Injection Molding Process Step by Step: Complete Guide

Step 1: What Is DFM Review and Why Does It Matter?

Your part geometry is frozen. The tooling quote is on your desk. Before steel cutting starts, there is one decision that determines first-shot success: Design for Manufacturability (DFM) review. We have run DFM checks on over 5,000 projects since 2005, and roughly 40% of first-shot failures trace back to wall thickness over 4mm with inadequate cooling. Fixing these after tooling costs ten times more. For more on matrijsontwerp fundamentals, see our mold guide.

The DFM checklist your engineer should present includes five non-negotiable items. For a broader overview of the entire spuitgieten workflow, see our complete guide. Wall thickness uniformity (target ±10% variation), draft angle adequacy (1-3° minimum), gate type and location rationale, material-specific shrinkage compensation, and cooling channel layout.

Your DFM sign-off should include specific measurements: nominal wall thickness with tolerances (±0.1mm for features under 3mm), expected shrinkage rates by material (0.5% for amorphous, 1.5-2.5% for semi-crystalline), gate size and location rationale, and cooling channel layout verification. If any of these are missing from the DFM report, request them before approving the mold build.

If you are comparing vendors or planning procurement, our injection molding supplier sourcing guide covers RFQ prep, qualification, and commercial risk checks.

Step 2: How Do You Dry and Prepare Materials for Injection Molding?

Droog resin pellets in een ontvochtigende hopper droger op materiaalspecifieke temperaturen (80–160 °C) voor 2–6 uur totdat vocht onder 0.02% valt, voer ze dan direct in de machinehopper via een gesloten droge lucht transferlijn. Gesloten zakken staan in warehouses, en hygroscopische resins absorberen vocht snel na opening—PA6 op 50% relatieve humidity bereikt 0.3% vocht in uren, veel boven de 0.02% threshold. Droging is niet optioneel voor engineering plastics; het is de eerste kwaliteitsgate.

Drying specifications depend on material type. PA6 requires 80-100°C for 4-6 hours. PC needs 120°C for 3-4 hours. PEEK demands 150-160°C for 4-6 hours. Monitor dew point of the drying air—below -30°C indicates properly functioning equipment. Above -10°C means your dryer needs service.

Non-hygroscopic materials like polypropylene and PE do not require aggressive drying, but surface moisture from condensation should still be removed with a brief 1-2 hour drying cycle at 60-80°C. Skip drying entirely only if the material has been stored in a climate-controlled environment.

Vochtgehalteverificatie voorkomt oppervlaktedefecten en structurele zwakte in gevormde onderdelen. Gebruik een halogenen-vochtmeter of Karl Fischer-titratie om te bevestigen dat het vochtgehalte van het hars onder de materiaalspecifieke drempel blijft voordat het in de trechter wordt geladen. Veelvoorkomende doelwaarden zijn PA6 en PA66 onder 0,2% vocht, polycarbonaat en PET op 0,02% of lager, en PBT onder 0,05%. In onze fabriek in Shanghai controleren we het vochtgehalte van elke productiebatch voordat de verwerking begint. Hergebruikt materiaal – zelfs wanneer het goed is opgeslagen in afgesloten containers – neemt omgevingsvocht sneller op dan nieuw hars, waardoor verificatie vooral cruciaal is bij het verwerken van hergebruiksmengsels. Het overslaan van deze controle leidt tot sproeimarkeringen, verminderde slagvastheid en dimensionale instabiliteit die geen na-bewerkingsparameteraanpassing kan omkeren.

Step 3: How Does Clamping and Mold Closing Work?

Het sluiten van de matrijs maakt gebruik van hydraulische of mechanische kracht, uitgedrukt in tonnen, om de scheidingslijn van de matrijs af te sluiten tegen injectiedrukken van 18.000–50.000 psi. De praktische regel is eenvoudig: bereken het geprojecteerde matrijsgebied, vermenigvuldig dit met de piek injectiedruk¹, voeg vervolgens een veiligheidsmarge van 10–20% toe. Correct sluiten voorkomt uitstulpingen, beschermt de geometrie van de scheidingslijn en zorgt voor herhaalbare afmetingen gedurende de productie.

