How Dunnage Contact Points Protect Fragile Industrial Components

Dunnage contact points protect a part by placing support on regions that can take load, holding the part away from sensitive surfaces, and limiting movement without forcing fit. The wrong contact geometry causes damage itself: it can scratch finished faces, concentrate pressure, or lock in a part that needs room. Validation with real parts confirms the layout.

What Does Dunnage Contact Point Design Mean?

Dunnage is the interior packaging inside a container. Contact point design decides exactly where the part and dunnage touch, and just as important, where they do not.

Four jobs are involved. Support carries the part’s weight. Restraint limits how far it can move. Cushioning reduces the shock that reaches it. Location control means the part sits in a repeatable position, which matters for robot picks and for inspection.

A single feature can do more than one of these, and the jobs can conflict. A pad that is soft enough to cushion well may not locate precisely. A rigid pocket may locate perfectly and transmit every shock.

Identifying Safe Support Surfaces

Start with the part, not the packaging, by sorting its surfaces into four groups.

• Structural regions: thick sections, ribs, and bosses that can take load
• Cosmetic surfaces: visible faces where a mark would make the part unsaleable
• Fragile protrusions: thin tabs, pins, and connectors that break under low force
• Precision-machined interfaces: sealing faces and bores where a scratch can cause a functional failure

Automotive trim offers a clear case. In US patent 5,178,279 for nestable dunnage, the design goal is to prevent contact between Class A surfaces and either other trim pieces or the dunnage itself, to avoid scratching or marring. Parts there are supported in an “almost floating relation” with all Class A surfaces out of contact. That is one design. A hidden surface is not suitable just because it is hidden, so choose a contact region after confirming that it can carry the load without damage.

How Contact Pressure Is Distributed

As a simplified concept, contact pressure is load divided by area. The numbers below are hypothetical, for a part weighing 6 lb on four identical pads.

Layout	Pad area (each)	Total area	Static pressure	Pressure at an assumed 3 g event
Generous pads	0.25 in²	1.00 in²	6 psi	18 psi
Very small pads	0.02 in²	0.08 in²	75 psi	225 psi

Same part, same shock, and pressure 12.5 times higher. The 3 g figure is a plain multiplier for illustration, not a measured transit acceleration, and neither pressure is a damage limit. The formula assumes the load shares evenly, which a real part on real pads may not do. A part resting on three of four pads puts its load on less area than the table shows.

Soft and rigid contact behave differently. A soft material spreads under load, enlarging the contact patch and lowering peak pressure, but it allows more movement. A rigid point holds position and concentrates force.

Local deformation and stress concentration follow from high pressure. A soft plastic part can take a permanent dent. A brittle part can crack starting at a small contact, even when overall load is modest.

Controlling Movement

• Lateral movement: side-to-side sliding, which can cause rubbing
• Vertical movement: bounce, which allows repeated impact
• Rotation: a part free to pivot can swing a protrusion into a wall
• Clearance: the gap that decides how far any of these can go

Every clearance is a travel distance, and travel builds up impact speed. Yet some clearance has to exist, because of the over-constraint problem that comes next.

Avoiding Over-Constraint

Fixed points on all sides can do damage even though they feel secure.

Temperature-dependent expansion is one source. Plastic dunnage and a metal part expand at different rates, so a snug fit in a cool warehouse may become a tight fit in a hot trailer.

Part-to-part tolerances are another. A pocket fit to a nominal part may pinch a part at the high end of tolerance and rattle a part at the low end. Design contact locations using the tolerance range, not the nominal dimension.

Insertion and removal forces belong here too. A tight pocket needs force to load and unload, and an operator who struggles with a pocket may pry, which is how a protective pocket becomes a source of damage.

Fit variability from wear completes the picture. After many cycles, a pocket that once fit may be loose, so check fit on used dunnage as well as new.

Contact Material Selection

Cushioning materials vary in stiffness and recovery. Some foams recover well after compression, and others take a set.

Abrasion is the concern on cosmetic parts. Dust trapped between a rough material and a finished face can act like a grinding compound. Chemical compatibility matters as well, since some materials can transfer plasticizers or stain sensitive surfaces, so check with the supplier and test on the actual finish.

Cleanliness and contamination are their own category. Fiber shedding, particle generation, and residue from repeated washing can disqualify a material for clean parts. A reusable system also has to tolerate the washing process it will go through.

Handling and Transportation Conditions

Vibration can do its damage slowly. A loose part moving a fraction of an inch against a contact point thousands of times can polish or wear through a finish.

Impact is the sudden case, and a drop at the corner of a container loads the contacts differently from a drop on the base.

Repeated placement is the operator’s contribution. Each load and unload slides the part against the dunnage, which can wear both.

Dunnage wear changes all of this over the life of a reusable system. Compressed foam loses cushioning, and worn pockets lose restraint. Inspection and replacement criteria belong to the design, not to an afterthought.

How to Validate a Dunnage Contact Layout

Use representative physical parts, including samples from both ends of the tolerance range. A 3D-printed model may not reveal how a finish responds.

Check sheet: tick each point as you work through it

1. Check loaded and unloaded fit at the expected temperature extremes.
2. Run handling trials with the operators who will load and unload.
3. Run transportation tests suited to the distribution route.
4. Inspect contact points on parts and on dunnage, at magnification where finish matters.

ASTM D4169, listed as D4169-23e1, offers a laboratory way to evaluate how shipping units withstand distribution, by subjecting them to a test plan consisting of a sequence of anticipated hazard elements. It evaluates a shipping unit under a plan. It does not give a formula for where to put dunnage contact points, and the result depends on the plan chosen.

This matrix ties those steps to the part. The entries are examples for a hypothetical metal or plastic part.

Part feature	Risk	Contact strategy	Validation
Cosmetic face	Scratching, rub marks	No contact; hold from another region	Inspect the face after a vibration test
Thick structural rib	Low risk	Primary support location	Check for pad marking
Thin tab	Breakage on impact	Clear space around tab	Drop test, then inspect tab
Machined bore	Scratch, contamination	Keep contact out of the bore	Gauge bore after transport test
Part with wide tolerance	Pinch or rattle	Compliant pad at contact	Fit check at both tolerance limits

Stack loads from above are a separate calculation, and how to estimate stacking loads on plastic totes sets it out. Tests on the finished product itself, without packaging, answer a different question, as explained in drop testing methods for finished plastic products.

Dunnage Contact FAQ

Can dunnage cause scratches even when a part is held securely?

Yes. Secure does not mean motionless, since vibration can still produce very small relative movement at a contact point, and trapped particles can turn that movement into abrasion. Keeping the contact off cosmetic surfaces is the direct remedy. Softer materials reduce the risk but do not remove it.

Layout Decisions in the Order They Should Happen

The order matters because each step limits the next. Support points can only come from surfaces that may be touched, so that map comes first. Contact pressure can only be calculated once those points and their pad areas exist. Restraint then has to leave room for tolerance and thermal growth, and contact materials are chosen with the part finish and the washing process in view. Reverse any of those and the design gets reworked. The closing test is a sample that someone tries to break, through handling trials, the shipping test plan, and inspection at the contact points. A layout passes when the part survives and the marks stay off the surfaces that matter.