Common Injection Molding Defects: Causes and Solutions

Cause

Injection pressure is too low.

Solution

Increase injection pressure.

Cause

Resin flowability is insufficient.

Solution

Raise the mold temperature,

Raise the resin temperature, and/or increase injection speed (especially effective for thin-walled sections).

Cause

Injection speed is too fast, preventing proper gas venting or clogging the vents.

Solution

Moderately reduce injection speed. If using multi-stage speed control, slow down the final stage.

Cause

Pressure drops as the resin travels from nozzle → sprue → gate → cavity.

Solution

Shorten the nozzle length, shorten the sprue length, and shorten the gate length while increasing its width or thickness (diameter).

Cause

A small nozzle diameter can prevent proper filling due to high pressure loss.

Solution

Increase the nozzle diameter.

Cause

The machine's injection capacity is equal to or smaller than the part's volume (weight).

Solution

Use a machine whose injection capacity is at least 50% greater than the part weight.

Cause

A worn check ring allows resin to flow backward during filling.

Solution

Since the check ring is a consumable part, inspect and replace it regularly.

Cause

With materials like POM or flame-retardant resins, parts may mold well initially but develop short shots later as contaminants clog the gas vents.

Solution

Clean the gas vents.

Cause

Flame-retardant resins contain UL-certified halogen gas additives that can clog vents with residue.

Solution

Maintain vent function through regular cleaning, maintenance, and inspection records.

Cause

In multi-cavity molds, poor gate balance causes cavities near the sprue to fill first and distant cavities to fill later, resulting in short shots.

Solution

Use a smaller gate cross-section near the sprue and a larger one farther away so all cavities fill simultaneously.

Cause

Runners, gates, or sprues are too small.

Solution

A wider gate cross-section generally gives the best balance — but if the gate is too wide, gate sealing takes longer, which can cause packing pressure issues and shrinkage defects. If the runner cross-section is too small, the molten resin loses flowability; increasing the runner size is usually more effective than enlarging the gate alone.

Cause

Poor resin flowability.

Solution

Use a higher-flow grade of the same resin type. Flowability can be evaluated using the L/t ratio (PS: L/t = 220–300; PC: L/t = 100–150). Resins with poor L/t values benefit from multiple gates, or from hot-runner systems with sequential valve gates.

Cause

Injection pressure is higher than necessary.

Solution

Lower the injection pressure (secondary/holding pressure)

Lower the resin temperature

Lower the mold temperature,

Reduce the shot size

Lower the final injection speed stage.

Cause

Insufficient clamping force (clamping force = mold internal pressure × projected area).

Solution

Use a machine with higher clamping tonnage.

Cause

Insufficient mold rigidity — a thin fixed-side mounting plate can bow under pressure. 

Support pin position differences between toggle-type and direct-pressure clamping units can cause the moving platen to deflect in opposite directions if the mold is moved between machine types.

Solution

Use a stronger mold material or increase mold plate thickness.
 

Cause

Poor machining precision

Solution

Improve mold machining precision (check tolerances).

Cause

Foreign matter trapped in the mold prevents proper mold closure.

Solution

Remove foreign matter from the mold.

Cause

Worn guide pins

Solution

Replace guide pins, or measure wear and confirm remaining clearance.

Cause

Weak mold material deforms over long-term use, or the mold has degraded from age.

Solution

Upgrade to a wear-resistant mold material.

Cause

Burr occurs repeatedly and consistently.

Solution

The mold needs to be reworked.

Cause

Injection pressure is too low.

Solution

Increase injection pressure.

Cause

Holding (packing) time is too short.

Solution

Extend the holding time until the gate freezes (i.e., until part weight stops changing).

Cause

Resin temperature is too high, causing excessive shrinkage.

Solution

Lower the resin temperature.

Cause

Mold temperature is too high, causing slow cooling and excessive shrinkage.

Solution

Lower the mold temperature.

