• Very shallow cavities (<5mm deep) — the post doesn’t have enough height to locate the device, and the device can lift off easily
• Devices that need high retention force (>6N) — posts provide lateral location but relatively low vertical retention; they rely on cavity depth for vertical hold

Demolding Considerations

Posts are the easiest retention feature to demold, but they have one unique failure mode: vacuum locking. Because a post is a deep, closed-end draw (the plastic wraps around a mold pin), air can get trapped between the plastic and the mold pin during forming, creating a vacuum that locks the part to the pin during ejection.

To prevent vacuum locking and ensure clean demolding:

• Always use 2–5° taper on the post: A straight post (0° draft) will vacuum-lock almost every time. Even 2° of taper breaks the vacuum seal and allows the part to release.
• Add vent holes or vent channels in the mold pin: For tall posts (>5mm), add a 0.5mm vent hole down the center of the mold pin, or a flat vent channel along one side, to allow air to enter during ejection and break the vacuum.
• Use a rounded dome tip, not a flat top: A flat-topped post creates a larger vacuum seal area at the tip. A rounded dome reduces the contact area and makes vacuum release easier.
• Ensure adequate post wall thickness: If the post wall is too thin (<0.3mm after forming), it will collapse inward during demolding, creating a permanent crease that both looks bad and can shed particulate. Calculate draw ratio: for a post with height H and diameter D, the draw ratio = H/D. Keep H/D < 2.0 for PETG, < 1.5 for APET, to avoid excessive thinning.
• Eject from the cavity floor, not from the post: Never put an ejector pin inside the post — it will mar the surface and create a stress point. Eject the tray from the flat cavity floor areas around the post base, and let the post release naturally as the part lifts.

How to Prevent Particulate Generation

Posts have the lowest particulate risk of any retention feature, because there’s no clamping or scraping action — the device simply slides on and off the post. However, particulate can still occur if the post is poorly designed:

• Keep post diameter at 90–95% of the device hole ID, not tighter: If the post is too tight (98%+ of hole ID), the device has to be forced on, and the post surface rubs against the hole interior, potentially shedding plastic fibers. A 5–10% clearance allows smooth insertion with zero friction.
• Round the post tip with a 0.5–1.0mm dome radius: A sharp or chisel-edged post tip can scratch the inner surface of the device hole during insertion, generating both plastic and device material particulate. A rounded dome guides the device on smoothly without cutting.
• Polish the mold pin to a mirror finish: The post surface is a direct replica of the mold pin surface. A rough mold pin (SPI B-3 or worse) creates a rough post surface that can shed micro-fibers when the device slides on and off. Polish to SPI A-2 or better for a smooth, low-friction surface.
• Avoid posts with sharp base corners: A sharp 90° corner at the post base creates a stress concentration that can develop a micro-crack during demolding or device insertion. This crack can propagate and shed particulate over the product shelf life. Use a minimum 0.5mm fillet radius at the post base.
• Don’t use posts for devices with rough or abrasive hole interiors: If the device hole is made of a rough material (e.g., unpolished metal, glass-filled polymer) or has burrs from machining, it will abrade the post surface every time the device is inserted or removed, generating particulate. In these cases, use a snap or undercut feature instead, which doesn’t contact the hole interior.

3. Undercut (Recess) Design: Highest Retention, Highest Complexity

Undercuts (also called recesses, pockets, or side-grips) are retention features where a portion of the cavity wall extends inward over the edge of the device, trapping it laterally. Unlike snaps, which clamp from above, undercuts hold the device by its side profile — the device slides into the undercut from the side during packaging, and can only be removed by lifting it up and out at an angle, or by flexing the tray wall outward.

Undercuts provide the highest retention force of any feature type, but they’re also the most complex to design, the hardest to demold, and the most likely to generate particulate if not designed correctly. They should be used only when snaps and posts can’t provide enough retention, or when the device geometry specifically calls for lateral retention.

