Dewey Wu General Manager & senior mechanical engineer at EPOC CRAFTER, 15 years in design engineering, quality, and metallurgy. Hands-on across CNC machining, metalwork, sheet metal, and prototyping (subtractive + 3D printing).
An injection molding machine with sufficient tonnage may still be unsuitable for your part. Before awarding a molded-part RFQ, check the machine’s shot size, screw and plasticizing capacity, injection pressure, mold mounting dimensions, and ejection travel against your material, part geometry, cavity count, and production requirements. These specifications establish whether a machine is a plausible match. They do not prove that the supplier can repeatedly meet your dimensional tolerances or production targets. That requires a separate review of process capability and molded-part inspection results.
1. Start With the Molded Part and RFQ Requirements
A machine capability sheet reports rated machine limits. Your molded-part RFQ defines the actual resin, mold, quality targets and production demand. Compare the two before accepting a supplier’s machine selection.
Check the part and mold first. Tonnage alone cannot establish compatibility.
| RFQ input | What to check | Why it matters |
| Resin grade | Polymer grade, reinforcement, and processing requirements | Changes melt behavior, pressure demand, shrinkage, and plasticizing conditions |
| Part dimensions | Overall geometry, wall thickness, and critical features | Influences flow length, filling behavior, cooling, and dimensional stability |
| Projected area | Total projected area of all cavities and runners at the parting plane | Establishes the area subjected to cavity pressure when estimating clamping force |
| Shot weight | Combined weight of molded parts and runners per cycle | Determines the material quantity the injection unit must deliver |
| Cavity count | Number of parts produced per molding cycle | Changes total shot weight, projected area, runner layout, and production output |
| Mold dimensions | Mold width, height, thickness, and mounting interfaces | Determines whether the mold can be installed and operated |
| Production quantity | Required output, cycle time, and production schedule | Determines whether the machine can support the planned production volume |
| Inspection requirements | Functional dimensions, tolerances, and acceptance conditions | Determines what must be verified beyond the machine’s nominal specifications |
For part geometry, gating, material selection and tooling requirements, review the plastic injection molding process alongside the injection molding DFM guidelines.
1.1. Part Projected Area, Material, Shot Volume and Cavity Count
Consider an electronics enclosure cover molded in a two-cavity cold-runner tool.
The production case uses CYCOLAC MG47 ABS, an unreinforced injection molding grade. Each cover measures 125 × 90 × 28 mm, has a nominal body wall thickness of 2.50 mm, and weighs 90.0 g. The cold-runner system contributes another 30.0 g per cycle.
Total shot weight = (number of cavities × part weight) + runner weight = (2 × 90.0) + 30.0 = 210.0 g.
The projected area for both cavities is 210.0 cm². Adding 20.0 cm² for the runner gives a total projected area of 230.0 cm².
These two values describe different machine requirements. The 210.0 g shot weight is relevant to injection-unit capacity. The 230.0 cm² projected area is an input to the clamping-force calculation.
The case uses a Haitian MA1600/540G-C injection molding machine with a rated clamping force of 1,600 kN, a 50 mm screw, and a nominal polystyrene shot capacity of 359 g. Its specified theoretical injection volume is 395 cm³.
Comparing the 210.0 g ABS shot directly against the 359 g polystyrene rating gives a mass ratio of approximately 58.5%. That comparison is only a preliminary screening figure. Different polymer densities mean the actual shot volume must be checked against the machine’s rated injection volume.
The mold also has a 400 × 400 mm footprint and a 320 mm mold height. Both dimensions must be checked against the machine’s available installation space. A suitable shot capacity does not guarantee that the mold will fit.
The production case reported a 42.0-second molding cycle. At two cavities per cycle, the corresponding theoretical gross production rate is approximately 171 parts per hour before downtime and rejected parts.
The RFQ needs separate checks for clamp capacity, injection volume, tool installation and repeatable output. Begin with injection molding tonnage calculated from projected area and an appropriate pressure basis.
