Manufacturing basics

Introduction to the Injection Molding Process from Mold Concept to First Article

This guide walks through the injection molding process from part design to a validated first article. It is written for design engineers and sourcing engineers who need to judge whether a part should be molded, how the tool will be built, and which checks catch problems before mass production.

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Injection molding process from mold design to first article comparison

On this page

1
What matters before you cut steel
2
How the injection molding process from pellet to part actually runs
3
Design rules that decide whether a part should be molded at all
4
Gates, runners and where the mold cavity gets filled
5
Cooling layout, cycle time and the checks that catch defects
6
Step by step: from CAD file to validated first article
7
Which process fits which part

Key takeaways

What matters before you cut steel

Wall thickness drives everythingUniform walls between 1.5 mm and 3.5 mm fill and cool evenly. Thick sections sink and warp.

Gate placement decides weld linesA gate at the thickest section fills the cavity. A gate at the thin end short-shots.

Cooling is most of the cycleRoughly 60–70% of cycle time is cooling. Channel layout matters more than injection speed.

Draft and shrink are not optional1–2° draft per side and the resin's shrink rate must be in the 3D model before tooling.

First article inspection closes the loopMeasure the molded part against the drawing, not against the CAD file.

Section 1

How the injection molding process from pellet to part actually runs

The injection molding process from resin pellet to finished part has four stages that repeat every cycle. The screw retracts and plasticizes pellets into a melt. The screw pushes forward and fills the cavity. Packing pressure holds the part while it cools and shrinks. The mold opens, ejector pins push the part out, and the cycle restarts.

Each stage has its own control variable. Melt temperature is set by barrel heaters. Fill speed is set by the injection profile. Packing pressure and time are set separately from fill. Cooling time is set by the mold, not the machine, which is why tool design has more influence on cycle time than any machine setting.

For a first-time reader, the useful mental model is this: the machine controls how fast plastic enters the mold, but the mold controls how fast heat leaves it. A part that fills easily but cools slowly will still be expensive to run.

Cycle time on a typical small part runs 20–40 seconds. Of that, cooling often accounts for 12–25 seconds. If your part has a 6 mm thick boss next to a 2 mm wall, the boss sets the cooling time for the whole part. Thin the boss to 60% of the adjacent wall and the cycle drops.

1
FillInjection profile pushes melt from the gate to the last-filled point.
2
PackExtra pressure compensates for volumetric shrink during cooling.
3
Cool60–70% of cycle time; set by wall thickness and channel layout.
4
EjectDraft angle and ejector pin placement decide whether the part releases cleanly.

Section 2

Design rules that decide whether a part should be molded at all

Not every part belongs in a mold. The injection molding process from design intent to tooling only pays off when the geometry repeats. If you need 50 parts, CNC machining or vacuum casting is usually cheaper and faster. If you need 5,000 parts a year for three years, the tool cost amortizes quickly.

The first screening rule is wall thickness. Keep nominal walls between 1.5 mm and 3.5 mm for most engineering resins. Below 1.2 mm, flow length becomes short and you need higher injection pressure. Above 4 mm, sink marks and long cooling times appear. ABS, PC and PA all follow roughly the same window, though PC needs the thicker end of it.

The second rule is uniformity. A part with a 2 mm wall and a 5 mm rib will show sink on the opposite surface. Ribs should be 50–60% of the nominal wall, with a draft of at least 0.5° per side and a radius at the base. Bosses should be 60% of the wall with a supporting rib.

The third rule is draft. Any surface parallel to the mold opening direction needs 1–2° of draft per side, or 0.5° minimum on textured surfaces. Zero-draft walls drag, scuff and sometimes crack on ejection. This is the single most common DFM comment we send back.

1
Good candidateUniform 1.5–3.5 mm walls, annual volume above 2,000 parts, stable geometry.
2
Poor candidateWalls over 5 mm, deep zero-draft pockets, annual volume under 500 parts.
3
Rib thickness50–60% of nominal wall, 0.5° draft minimum, radius at the base.
4
Boss thickness60% of nominal wall, tied to a rib or wall to avoid sink.

Section 3

Gates, runners and where the mold cavity gets filled

Gate location decides fill pattern, weld line position and shrinkage direction. Put the gate at the thickest section of the part so the melt front moves from thick to thin. A gate at the thin end starves the thick section and creates a short shot or a void.

For a single-cavity tool, a direct or edge gate is simple and cheap to cut. For multi-cavity tools, a balanced runner system keeps fill pressure equal across cavities. An unbalanced runner fills one cavity first and over-packs it, which shows up as dimensional scatter between cavities.

Weld lines form where two flow fronts meet. They are mechanically weaker than the surrounding material, often 10–20% lower in tensile strength. If a weld line lands on a structural rib or a snap-fit, move the gate. A round part with a center gate has a circular weld line that is usually acceptable. A rectangular part with two gates has a weld line down the middle.

Hot runner systems remove the runner scrap and reduce cycle time, but they add cost and maintenance. Use a hot runner when the resin is expensive, when runner regrind is not allowed, or when the runner is more than 30% of the shot weight.

1
Gate at thick sectionMelt flows thick to thin, avoiding voids and short shots.
2
Weld line riskKeep weld lines away from snap-fits, ribs and sealing surfaces.
3
Balanced runnersEqual flow length to every cavity keeps dimensions consistent.

Section 4

Cooling layout, cycle time and the checks that catch defects

Cooling channels should follow the part contour at a constant distance from the surface, typically 1.5 to 2.5 times the channel diameter. A 10 mm channel sits 15–25 mm from the cavity wall. Channel spacing of 2.5 to 3 times the diameter gives even heat removal.