Design Specifications and Reference Dimensions

Applicable Scenarios

Undercuts are the best choice when:

• The device is flat or irregularly shaped (instrument jaws, blade handles, flat implants, connector housings) and doesn’t have a round feature for a snap or a hole for a post
• You need high retention force (5–15N) for heavy or high-value devices that must not move during shipping
• The device will be removed by tilting and lifting, not straight vertical pull — undercuts are designed for angled removal
• The tray has relatively deep cavity walls (>10mm) that can flex outward to release the device — shallow cavities can’t flex enough
• The device has a consistent side profile with a shoulder or edge that the undercut can catch on — undercuts need a defined edge to work

Undercuts are not ideal for:

• Round or cylindrical devices — use a snap instead, which is simpler and cleaner
• Devices with holes or lumens — use a post instead, which has lower particulate risk
• Shallow trays (<8mm cavity depth) — the walls can’t flex enough to release the device, and the undercut geometry is too small to form reliably
• High-volume production (>1M units/year) — undercuts increase mold complexity and cycle time, and have higher scrap rates than snaps or posts
• Devices that need to be removed quickly or with one hand in emergency situations — undercuts require a two-step tilt-and-lift motion that’s slower than a straight pull

Demolding Considerations

Undercuts are the most difficult retention feature to demold, because by definition they create a negative draft angle on the inner cavity surface — the plastic wraps around a protruding feature on the mold core, and you can’t simply lift the part straight up. There are three demolding strategies for undercuts:

Strategy 1: Flex Release (for shallow undercuts <1.0mm depth)

For shallow undercuts with depth ≤1.0mm and tray wall thickness ≥0.5mm, the plastic can flex enough to release from the mold core during ejection without any special mold action. This works because PETG and APET have good flexibility when warm. The key is to eject the part while it’s still warm (but below heat distortion temperature) so the walls can flex outward over the mold core undercut without cracking. This is the simplest and most common approach for medical blister trays.

Strategy 2: Collapsible Core (for undercuts 1.0–2.0mm depth)

For deeper undercuts, a collapsible core (also called a split core or segmented core) is used. The mold core is made of multiple segments that retract inward during ejection, reducing the core diameter and releasing the undercut. Collapsible cores are more expensive and complex than standard cores, but they allow deeper undercuts without relying on plastic flex. They’re commonly used for instrument tray cavities with multiple undercut features.

Strategy 3: Side-Action (for undercuts >2.0mm depth or complex geometries)

For very deep undercuts or undercuts on multiple sides, side-action mold components (also called side pulls or cams) are used. These are mold inserts that retract horizontally (outward from the undercut) before the part is ejected. Side-actions are the most expensive and complex mold solution, and they add cycle time, but they allow almost any undercut geometry to be molded. They’re rarely needed for standard medical blister trays — most undercut requirements can be met with flex release or collapsible cores.

Regardless of strategy, these demolding rules apply to all undercuts:

• Always use 5–10° draft on the outer cavity wall surface: The outer surface (the surface that contacts the mold cavity, not the core) needs proper draft to release from the cavity half. If the outer surface sticks, the part will tear during ejection regardless of how well the undercut releases.
• Ensure undercut wall thickness ≥0.5mm after forming: The undercut lip is often the thinnest point in the entire tray because it’s a sharp draw feature. If it’s too thin, it will fold over or crack during demolding. Calculate draw ratio and verify with prototype parts.
• Round all undercut edges with minimum 0.3mm radius: Sharp edges on the mold core create sharp edges on the plastic, which are stress concentration points that crack during flex release. Rounded edges distribute stress and allow the plastic to slide over the mold core more easily.
• Polish the mold core undercut surface in the ejection direction: The undercut surface should be polished along the direction of ejection (vertical), not across it. Cross-direction polishing creates micro-grooves that catch on the plastic during release and cause tearing. SPI A-2 finish, polished in the draw direction, is standard.

How to Prevent Particulate Generation

Undercuts have the highest particulate risk of any retention feature, because the device edge rubs against the undercut lip during both insertion and removal, and the undercut wall flexes every time the device is removed. To minimize particulate:

