A one-piece EVA tote is made in a single injection cycle of 90 to 150 seconds: compounded pellets are dried, melted at 160 to 190 degrees, injected into a closed tool at 60 to 120 megapascals, held under packing pressure while the body cools to 40 to 60 degrees, then ejected with its handles, base tread and perforations already formed. No seams, welds or glued joints exist anywhere in the finished body, which is why it can be hosed down and left to drain indefinitely. The holes are formed by pins in the tool rather than cut afterwards. QUANZHOU JUNYUAN BAGS runs this process from 500 pieces per design and colour.
From Pellet to Finished Body in Seven Stages
Injection moulding of ethylene-vinyl acetate is a cyclic process: everything in one cycle has to happen before the next can begin, and the cycle time is what sets both the output and the unit price. On a body of this size the cycle runs 90 to 150 seconds, of which cooling is the largest share at 40 to 70 seconds. The whole sequence is controlled by four numbers that an operator watches and adjusts — melt temperature, injection pressure, packing pressure and cooling time — and almost every defect in the finished body traces back to one of them.
| Stage | What happens | Key value | Typical duration |
|---|---|---|---|
| Compounding | Base resin, pigment and additives are blended | 12 to 18 percent vinyl acetate | Before the run |
| Drying | Moisture is removed from the pellets | Below 0.05 percent moisture | 2 to 4 hours |
| Plastification | Pellets are melted and homogenised | 160 to 190 degrees | Within the cycle |
| Injection | Melt is driven into the closed cavity | 60 to 120 megapascals | 3 to 8 seconds |
| Packing and holding | Pressure is held as the material shrinks | 40 to 70 percent of injection pressure | 10 to 25 seconds |
| Cooling | The body solidifies in the tool | Tool at 20 to 40 degrees | 40 to 70 seconds |
| Ejection and deflashing | The body is pushed out and the sprue removed | Body at 40 to 60 degrees | 5 to 15 seconds |
Reading that table as a buyer, the two lines worth arguing about are packing pressure and cooling time. Cutting cooling time is the fastest way to raise output and the most reliable way to produce a body that warps after it leaves the tool, because EVA is still soft above about 70 degrees. Cutting packing pressure saves a few seconds and produces sink marks and short shots at the thick sections, which are exactly the handle roots and the base corners that carry the load.
Compounding: Where Colour and Feel Are Set
EVA arrives as a base resin and is compounded before it ever reaches the machine. The compound carries the pigment, a stabiliser package against ultraviolet degradation, a processing aid and sometimes a small proportion of regrind from the same run. The vinyl acetate content of the base resin is the decision that sets the feel: 12 to 18 percent gives the firm, rubbery body that stands up on its own, while a higher content gives a softer, more compressible article that reads as less rigid on a shelf.
Colour is matched in this stage and not later, which is why a colour change is a material event rather than a machine setting. Matching takes 2 to 3 working days and produces a physical drawdown, and the tolerance worth agreeing is a delta E of 2.0 or less against the master. Colour measurement methods for coloured articles are published by AATCC, and agreeing the number at this stage is what prevents a bulk delivery being argued about three weeks later.
Drying and Feeding
EVA is only mildly hygroscopic, but on a wall thickness of 3 to 5 millimetres even mild moisture shows. Pellets are dried for 2 to 4 hours to below about 0.05 percent moisture before feeding, and the reason is physical rather than procedural: water in the melt flashes to steam at injection temperature and leaves bubbles, streaks or a rough surface in the finished wall. On a body that will be sold on its appearance, that is a reject rather than a blemish.
Feeding is metered by screw rotation and the shot size is set by the volume of the cavity plus a cushion, normally 3 to 8 millimetres of melt left in front of the screw at the end of injection. The cushion is what allows holding pressure to be transmitted into the cavity as the material shrinks; without it, the screw bottoms out and the body is under-packed regardless of what the pressure gauge reads. An operator who understands that relationship produces consistent bodies, and one who does not produces a run where the first hundred pieces are good and the rest are not.
