Manufacturing · Stage 03

Injection Molding: Preforms, Closures and Dispensers

Injection molding sets the tolerances that the rest of the bottle inherits. 10+ injection-molding machines produce every PET preform we blow, and every cap, pump, sprayer, dropper, trigger and foamer we fit.

Quick answer

Injection molding is where the neck is decided, and the neck is where a pack leaks

Resin is dried, metered into a heated barrel, injected under pressure into a closed steel mold, held while it cools, and ejected as a finished part.

Two very different families of part come off those machines. The first is the PET preform — a thick-walled test tube that is later reheated and blown into a bottle. The second is the closure or dispenser — caps, lotion pumps, mist sprayers, droppers, trigger sprayers and foamers. Both are injection molded because both need dimensions that blow molding cannot hold.

10+

Injection-molding machines

±2%

Weight tolerance

4-point

Wall thickness check per cavity

646

PET molds

427

PE molds

ISO 9001

Quality system

Scope of this page

This page explains injection molding as a production stage. If your question is which neck finish to choose or how to match a pump to a bottle, that is an engineering question — see neck and closure. If it is how tooling is built and owned, see tooling and molds.

Six questions

Injection molding, answered in six questions

The same six questions apply to every manufacturing process on this site, so the answers can be compared stage by stage.

What is the process?

Resin is dried to a controlled moisture level, fed into a heated barrel, plasticised by a screw and injected under pressure into a closed, water-cooled steel mold. The part is held under pressure while it cools, then the mold opens and the part is ejected. The cycle then repeats, which is why injection is described by its cycle and its cavity count rather than by its output alone.

Why does it matter?

A preform is not a rough blank — it is the part that carries the neck finish, the thread and the sealing surface all the way through to the finished bottle, because the neck is not re-formed during blowing. If the preform neck is out of tolerance, the finished bottle will not seal, and no amount of blow-tuning will recover it. The same logic applies to closures: a cap that is a tenth of a millimetre off becomes a leak in a carton, six weeks later, in a warehouse you do not control.

What packaging does it produce?

Preforms for the PET stretch-blow route, in the 5–1,000 ml finished-bottle range. Caps in screw, flip-top and disc-top formats. Dispensers: lotion pumps, mist sprayers, droppers, trigger sprayers and foamers. Heavy-wall parts such as jars and thick-base cosmetic components. Applicator tools and other small technical moldings.

What quality issues should buyers watch?

Neck and thread dimensions; weight outside the ±2% band; short shots, flash and sink marks; haze or brittleness from moisture left in the resin; colour drifting from the approved chip; closure fit and release torque; and leaks that only appear once the closure is on the bottle rather than on a bare part. Each of these is checked at a named stage — the table below says which.

What information does the buyer need to provide?

A drawing or a physical sample of the bottle or closure; the neck finish if it is already fixed (for example 24/410); the material or recycled-content target; a colour reference; the decoration you intend to apply; the fill product and its chemistry; annual volume; and the destination market, because that decides which documentation has to be prepared.

What happens after this process?

The two families split. Preforms go to ISBM, where they are reheated and blown — see stretch blow. Closures and dispensers go straight to closure matching, where they are fitted to a bottle and torque and leak tested. Both paths then meet at decoration, final QC, packing and export.

The cycle

The injection cycle, step by step

This is the sequence a preform or a closure actually follows. The drying step is listed first because it is the one most often skipped by suppliers and the one most expensive to get wrong.

Resin drying and handling

PET and PE resin dried at 170–180 °C for 3–4 h; masterbatch at 100–120 °C for 2 h. Regrind is controlled and never used as a silent substitute for virgin resin.

Dosing and plasticising

Dried resin and masterbatch are dosed at the agreed ratio and plasticised by the screw. Masterbatch colour is matched against the approved standard chip, not against a screen.

Injection

Melt is injected into the closed mold at a pressure profile set for that part. For PET preforms the profile controls wall distribution, which decides how the bottle blows.

Hold and cooling

Packing pressure is held while the part freezes. Cooling layout is designed into the tool, and it sets both the dimensional stability of the part and the cycle time.

Ejection and first-article sign-off

Parts are ejected and, before mass production is released, the first article is signed off against the approved drawing or master sample.

In-process inspection

Weight against the ±2% band, wall thickness at four points per cavity, neck and thread dimensions, and visual defect inspection every 30 minutes while the run continues.

Handover

Preforms are routed to stretch blow. Closures and dispensers are routed to assembly and closure matching. Both carry the batch record with them.

Defects

What a buyer should watch for, and how we control it

These are the injection defects that reach a customer if they are not controlled. Each row names the symptom a buyer can actually see or measure, the usual cause, and the control that catches it.

Symptom	What it usually means	How we control it
Cap, pump or sprayer will not seal; leaking in transit	Neck or thread dimension out of tolerance, or a sealing land that is not concentric. This is the most expensive defect in the category because it appears after filling, not before.	Neck finish and thread fit tested against your actual closure; air-pressure leak test on the assembled unit at 0.6–0.8 MPa, inverted, at OQC
Weight outside the agreed band	Inconsistent shot size, drifting barrel temperature or a worn non-return valve.	Weight is inspected in process against ±2% and spot-checked against the approved specification
Thin wall on one side; a preform that blows unevenly	Uneven cavity cooling, or a melt profile that fills one side faster than the other.	Wall thickness measured at four points per cavity in process, and the preform wall distribution is set at first-article sign-off
Haze, white streaks or brittle parts in PET	Moisture left in the resin. Residual moisture hydrolyses PET during processing and the damage cannot be reversed downstream.	Moisture content checked at IQC before drying, and a drying record kept against the batch
Colour differs from the approved sample	Masterbatch dose drift, or a colour approved from a screen or a paper reference rather than a physical sample.	Colour matched against the approved physical master sample under standard light, checked at IQC and again in process
Short shots, flash or sink marks	Incomplete fill, worn shut-offs, or insufficient packing pressure. Flash on a sealing surface becomes a leak.	First-article sign-off before the run, then visual defect inspection every 30 minutes and AQL 2.5 sampling on the finished lot
Ejection marks or scuffing on a visible surface	Ejector pin layout or part removal geometry, which is a tooling issue rather than a machine setting.	Appearance is agreed against a physical master sample at first-article stage, so a visible area is defined before the tool is cut

A defect found at first article costs minutes. The same defect found at final inspection costs the run, and the same defect found in your warehouse costs a relationship.