Machine tonnage ratings defineren het maximum clamp force² Beschikbaar. Een matrijs draaien op 60-80% van de rated tonnage geeft optimale energie-efficiëntie terwijl een adequate veiligheidsmarge wordt gehouden voor drukpieken tijdens injectie en packing fasen.

Mold closing speed follows a two-stage profile. Approach stage: rapid closing from 200-300 mm/s until the mold faces are within 5-10mm of contact. Positioning stage: slow closing from 5-10 mm/s for final closure to protect mold surfaces and alignment features. Some modern machines add a third low-pressure protection stage at 1-2 mm/s with 5-10 bar clamping pressure before full clamp engagement. This prevents damage if foreign material or a stuck part remains in the mold cavity.

Montagetest

Step 4: How Does Plastic Melting and Injection Work?

Injectie smelt resin pellets in een verwarmde barrel via een roterende screw, forceert dan de homogenized melt in de matrijs cavity op 50–200 mm/s. Granules enter de hopper, bewegen door de verwarmde barrel, en worden gesheared door de roterende screw. De feed, compression, en metering zones transport, melt, homogenize, en meter het materiaal zodat viscosity stabiel blijft tijdens filling.

Screw rotation speed affects melt quality and throughput. Too slow: insufficient shear heating creates unmelted pellets. Too fast: excessive shear degrades the polymer and causes discoloration. Most engineering resins perform best at 50-120 RPM, with the speed adjusted based on screw diameter and material viscosity.

Injection begins when the screw stops rotating and moves forward as a plunger, forcing the accumulated melt through the nozzle, sprue, runner system, and into the cavity. Injection speed controls surface finish and weld line strength. Fast fill reduces temperature loss but can trap air. Slow fill improves venting but may cause premature freeze-off.

Step 5: What Is Packing and Holding Pressure?

The mold is 95-98% full. The cavity is mostly filled but not packed. Packing pressure compensates for volumetric shrinkage as the plastic cools from melt temperature to ejection temperature—typically 10-15% volumetric shrinkage for semi-crystalline materials. Without adequate packing, parts show sink marks, voids, and dimensional variation that pushes them out of tolerance.

The critical decision point is gate freeze-off time. The gate must solidify before holding pressure is released, or material flows back out of the cavity. Typical gate freeze times range from 1-3 seconds for edge gates to 0.3-0.8 seconds for sub-gates. Monitor cavity pressure curves—a sharp pressure drop after packing indicates premature gate unfreeze.

Packing pressure profile can be staged rather than constant. Stage 1: High pressure (80-100% of injection) for 20-30% of packing time to drive material into thick sections and corners. Stage 2: Reduced pressure (50-70% of injection) for the remaining time to maintain density without over-packing. This profile reduces sink marks while minimizing flash risk. The transition point is determined by observing the part weight curve and visual inspection of thick sections for sink marks under different pressure levels.

Step 6: How Does Cooling and Solidification Work?

The gate is frozen. The material is packed. The part is dimensionally stable enough to survive ejection but needs to solidify fully before the mold opens. Cooling time dominates cycle time at 60-80% of the total. A 10-second reduction in cooling time on a 25-second cycle is a 40% productivity gain. This is where engineering pays for itself.

Conventionele koeling gebruikt recht geboorde kanalen met 8–12 mm diameter, spacing 3–5 keer de diameter, en afstand van onderdeeloppervlak tot kanaal centerline van 2–3 diameters. Dit werkt voor uniforme wanddikte onderdelen met simpele geometrie. Wanneer je bosses, ribs, of variërende wanddikte hebt, wordt uniforme koeling moeilijk—dikke secties koelen langzamer, veroorzaken differentiële shrinkage, warpage, en restspanning.

Coolant temperature should be 10-20°C below the material’s heat deflection temperature. For PC, set mold temperature at 80-100°C. For PP, 20-40°C works. Higher mold temperatures improve surface finish and crystallinity but extend cycle time. The tradeoff is always cosmetic quality versus throughput.

