Cycle time directly determines the capacity of an injection molding plant. Every second gained shows up at the end of the day in the part count, and at the end of the month in the cost per machine.
However, most cycle-shortening efforts start in the wrong place: the injection speed is raised, holding is shortened, and in the end quality suffers and the seconds gained are paid back as scrap. The right approach is first to see where you are spending the time.
What makes up the cycle?
| Stage | Typical share |
|---|---|
| Mold closing | 5 – 10% |
| Filling (injection) | 5 – 10% |
| Holding (holding pressure) | 10 – 20% |
| Cooling | 50 – 70% |
| Mold opening + part ejection | 10 – 15% |
The table says one thing: the gain is in cooling. Doubling the injection speed gains a few percent of the total cycle, whereas a second gained from cooling is a full second.
1. Cooling time and the square effect of wall thickness
Cooling time depends on the thickest section of the part and increases with the square of the thickness:
t = (s² / (2 π² α)) × ln[ (8 / π²) × (Tm − Tw) / (Te − Tw) ]
- s = wall thickness (mm)
- α = thermal diffusivity of the material (mm²/s)
- Tm = melt temperature · Tw = mold wall temperature · Te = ejection temperature
Example calculation
PP part, α = 0.07 mm²/s, melt 230 °C, mold 30 °C, ejection 80 °C:
| Wall thickness | Cooling time |
|---|---|
| 2.0 mm | 3.4 seconds |
| 2.5 mm | 5.3 seconds |
| 3.0 mm | 7.7 seconds |
When the thickness increases by 25%, the cooling time increases by 56%. The reverse is also true: reducing the wall from 2.5 to 2.0 mm in the part design shortens cooling by about 35%.
The practical conclusion: The source of a cycle time problem is very often not in the machine but in thick sections in the part design. In a part with unbalanced wall thickness, the thickest point determines the whole cycle.
2. Mold temperature — a two-way balance
Tw (mold temperature) in the formula directly affects the cooling time. For the same PP part:
| Mold temperature | Cooling time |
|---|---|
| 30 °C | 3.4 seconds |
| 45 °C | 4.2 seconds |
| 60 °C | 5.6 seconds |
Practical rule: every 10 °C increase in mold wall temperature lengthens the cooling time by about 20%. This is an established rule of thumb in the industry; the table above confirms it (a 24% increase from 30 → 45 °C and 33% from 45 → 60 °C). When deciding to raise the mold temperature, calculate the gain together with this cost.
Cooling the mold shortens the cycle — but not without a price:
- Too cold a mold → short shots, pronounced weld lines, a matte surface, increased internal stress
- In semi-crystalline materials (PA, POM, PBT) a low mold temperature lowers the degree of crystallinity; the part is weaker and undergoes post-shrinkage over time
- Unbalanced cooling (a difference between the mold halves) directly produces warpage
The right approach is not to cool the mold as much as possible but to keep it stable at the temperature the material requires. The temperature fluctuating from cycle to cycle is more harmful than the average value itself.
The equipment that achieves this is mold temperature control units. A unit with insufficient capacity cannot hold the mold at target as production heats up; the cycle time lengthens during the shift. Details: What is mold temperature control, the water vs oil difference
Target mold temperatures by material: Material processing values table
3. Cooling channel design and flow rate
However good the temperature control unit is, it is the water flow rate that draws the heat out of the mold. Common problems:
- Insufficient flow — a large temperature difference forms between the channel inlet and outlet, and one side of the mold stays hotter than the other
- Scaled-up channels — over the years the cross-section narrows and heat transfer falls; in most plants this goes unnoticed
- Many circuits connected in series — because the water heats up as it travels, the last circuit runs far above target
- Channels that do not reach hot areas — cores and deep sections cannot be cooled
Turbulent flow draws many times more heat than laminar flow. Increasing the flow rate is often a cheaper gain than re-machining the mold.
4. Determine holding time by measurement, not by guesswork
In most plants the holding (holding pressure) time is kept longer than necessary to be on the safe side. Yet pressure applied after the gate has frozen contributes nothing to the part — it only lengthens the cycle.
The correct time is determined by weighing: the holding time is increased step by step and the part is weighed at each step. The point at which the weight stops increasing is the moment the gate freezes. Every second beyond that is lost.
A step-by-step application of the method: Process diagnostic methods
Exception: With TPE and some elastomers, weight measurement does not give a clear plateau; for these materials dimensional measurement is used as the basis.
5. Time gained with a hot runner
In a cold runner the runner channels also have to cool like the part. Because the runner is often thicker than the part, it determines the cycle — the square effect applies here too.
With a hot runner system:
- The runner's cooling time disappears completely
- Granulating, conveying and re-feeding the runner come to an end
- Material loss and quality variation caused by regrind are eliminated
In large parts, cascade (sequential) injection shortens the flow path and so also lowers the fill time and the required pressure.
6. Run plasticizing in parallel with cooling
The screw should prepare the next shot while the part is cooling. If plasticizing takes longer than cooling, the screw is determining the cycle.
Things to check:
- Screw speed — is it below the peripheral speed limit? (excessive speed degrades the material)
- Back pressure — back pressure higher than necessary lengthens plasticizing
- Screw diameter — the shot weight should be within 20 – 80% of the screw volume
To calculate speed, peripheral speed and cooling time with your own values: calculators · Ready-made tables: injection molding calculation tables · The effect of parameters: Parameter effects
7. Mold opening and part ejection
On most machines the opening and ejection movements are set conservatively. Points where time can be gained:
- The opening stroke should be reduced to the minimum value required for the part to come out
- The ejector stroke and dwell time should be reviewed
- Robot or conveyor synchronization — instead of waiting for the part to drop, work in parallel with a conveyor
- On servo-driven machines the movement speeds are markedly higher than with hydraulics: The servo, hydraulic and all-electric difference
8. The hidden cause that lengthens the cycle: raw material
Insufficiently dried raw material does not lengthen the cycle directly — but it produces scrap. A line that stops because of silver streaks, bubbles and dimensional deviation is more expensive than the best-optimized cycle.
In the same way, dosing deviation produces color differences and changes in mechanical properties. Before doing cycle work, make sure the raw material side is stable: Raw material drying · Gravimetric dosing
Order of application
Carry out cycle-shortening work in this order — starting from the end wastes time:
1. Measure — record the stage times of the current cycle and find the longest stage 2. Stabilize the raw material — if drying and dosing are not working properly, optimization is meaningless 3. Determine the holding time by weighing — usually the fastest and cost-free gain 4. Fix the mold temperature at target — eliminate fluctuation 5. Check the cooling flow rate and channel condition 6. Verify that plasticizing does not exceed cooling 7. Tighten the opening and ejection movements 8. Plan structural improvements — hot runner, channel revision, part wall thickness
At each step change a single parameter and measure the result. When several changes are made at once, it is impossible to tell where the gain came from; and when a problem arises, there is no way back.
For quality problems that arise: Defect troubleshooting guide
For equipment and process support for your line, you can consult our technical support team.
Cycle time can also be shortened on the hardware side; electric pre-plasticizing and high injection speed are the two most obvious ways. You can see which series offers which in the table on the plastic injection molding machines page.