2. Injection Molding Tonnage and Clamping Force
Injection molding tonnage and injection molding clamping force are the rated closing force available to keep the mold shut. It is not a rating for part weight, shot size, dimensional accuracy, or the pressure that the screw can generate. A larger press can still have an unsuitable injection unit.
The opening load depends on cavity and runner projected area and the pressure acting across the parting plane. The machine must also accommodate the tool mechanically. Do not use screw injection-pressure ratings as a substitute for cavity pressure in a clamping calculation.
2.1. Clamping Force Calculation: What Inputs Are Necessary?
For a preliminary clamping force calculation, clamp force (kN) = projected area (cm²) × assumed effective cavity pressure (MPa) × 0.1. The 0.1 converts cm² × MPa to kN. The result is sensitive to the assumed pressure, which must suit the resin, geometry, gate design, flow path and processing conditions.
For project ZS-44-01, the case records a total projected area of 230.0 cm² and an effective parting-plane pressure assumption of 45 MPa. The calculated opening-force estimate is 230.0 × 45 × 0.1 = 1,035 kN. The selected press is rated at 1,600 kN, a difference of 565 kN. The ratio 1,600/1,035 is approximately 1.55 for these stated assumptions; it is not a universal design factor.
The 45 MPa value is an effective parting-plane pressure assumption in this case, not a sensor reading. Gate design, melt flow, material and processing conditions alter actual pressure. Treat the 1,035 kN result as a screening calculation and verify it against the intended tool and molding conditions.
Suggested figure: white-background four-column diagram linking projected area, material shot, mold envelope and inspection requirements to corresponding machine checks; conceptual, not a machine datasheet.
3. Shot Size, Screw and Plasticizing Capacity
Injection molding shot size must cover each cavity and runner. The injection unit must deliver the required melt, recover the next shot within the cycle and avoid unsuitable resin residence. An injection molding machine capacity rating alone does not establish all three.
Check the screw diameter, L/D ratio, usable injection stroke, theoretical injection volume, material-specific shot weight, plasticizing output, and maximum injection pressure. A nominal shot-weight specification often refers to a reference polymer. Converting that mass rating to another resin requires the applicable melt-density basis and equipment documentation, not a direct gram-for-gram assumption.
The case equipment lists a 50 mm injection molding screw with L/D 18:1, theoretical volume of 395 cm³, PS reference shot weight of 359 g, and nominal plasticizing capacity of 90 kg/h. These describe the specified unit; none alone establishes the validated processing window for CYCOLAC MG47 ABS.
3.1. Shot Size Calculation and Material Residence Time
For the two-cavity cover, shot weight is 210.0 g including cold runner. The documented 58.5% ratio is 210/359 using a PS reference mass. It should not be presented as an ABS shot-utilization percentage. A volumetric check would require an ABS melt-density estimate valid for the actual processing conditions and the usable screw-stroke limit.
The case records 11.0 seconds of screw recovery during a 42.0-second cycle. That is an observed operating condition, not a guarantee for another resin, mold or shot weight. The nominal 90 kg/h plasticizing capacity is a machine rating; actual recovery must be checked at the specified settings.
Material residence time depends on retained melt volume, throughput, screw geometry and interruptions. Excessive residence can degrade heat-sensitive polymers; too little melt residence or unstable recovery may affect consistency. Check the plastic materials and properties relevant to the RFQ before selecting barrel temperatures and assessing residence time.
For recycled plastic injection molding, batch-to-batch melt behavior can change recovery and filling even when nominal shot weight stays similar. The supplier should explain how its injection molding process parameters are established when resin viscosity or regrind content changes.
4. Injection Pressure, Speed and Cycle-Time Constraints
Maximum injection molding injection pressure is a machine rating, not the measured pressure inside the tool. Confirm the rate and pressure available at the required injection speed, then examine transfer and holding behavior under production conditions.
The case lists a rated maximum injection pressure of 137 MPa and rated screw linear speed of 172 mm/s. Recorded settings used injection stages of 95 and 65 mm/s, followed by position-based transfer and 55 MPa holding pressure for 7.0 seconds. The machine-side injection-pressure peak was 105 MPa. These figures describe this production setup, not a universal recipe.