Koelingoptimalisatie kan cyclustijd verminderen met 20-35% op bestaande matrijzen zonder hardware wijzigingen. Procesaanpassingen: verminder packing tijd tot het minimum dat onderdeelgewicht houdt, verhoog coolant flow rate binnen pomp limieten, en verlaag matrijs temperatuur tot het minimum dat vervorming voorkomt. Matrijs modificaties: add baffles om diepe cores te koelen, reposition kanalen dichter bij dikke secties, en install conformal cooling voor complexe geometrieën. ROI wordt typisch gerealiseerd binnen 1000-5000 onderdelen.

Step 7: How Does Mold Opening and Part Ejection Work?

The part is solidified. The cooling time has elapsed. The mold opens. This seems straightforward, but ejection is where 20-30% of injection molding defects occur. Ejection requires overcoming two forces: adhesion of the cooled plastic to the mold steel and mechanical interlocking due to undercuts or insufficient draft. The ejection system must apply enough force to overcome these factors without distorting the part, creating ejector pin marks, or causing part stick-back on the core side.

Ejection system selection depends on part geometry. Straight ejection uses ejector pins for simple geometries. Sleeve ejection handles bosses and cylindrical features. Stripper plate ejection works best for thin-wall cups and caps. For undercuts, you need lifters or angled pins. Choosing the wrong system causes part deformation, sticking, or tooling damage that compounds over thousands of cycles.

Ejector pin placement follows specific guidelines. Place pins in thick sections and rib intersections where ejection resistance is highest. Space pins evenly along the part perimeter to distribute force. Pin diameter should be at least 1.5x the pin length to prevent bending. For polished or textured surfaces, avoid placing pins on visible cosmetic areas.

Mold opening speed affects ejection quality. The opening profile: slow initial opening (5-10 mm/s) for first 10-20mm to allow part separation from core without stress. Rapid opening (100-200 mm/s) for the majority of the stroke to minimize cycle time. Deceleration (20-50 mm/s) for final 50-100mm to avoid slamming the mold open and reducing wear on guide pins and bushings. The deceleration is particularly important for molds with stripper plates or complex lifters that need controlled opening sequences.

Step 8: How Do You Inspect Quality and Monitor the Process?

The part is ejected. It lands in the chute or is robotically removed. Now what? If you assume the process is set and let it run, you will discover defects hours or days later when your customer rejects the shipment. Quality inspection must happen at every shift start, after every material change, and at defined intervals during production. The inspection hierarchy: first article inspection (FAI) on startup, in-process inspection every 50-100 parts, final inspection on each shipment lot.

Visual inspection catches 60-70% of defects. Burn marks, flash, short shots, sink marks, and surface blemishes are immediately visible. Train operators to inspect critical cosmetic zones first, then structural features. Use backlit inspection stations for transparent parts and polarized light for birefringence detection in optical components.

Dimensional inspection verifies parts meet print requirements. Critical dimensions use CMM (coordinate measuring machine) measurement with ±0.01mm accuracy. Standard dimensions get caliper or go/no-go gauge checks. Sample 5 parts per 100-shot cycle for statistical process control, tracking Cp and Cpk values.

Moderne machines tracken injectiedruk, pack druk, melt temperatuur, matrijs temperatuur, cyclustijd, en screw recovery tijd real-time. Alarmen op ±10–20% van targetwaarden triggeren automatische pauses, segregeren aangetaste onderdelen, en alert operators—vangen proces drift op voordat out-of-spec onderdelen accumuleren.

What Are Common Injection Molding Issues and How Do You Troubleshoot Them?

Veelvoorkomende spuitgietdefecten – zinkplekken, uitstulpingen en onvolledige onderdelen – komen zelfs in goed afgestelde processen voor. Hier zijn de oorzaken en oplossingen.

Sink marks occur when thick sections cool slower than adjacent thin sections, creating surface depressions. The root cause is differential shrinkage. Troubleshooting path: first check wall thickness ratio—if it exceeds 3:1, redesign is required. If wall thickness is acceptable, increase packing pressure in 10% increments while monitoring for flash. Add baffles or bubblers to cool thick sections faster. Reduce melt temperature 5-10°C to minimize initial shrinkage. In severe cases, add external core-outs or gas-assisted molding to eliminate thick sections entirely.

