A part the size of a matchbox can still flash on a 100-ton press. That surprises many first-time buyers. Size alone does not set tonnage. Instead, projected area, resin flow and cavity count work together to decide the force you need.
Every small injection molding machine has one main job at the parting line. Its clamp must hold the mold shut while molten plastic pushes hard to open it. If the clamp is too weak, plastic escapes and you get flash. If it is too strong, you pay for steel, power and floor space you never use.
Buyers in the USA, the UK, Türkiye, Malaysia, Saudi Arabia, the UAE, Qatar, South Africa and Morocco ask the same question. How many tons is enough for a small injection molding machine? The answer comes from a simple formula, plus three checks that most guides skip.
This article covers the tonnage formula, clamp factors for common resins, a worked example, and the shot size and mold-fit checks that decide the final choice.
Quick Answer: To size a small injection molding machine, multiply the total projected area of all cavities plus the cold runner (in²) by the resin’s clamp factor, usually 2–5 tons/in². Then add a 10–20% safety margin. Finally, confirm the shot uses 20–80% of barrel capacity and the mold fits between the tie bars.
What Does Tonnage Mean on a Small Injection Molding Machine?
Tonnage is the maximum clamping force a machine can apply to keep the mold closed. On a small injection molding machine, this force usually falls between 30 and 300 tons. It must exceed the opening force, which equals cavity pressure multiplied by the part’s projected area.
Think of the clamp as a heavy vise. During injection, melt pressure inside the cavity can reach several hundred bar. That pressure acts on every square inch of the part facing the platens. So a wide, flat lid needs more force than a tall, narrow tube of the same weight.
Tons, Metric Tonnes and kN
Machine makers do not all use the same unit. US suppliers often quote US short tons, while European and Asian makers quote kN or metric tonnes. One metric tonne-force equals about 9.81 kN, and one US ton equals about 8.9 kN. Therefore, a “100-ton” machine from Europe may list 980 kN on its data sheet. Always convert to one unit before you compare IM machines side by side.
Why Oversizing Also Costs You
Many buyers add tonnage “just in case.” However, an oversized clamp brings real costs. The machine price rises, the hydraulic system draws more power, and the larger barrel often holds too much melt for small shots. That last point matters most, because long residence time can burn heat-sensitive resins. A right-sized press saves money on day one and on every cycle after.
How to Calculate Clamping Force for a Small Injection Molding Machine
The standard formula is: Tonnage = Projected Area (in²) × Clamp Factor (tons/in²) × Safety Factor. You find the projected area from the part drawing, pick the clamp factor from the resin type, and add a 10–20% margin. This method works for parts from 30 to 300 tons.
The Society of Plastics Engineers notes that tonnage factors typically range from 2 to 8 tons/in², depending on the material and the project. The three steps below show how to apply that range in practice.
Step 1: Find the Total Projected Area
Projected area is the part’s shadow when you look straight down the clamp direction. For a rectangle, multiply length by width. For a round part, use π × r². Next, multiply by the number of cavities. Then add the cold runner and sprue, since they also face the platens. Hot runners add very little, because the melt stays inside the manifold. Finally, subtract only true through-holes where steel shuts off the flow.
Step 2: Choose the Clamp Factor
The clamp factor reflects how hard the resin is to push. Easy-flow resins such as PP and PE need about 2–3 tons/in². Stiffer resins such as PC and nylon need 4–5 tons/in². Thin walls and long flow paths push the factor higher, so move to the top of the range when your wall is under 1 mm.
Step 3: Add a Safety Margin
Melt viscosity shifts from lot to lot, and mold temperature drifts during a shift. For this reason, add 10–15% for stable, high-volume jobs. For prototype molds or new resins, use 15–20%. Then round up to the next standard machine size.
Clamp Factors by Resin for a Small Injection Molding Machine
Clamp factors run from about 2 tons/in² for polypropylene to 5 tons/in² for polycarbonate on a small injection molding machine. These figures are starting points, not fixed rules. Always confirm with the resin supplier’s data sheet, because grade, filler and wall thickness all move the number.