4.1. Injection Pressure Versus Cavity Pressure
Injection molding pressure is not one single quantity. Machine injection pressure is measured or calculated on the injection side. Cavity pressure describes the pressure inside the mold. Pressure losses through the nozzle, runner and gate mean the two quantities are not interchangeable. Holding pressure and injection molding back pressure are different controls again. Label every pressure reading by measurement location and function.
In the case, 105 MPa is the machine-side peak, while 45 MPa is the effective parting-plane pressure assumed in the clamp calculation. There is no documented cavity-pressure sensor trace. The two numbers serve different purposes and should not be compared as though measured at the same location.
Pressure or flow limits may show up as incomplete fill, flashing, or a narrow processing window. For defect diagnosis rather than machine screening, Use the injection molding defects guide to investigate incomplete fill, flash and other molding defects.
4.2. Actual Process Control Versus Nameplate Ratings
A useful injection molding process control review looks at repeatable screw position, transfer behavior, pressure trends, recovery time and cycle-time variation. Injection molding process monitoring can use machine trend logs or, where installed, direct cavity instrumentation. A single peak value does not describe every shot.
Closed-loop injection molding functions can compensate for some material or process variation. OEM systems from suppliers such as ENGEL and ARBURG illustrate pressure-based adjustments and monitoring. The specific feature, sensor location and validation method matter more than a controller marketing label. An AI anomaly alert does not demonstrate compliance with a molded-part tolerance by itself.
At two cavities and 42.0 seconds per cycle, gross output is 2 × 3,600 ÷ 42 = 171.4 parts/hour. The case ran 500 cycles and molded 1,000 pieces; 9 visual rejects were recorded. Gross cycle capacity does not include downtime or additional production loss.
5. Mold Fit: Tie-Bar Spacing, Platen Size and Ejector Stroke
A mold can pass the clamp-force calculation and still fail installation. Compare injection mold dimensions and injection molding mold size with the actual machine envelope with tie-bar spacing, platen mounting area, allowable mold height, machine opening stroke, ejector travel and any robotic or core-pull interfaces. Injection molding machine sizing must consider each of these limits.
| Machine or mold item | Case value | Practical screening result |
| Tie-bar clearance | 455 × 455 mm | 400 × 400 mm mold fits inside stated clear opening |
| Mold footprint | 400 × 400 mm | Also verify platen clamps, mounting holes and load support |
| Mold thickness | 320 mm | Within stated 180 to 500 mm allowable range |
| Opening stroke | 420 mm | Confirm part removal and tool geometry fit within usable opening |
| Ejector stroke | 140 mm rated; 60 mm setting | Confirm ejector connection, stroke and interference clearance |
The listed injection molding platen size is not a substitute for the actual effective mounting area. The case records 455 × 455 mm tie-bar clearance but describes platen clamping as matched to the tool, rather than supplying a numeric full platen envelope. The RFQ review should therefore address the actual mounting interface, not infer full platen dimensions from tie-bar spacing.
5.1. Mold Height, Opening Stroke and Mold Installation
A 320 mm mold lies within the machine’s stated 180 to 500 mm mold-height range. Its 400 × 400 mm footprint is smaller than the 455 × 455 mm tie-bar opening in both directions. The case reports a 420 mm opening stroke and 140 mm rated ejector stroke, with the ejector set to 60 mm to avoid interference.
This is a dimensional compatibility check, not proof of automatic demolding. Part height, ejector plate travel, protruding cores and handling clearance still control whether the part can be removed safely. Include mold dimensions, mounting details and handling clearances in the production tooling requirements supplied with the RFQ.
Suggested figure: plan and side-view dimensioned educational schematic for 455 × 455 mm opening, 400 × 400 mm tool, 320 mm mold height and distinct opening/ejection strokes; explicitly labeled case schematic.