Flash appears at the parting line, around ejector pins, or in vent gaps when material escapes the cavity under excessive injection or packing pressure. Contributing factors include worn mold surfaces, insufficient clamp force, and high melt temperatures that reduce viscosity. Fix flash by increasing clamp force first, then reducing packing pressure, and finally checking mold surface alignment if the problem persists across multiple cavities.

Short shots occur when the cavity is not completely filled, leaving incomplete parts. Common causes include insufficient injection pressure, blocked vents preventing air escape, low melt temperature increasing viscosity, or inadequate shot size. Diagnose by checking injection pressure curves first—most short shots resolve by raising injection speed or pressure by 10-15%. If venting is the issue, clean or deepen vent channels to 0.01-0.02mm depth.

When Should You Adjust vs. Redesign Your Injection Molding Process?

Ga over naar redesign wanneer drie of meer ±20% parameterwijzigingen falen, of wanneer de oorzaken wandratio's boven 3:1 of inadequate ontwerphellingen bevatten. De vuistregel: als je drie parameters hebt aangepast met ±20% en het defect blijft, is het probleem waarschijnlijk design-gerelateerd. Verder aanpassen na dit punt verspilt materiaal en cyclustijd zonder het probleem op te lossen.

Design issues that resist process adjustment fall into five categories: wall thickness non-uniformity (causes sink and warp), inadequate draft angles (causes sticking), incorrect gate type or location (causes flow lines and weld lines), insufficient coring (wastes material and cycle time), and sharp corners without fillets (creates stress concentrators). Each of these requires a mold modification, not a parameter tweak.

Redesign kost 5.000–15.000 USD voor engineering, modificatie en re-validatie, maar het produceren van defecte onderdelen met 5–15% afvalpercentages op een run van 100.000 onderdelen kost veel meer.

How Do You Optimize Injection Molding for Production Efficiency?

Target koeling eerst—het domineert 60–80% van elke cyclus—via conformal kanalen en turbulente flow, minimaliseer dan packing en ejectie tijden. Cyclustijd is de som van injectietijd (5–10%), packing en holding tijd (10–20%), koeltijd (60–80%), matrijs opening en sluiting tijd (5–10%), en ejectietijd (2–5%). Koeling is de dominante factor, dus optimalisatie moet daar eerst focussen, dan door de andere componenten gaan.

Cooling optimization targets three areas: channel placement, coolant parameters, and mold material selection. Conformal cooling channels follow the part contour, reducing distance to the cavity surface from 15-25mm (drilled) to 3-8mm (conformal). Coolant flow rate must maintain turbulent flow (Reynolds number above 5,000) for effective heat transfer. Mold materials with higher thermal conductivity like beryllium copper inserts in hot spots can cut local cooling time by 30-40%.

Injection optimization focuses on fill time and melt quality. Fill time optimization: reduce injection time until you see burn marks (too fast) or short shots (too slow), then back off 10%. Velocity-to-pressure switchover point should trigger at 95-98% fill to avoid overshooting. Melt temperature profiling across barrel zones prevents degradation while ensuring complete melting.

ROI hangt af van de onderdeelwaarde en volume. Terugverdientijd onder 1.000 onderdelen moet direct worden geïmplementeerd; 1.000–5.000 onderdelen vereist evaluatie; meer dan 5.000 onderdelen vereist strategische rechtvaardiging.

Veelgestelde vragen over het spuitgietproces

Veelgestelde vragen

What are the 7 steps of injection molding?

The seven steps of injection molding are: (1) clamping and mold closing, where the machine secures the two mold halves together under high pressure; (2) plastic melting and injection, where heated pellets become molten and are forced into the cavity; (3) packing and holding, where additional material compensates for shrinkage; (4) cooling and solidification, where the part hardens inside the mold; (5) mold opening and part ejection, where the finished part is removed; (6) quality inspection, which covers visual, dimensional, and functional checks; and (7) process monitoring and adjustment, ensuring consistent output throughout production runs.