Cause

Cushion amount is too large or too small.

Solution

Adjust the cushion amount to an appropriate level (around 10 mm).
 

Cause

A small nozzle diameter causes high pressure loss, so pressure isn't transmitted effectively.

Solution

Increase the nozzle diameter.

Cause

With materials like POM or flame-retardant resins, parts may mold well initially but develop defects later as contaminants clog the gas vents.

Solution

Clean the gas vents.

Cause

Flame-retardant resins contain UL-certified halogen gas additives that can clog vents with residue.

Solution

Maintain vent function through regular cleaning, maintenance, and inspection records.

Cause

In multi-cavity molds, poor gate balance causes cavities near the sprue to fill first and distant cavities to fill later.

Solution

Use a smaller gate cross-section near the sprue and a larger one farther away so all cavities fill simultaneously.

Cause

Runners, gates, or sprues are too small.

Solution

A wider gate cross-section gives the best balance, though an excessively wide gate delays gate sealing and can cause packing/shrinkage issues.

A larger runner cross-section is usually more effective than enlarging the gate alone.

Cause

Low-shrinkage resins produce smaller sink marks.

Solution

Use a lower-shrinkage grade within the same resin family.

Cause

PE, PP, and POM shrink significantly.

Solution

If a material substitution is possible, switch to an amorphous resin such as PC or ABS.

Cause

Resin temperature is too low, causing a solidified layer to form quickly and reducing flowability.

Solution

Increase the resin temperature.

Cause

Mold temperature is too low, causing the same early solidification effect.

Solution

Increase the mold temperature.

Cause

Injection speed is too slow, causing the same early solidification effect.

Solution

Increase injection speed.

Use multi-stage speed control to precisely manage the flow rate near the gate.

Cause

Pressure doesn't transmit easily to the flow end.

Solution

Increase holding pressure, since slow flow is the underlying cause.

Cause

Resin that solidifies near the nozzle gets injected into the cavity, forming a cold slug.

Solution

Add a cold slug well for indirect gates.

Cause

With direct gates, particularly for PC, heat loss increases viscosity.

Solution

Avoid "touch molding" and use "untouch molding" instead.

Cause

Poor gas venting increases flow resistance.

Solution

Ensure gas vents function properly.

Cause

In multi-cavity molds, resin cools while traveling through long runners, producing flow marks.

Solution

Improve gas venting.

Cause

Insufficient back pressure during plasticization draws in air; this air, surrounded by resin, ruptures inside the cavity and appears on the mold surface (occurs regardless of whether the resin is crystalline or amorphous).

Solution

Increase back pressure (PB) to prevent air intake during plasticization. 

(Note: excessive suck-back, even with higher back pressure, can increase silver streaking.)

Cause

Frictional heat during metering or molding thermally decomposes the resin into gas, which ruptures inside the mold.

Solution

Keep screw RPM within an appropriate range.

Cause

Shear friction inside the mold.

Solution

Avoid overly thin wall sections or overly complex mold geometry.

Cause

Small runner, gate, or sprue cross-sections.

Solution

Maintain vent function through regular cleaning, maintenance, and inspection records.

Cause

Uneven wall thickness within the mold.

Solution

Design the part to avoid abrupt thickness changes.

Cause

Poor gas vent exhaust.

Solution

For flame-retardant resins or large-volume parts that generate more gas, design vents thoroughly, and clean or widen vents when they become clogged or lose effectiveness.

Cause

Insufficient resin drying leaves excess moisture in the material.

Solution

Ensure resin is fully dried in a hopper dryer.

Cause

Mixing of dissimilar resins.

Solution

Take care to avoid cross-contamination between different resin types.

Cause

Resin fails to properly transfer/replicate against the mold surface.

Solution

Adjust mold temperature. 

If gas generation is causing the cloudy surface, lower the mold temperature.