5.2. Mold Filling Simulation and Machine Limits
Injection molding simulation or mold filling simulation can help estimate filling behavior, pressure demand and imbalance between cavities before a mold is committed to production. It does not replace an actual machine trial. Model outputs depend on mesh, resin properties, thermal boundary conditions and gate assumptions. Thin wall injection molding and high speed injection molding can make injection-rate and pressure constraints especially important.
Use the simulation to ask a narrower question: does the machine have enough injection rate and pressure capability for this part at the specified gate and material conditions? Validate the answer with tooling and trial information before accepting a supplier’s claim.
6. Supplier Machine-Capability Checklist for Molded-Part RFQs
For a molded-part RFQ, ask the proposed injection molding supplier to name the machine configuration and show how its clamp, shot, screw, pressure, mold fit and production settings meet the job. Machine availability alone is not evidence of compatibility. This applies whether you are comparing injection molding companies or reviewing a custom injection molding quote.
| RFQ check | Record from supplier | Decision question |
| Machine identification | Exact model, screw/injection-unit configuration and clamp rating | Is the quoted configuration the one planned for production? |
| Clamping | Projected area, pressure basis and rated clamp force | Is opening-force risk assessed for this cavity layout? |
| Shot and plasticizing | Usable volume, reference-resin rating, screw configuration and recovery conditions | Can the resin be delivered and recovered in the target cycle? |
| Injection | Pressure/rate capability and process settings | Can the part fill without approaching an unsuitable operating limit? |
| Mold installation | Tie bars, platen interfaces, allowable mold height and opening travel | Can the actual tool mount, open and release parts? |
| Ejection and auxiliaries | Ejector travel, interfaces, cooling and core pulls when applicable | Will the tool function with this production cell? |
| Production validation | Cycle observation, sample inspection and material/lot details | Is the claim supported beyond nominal ratings? |
| Acceptance | Critical features, datum, inspection method and acceptance timing | Is finished-part conformity defined independently? |
6.1. Required OEM Datasheet and Machine-Specific Evidence
Check the supplier’s rapid injection molding capabilities against the exact mold, resin and production requirements.
Ask for the selected OEM machine specification and the actual screw/injection-unit configuration. Machines with the same clamping force may carry different injection units. Confirm that every stated injection molding machine specifications value belongs to the quoted configuration.
For a production RFQ, the supplier should be able to state its selected equipment, the limiting parameter for the proposed part, the expected injection molding production capacity and the acceptance method. For small production runs, include tool changeover and lot size in low volume production planning because both influence cost.
A press rated for more tonnage is not automatically better. An electric injection molding machine may offer a different control and energy profile from a hydraulic press, but injection molding energy consumption must be compared at an equivalent mold, cycle, throughput and measurement boundary. Do not choose a supplier by energy-claim percentages without an applicable basis.
6.2. Cavity Pressure, Closed-Loop Control and Validation
A cavity pressure sensor, where installed, can show what happens inside the mold rather than solely at the injection screw. Cavity pressure monitoring is particularly useful for transfer behavior, shot consistency and certain critical features. Where a direct sensor is absent, do not label a machine-side trace as cavity-pressure evidence.
Tie process records to resin lots and inspection results. For precision injection molding, verify how the supplier detects excursions and isolates affected parts. Instrumentation and automated alerts support control but do not themselves prove compliance.
A practical illustration comes from ZS-44-01: the selected 1,600 kN machine satisfied the documented clamp estimate and mold-installation screening. Over 500 cycles, the case reports 50 dimensionally inspected parts, 25 from each cavity, and 9 visual rejects among 1,000 molded pieces. That supplies separate evidence for machine matching, sampled dimensions and observed defect rate; it does not prove an unrestricted process-capability index.
Suggested figure: white technical evidence summary with the 1,035 kN calculated opening-load assumption, 1,600 kN rated clamp, 400 mm versus 455 mm mold clearance, and dimensions explicitly marked as case data; no fabricated inspection photo.