Uneven cooling causes differential shrinkage, and differential shrinkage causes warp. If one side of the part cools faster than the other, the part curls toward the hot side. Add more channels on the thick side, or use a baffle or bubbler in deep cores.

Mold temperature also matters. ABS runs at 50–80 °C mold temperature. PC runs hotter, 80–110 °C. PA66 with glass fiber runs 80–100 °C. Running a PC part in a cold mold produces high internal stress and stress-crack failures in service.

First article inspection is where the design intent meets reality. Measure wall thickness, critical dimensions, draft-related features and flatness. Check for sink, short shots, flash, weld line position and warpage. Compare the molded part to the drawing, not to the CAD file, because the drawing carries the tolerances that matter.

1
Channel distance1.5–2.5 times channel diameter from the cavity surface.
2
Channel spacing2.5–3 times diameter for even heat removal.
3
Mold temperatureABS 50–80 °C, PC 80–110 °C, PA66-GF 80–100 °C.

How to run it

Step by step: from CAD file to validated first article

Follow this order. Skipping step 2 or step 5 is the most common cause of tool rework.

1
1. Screen the part for moldabilityCheck nominal wall (1.5–3.5 mm), wall uniformity, draft (1–2° per side) and annual volume. If volume is under 500 parts, stop and price CNC machining instead.
2
2. Run a DFM review before quotingSend the 3D file and drawing. We return a DFM report within 12 hours covering wall thickness, draft, gate location, ejector placement and any features that will not fill. Fix the model now, not after the tool is cut.
3
3. Confirm material and shrink ratePick the resin and its shrink rate: ABS 0.4–0.7%, PC 0.5–0.7%, PA66-GF30 0.3–0.5%. Scale the cavity to the shrink rate. For tight tolerances, cut steel on the low side and adjust after first article.
4
4. Cut the tool and set the process windowTool steel is machined on our 5-axis and mill-turn centers to ±0.005 mm where the fit demands it. Set melt temperature, fill speed, packing pressure and cooling time, then record the window, not just one set of numbers.
5
5. Mold the first article and inspect itRun 20–30 shots to stabilize, then measure. Check critical dimensions, wall thickness, flatness, sink and weld line position. Compare against the drawing. Request a dimensional report if the part is going into an assembly.
6
6. Adjust steel only after the data is inIf dimensions drift, adjust packing pressure and cooling first. Weld or re-cut steel only when the process window is already correct. Every steel change costs a cycle of tooling and re-validation.
7
7. Lock the process and document itRecord melt temperature, mold temperature, fill time, packing pressure, packing time and cooling time. Those six numbers are the process. Without them, the next run will not match the first.

Decision table

Which process fits which part

Use this table when you are deciding between injection molding and a lower-volume process.

Part situation	Recommended process	Why
1–50 parts, design still changing	CNC machining or 3D printing	No tool cost, changes are free
50–500 parts, needs molded look	Vacuum casting	Silicone tool, low setup cost
500–2,000 parts per year	Aluminum bridge tool	Cheaper tool, shorter life
2,000+ parts per year, stable design	Steel injection mold	Tool cost amortizes over volume
Wall under 1.2 mm, long flow path	Re-design or high-pressure molding	Thin walls need higher injection pressure
Wall over 5 mm, thick section	Re-design with ribs	Thick sections sink and cool slowly
Annual volume under 500, tight tolerance	5-axis CNC machining	±0.005 mm without tool cost
Two-shot or overmold requirement	Injection molding, two-shot tool	Only molding bonds the two materials

FAQs

Questions engineers ask before tooling

How do I know if my part is too thick for injection molding?

Measure the thickest section. If it is more than 4 mm and surrounded by thinner walls, it will sink on the opposite surface and set the cooling time for the whole part.

Core out the thick section or replace it with ribs at 50–60% of the nominal wall. If the thick section is structural and cannot be cored, molding may still work, but expect a longer cycle and a higher piece price.

What draft angle do I need on textured surfaces?

Smooth walls need 1–2° per side. Textured walls need more because the texture adds friction during ejection.

A light matte texture needs about 1.5° per side. A heavy grain or leather texture needs 3° or more. If the draft is under 0.5°, expect scuffing and possible cracking at the corners.

How does shrink rate affect my dimensions?

The cavity is cut larger than the part by the resin's shrink rate. ABS shrinks 0.4–0.7%, PC 0.5–0.7%, and PA66-GF30 0.3–0.5%.

Shrink is not uniform in all directions, especially with glass-filled resins. Flow direction shrinks less than cross-flow direction. For tight tolerances, cut steel on the low side and adjust after first article measurement.

When should I use a hot runner instead of a cold runner?

Use a hot runner when the resin is expensive, when regrind is not allowed, or when the runner weight is more than 30% of the shot weight.

Cold runners are simpler and cheaper to maintain. For a low-volume tool or a single-cavity part with a short runner, a cold runner is usually the better choice.

What causes weld lines, and when do they matter?

Weld lines form where two melt fronts meet. They are mechanically weaker than the surrounding material, often 10–20% lower in tensile strength.

They matter when they land on a structural rib, a snap-fit, or a sealing surface. Move the gate or add a flow leader to relocate the weld line to a non-critical area.

Can the same tool run two different resins?

Sometimes, if the shrink rates are close and the mold temperature windows overlap. ABS and PC are not interchangeable in the same cavity without dimension changes.

Changing resin usually means re-validating dimensions and possibly adjusting gate size. Tell us the resin before the tool is cut so the gate and cooling can be sized for it.

Send us the 3D file and get a DFM report in 12 hours

We review wall thickness, draft, gate location and cooling before any steel is cut. Uploads are secure and confidential, and an NDA is available on request.

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