How long does an injection molding cycle take?

Cycle time ranges from as short as 5 seconds for small thin-wall parts to over 120 seconds for large, thick-wall components. For a typical engineering plastic part with 3mm wall thickness, expect 15-25 seconds per cycle. Cooling dominates the timeline, accounting for 60-80% of total cycle time, while injection fills the cavity in just 0.5-2 seconds. Reducing cooling time through conformal channels or optimized coolant flow is the single most effective way to increase throughput, often cutting cycle time by 20-35% on existing molds.

What is the difference between injection and packing?

Injection is the high-pressure fill phase where molten plastic is forced into the mold cavity at speeds designed to fill 95-98% of the volume, typically completing in 0.5-2 seconds. Packing (or holding) follows immediately at lower pressure, pushing additional material into the cavity to compensate for thermal shrinkage as the plastic cools and contracts. Packing continues until the gate freezes off, usually 2-6 seconds. Think of injection as getting the material into the mold, and packing as keeping it dimensionally accurate as it solidifies.

Why do I need to dry plastic before injection molding?

Hygroscopic materials such as PA6, PC, PET, and PEEK absorb moisture from ambient air over time. During injection molding, this trapped moisture vaporizes instantly at melt temperatures (often above 250°C), causing visible bubbles (splay marks), surface streaks, reduced mechanical strength, and dimensional instability in the finished part. Proper drying at material-specific temperatures (80-160°C) for 3-6 hours reduces moisture content below the critical 0.02% threshold required for defect-free molding. Skipping the drying step remains one of the most common and costly causes of rejected parts in production.

What temperature is used for injection molding?

Injection molding temperatures vary significantly by material type. Polypropylene processes at 180-220°C, ABS at 210-250°C, polycarbonate at 280-320°C, and high-performance PEEK requires 380-420°C. The barrel maintains a temperature gradient from the feed zone (coolest) through compression to the metering zone (hottest), typically with a 20-40°C rise. Mold temperature also plays a critical role: colder molds speed up cycle time but can increase residual stress, while heated molds (60-150°C depending on resin) improve surface finish, crystallinity, and dimensional stability for engineering-grade materials.

How much pressure is needed for injection molding?

Injection pressure typically ranges from 18,000 to 25,000 psi for standard engineering thermoplastics. High-viscosity or glass-filled materials like PEEK or PA66-GF30 can require up to 35,000-50,000 psi. Packing pressure runs at 50-80% of injection pressure. To determine required clamp force, multiply the projected part area (in square inches) by injection pressure, then add a 20-30% safety margin. For example, a 10 square inch part at 18,000 psi needs roughly 90 tons of clamp force, so a 110-115 ton machine provides adequate headroom.

What causes sink marks in injection molding?

Sink marks form when thick wall sections cool more slowly than adjacent thin sections, creating differential shrinkage that physically pulls the surface material inward. The primary causes include wall thickness ratios exceeding 3:1, insufficient packing pressure or hold time, and inadequate cooling channel placement near heavy cross-sections. Practical fixes include coring out thick sections during the DFM stage, increasing packing pressure and extending hold time until gate freeze, and redesigning cooling channels to target thick areas. Process adjustments can resolve mild cases, but severe recurring sinks usually require a mold modification.

How do you calculate clamp force for injection molding?

Beheers het spuitgietproces stap voor stap: van DFM-beoordeling via klemmen, inspuiten, naspuiten, koelen en uitwerpen tot kwaliteitsinspectie.

1. injectiedruk: Injectiedruk verwijst naar de hydraulische druk die op de schroef wordt uitgeoefend om gesmolten plastic in de matrijs te persen, doorgaans variërend van 35.000 tot 50.000 psi. ↩

2. clamp force: Klemkracht is de hydraulische of mechanische kracht die de matrijs gesloten houdt tijdens injectie, gemeten in ton, waarbij 90T tot 1850T gebruikelijke ranges zijn. ↩

3. cyclustijd: Cyclustijd is de totale tijd nodig om één spuitgietcyclus te voltooien, gemeten in seconden, van matrijs sluiten tot de start van de volgende cyclus. ↩