Solving Fracture and Cracking in Injection Molding

Illustration — MobiusDaXter / Wikimedia Commons (CC BY-SA 3.0)

A cracked moulding is rarely a single-cause problem, which is why the same defect keeps returning after a fix that addressed only one symptom. Fracture is the end of a chain that usually starts with material condition, passes through melt and shear history, and finishes at a stress riser or a weld line where the part was already weak. Made-in-Dayin's injection molding guidance. This guide works through the chain in the order the physics occurs, with the countermeasures that address cause rather than symptom.

Key takeaways

• Most fractures trace to one of four causes: moisture or overheating degrading the polymer, excessive residual stress, a geometric stress riser, or a weak weld line.
• Hygroscopic polymers such as PC, PA, PET and PBT must be dried to a specified moisture content before processing, or they hydrolyse and lose impact strength.
• Internal sharp corners concentrate stress; radii of at least half the wall thickness are the usual design fix.
• A part that passes first-article inspection and cracks in the field usually has a residual-stress or environmental-stress-cracking problem, not a material one.

Follow the chain, not the symptom

Fracture appears at the weakest point of the finished part, but the weakness was created earlier. Polymer degradation happens in the dryer and the barrel; residual stress is frozen in during packing and cooling; a stress riser is designed into the geometry; a weld line is created where two flow fronts meet. Treating the visible crack without addressing the earlier link guarantees recurrence.

The diagnostic discipline is to work backwards. Establish whether the fracture is brittle or ductile — a brittle break with no yielding suggests degradation or a weld line, while a ductile break suggests an overload or a stress riser. That single observation narrows the cause substantially.

• Brittle break: suspect degradation or a weld line.
• Ductile break: suspect overload or a geometric stress riser.
• Cracks that appear weeks later: suspect residual or environmental stress.
• Always establish the failure mode before changing parameters.

Moisture and degradation

Hygroscopic polymers absorb atmospheric moisture, and if they are processed wet the water hydrolyses the polymer chains at melt temperature. The molecular weight falls, and impact strength falls with it — often dramatically, while tensile strength and appearance barely change. This is why an undried batch of polycarbonate can produce parts that look perfect and shatter.

Each material has a drying specification: a temperature and a time that bring the moisture content below a limit. Polycarbonate is typically dried around 120 degrees for three to four hours, and polyamide and PBT have their own windows. Drying is not optional and cannot be replaced by a longer cycle; a desiccant dryer and a moisture check are the controls.

Material	Hygroscopic	Typical drying need
PC	Yes	~120 °C, 3–4 h, moisture <0.02%
PA (nylon)	Yes	~80–90 °C, moisture <0.1%
PET / PBT	Yes	Specified per grade, low moisture
ABS	Mildly	Short dry, moderate temperature
PP / PE	No	Drying not usually required

Overheating and residence time

Degradation does not require moisture. Excessive melt temperature, a long residence time in the barrel, or a large shot in a small barrel all overheat the polymer and break the chains. The signs are a brittle part, a smell, silver streaking or discolouration, and a drop in impact strength measured on a test bar.

Residence time is the parameter most often ignored. A machine sized for a large part, running a small shot, holds the material in the barrel far longer than the material's thermal history allows. Matching barrel capacity to the shot weight, and lowering the melt temperature to the minimum that fills the part, are the two most effective countermeasures.

• Excessive melt temperature degrades the polymer.
• Long residence time in an oversized barrel does the same.
• Symptoms: brittleness, odour, silver streaks, discolouration.
• Match barrel capacity to shot size and minimise melt temperature.

Residual stress and packing

Residual stress is stress locked into the part when it cools unevenly or when it is packed at the wrong pressure. Thick sections cool more slowly than thin ones and shrink more, so the part is internally fighting itself before it is ever loaded. The result is a part that cracks under a load it should easily survive, or crazes spontaneously over time.

Packing pressure and time, hold profile, and cooling uniformity are the levers. Insufficient packing leaves voids and sinks and creates a weak core; excessive packing over-packs the gate region and creates high frozen-in stress. A pressure trace and a mould-temperature survey are how you see what is actually happening rather than guessing.

• Uneven cooling locks stress into the part.
• Insufficient packing creates weak, voided cores.
• Excessive packing creates high frozen-in stress.
• Use pressure traces and mould-temperature surveys to diagnose.

Geometry: the stress riser

Sharp internal corners concentrate stress, and the concentration factor rises sharply as the radius shrinks. The standard design rule is an internal radius of at least half the nominal wall thickness, and more where the part carries load. A radius that looks generous on a drawing is often the cheapest change available.

The other geometric offenders are abrupt wall-thickness changes, unradiused rib roots and boss junctions. Where a rib meets a wall, the junction thickness should be well under the wall thickness to avoid a thick, slowly cooling mass; the usual rule is around half to two thirds of the wall. BOSSes should be supported with gussets rather than thickened.

• Internal radius of at least half the wall thickness.
• Avoid abrupt wall-thickness transitions.
• Keep rib roots well under the wall thickness.
• Support bosses with gussets rather than thick walls.

Weld lines and flow

When two flow fronts meet, the material at the interface is already cooling and the chains do not entangle across the boundary. The result is a weld line with perhaps a fraction of the base material's strength, and if it sits at a stressed location the part will fail there every time. Flow simulation shows where weld lines will form before the tool is cut.

Countermeasures are to move the weld line away from stressed regions, raise the melt and mould temperature to improve entanglement, increase injection speed to reduce cooling at the front, and redesign the gate so the fronts meet at a favourable angle. Venting also matters, because trapped gas at the meeting point weakens the joint further.

