- DFM review before tooling prevents 80% of first-shot failures
- Material drying is mandatory for hygroscopic plastics like PA6 and PC
- Cooling accounts for 60–80% of total cycle time
- Clamp force must exceed projected injection pressure by 20–30%
- Quality inspection at every stage catches defects early
🔗 Related Guides: See our complete injection molding guide[1] и injection mold overview for broader context.
What Are the Core Steps of the Injection Molding Process?
The injection molding process consists of eight sequential steps, each controlling a specific aspect of part quality. Skipping or rushing any step leads to defects that compound downstream, increasing scrap rates and delivery delays. From our experience running 45 machines across hundreds of materials at our Shanghai facility, the most reliable production runs follow this sequence precisely every time.
The eight steps are: (1) DFM review and mold design validation, (2) material drying and preparation, (3) clamping and mold closing, (4) plastic melting and injection, (5) packing and holding pressure, (6) cooling and solidification, (7) ejection, and (8) quality inspection. Each step has specific parameters that directly affect dimensional accuracy, surface finish, and structural integrity of the final part.
How Does DFM Review Prevent Injection Molding Defects?
Design for Manufacturability (DFM) review is the single most cost-effective step in the entire process. Fixing a wall thickness issue during DFM costs nothing. Fixing it after mold steel is cut costs $5,000–$50,000 per modification. We have run DFM checks on thousands of projects since 2005, and roughly 40% of first-shot failures trace back to geometry problems that should have been flagged during this review.
Our 8 senior mold engineers review every new part for wall thickness uniformity, gate placement, and cooling channel efficiency before tooling approval. We build 100+ molds per month and process 400+ materials, so we see design-tradeoff patterns across industries.
The DFM checklist must cover five items: wall thickness ratio under 3:1, draft angles at least 1° for shallow features and 2° for deep ribs, gate location avoiding weld lines in cosmetic areas, cooling channels following cavity contours, and ejection system accounting for undercuts. If any parameter is marked “TBD,” send the review back.
“DFM review eliminates 80% of potential injection molding defects.”Правда
Catching wall thickness variations, insufficient draft angles, and gate location issues before steel cutting prevents sink marks, warpage, and short shots that typically require costly mold modifications.
“All wall thickness variations require mold modification.”Ложь
Small variations within a 2:1 ratio can be compensated with processing adjustments. Major variations exceeding 3:1 or causing chronic defects do require mold redesign.
Why Is Material Drying Critical Before Injection?
Hygroscopic materials like nylon (PA6), polycarbonate (PC), and PEEK absorb moisture from the air. If you inject these materials without drying, the water vaporizes inside the barrel at 200–300°C, causing splay marks, reduced molecular weight, and brittle parts. This is not a suggestion — it is a hard requirement for every hygroscopic polymer.
| Материал | Temp (°C) | Time (hrs) | Max Moisture (%) |
|---|---|---|---|
| PA6 | 80–100 | 4–6 | 0.02 |
| ПК | 120 | 3–4 | 0.02 |
| PEEK | 150–160 | 4–6 | 0.01 |
| ABS | 80–85 | 2–3 | 0.02 |
The critical metric is final moisture content[2], not drying time. Use Karl Fischer titration for lab verification or in-line sensors on hopper dryers for production monitoring. Target below 0.02% for engineering plastics and below 0.01% for high-performance materials like PEEK.
“Non-hygroscopic materials like PP and PE do not require pre-drying.”Правда
Polypropylene and polyethylene do not absorb moisture into their molecular structure, so they can be processed directly from sealed bags without drying in most cases.
“Longer drying time always produces better results.”Ложь
Over-drying degrades some materials. PA6 dried above 110°C for over 8 hours oxidizes and yellows. Always follow manufacturer data sheets for temperature and duration limits.
How Does the Clamping Unit Secure the Mold?
The clamping unit closes the two mold halves and holds them together against injection pressure. If clamp force is insufficient, the mold opens slightly during injection, producing flash — thin fins of plastic along the parting line that require secondary trimming and indicate a process running on the edge of failure. The required clamp force equals the projected area of the part multiplied by injection pressure, plus a 20–30% safety margin.
Two main clamp designs exist: toggle (mechanical) and hydraulic. Toggle clamps are faster — typical clamp-open-close cycles run 1–3 seconds — but offer limited tonnage adjustment. Hydraulic clamps provide precise, repeatable force control and are preferred for thin-wall parts requiring consistent locking pressure across the full mold face.
Our Shanghai factory operates 45 injection molding machines ranging from 90T to 1850T. This tonnage range covers everything from small precision electronics components to large automotive housings weighing up to 10 kg. In 2024, we added three two-shot machines and a 1850T large-tonnage press to handle increasingly complex multi-material parts.
What Happens During Plastic Melting and Injection?
Сайт термопластик[3] granules enter the barrel through the hopper. The screw rotates, conveying material forward through three zones: feed (softening), compression (melting and air removal), and metering (homogenization). The melt accumulates in front of the screw, which then stops rotating and moves forward as a plunger to inject the material into the mold cavity.
Injection speed, pressure, and temperature must work together. Too slow, and the melt freezes before filling the cavity — causing a short shot that produces an incomplete part. Too fast, and shear heating degrades the material or traps air, causing burn marks or gas pockets. Typical injection pressures range from 70–140 MPa depending on material viscosity and part geometry.
“Injection speed affects both surface finish and dimensional accuracy.”Правда
Higher injection speeds improve surface finish by preventing premature freezing but can cause jetting or flash if not controlled. Profiled injection speed (slow-fast-slow) is standard practice for complex geometries.