Plastification and Injection
The melt is prepared in a heated barrel with a reciprocating screw, brought to 160 to 190 degrees and homogenised by the shear of the screw itself. Temperature control matters in both directions: too cool and the melt is too viscous to fill the thin sections at the hole pins, producing short shots at the perforations; too hot and the material begins to degrade, releasing acetic acid, which shows as a sharp smell and a brown streak in the wall. The working window is roughly 30 degrees wide and a well-set process sits in the middle of it.
Injection itself is fast: the melt is driven into the closed cavity at 60 to 120 megapascals in 3 to 8 seconds. Fill speed is the setting that decides surface finish and the position of the flow front, because EVA freezes against the cool tool surface as it flows and a slow fill produces a visible flow line where two fronts meet. On a large flat panel such as the base of a 54 centimetre body, the gate position and the fill speed together decide whether that line is visible, which is why the same tool produces a better surface on one machine than on another.
Packing, Cooling and Ejection
EVA shrinks by 1.5 to 2.5 percent as it solidifies, and packing is the answer to that. Holding pressure at 40 to 70 percent of the injection figure is maintained for 10 to 25 seconds while the gate freezes, pushing additional melt into the cavity to compensate for the shrinkage. Get it right and the body dimensions land within plus or minus 2 millimetres across a run. Get it wrong and the symptoms are specific and readable: sink marks at thick sections mean too little packing, and flash at the parting line means too much.
Cooling then runs 40 to 70 seconds against a tool held at 20 to 40 degrees, and this is the part of the cycle that sets how many bodies a day can be made. A thicker wall cools with the square of its thickness, so moving from a 3 millimetre wall to a 5 millimetre one roughly triples the cooling time rather than increasing it by two thirds. That is the real cost of a rigid body, and it is why wall thickness is a price decision as much as a performance one.
Ejection happens at 40 to 60 degrees, when the body is solid enough to hold its shape but still slightly warm. Ejector pins push it off the core, and the design detail that matters here is where those pins sit: they leave small round marks, and a tool designed well puts them on the base or inside the handle where nobody looks. A body ejected too hot deforms under its own weight while it cools on the bench, which is why a rack rather than a pile is the correct place for a body to go next.
How the Holes Are Formed
Perforations are formed in the tool rather than cut afterwards, and this is the single feature that distinguishes a moulded tote from a welded or sewn one. The cavity carries an array of pins, typically 8 to 14 millimetres in diameter and anywhere from 60 to several hundred in number depending on the layout. Melt flows around each pin and rejoins behind it, leaving a weld line on the downstream side of every hole. Those weld lines are slightly weaker than the surrounding wall, and they are where a crack starts if the body is flexed hard at a hole.
| Perforation variable | Typical value | Effect on the body | Effect on the process |
|---|---|---|---|
| Hole diameter | 8 to 14 millimetres | Sets drainage speed and charm fit | Larger holes are easier to fill |
| Hole count | 60 to 300 plus | Sets open area and drain time | More pins means more weld lines |
| Pin length | Matches wall thickness | Long pins need more packing pressure | Long pins deflect and can break |
| Layout | Full, partial or base only | Sets print area and drain time | A solid panel fills more easily |
| Spacing | 12 to 20 millimetres between holes | Sets rigidity between holes | Tight spacing weakens the panel |
Two consequences follow for a buyer. A body with a very large number of small holes drains more slowly than one with fewer, larger holes of the same total open area, because surface tension holds water in a small aperture. And a layout with a solid upper panel, chosen for printing, fills more reliably and produces a stronger body, because the melt flows through an uninterrupted panel instead of rejoining behind three hundred pins. Semi-perforation is a print decision that happens to be a quality decision as well.
Defects and What Causes Them
Every defect in a moulded EVA body has a cause in one of the four controlled numbers, and reading them is a skill worth having because it tells a buyer whether a problem is a one-off or systemic. A single defective piece in a hundred is a start-up scrap issue; the same defect appearing at piece 3,000 means the process drifted and the whole run needs looking at.