If the surface finish transfer is poor due to low mold temperature, raise it instead. 

When cloudy surface occurs, try lowering the mold temperature first, and raise it if that doesn't help (raising temperature generally takes less time than lowering it).

Cause

Rough mold surface finish.

Solution

Polish the mold surface to improve cloudy surface issues.

Cause

Degraded chrome plating that has partially worn away.

Solution

Fully strip and reapply the chrome plating.

Cause

Lubricant or release agent residue on the mold surface.

Solution

Clean the lubricant and release agent from the mold surface.

Cause

Poor venting.

Solution

Eliminate trapped air and ensure gas vents are functioning properly.

Cause

Injection speed is too slow, so the resin loses heat to the mold before the two flow fronts meet.

Solution

Increase injection speed.

Cause

Injection speed is too fast, preventing adequate gas venting; trapped air compresses adiabatically and delays flow, and the resin loses heat to the mold before the flow fronts meet — this often occurs at the end of fill.

Solution

Check venting near the weld line area and adjust injection speed.

Cause

Resin temperature is too low where the two flow fronts meet.

Solution

Increase resin and/or mold temperature.

Cause

Injection pressure is too low to maintain sufficient flow speed.

Solution

Increase injection pressure.

Cause

Poor venting can cause weld lines to form.

Solution

Add a "tub" feature to relocate the weld line into a vented pocket, then remove the tub after molding to eliminate the visible weld.
 

Cause

Poor venting creates air resistance that impedes and delays resin flow, causing it to cool and form a weld line.

Solution

Modify or clean the mold to improve venting — use parting lines, slide pins, core pins, or porous steel inserts for venting.

Cause

Parts with holes or multiple gates create weld lines where flow fronts meet.

Solution

Adjust gate position and count to relocate the weld line elsewhere. Alternatively, use heating/cooling (heating the weld area to ~200°C with a halogen lamp, then cooling — note this changes mold dimensions and requires durable materials due to repeated thermal cycling), or use in-mold machining (venting via a pin hole under holding pressure after fill) to eliminate the weld line.

Cause

Bubble formation from trapped air.

Solution

Increase back pressure and lower resin temperature.

Cause

Insufficient resin is supplied to compensate for volumetric shrinkage in thick sections, creating voids.

Solution

Increase injection pressure, increase holding pressure, extend holding time, and/or increase injection speed.

Cause

Even with sufficient holding pressure, if resin and mold temperatures are too low, pressure loss along the flow path prevents enough resin from reaching thick sections to compensate for shrinkage.

Solution

Increase resin temperature and mold temperature.

Cause

Abrupt wall thickness changes prevent even pressure transmission, causing microscopic voids as shrinkage occurs or trapped gas bubbles merge and grow. 

Solution

Clean gas vents and design the part to avoid abrupt wall thickness changes.

Cause

Abrupt wall thickness changes cause the surface to solidify quickly while the interior shrinks significantly.

Solution

Design the part to avoid abrupt wall thickness changes.

Cause

Even with sufficient holding pressure, a thin gate location makes pressure transmission difficult, preventing enough resin from compensating for shrinkage.

Solution

Position the gate at a thicker section of the part.

Cause

Small runner and gate cross-sections cause high pressure loss, preventing holding pressure from transmitting effectively.

Solution

Increase runner and gate size.

Cause

Crystalline resins with slow crystallization rates solidify quickly at the surface while the hotter interior solidifies more slowly, creating internal voids.

Solution

Use a resin with a faster crystallization rate.

Cause

Carbonization from frictional heat at the gate. 

Solution

Lower injection speed and injection pressure.

Cause

Using the same resin after a shutdown without purging causes repeated heating/cooling/reheating cycles that carbonize residual resin.

Solution

Keep the barrel warm during downtime. For example, acrylic held at 220°C that cools to room temperature over a break tends to develop black spots; keeping it warm can reduce or eliminate this on the next run. PP (food-grade, additive-free), PE (medical-grade, additive-free), and PMMA also benefit from being kept warm, though it may not eliminate the issue entirely.

