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  • Brandon Henderson (NC) • Journeyman Tool & Die (CPSP)
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Design Principles

Core Injection Molding DFM Principles

Most molding problems are designed in, not processed in. Sink marks, weak weld lines, parts sticking in the cavity and long cycle times usually trace back to wall thickness, draft, rib or gate decisions made before the mold was ordered. A DFM review catches them while the fix is a CAD change, not a steel change.

Primary Geometry Checks

  • Uniform wall thickness: thick and thin areas cool and shrink at different rates, which causes warp, sink and molded in stress. Transitions should be gradual.
  • Draft: about 1 degree per side on polished walls, more on deep cores. Textured walls need about 1.5 degrees for every 0.001 in (0.025 mm) of texture depth.
  • Ribs: about 40 to 60 percent of the wall they join, depending on resin and cosmetic needs, no taller than about 3 times the wall, and spaced at least 2 walls apart.
  • Radii: sharp inside corners concentrate stress and block flow. An inside radius of about half the wall, with the outside radius one wall larger, keeps the corner the same thickness.

Typical design guidelines. TK confirms the right values for your resin and part in the DFM report.

TK DFM example comparing the original sharp rib design with the radiused rib after DFM review
TK DFM example: the sharp rib transition (left) is radiused after DFM review (right) to prevent sink and stress.
Moldflow Simulation

Moldflow Finite Element Simulation Capabilities

Figure 1: Moldflow Cavity Pressure and Thermal Distribution Simulation
The melt front advances from the gate, splits around a core and rejoins at a weld line. Colors show cavity pressure, highest at the gate and dropping to zero at the melt front, through fill, pack and hold, and cooling.
1 FILL2 PACK & HOLD3 COOLGateCore PinWeld LineLast to Fill: Vents PlacedGate Pressure (MPa)80400V/P SwitchoverTime0 MPa80 MPa (High)Cavity pressure: highest at the gate, dropping to zero at the advancing melt front

Key Moldflow Deliverables

TK’s certified analysts run Autodesk Moldflow on the part and the mold layout, and report:

  • Fill and pack: fill pattern, pressure drop, hesitation in thin ribs, short shot risk and the clamp tonnage needed.
  • Weld lines and air traps: where melt fronts meet and where gas gets trapped, so gates move weld lines off cosmetic and loaded areas and vents go where they are needed. Vent depth is set by the resin, often 0.0005 to 0.002 in.
  • Mold temperature and cooling: hot spots in the steel and how long each area takes to cool.
  • Warpage: predicted deflection from uneven shrinkage, and for glass filled resins, fiber orientation.
Moldflow fill time result for a round housing filled from a center sprue, blue at the gate to red at the last areas to fill, 2.316 seconds
Moldflow fill time result from a TK project, played back in fill order: the part fills from the sprue (blue) to the last areas to fill (red) in 2.316 s.
Shrinkage and Warp

Warpage and Volumetric Shrinkage Analysis

Every plastic shrinks as it cools from melt temperature. When one area shrinks more than another, because of a thick section, a hot spot in the mold or fibers lined up in one direction, the part twists or bows. Moldflow predicts that deflection so it can be designed out or built into the steel.

Resin TypeTypical Linear Mold ShrinkageWarpage RiskMain DFM Lever
Amorphous (ABS, PC, PMMA)0.4 to 0.7%LowUniform walls, generous radii, balanced packing
Semicrystalline (PP, PE, POM)1.5 to 3.0% (PP and PE), about 2% (POM)HighUniform walls and even cooling, including conformal cooling
Glass filled (PA66 GF30, PBT GF)About 0.2 to 0.5% with flow, 0.8 to 1.2% across flowHigh, direction dependentGate location to control fiber direction
High temperature (PEEK, PPS GF)1.0 to 1.5% (PEEK), 0.2 to 0.5% (PPS GF)Moderate to highHot oil or electric mold heating, 135 to 200 °C

Typical ranges from resin supplier data sheets. Actual shrinkage depends on grade, wall, pack pressure and mold temperature.

For hard to cool areas, see our conformal cooling technology, or our custom injection mold tooling capabilities.

Runner Design

Gate Location Optimization and Cold vs Hot Runners

TK multi cavity infusion component mold with a hot runner and valve gates
TK infusion component mold with a hot runner and valve gates.