7. Part Tolerances and Acceptance: ISO 20457:2026
Machine fit and molded-part acceptance are separate questions. ISO 20457:2026, Plastics moulded parts — Tolerances and acceptance conditions, addresses dimensional and geometrical tolerancing and acceptance of applicable molded plastic parts. It does not prescribe injection molding machine tonnage, shot-size utilization, screw geometry or machine-selection factors.
The standard considers material stiffness, shrinkage, shrinkage anisotropy, part design, mold and production/quality effort when describing technically achievable accuracy. General tolerances do not replace individually specified functional dimensions. Material shrinkage and datum selection therefore belong in the dimensional agreement rather than being inferred from machine nameplate capacity.
For the ABS cover case, the acceptance ranges are 124.80 to 125.20 mm for length, 89.80 to 90.20 mm for width, and 59.85 to 60.15 mm for snap spacing. The sampled extremes were 124.91 to 125.10 mm, 89.90 to 90.11 mm and 59.92 to 60.09 mm respectively. All 50 reported dimensional samples were inside those specified ranges.
The case separately reports three short shots, two flash defects and four visible flow marks among 1,000 molded parts, totaling nine visual rejects or 0.90%, below its stated 1.00% visual-defect limit. ISO 20457 does not itself define these cosmetic defect acceptance categories. They belong to the project’s separate acceptance criteria.
ISO 20457:2026 specifies reference conditions of 23 ± 2 °C and 50 ± 10% relative humidity. Its normal acceptance window runs from 16 to 72 hours after molding, unless another condition is agreed. The case documents dimensional inspection after at least 48 hours under these temperature and humidity conditions. These are acceptance controls, not equipment ratings.
ISO 20457:2026 identifies reference environmental conditions of 23 ± 2 °C and 50 ± 10% relative humidity. Its normative acceptance window is not earlier than 16 hours and not later than 72 hours after molding, unless agreed otherwise. In the case, dimensional checks were conducted no earlier than 48 hours after molding at 23 ± 2 °C and 50 ± 10% relative humidity, using 0.01 mm-resolution calipers and cavity-stratified samples.
The reported 50 dimensional samples all met the specified limits. Minima, maxima and means alone do not establish Cp or Cpk. Statistical capability would require the individual observations and a defined assessment method. Annex D of ISO 20457:2026 treats machine, process and measurement capability separately and is informative.
For RFQ acceptance, define critical features, their references, method, timing and environment. Specify the applicable manufacturing tolerances and standards in the RFQ so the supplier can quote against defined acceptance criteria. Consistent dimensional results should also be evaluated against lot-to-lot changes, mold condition and resin shrinkage before making broad long-term claims.
8. FAQ: Machine Selection, Tonnage and Shot Size
8.1. How Do You Calculate Injection Molding Tonnage?
Use total projected cavity and runner area multiplied by a justified effective cavity-pressure assumption, with consistent units. For cm² and MPa, multiply by 0.1 to obtain kN. Verify that the assumption matches the material and mold. Do not insert the machine’s maximum injection pressure into the equation as though it were measured cavity pressure.
8.2. What Is Shot Size in Injection Molding?
Shot size is the melt delivered per molding cycle. Include all cavity parts and runner material. Shot size calculation by mass is useful for an RFQ, but a reference-polymer shot-weight rating must be converted using the relevant melt-volume basis before it is compared with a different polymer.
8.3. How Do You Choose an Injection Molding Machine for a Specific Mold?
Check projected area and clamp requirement, shot volume and recovery, injection pressure and rate, tie-bar clearance, platen installation, mold-height range, opening stroke, ejection interfaces and cycle performance. These are independent gates; passing one does not cancel a failure in another.
8.4. Does a Sufficient Clamp Rating Guarantee Tight Tolerances?
No. Injection molding tolerances and dimensional stability depend on the material, shrinkage, mold, processing and inspection conditions. The appropriate capability evidence is a defined part inspection and process-monitoring record under agreed production conditions, not the clamp number.
Before awarding the molded-part RFQ, identify the selected press and mold, check each limiting machine parameter, then compare production and inspection results with the stated acceptance criteria. The electronics enclosure case illustrates why an acceptable clamp calculation is only one part of this decision.
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