Cause

Resin lingering in gaps near the nozzle inside the cylinder carbonizes and gets carried into the part.

Solution

Clean the internal nozzle gaps frequently.

Cause

Resin trapped for extended periods around the screw's check ring carbonizes and enters the part.

Solution

Clean the check ring area frequently.

Cause

Screw surface damage traps resin in the flow channel, where it stagnates, carbonizes, and enters the part.

Solution

Repair the screw surface, and apply coating if needed to reduce resin adhesion.

Cause

Using a screw with a short compression zone or long metering zone for easy-to-melt resins causes overheating/decomposition that enters the part.

Solution

Replace the screw to avoid overheating in the compression and metering zones during plasticization.

Cause

Poor venting causes resin to carbonize inside the mold.

Solution

Clean or enlarge gas vents to improve venting.

Cause

Mold lubricant residue on the mold surface carbonizes when contacted by incoming resin.

Solution

Clean the mold frequently.

Cause

Switching from ABS to PC/ABS without proper purging can cause defect rates above 10% due to black spotting (at high temperatures, PC/ABS can carbonize resin residue left in the screw, or resin that underwent high-temperature exposure can stick to the screw and carbonize, shedding debris into subsequent shots).

Solution

When switching between PC/ABS and ABS, thoroughly clean the barrel interior using a high-viscosity PE purge compound.

Cause

Excessive injection pressure causes significant shear stress and residual stress.

Solution

Lower injection pressure, lower holding pressure, reduce holding time, increase resin temperature, increase mold temperature, and/or apply annealing to prevent cracking/crazing (anneal at 15–20°C below the heat deflection temperature).

Cause

Insufficient resin flowability.

Solution

Raise mold temperature, raise resin temperature, and/or increase injection speed (especially effective for thin-walled sections).

Cause

Injection speed is too fast, preventing proper gas venting or clogging the vents.

Solution

Moderately reduce injection speed; if using multi-stage speed control, slow down the final stage.

Cause

In insert molding, differences in thermal expansion (shrinkage rate) between the metal insert and resin cause cracking.

Solution

Preheat the metal insert to 50–60°C (sometimes up to 100°C) before injection.

Cause

Very low resin or mold temperature causes the resin to solidify immediately upon contacting the mold surface.

Solution

Lower injection pressure, raise resin and mold temperature, adjust back pressure and screw RPM, and dry the resin thoroughly.

Cause

Too few runners/gates, or a thin and complex part geometry, creates excessive shear stress during filling.

Solution

Enlarge runners and gates, and design the mold with shear stress during filling in mind.

Cause

Mixing incompatible dissimilar resins — often from incomplete purging (mixing inside the cylinder during molding) or contaminated raw material.

Solution

Purge thoroughly, and clean the cylinder interior when necessary.

Cause

Fast resin flow combined with the inertia of injection speed and the elasticity of the molten resin at the gate.

Solution

Reduce injection speed near the gate so resin fills the gate area sufficiently before speeding up again. Once the area around the gate is filled, increasing speed afterward won't cause jetting, since the increased viscosity of the melt and friction between the solidified and molten layers create enough flow resistance to prevent it.

Cause

No feature near the gate to slow down flow velocity.

Solution

Use a side gate to prevent jetting — this allows higher injection speed near the gate without causing the defect. Alternatively, install a core pin in front of the gate.

Cause

Uneven wall thickness causes greater shrinkage in thick sections and less shrinkage in thin sections, leading to warpage from differential shrinkage.

Solution

Design the part to minimize thickness variation.

Cause

Solution

Rapidly cool the inside of the box and slowly cool the outside to correct inward warping — but note that over-cooling the inside to fix inward warp can introduce outward warp instead.
 