Inputs

What we need from you before injection can start

Every item on this list removes a round of sampling or a rework. Send what you have and mark the rest as open — that is a normal way for a project to start.

Injection project inputs

Bottle or closure drawing

A dimensioned drawing, a 3D file, or a physical sample we can measure. A sample is often faster than a drawing when the design is already fixed.

Neck finish

The finish code if it is already decided, for example 24/410 or 28/410. If it is open, we will propose one from the closure you intend to use.

The closure or dispenser

A physical sample of the cap, pump, sprayer or dropper if you have one. Fit, torque and leak behaviour are tested on the assembled unit, so the real closure matters more than its nominal size.

Material and recycled content

PET, HDPE, PE, PETG, PP or a PCR grade, plus any recycled-content percentage you have to meet.

Colour reference

A Pantone reference and, where possible, a physical sample. Production is matched against the physical sample.

Fill product

What goes in the pack, and its chemistry. This decides whether the material you have chosen is the right one before a tool is cut.

Decoration plan

Which process carries the artwork, how many colours, and whether the artwork is final.

Fill temperature

The temperature at your filler. It constrains the material and sometimes the wall thickness.

Volumes

First order, annual volume and how the demand is spread across the year.

Destination market

Because it decides which documentation has to be prepared alongside production.

Injection is normally the first stage where a project stops being a drawing and becomes a real part, so it is also the cheapest moment to change something. See bottle design for the decisions that should be settled before this point.

FAQ

Injection molding FAQ

Can you mold a preform to fit a bottle design we already have?+

Yes, if the design respects the blow ratio the shape implies. Send the drawing or a sample and we will come back with the preform specification, the tooling position and the sampling window. If the geometry cannot be blown reliably, we will say so before a tool is cut rather than after.

Do you make both the bottle and the closure?+

Yes. Preforms and closures are both injection molded here, and the two are matched on the assembled unit rather than checked separately. That is what lets us test fit, torque and leak behaviour as one assembly.

Which neck sizes can you mold?+

Our standard range runs through 18/415, 20/410, 24/410, 28/410 and 28/415, covering 18 mm to 28 mm necks. Custom finishes are possible on a private mold. See neck and closure for what the finish codes mean.

What is your weight tolerance?+

±2%, inspected in process and spot-checked against the approved specification. It is measured on the actual part, not on a resin sample.

How do you stop moisture from ruining a PET part?+

Moisture content is checked at IQC before drying, PET and PE resin dried at 170–180 °C for 3–4 h; masterbatch at 100–120 °C for 2 h, and the drying record is kept against the batch. Hydrolysis cannot be reversed at the blow stage, so it has to be prevented at this one.

Can we supply our own closures for matching?+

Yes, and we recommend it. Send a physical closure sample and we will test fit, torque and leak behaviour on the assembled unit and report the result. A nominal neck size does not guarantee a seal, because thread profiles differ between suppliers.

What happens if a batch fails inspection?+

The batch is segregated and graded, the cause is investigated rather than the symptom corrected, and the corrected batch is re-inspected before it ships. The record is closed with the action that prevents recurrence. See quality & compliance.

Where injection molding sits in the chain

Injection is stage 03 of eleven. The stage before it prepares the resin; the stage after it is blow molding, or closure matching when the part is a cap or a dispenser.

Manufacturing

Manufacturing hub →Injection molding →ISBM — stretch blow →EBM — extrusion blow →Tooling and molds →Decoration line →

Materials and engineering

Materials hub →Material comparison →Packaging engineering →Bottle design →Neck and closure →Formula compatibility →

Quality, process and reading

Quality & compliance →Quality & compliance — certifications →One-stop capability →How to choose between PET, HDPE and PP bottles →PET vs HDPE vs PP vs PETG: material guide →Virgin resin vs recycled resin: what to verify →

Send us the neck, the closure and the fill.

We will tell you whether the geometry can be injection molded as drawn, and what the tooling and sampling position looks like.

Talk to Engineering Request a Quote

Part of Plastic Packaging Manufacturing: From Resin Pellet to Export | Shijin

Keep reading

Material, engineering and buying pages that this one sits next to.

Bottle Decoration: Printing, Labelling and Coating | Shijin EBM: Extrusion Blow Molding for HDPE, PE and PP Bottles | Shijin ISBM: Injection Stretch Blow Molding for PET Bottles | Shijin Tooling: Stock Mold vs Custom Mold Explained | Shijin Quality & Compliance: ISO 9001, Testing and Documentation | Shijin Packaging Engineering: Bottles, Closures and Decoration | Shijin Custom Bottles, Jars & Dispensers — Wholesale

Products behind this page

Real items from the Shijin stock-mold library. Shown specifications are the published ones; a quotation is built from your own drawing and fill volume.

PET | 15 ml | SJT0484-15-05