“Higher barrel temperature always produces better melt quality.”Ложь
Excessive barrel temperature causes thermal degradation, gas generation, and reduced molecular weight. Each material has an optimal processing window — exceeding it by even 20°C can degrade sensitive polymers like POM or PVC.
Why Are Packing and Holding Pressure Important?
After the cavity fills, the screw does not simply retract. It maintains pressure — called packing pressure — to push additional material into the cavity as the plastic cools and shrinks. Packing compensates for volumetric shrinkage, which can reach 2–3% for semi-crystalline materials like PA6 and POM. Without adequate packing, the finished part will show sink marks and voids.
Packing pressure typically runs at 50–80% of injection pressure. Holding pressure then maintains that force until the gate freezes, which takes 2–6 seconds for most parts. Once the gate solidifies, no more material can enter the cavity, so continuing to hold pressure beyond gate freeze time wastes energy without improving part quality. Determining the exact gate freeze point is critical — too short and you get sink marks, too long and you waste cycle time.
With 45 machines running daily, we use in-mold pressure sensors to determine the exact gate freeze time for each part. This data-driven approach eliminates guesswork and reduces sink mark defects by over 90% compared to time-based holding methods.
How Does Cooling Affect Part Quality?
Cooling is the longest phase in the cycle, accounting for 60–80% of total cycle time. Proper cooling design is therefore the highest-leverage factor for both part quality and production efficiency. Uneven cooling causes differential shrinkage, which leads to warpage, internal stresses, and dimensional instability.
Conventional drilling limits cooling channels to straight paths that cannot follow complex cavity shapes. Conformal cooling channels, made possible by metal additive manufacturing, follow the cavity contour and reduce cycle time by 20–35% while improving dimensional uniformity across the entire part. The temperature differential across the cavity surface should stay under 5°C for tight-tolerance parts.
Cooling time depends on wall thickness — thicker walls need exponentially longer to solidify. A part with 3mm walls typically needs 15–25 seconds, while a 1mm wall may solidify in 5–8 seconds. This is why maintaining uniform толщина стенки[4] during DFM review directly affects production economics.
What Makes Ejection and Quality Inspection Reliable?
Once the part has cooled enough to maintain its shape, the mold opens and the ejection system pushes the part out of the cavity. Common ejection methods include ejector pins, stripper plates, and air blast. Sleeve ejectors are used for thin-walled cylindrical features. Ejection force depends on shrinkage around cores, draft angle adequacy, and part geometry. Standard practice requires 1.5–2 times the calculated ejection force to account for friction variation and mold wear over time.
Quality inspection follows each shot. At minimum, the process includes visual inspection for surface defects such as flash, sink marks, and discoloration, dimensional measurement against tolerances using calipers and CMM, and functional testing where applicable. Our six-step quality control process — from Incoming Quality Control through Out-going Quality Control — ensures full traceability from raw material to shipped part.
“Statistical process control (SPC) reduces defect rates by identifying trends before they produce out-of-spec parts.”Правда
SPC monitors key process parameters like injection pressure, cycle time, and part weight in real time, allowing operators to adjust before defects occur rather than inspecting after the fact.
“A part that looks good visually is always dimensionally correct.”Ложь
Internal stresses from uneven cooling, post-mold shrinkage, and creep can cause dimensional drift hours or days after molding. Critical dimensions require measurement with calibrated instruments, not visual inspection alone.
ЧАСТО ЗАДАВАЕМЫЕ ВОПРОСЫ
What are the 7 steps of injection molding?
The seven steps are: (1) DFM review, (2) material drying, (3) clamping, (4) melting and injection, (5) packing and holding, (6) cooling, and (7) ejection with quality inspection.
Why is DFM review important before injection molding?
DFM review catches geometry issues like uneven wall thickness and insufficient draft angles before tooling begins. Fixing these during design costs nothing; fixing after steel cutting costs $5,000–$50,000 per change.
How long should plastic pellets be dried?
Drying time varies by material: PA6 needs 4–6 hours at 80–100°C, PC needs 3–4 hours at 120°C, and PEEK requires 4–6 hours at 150–160°C. Always verify final moisture content is below 0.02%.
What is the difference between packing and holding pressure?
Packing pressure (50–80% of injection pressure) pushes additional material into the cavity to compensate for shrinkage. Holding pressure maintains that force until the gate freezes, typically 2–6 seconds.
How much clamp force is needed for injection molding?
Calculate projected area × injection pressure, then add 20–30% safety margin. For a 100 cm² part at 140 MPa injection pressure, you need approximately 200 tons of clamp force.
What percentage of cycle time is cooling?
Cooling typically accounts for 60–80% of total cycle time. Optimizing cooling channel design is the most effective way to reduce cycle time and improve part quality simultaneously.
Заключение
Mastering the injection molding process step by step means controlling each variable — from DFM review through final inspection — with data, not guesswork. Whether you are producing precision medical components or large automotive panels, the same fundamental principles apply: validate the design, prepare the material correctly, optimize each processing parameter, and inspect rigorously. Our 20+ years of experience at ZetarMold’s Shanghai facility have shown that systematic process control consistently outperforms trial-and-error approaches by a wide margin. The difference between a profitable production run and weeks of costly rework comes down to following these eight steps with discipline and precision.
Need help with your injection molding project? Get a free quote from our engineering team — we respond within 24 hours.
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A comprehensive manufacturing process in which molten material is injected into a mold cavity to produce precision parts in high volumes. ↩
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Karl Fischer titration is an analytical method for determining water content in materials, widely used in plastics processing to verify drying adequacy. ↩
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Thermoplastic polymers soften when heated and harden when cooled, allowing repeated processing cycles without chemical change. ↩
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Cooling time scales with the square of wall thickness, meaning a doubling of wall thickness roughly quadruples the required cooling time. ↩
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