Cause

Orientation of polymer chains or reinforcing fibers along the flow direction creates differential shrinkage between the flow direction and the cross-flow direction, causing warpage.

Solution

Apply separate shrinkage rates for the flow direction and cross-flow direction during mold design to achieve the desired final dimensions.

Cause

Color variation arises from differences in molding temperature and residence time in the injection cylinder. Even with the same resin, color can vary due to resin temperature, residence time, or differences between machine models; gloss differences can also contribute to perceived color variation.

Solution

Monitor actual resin temperature rather than the set temperature, and keep cylinder residence time consistent. Maintain consistent injection speed, back pressure, and other process conditions to keep resin temperature stable.

Cause

Pigment is not uniformly dispersed in the resin (typically near the gate).

Solution

Use pre-colored (compounded) pellets, and adjust back pressure.

Cause

Insufficient thermal stability of the colorant (appears across the entire surface).

Solution

Evaluate alternative colorants, and account for the resin's thermal stability relative to cylinder residence time.

Cause

Crystalline resins can show color variation due to cooling rate differences.

Solution

Set the mold temperature appropriately for the specific resin.

Cause

Gloss level varies with molding conditions. 

Solution

Higher injection speed (pressure) generally increases gloss; higher mold temperature improves gloss; resin temperature also affects gloss (higher resin temperature generally increases flowability and gloss). Poor resin drying can also degrade gloss.

Cause

Poor mold surface polishing. 

Solution

Improve mold surface polishing (for fully transparent parts, plating the mold surface can further improve gloss).

Cause

Release agent residue or other contamination on the mold surface.

Solution

Clean the mold surface and strictly control release agent use.

Cause

Insufficient resin flowability (reduces gloss). 

Solution

Increase resin temperature.

Cause

Volatile gas content in the resin reduces gloss (thermal decomposition of the resin increases gas generation).

Solution

Dry the resin thoroughly to suppress gas generation, and mold at an appropriate temperature with short cylinder residence time.

Cause

Overpacking creates high cavity pressure near the front of the cavity, generating residual stress.

Solution

Gradually reduce injection speed and holding pressure/time.

Cause

Poor ejection/release.

Solution

Adjust ejector speed.

Cause

Injection speed is too fast, preventing proper gas venting or clogging the vents.

Solution

Moderately reduce injection speed; if using multi-stage speed control, slow down the final stage.

Cause

Insufficient mold strength.

Solution

Upgrade to a higher-grade, wear-resistant mold material.

Cause

Poor ejection/release.

Solution

Increase the draft angle and improve surface finish where whitening occurs, add fillets at rib/boss/edge areas, and reposition ejector pins (redesigning the part if needed).

Cause

성Overpacking during molding causes less-than-normal shrinkage, generating force in the opposite direction of ejection.

Solution

Adjust resin temperature, pressure (holding/injection pressure), holding time, and holding pressure transition point to prevent overpacking.

Cause

Uneven shot metering.

Solution

Inspect the screw check ring and replace if needed; set an appropriate back pressure.

Cause

Ejection force acts against the natural release direction of the part.

Solution

Increase the draft angle and improve surface finish, eliminate mold undercuts, add fillets at rib/boss/edge areas, and reposition or add ejector pins (redesigning the part if needed).

Cause

Small ejector pin contact area concentrates force per unit area, causing whitening or cracking during ejection. S

Solution

Increase ejector pin contact area (add more pins and/or reposition as needed).

Cause

The part sticks to the mold.

Solution

Introduce air between the mold and part during ejection.

Cause

The part hasn't cooled/solidified sufficiently before ejection, causing deformation and release problems.

Solution

Improve mold cooling — add cooling lines or lower the mold temperature.

Cause

Poor sprue release. 

Solution

Remove sprue undercuts and improve surface finish; confirm the mold is aligned with the nozzle center (sprue bushing radius > nozzle tip radius); if the sprue's internal angle is insufficient, increase cooling time for efficient sprue cooling.