The table below lists typical ranges for resins molded on small machines. It also gives metric values for buyers who work in cm².
| Resin | Clamp Factor (tons/in²) | Approx. (tonnes/cm²) | Typical Small Parts |
|---|---|---|---|
| PP (polypropylene) | 2.0 – 3.0 | 0.31 – 0.47 | Caps, lids, food containers |
| HDPE / LDPE | 2.0 – 3.0 | 0.31 – 0.47 | Closures, plugs, household items |
| PS / HIPS | 2.0 – 2.5 | 0.31 – 0.39 | Cups, disposable cutlery, trays |
| ABS | 3.0 – 4.0 | 0.47 – 0.62 | Electronics housings, toy parts |
| PMMA (acrylic) | 3.0 – 4.0 | 0.47 – 0.62 | Lenses, light covers |
| POM (acetal) | 4.0 – 5.0 | 0.62 – 0.78 | Gears, clips, fasteners |
| PA6 / PA66 (nylon) | 4.0 – 5.0 | 0.62 – 0.78 | Connectors, cable ties |
| PC (polycarbonate) | 4.0 – 5.0 | 0.62 – 0.78 | Medical parts, clear housings |
| Glass-filled grades | Add 0.5 – 1.0 | Add 0.08 – 0.16 | Structural brackets |
As you can see, the same part in PC needs roughly twice the clamp force of that part in PP. So pick your resin before you pick your machine.
Worked Example: Sizing a Small Moulding Machine for an ABS Housing
A 2-cavity ABS electronics housing measuring 3 × 2 inches needs about 65 tons, so an 80-ton small moulding machine is the right fit. The steps below show the math. They also show why the shot size check confirms the choice.
This example mirrors a common job in electronics industry molding, where small, precise housings run in high volume.
The Tonnage Math
- Part area: 3 in × 2 in = 6 in²
- Two cavities: 6 × 2 = 12 in²
- Cold runner: 2 in²
- Total projected area: 14 in²
- Clamp factor for ABS: 4 tons/in² → 14 × 4 = 56 tons
- Safety margin of 15%: 56 × 1.15 = 64.4 tons
The next standard size above 64 tons is usually 80 tons. Therefore, an 80-ton small injection molding machine gives a clean margin without wasted capacity.
The Shot Size Check
Each housing weighs 18 g, and the runner weighs 6 g. So the full shot weighs 42 g. An 80-ton machine with a typical 100 g shot capacity runs this job at 42% utilization. That sits inside the 30–50% window that Plastics Machinery & Manufacturing recommends for engineering resins like ABS and PC. As a result, both the clamp and the barrel match the job.
Checks Beyond Tonnage for a Small Injection Molding Machine
Tonnage alone does not confirm a fit. A small injection molding machine must also pass three checks: shot size within 20–80% of barrel capacity, tie-bar spacing wider than the mold, and mold height inside the machine’s minimum and maximum range. Skip any one, and the press may not run your tool.
Shot Size and Barrel Capacity
Data sheets for a small injection molding machine list shot capacity in GPPS (general-purpose polystyrene). Resins with lower density give a lighter shot, so adjust for your material. RJG, a leading process training firm, advises running shots between 20 and 80 percent of barrel capacity, ideally 40 to 60. Below 20%, melt sits too long and degrades. Above 80%, the screw cannot melt the resin evenly.
Tie-Bar Spacing
The mold must slide between the four tie bars. Measure the mold’s width and height, then compare them with the machine’s horizontal and vertical tie-bar spacing. Many buyers find that a large, low-pressure mold needs a bigger frame than tonnage suggests. In that case, the frame size, not the clamp force, sets the class of small injection molding machine you need.
Mold Height and Opening Stroke
Every machine has a minimum and maximum mold height. Your mold thickness must sit between them. Also check the opening stroke. As a rule, the stroke should be at least twice the part depth, plus room for ejection and robot pick-up.