Gating Best Practices

Gate into the thickest section, so pack pressure can keep feeding the part while thinner areas freeze. Place the gate so weld lines and flow marks land in hidden or low stress areas, and so the flow fills the part evenly.

Cold runners with tunnel or edge gates suit family molds and lower volumes. Hot runners remove runner scrap and shorten cycles, and valve gates give a clean gate mark with controlled opening, as on TK’s full hot runner valve gate medical and packaging molds. Learn more on our export molds for U.S. presses page.

Cooling and Trial

Cooling Analysis and Trial Verification

Cooling is usually the longest part of the cycle, so TK runs mold temperature and cooling analysis with the fill study. Where straight drilled lines cannot reach, 3D printed conformal cooling gives about 30% higher cooling efficiency and less warpage. The simulation is then proven on a real press: TK’s trial presses run from 6 to 1,500 tons, and every new mold goes through TK’s ten step standardized debug, with closed loop tuning of injection, hold and temperature.

Figure 2: What the trial tunes, phase by phase
Typical curve shapes during one shot, not to scale. Cavity pressure at the end of fill rises later and lower than at the gate, and both fall to zero as the part cools.
INJECTPACK AND HOLDCOOLING Pressure Time
Hydraulic pressureCavity pressure at gateCavity pressure at end of fillBack pressure during screw recovery

From RFQ to Production

TK’s new product introduction process
  1. 1
    RFQBusiness and cost review, capacity analysis
  2. 2
    ESIDesign optimization, tooling and process DFM, Moldflow, program plan
  3. 3
    ProtoWeekly tooling progress reports, DOE verification
  4. 4
    EVT, DVT, PVTTooling trial, FAI, ECN, product and process qualification, IQ, OQ, PQ for medical
  5. 5
    Ramp upRelease to mass production, yield, cycle time and stability improvement
Interactive Verification

Interactive DFM Guideline Checker

Pick your resin and enter the wall, texture and draw depth. The checker gives the typical draft, rib, radius and wall guidelines, draws the rib section to scale, and estimates how long the wall takes to cool. Open “Show the math” to see every step with your numbers.

1Material and part
0 for a polished wall. Light textures are about 0.025 mm (0.001 in).
Rib section, to scale
Estimated cooling time for this wall
Show the math
    Send my part for a DFM review

    These are typical design guidelines and typical resin data (melt, mold and ejection temperature, thermal diffusivity). Cooling time is the theoretical time for the wall center to reach ejection temperature; the full cycle adds fill, pack, mold open and ejection. TK confirms every value for your exact grade in the DFM report.

    Frequently Asked Questions

    Technical Sourcing FAQ: DFM & Moldflow

    What CAD formats are accepted for DFM analysis?+
    Send 3D models as STEP (.stp or .step) or IGES (.igs) files, with 2D drawings that show critical dimensions and tolerances. TK designs in Siemens UG NX, so neutral STEP files work best.
    What does a TK DFM and Moldflow report cover?+
    Wall thickness, draft, ribs, radii, undercuts, parting line and gate location, plus Moldflow fill, pack, mold temperature, cooling and warpage results, with recommended design changes before the mold design is released.
    When should DFM happen?+
    As early as possible. In TK’s process, tooling and process DFM and Moldflow happen at the early supplier involvement (ESI) stage, before the prototype and before the mold design is frozen, when changes are still CAD changes.
    How is the simulation checked against the real mold?+
    Every new mold is trialed on TK’s own presses (6 to 1,500 tons) using a ten step standardized debug process, then measured with CMM in TK’s lab before it ships. On some programs, TK also uses RJG in mold sensing to control quality in production.
    How do I start a DFM review?+
    Send your STEP files, drawings, resin and annual volume to Brandon. He will set up the review with TK’s engineering team and walk you through the findings.

    Get a Comprehensive DFM Review on Your Part CAD

    Send your 3D CAD to Brandon Henderson, Technical Sales Support Manager for the Americas. TK’s engineers will flag molding risks, suggest wall and gate changes, and propose a mold layout before you commit to steel.

    Submit 3D CAD for DFM Review Call Brandon: (336) 953-9664