Cause

Overpacking creates excessive cavity pressure near the front.

Solution

Adjust resin temperature, pressure (holding/injection pressure), holding time, and holding pressure transition point to prevent overpacking.

Cause

Cooling time is too short — the part hasn't fully solidified before ejection.

Solution

Extend cooling time, lower mold temperature, and inspect cooling lines to improve cooling efficiency; add cooling lines if needed.

Cause

Uneven mold temperature.

Solution

Inspect cooling lines to ensure proper mold cooling; add cooling lines if needed to make mold cooling more uniform.

Cause

Small ejector pin contact area concentrates force per unit area, causing ejection defects (cracking, deformation, whitening).

Solution

Increase ejector pin contact area, and add more pins or reposition them as needed.

Cause

Overpacking creates excessive cavity pressure.

Solution

Adjust resin temperature, pressure (holding/injection pressure), holding time, and holding pressure transition point to prevent overpacking.

Cause

Resin temperature is too high, or extended residence time in the cylinder weakens part strength, causing breakage during ejection.

Solution

Lower resin temperature and reduce residence time; consider a machine with a barrel capacity better matched to part weight if needed.

Cause

Cooling time is too short — the part hasn't fully solidified before ejection.

Solution

Extend cooling time, lower mold temperature, and inspect cooling lines to improve cooling efficiency; add cooling lines if needed.

Cause

Poor resin drying is a root cause of weak part strength.

Solution

Dry the resin thoroughly. Take extra care with raw material storage during humid seasons — moisture-saturated resin from improper storage may require dehumidifying dryers and significantly longer drying times to recover.

Cause

Excessively high resin temperature causes thermal decomposition and increased gas generation, which accumulates on the mold surface and damages gloss over time.

Solution

Reduce cylinder residence time, dry the resin thoroughly, lower resin temperature, and inspect and add gas vents as needed. Molds used with flame-retardant resins should be disassembled and cleaned regularly, since tar-like residue in the gas can clog vents or damage the mold over time.

Cause

Insufficient mold strength.

Solution

Use a higher-grade, corrosion- and wear-resistant mold material; plate the cavity/core with corrosion- and wear-resistant coating if the resin requires it.

Cause

Improper mold storage leads to oxidation from moisture exposure.

Solution

Store molds properly (apply a rust preventive to the mold surface before storage).

Cause

Excessive cooling at the gate or thin sections causes flow stagnation (from low molding temperature, slow injection speed, or low mold temperature). If the incoming molten resin is cooler than the mold or passes through a thin section, premature solidification reduces flow, typically occurring at the smaller-area gate section.

Solution

Raise molding temperature, increase injection speed (note: too fast can cause jetting), and raise mold temperature. (This defect typically occurs at gates with small cross-sections, so it should be distinguished from jetting/whitening at the gate.)

Cause

Gate size is too small.

Solution

Increase gate size.

Cause

Screw damage.

Solution

Inspect the check ring and replace if needed.

Cause

Uneven metering / inconsistent injection stroke.

Solution

Set an appropriate back pressure, and ensure a sufficient screw cushion (melt cushion) remains during molding.

Cause

Malfunctioning temperature controls (cylinder heaters/sensors and mold temperature controller).

Solution

Inspect the injection molding machine. Uneven metering is closely tied to part weight and dimensional consistency, so when this issue occurs, check the screw check ring for damage first. Check ring wear is often too subtle to detect visually, so it's best to inspect it whenever metering becomes inconsistent, or replace it on a regular schedule. To check for check ring wear: as the screw rotates and advances during injection, weight should increase proportionally with any increase in shot size (stroke) — if the part weight doesn't increase to match the increased metered volume, the check ring may be worn.

Cause

Inconsistent injection stroke.

Solution

If using regrind/scrap material, keep pellet size consistent, and dry the resin thoroughly.