• Weld lines form where flow fronts meet.
• Strength at a weld line is well below the base material.
• Move weld lines away from stressed regions.
• Raise temperatures and improve venting to strengthen the joint.

Environmental stress cracking

Some failures occur with no apparent load at all, weeks after assembly. Environmental stress cracking happens when a stressed polymer is exposed to a chemical — a mould release agent, a cleaning solvent, an adhesive, even some oils — that accelerates crack growth. The part is already stressed from processing, and the chemical is the trigger rather than the cause.

The diagnosis is to look for a stress raiser and a chemical together. The countermeasures are to reduce residual stress (annealing the part can help), change the mould release or the cleaning agent, and avoid assembling parts into a stressed condition. Annealing at a controlled temperature below the glass transition relieves moulded-in stress and often eliminates the problem outright.

• No visible load, failure appears weeks later.
• Requires a stressed polymer plus a chemical trigger.
• Remove or change the release agent, solvent or adhesive.
• Annealing below the glass transition relieves moulded-in stress.

Regrind, contamination and material handling

Regrind shortens the polymer chains every time it is reprocessed, so a high regrind ratio produces a brittle part even when everything else is correct. The permitted ratio belongs in the specification and should be recorded per batch; without that discipline, regrind accumulates invisibly and impact strength drifts downward.

Contamination has the same effect. Dust, a different polymer from a previous job, or moisture from an open container all degrade the melt. Closed conveying, purging between materials and labelled, sealed containers are basic housekeeping that prevents a whole class of fracture complaints.

• Regrind degrades with every heat cycle; cap the ratio.
• Record the regrind ratio per batch.
• Purge and clean between different polymers.
• Keep material sealed to prevent moisture uptake.

Buyer's specification checklist

• Establish whether the fracture is brittle or ductile.
• Confirm the material was dried to its specified moisture limit.
• Check melt temperature against the material's processing window.
• Check barrel capacity against shot weight for residence time.
• Cap and record the regrind ratio.
• Review packing pressure, hold profile and cooling uniformity.
• Bring internal radii to at least half the wall thickness.
• Check rib and boss junctions for thick, slow-cooling masses.
• Locate weld lines and move them away from stressed regions.
• Test for environmental stress cracking with a chemical trigger.
• Consider annealing to relieve moulded-in stress.

Frequently asked questions

Why do my parts look fine but break in service?

Appearance does not track impact strength. Undried hygroscopic material, long residence time and high regrind all reduce impact strength while leaving surface finish intact, so the defect only appears under load.

How do I know if moisture is the problem?

Dry the material to specification, mould a test bar and compare impact strength before and after. A large improvement confirms hydrolysis, and a moisture meter on the dryer feed is the ongoing control.

What radius should internal corners have?

At least half the nominal wall thickness, and more for load-bearing corners. Stress concentration rises sharply as the radius shrinks, so this is usually the cheapest fix available.

Why does my part crack at the weld line?

Because the two flow fronts meet when the material is already cooling and the chains do not entangle across the boundary. The weld line has a fraction of the base strength, so move it away from stressed regions or raise temperatures to improve bonding.

What causes cracking weeks after assembly?

Environmental stress cracking — a stressed part combined with a chemical trigger such as a mould release, solvent or adhesive. Reduce residual stress, change the chemical, or anneal the part.

Does annealing help?

Often, yes. Heating the part to a controlled temperature below its glass transition and cooling it slowly relieves moulded-in stress, which is frequently enough to eliminate delayed cracking.

How much regrind is acceptable?

It is material-specific but usually a modest percentage, because each heat cycle shortens the chains. Put the number in the specification and record it per batch rather than leaving it to the operator.

What is the difference between a sink mark and a crack?

A sink mark is a cosmetic shrinkage defect on a rib or boss; a crack is a structural failure. They share a cause in uneven cooling, but a sink mark is graded on appearance while a crack is a functional defect.

Should I change material to fix cracking?

Only after you have ruled out moisture, degradation, stress and geometry. Changing material masks a process problem and usually introduces a new one, at higher cost.

How do I diagnose a fracture properly?

Establish brittle or ductile, then check the material's thermal history, then the packing and cooling profile, then the geometry and weld-line position. Work in that order rather than changing parameters at random.

How do I test for residual stress?

Immerse the part in a suitable solvent or stress-cracking medium and observe the time to craze, or compare shrinkage and warpage against a moulded standard. Destructive comparison against a known-good part is often the fastest evidence.

Can changing the gate fix cracking?

Sometimes. Gate position determines weld-line location and shear history, so a change can remove a weak point. It is usually a tool modification, so confirm the diagnosis before committing.

Why did the defect appear only in winter?

Because cooler ambient conditions change the mould temperature and the cooling rate, which changes residual stress and weld-line strength. Seasonal drift is a common reason a defect appears, disappears and returns.

Is a thicker wall stronger?

Not necessarily. Thick sections cool slowly, sink and warp, and can be weaker because of internal voiding. Uniform walls with ribs for stiffness usually produce a stronger and more consistent part.

Illustration — Internet Archive Book Images / Wikimedia Commons (No restrictions)

References and standards

• Polymer drying specifications — moisture limits and drying windows for PC, PA, PET and PBT.
• Design-for-manufacture rules — internal radii, wall uniformity and rib-to-wall ratios.
• Environmental stress cracking — chemical-transition-accelerated failure of stressed polymers.
• Weld-line mechanics — chain entanglement across meeting flow fronts and its effect on strength.

Standards and references. ASTM International; nondestructive testing; injection moulding