Typical Tonnage Classes and Applications
The table below shows how small machine classes map to common parts.
| Tonnage Class | Typical Shot Weight (GPPS) | Common Applications | Usual Drive Type |
|---|---|---|---|
| 30 – 60 tons | 20 – 80 g | Micro medical parts, connectors, seals | All-electric or servo-hydraulic |
| 60 – 120 tons | 60 – 200 g | Caps, closures, toy parts, small housings | Servo-hydraulic or hybrid |
| 120 – 200 tons | 150 – 450 g | Food containers, kitchenware, phone cases | Servo-hydraulic or hybrid |
| 200 – 300 tons | 350 – 900 g | Buckets, small crates, auto interior clips | Servo-hydraulic or two-platen |
Shot weights vary by screw size, so treat these ranges as a guide only. Moreover, check that any machine meets EN ISO 20430 (UK, EU and Türkiye) or ANSI/PLASTICS B151.1 (USA) safety requirements before you buy.
Drive Type, Energy Use and Total Cost
Drive type changes running cost more than tonnage does on a small injection molding machine. Servo-hydraulic machines suit most jobs from 60 to 300 tons. All-electric machines suit precise, clean parts under 100 tons. Hybrid machines pair an electric injection unit with a hydraulic clamp for a balance of speed and price.
Energy matters in every target market. Electricity prices in the UK and South Africa make each kilowatt-hour count. Meanwhile, Gulf buyers in Saudi Arabia, Qatar and the UAE need machines that hold oil temperature in high ambient heat. A servo pump cuts energy use because it only runs when the machine moves. You can read more about eco-friendly machine technologies that lower power bills.
Automation also shifts the math. A robot on a small press can turn a one-operator cell into a lights-out cell. For this reason, plan the tie-bar spacing and opening stroke for a robot from the start. The latest automation trends in plastic molding equipment show how small plants now run longer shifts with fewer staff.
Get Tonnage Right the First Time
The right tonnage protects your mold, your parts and your margins. Calculate the projected area, apply the resin’s clamp factor, and add a sensible margin. Then confirm shot size, tie-bar spacing and mold height. Small errors add up fast. A press that is 20 tons short will flash on every shot, while one that is 100 tons too big burns extra power for years. Together, these four steps cut most sizing mistakes before you order a small injection molding machine.
Daoben builds machines from 30 to 4,000 tons, with servo-hydraulic, hybrid and two-platen options. The range includes the UNIVERSAL, M8SII, M7, EV Hybrid and DU two-platen series. Screw and barrel options let you match the injection unit to your shot, not just your clamp.
Send us your part drawing, resin and annual volume. Our engineers will run the tonnage, shot size and mold-fit checks for you. Explore the full range of Daoben Molding Machines to find the model that fits your job.
Frequently Asked Questions
How many tons do I need for a small injection molding machine?
Most small parts need between 30 and 300 tons. To find your number, multiply the total projected area by the resin’s clamp factor, then add 10–20%. For example, a 14 in² ABS shot needs about 65 tons, so an 80-ton small injection molding machine fits well. Always confirm the result with a shot size check.
What is the tonnage formula for injection molding?
The formula is projected area in square inches, times the clamp factor in tons per square inch, times a safety factor. Clamp factors run from about 2 for PP to 5 for PC. This formula gives a reliable starting point when you size a small injection molding machine. Mold flow software then refines the figure.
What happens if the clamping force is too low?
When clamping force is too low, cavity pressure pushes the mold halves apart. Plastic then escapes at the parting line and forms flash. Flash adds trimming work, wears the mold edges and can cause short shots. Choosing a small injection molding machine with a 10–20% tonnage margin prevents this problem in most cases.
Can a 50-ton machine run a multi-cavity mold?
Yes, if the total projected area stays low enough. A 50-ton press can run a four-cavity PP cap mold when the combined area stays under about 18 in². However, the mold must also fit between the tie bars. A small injection molding machine with wider tie-bar spacing gives more room for multi-cavity tools.
Is a small moulding machine cheaper to run than a larger one?
Usually, yes. A right-sized press uses less power, less cooling water and less floor space. It also keeps the shot within the ideal barrel range, which lowers scrap. A servo-driven small injection molding machine can use far less energy than an older fixed-pump press of the same size.
Which is better for small parts: electric or hydraulic IM machines?
Electric IM machines suit precise, clean parts under about 100 tons, such as medical and electronics components. Servo-hydraulic machines cost less and handle a wider range of molds and resins. For most buyers, a servo-hydraulic or hybrid small injection molding machine offers the best balance of price, speed and accuracy.