• Keep undercut depth ≤1.0mm whenever possible: Deeper undercuts require more wall flex to release, and the device edge rubs harder against the lip, increasing both plastic wear and device abrasion. If you need more than 1.0mm depth, consider using two undercuts with 0.8mm depth each instead of one with 1.6mm.
• Round the undercut lip inner edge with 0.3–0.5mm radius: This is the single most important particulate prevention measure for undercuts. A sharp 90° lip edge will scrape the device side every time it’s removed, generating micro-particulate from both the plastic and the device. A rounded lip glides over the device edge without cutting.
• Use a 20–30° lead-in ramp on the top of the undercut: A steep lead-in (45°+) creates a sharp shoulder that catches on the device edge during insertion and can chip plastic. A gradual 20–30° ramp lets the device slide into place smoothly, and also makes removal easier because the device can ride up the ramp as it’s lifted.
• Ensure undercut wall thickness ≥0.5mm after forming: Thin undercut walls develop fatigue cracks at the base after repeated flexing (even just the flex during demolding). These cracks shed particulate over time. Thicker walls distribute flex stress more evenly and resist fatigue. If the wall is too thin due to draw, either reduce the undercut depth or increase the nominal sheet thickness.
• Design for angled removal, not straight vertical pull: Undercuts are meant to be removed by tilting the device at 15–30° and lifting it out, which lets one edge clear the undercut first, then the other. If a user tries to pull straight up, both undercuts have to flex simultaneously, and the device edge drags hard against both lips, generating maximum particulate. Add a finger recess or lift tab on one end of the device cavity to encourage the correct angled removal motion.
• Polish the undercut contact surface to SPI A-2 or better: The surface that contacts the device during removal should be as smooth as possible to minimize friction and abrasion. A rough surface (SPI B-3 or worse) will act like sandpaper against the device edge, generating particulate with every removal. Mirror-polish the mold core at all undercut contact surfaces.

How to Choose the Right Retention Feature for Your Device

With three feature types to choose from, how do you decide which one to use? Follow this decision tree, in order of priority:

1. Does the device have an existing hole, lumen, or bore? If yes, use a post. It’s the cleanest, lowest-particulate option, and easiest to demold. Only use something else if the post doesn’t provide enough retention.
2. Does the device have a cylindrical or rounded feature (handle, shaft, tube)? If yes, use a snap. It provides predictable, moderate retention with clean release, and is well-understood by both designers and production staff.
3. Is the device flat, irregular, or does it have a side shoulder/edge but no round feature or hole? If yes, use an undercut. It’s the most complex option, but it’s the only one that works for these geometries.
4. Do you need retention force >10N? If yes, use an undercut (or multiple undercuts). Snaps and posts rarely provide more than 8N reliably.
5. Is particulate risk a critical concern (e.g., implantable devices, sterile field use)? If yes, prioritize posts > snaps > undercuts. Choose the cleanest option that meets your retention requirements.

5 Common Retention Feature Design Mistakes

1. Designing based on nominal sheet thickness, not actual formed thickness. This is the #1 mistake. A snap arm designed for 0.5mm thickness may be only 0.25mm after forming in a deep draw area, causing it to break or not retain. Always calculate draw ratio and measure actual thickness at the feature location on prototype parts.
2. Making retention too strong “to be safe.” If the device requires 15N to remove, the user will jerk it out, and the sudden release will either launch the device across the room or cause the retention feature to chip and shed particulate. Target 3–8N for most applications, 10N maximum. Test with actual gloved users.
3. Ignoring demolding during design. A feature that works perfectly in CAD but can’t be removed from the mold is useless. Always consider demolding strategy (flex, collapsible core, side-action) before finalizing the feature geometry, and ensure proper draft on all non-undercut surfaces.
4. Sharp edges on all contact surfaces. Any edge that contacts the device during insertion or removal should be rounded with minimum 0.3mm radius. Sharp edges scrape, chip, and generate particulate. This applies to snap tips, post tops, and undercut lips — all of them.
5. Not testing retention after sterilization and aging. Retention force changes after sterilization (gamma makes PETG slightly more brittle, EO can affect surface friction) and after shelf aging (plastic stress relaxation reduces snap clamping force over time). A feature that holds perfectly when new may be loose after 2 years on the shelf, or too tight after gamma. Always test retention force after sterilization and accelerated aging.

Retention Feature Design Checklist

Final Thoughts

Retention features are small details that have an outsized impact on the success of a medical blister tray. A tray with poorly designed retention features will either fail in shipping (device movement, lid puncture, sterility breach) or fail at the point of use (hard to open, particulate generation, user frustration). Getting retention right requires attention to detail — actual formed thickness, demolding strategy, edge radii, release force — but it’s not rocket science. Follow the design guidelines in this article, prototype early, test with real users and real devices, and you’ll get it right.

If you’re designing a new blister tray and want help optimizing the retention features, or if you have an existing tray with retention problems (loose devices, particulate, hard removal), reach out to our engineering team. We’ve designed hundreds of retention features for medical blister trays, and we can help you avoid the common mistakes and get it right the first time.