| Defect | Appearance | Cause | Correction |
|---|---|---|---|
| Short shot | Holes not fully formed, edge incomplete | Melt too cool or fill too slow | Raise melt temperature or fill speed |
| Sink mark | Shallow depression at a thick section | Packing pressure too low or held too briefly | Raise holding pressure or extend hold time |
| Flash | Thin film of material at the parting line | Packing pressure too high or clamp force too low | Reduce pressure or raise clamp force |
| Flow line | Visible arc on a large flat panel | Fill speed too slow for the gate layout | Raise fill speed or add a gate |
| Void or bubble | Internal cavity, sometimes visible as a bump | Moisture in the pellets or insufficient packing | Dry longer or extend the hold |
| Warp | Base not flat, body rocks when set down | Ejected too hot or cooled unevenly | Extend cooling, cool on a rack |
The one defect that matters most in this category is warp, because a base that is not flat is a bag that tips, and tipping is the complaint that ends a product line rather than a single order. A flat base is checked by setting the empty body on a level surface and looking for rocking, and it is worth making an explicit line in the specification: the empty body should stand without rocking on a level surface and should remain upright on a slope of 10 degrees.
Inspection and Measurement
Inspection on this category is measurement rather than looking. Three figures catch almost everything: wall thickness at the rim, the wall and the base; hole diameter across three holes taking the smallest; and empty weight on a scale. Weight is the most valuable of the three because it is the hardest to fake and the fastest to take — a body that has drifted two tenths of a millimetre thinner passes every visual check and silently removes resin from every piece in the run.
Physical test methods for the dimensional, hardness and load checks used here are published by ASTM International, and documented inspection and release practice aligns with the International Organization for Standardization. On a bulk order the checks are applied to a random sample drawn from the finished cartons at AQL 2.5, with zero acceptance for critical defects such as a strap anchor that separates under load, and the results are recorded with photographs so that a disagreement can be settled on evidence rather than on memory.
Why One Piece Matters
The absence of joints is the property that everything else in the category rests on. A welded or sewn bag has a seam, and a seam is a place where two materials meet, where an adhesive or a weld has to hold, and where water eventually gets in. A moulded body has none of that: the wall, the base, the tread, the handles and the perforation are all the same continuous piece of material, formed in the same instant, with no interface anywhere.
Three practical consequences follow, and they are the three reasons a buyer chooses this construction. The body can be hosed at tap pressure and left to drain, because there is no adhesive to soften and no stitch hole to wick. It absorbs no water at all, because there is no textile layer to hold it. And it stands on wet ground without the base rotting or delaminating, because the base is the same material as the wall rather than a bonded-on panel. Every one of those is a consequence of one piece rather than several.
What the Process Cannot Do
Moulding has real limits and knowing them prevents a buyer specifying something that cannot be made. The list below is short and each item has a reason rather than a preference behind it.
- Very thin walls: below about 2.5 millimetres the melt freezes before it fills a body this size, so a light but rigid large body is not achievable.
- Sharp internal corners: every corner needs a radius, typically 1.5 to 3 millimetres, or the tool cannot be machined and the part will crack at the corner.
- Undercuts without side actions: a shape that traps the body in the tool needs a moving part in the mould, which adds 2,000 to 6,000 dollars to the tool.
- Two colours in one shot without a second operation: a two-tone body needs either a second injection or an applied panel.
- Perfect dimensional repeatability: shrinkage varies by 1.5 to 2.5 percent, so tolerances tighter than about plus or minus 2 millimetres are not realistic.
- A perfectly flat base by default: it requires correct cooling and a rack, not a pile, and it should be written into the specification.
None of those is a reason to avoid the process. All of them are reasons to design with it rather than against it, and a buyer who brings a drawing that respects them will get a quotation faster, a cheaper tool and a body that comes out right on the first shot.
Regrind, Start-Up Scrap and Tool Life
Two things about the process affect a buyer's cost and neither appears on a quotation. The first is start-up scrap: the first 10 to 30 bodies of any run are shot while the four controlled numbers are being set, and they are not saleable. On a 500 piece order that is up to six percent of the run; on a 10,000 piece order it is a rounding error. This is one more reason the economic minimum sits where it does, and it is why a buyer asking for 200 pieces is quoted a price that looks disproportionate.
The second is regrind. Sprues, runners and rejected bodies can be ground and returned to the same programme, commonly at 10 to 20 percent of the shot, without a measurable change in the properties a tote needs. What should not happen is regrind from a different colour or a different compound entering a run, because it produces colour streaks and unpredictable shrinkage. A buyer is entitled to ask what proportion of regrind is used and whether it is from the same run, and a programme that answers that question in writing is one that is controlling its process rather than hoping.
