Blow molded
- Narrow-neck bottles and closed hollow shapes
- Walls vary with how the plastic stretches in the mold
- The neck is the only closely held area
Molded part guide: containers
Injection Molded Containers
A molded tub or jar is formed over a long steel core. The core has to stay centered while the plastic flows around it, and the part has to come off it again after shrinking tight onto it. Those two facts shape the gate, the walls, and the tool more than the outside shape does.
Around the core
Open-top containers are built around one long core
Food tubs, cosmetic and supplement jars, storage boxes, bins, and pails.
Injection molding suits containers with an open top: tubs, wide-mouth jars, boxes, and bins. The inside is formed by a core, the outside by a cavity, and the rim, any thread, and any lid bead come straight from the steel. Narrow-neck bottles are a different process, blow molding, because a core cannot be pulled out through a small neck.
The core sets the first rule. It is a long post of steel that the melt surrounds. If plastic reaches one side of it first, the pressure pushes the core toward the other side, that side fills thinner, and the imbalance feeds itself. Most containers are gated at the center of the base so the melt spreads evenly down every side.
The second rule is release. The plastic shrinks onto the core as it cools, and a deep, closed-bottom shape pulls a vacuum as it slides off. Draft on the walls, a polished core, and a way to let air in behind the base, often a small air valve in the core, keep the part from sticking or collapsing on ejection.
Thin walls are what make these parts hard to fill. The thinner the wall and the deeper the part, the faster the melt must travel before it freezes. Thin wall containers need high filling speed, a resin grade made for easy flow, and an even section from base to rim.
Design choices
Where a container design earns its keep
Each row is a decision that shows up in the tool.
| Feature | Why it matters | Design response |
|---|---|---|
| Gate at the center of the base | Melt reaches every side together and keeps the core centered | Keep the base section even around the gate |
| Draft on deep walls | Deep parts grip the core as they shrink | Generous draft, and more if the outside is textured |
| Air release on ejection | A deep closed shape pulls a vacuum on the core | An air valve in the core lets air in behind the base |
| Nesting taper | Empty containers ship stacked inside each other | Stacking lugs stop nested parts from jamming |
| Top load | Filled containers are stacked on pallets | Rim shape and sidewall steps carry the load |
Service tiers
Container service tiers and quote path
Part focus: Thin tubs: core shift, center gate, vacuum on ejection. Upload CAD with critical fits and environment in notes when not modeled.
| Tier | Environment | What matters most | Typical molded route | Quote path |
|---|---|---|---|---|
| 1 Center gate | An open tub, jar, or box | Melt reaches every side together | PP | Instant quote; field PP |
| 2 Deep release | The part shrinks onto the core | Draft, a polished core, air behind the base | PP | Upload CAD; note the depth |
| 3 Thin wall | A deep part that must fill fast | An even section from base to rim | An easy-flow grade | Upload CAD; expect thin-wall DFM |
| 4 Nesting | Empties ship stacked | Stacking lugs so they do not jam | PP | Instant quote; note whether empties nest |
| 5 Clear | You need to see the contents | A clear grade and a polished tool | Clear PP or the grade you name | Instant quote; note clarity |
Materials
Resin families: quick reference for quote notes
One primary family per line item. Put commercial grade, fillers, and environment filters in comments.
| Family | Typical use on this part type | Notes for instant quote |
|---|---|---|
| PP | Tubs, jars, boxes, and bins | Field PP; note if the grade is for easy flow |
| Clear PP | Containers you need to see through | Field the clear grade; polish the tool surface |
Core shift
Why one side of the tub comes out thinner
The core is a long steel post, and the melt pushes on it.
- If melt reaches one side of the core first, the pressure difference bends the core toward the other side
- That side then fills thinner and later, and the imbalance grows
- A center gate on the base with an even base section keeps the flow balanced around the core
- Deep, slender cores may also need support or interlocks in the tool, which DFM review checks
Process choice
Blow molded or injection molded container
The mouth of the container usually decides the process.
Injection molded
- Wide-mouth jars, tubs, boxes, and bins
- Every wall, rim, and thread is formed against steel
- Stacking lugs, handles, and label panels are molded in
Avoid these
Common design mistakes on molded containers
Two choices that show up as a thin wall, or a tub that sticks on the core.
Gating the side wall so one side fills first. The pressure bends the core, and that side comes out thinner. Gate at the center of the base, and keep the base section even around the gate.
A deep tub with little draft and no air behind the base. The part shrinks onto the core and pulls a vacuum as it comes off. Draft, a polished core, and air behind the base keep it from sticking.
Drawing
What to send with a container
Fill, stacking, and the lid matter as much as the shape.
Container CAD with the rim, any thread, and any lid bead
Fill volume and the product, including hot filling or freezing
Stack height when filled, and whether empties must nest
The lid or cap design, or a request to design both together
Drop and handling requirements
Color, clarity, and any print or label area
Keep reading
Related guides
Lids and caps for containers, and the rules behind deep parts.
Lids
Snap-on lids and rim fit.
LidsBottle caps
Screw caps built to a neck finish.
Bottle capsThreaded caps
Caps for threaded jars.
Threaded capsTrays
Open, shallow parts with pockets.
TraysInjection vs blow molding
Which containers suit which process.
Injection vs blow moldingDraft angle
Release for deep walls.
Draft angleSticking parts
Parts that hang on the core.
Sticking partsShort shot
Thin walls that do not fill.
Short shotMolded parts library
Every part-type guide, grouped by family.
Molded parts libraryFAQ
Questions about molded containers
Can you mold a bottle with a narrow neck?
Narrow-neck bottles are blow molded, which NetProto does not offer. Wide-mouth jars and tubs are injection molded, and so are the caps and lids for either.
Why does the tub stick on the core?
Deep parts shrink onto the core and pull a vacuum as they come off. More draft, a polished core, and a way to let air in behind the base solve most sticking.
How thin can the walls be?
It depends on flow length, resin grade, and depth. Thin wall containers need fast filling and an easy-flow grade, and DFM review checks fill on your actual design.
Can the container be clear?
Yes, in clear PP or other clear grades. Clarity depends on the resin and on a polished tool surface.
What keeps empty containers from jamming when they nest?
The same taper that lets the part off the core also lets empties stack inside each other. Stacking lugs hold them a little apart so the stack does not lock.
Are the rim, thread, and lid bead molded with the container?
On an open-top tub, jar, or box, yes. They come straight from the steel. Send the rim, any thread, and the lid or cap with the container CAD.
Next step
Quote the container and its lid together
Upload the container, and the lid or cap if you have it. DFM feedback covers gating, core balance, release, fill, and stacking.