Programme Notes
Material properties sit in the EVA tote materials guide, wall thickness and rigidity in the EVA tote wall thickness guide, and the finished range in the injection-molded EVA tote guide. Buyers comparing a welded or sewn construction should read the EVA versus PVC comparison for the alternatives. QUANZHOU JUNYUAN BAGS has run waterproof bag programmes since 2014 with a founding team in this trade since 2004, at monthly output of 200,000 units verified by SGS, under BSCI and ISO 9001, with materials screened for REACH and California Prop 65 and every bulk shipment inspected at AQL 2.5 before release.
Send the body dimensions, wall thickness target, perforation layout, colourway count and quantity to service@junyuanbags.com, and ask whether an existing tool fits the dimensions before specifying a new one, because that single question decides both the tooling charge and twenty-five to fifty days of the schedule. Sampling runs 6 to 10 working days and bulk 35 to 50 days.
People Also Ask
How is an EVA tote made?
By injection molding: pellets are dried, melted at 160 to 190 degrees, injected at 60 to 120 megapascals into a closed tool and cooled for 40 to 70 seconds, one body per cycle.
How long is one molding cycle?
90 to 150 seconds for a body this size, of which cooling takes 40 to 70 seconds and injection only 3 to 8.
Are the holes cut or molded?
Molded. The cavity carries pins of 8 to 14 millimetres that the melt flows around, so the perforations are formed in the same instant as the body.
Why does a molded tote have no seams?
Because the wall, base, tread, handles and perforations are all one continuous piece formed in a single cycle, with no joint, weld or adhesive anywhere.
What causes sink marks on an EVA tote?
Packing pressure held too low or too briefly. Shrinkage of 1.5 to 2.5 percent is not compensated, leaving a shallow depression at thick sections.
Can a molded EVA body have sharp corners?
No. Every corner needs a radius of 1.5 to 3 millimetres or the tool cannot be machined and the body will crack at that point.
Frequently Asked Questions
Q1. What temperature is EVA molded at?
A melt of 160 to 190 degrees into a tool held at 20 to 40 degrees, with the body ejected at 40 to 60 degrees once it is solid enough to hold shape.
Q2. How long does cooling take?
40 to 70 seconds, and it scales with the square of the wall thickness, so a 5 millimetre wall takes roughly three times as long as a 3 millimetre one.
Q3. How much does EVA shrink?
1.5 to 2.5 percent. Holding pressure compensates for it, which is why tolerances tighter than about plus or minus 2 millimetres are not realistic.
Q4. Why are the pellets dried first?
Moisture above about 0.05 percent flashes to steam in the melt and leaves bubbles, voids or a rough surface in the wall.
Q5. What is packing pressure for?
It compensates for shrinkage. Held at 40 to 70 percent of injection pressure for 10 to 25 seconds, it keeps the body within 2 millimetres across a run.
Q6. How many holes can a body have?
Anywhere from 60 to several hundred, at 8 to 14 millimetres each, depending on the layout chosen for drainage and print area.
Q7. Do more holes make the body weaker?
Yes, mildly. Each hole leaves a weld line on its downstream side, and a crack starts there if the body is flexed hard at a hole.
Q8. What is a short shot?
A body where the melt froze before the cavity filled, leaving incomplete holes or an unfinished edge. It means the melt was too cool or the fill too slow.
Q9. What causes a base that rocks?
Warp, usually from ejecting the body too hot or cooling it unevenly. The fix is longer cooling and cooling on a rack rather than in a pile.
Q10. Can two colours be molded in one shot?
Not on a standard tool. A two-tone body needs a second injection or an applied panel, both of which are separate operations.
Q11. What wall thickness is achievable?
Typically 3 to 5 millimetres. Below about 2.5 the melt freezes before it fills a body this size, and the part cannot be made reliably.
Q12. How is the finished body checked?
By measurement: wall thickness, hole diameter and empty weight, applied to a random sample from the finished cartons at AQL 2.5 with zero acceptance for critical defects.