Professor Joseph Greene All rights reserved Copyright 2000 Mold Layout Chapter 6 Professor Joseph Greene All rights reserved Copyright 2000

Mold Layout Rules Balancing Lateral Forces Balancing of Runners Arrangement of Cavities Two-plate molds Three-plate molds Hot runner layout

Mold Layout Rules Rules Before getting started, be sure to have the following: Approved fully detailed and toleranced product drawing Molding material and shrinkage known Mold machine specs and anticipated production volume DFM Guidelines for cavities, runner and ejection systems First Considerations Precedent for similar job. Look at lessons learned and components No precedent. Look for other jobs that had mistakes and learn from them

Mold Layout Rules Mold Stack Design Step 1: Product drawing assists in the determination: Injection location (use flow analysis) on A side (stationary platen) Ejection method (select locations that are possible) on B side (moving side) Side coring (slides) if required Cooling Gating and venting Step 2: Will product pull out of cavity? If yes, proceed to step 3 If no, then add a slide or lifter Avoid side cores (pins of the side walls) on the cavity side. Side cores on core side (usually the moving half). During mold opening, the side core opens and allows part removal Reentrant (false bottom) of the product might require 2-stage motion if there is little draft (less than 5° per side) B A Ejection Injection Plastic part

Mold Layout Rules Mold Stack Design Step 3: Will the product pull off the core If yes, proceed to step 3 If no, then consider these options: stripping, 2-stage action, unscrewing of internal threads, collapsible cores, or internal side cores. Stripping- pushing plastic out of the grooves on demold. Depends on: Shape of grooves so as not to deform plastic Type of plastic. Material needs to be somewhat elastic like PE and PP Two-stage action with part of the core moves in relation to the other Unscrewing of internal threads (Chap 12) Collapsible cores (Chap 12) Internal side cores

Mold Layout Rules Mold Stack Design Step 4: Establish the Parting Line (P/L) Usually obvious choice for location. For not obvious choice consider, Ejection: to insure product will eject Shut-offs location to insure no sliding of cavity and core. Strength of P/L to support clamp force Bursting strength of cavity walls to resist cavity pressure Machining of P/L: how P/L will be machine to produce flash free part. Preferred design is P/L in one plane. Fig 6.4 Step 5: Is the product Balanced? Balance is when the injection forces are pushing right angles to the center line of the mold If not, add wedges to balance forces

Mold Layout Rules Mold Stack Design Step 6: Gating Cold runner, hot runner, three-plate, edge, tunnel, etc. Use flow analysis to design runner system if new. Use precedents Step 7: Inserts and Core Pins Are used where it is not practical to machine the cavity or core from one piece and not cause small, deep, narrow grooves and holes in product Required when have openings in product which are at right angles to the mold movement. Reduce wear of shut-offs

Mold Layout Rules Mold Stack Design Step 8: Venting Rule 1- vent to suit anticipated flow of plastic at all points remote from gate. Rule 2- vent at closed end of all pockets, any area not in the direct plastic stream. Ejector pins, slides, and the parting line provide natural vent locations. Rule 3- vents should be large as possible, without causing flash. Rule 4- vents are needed in areas where flow changes direction. Ribs and bosses are particularly troublesome for trapped air and need to be vented with the insert or slide used. Rule 5- vent should be moving and open with every time opens, e.g., ejector pins. Good practice to pressurize stationary vent pins or slots to flow air. Rule 6- vent in land area between cavity and core may be blocked by clamp forces deforming metal Rule 7- Every vent must vent to the outside. If not, then use of vent grooves and channels must be provided.

Mold Layout Rules Mold Stack Design Step 9: Ejection (Chap 12) Select and detail method, i.e., stripper, ejector, pins, sleeves, air poppets. Step 10: Cooling (Chap 13) Mold is an heat exchanger Want even distribution of the heat in the mold during cooling Select appropriate number, diameter, and location of cooling channels Step 11: Alignment Mold cavity and core close together Use mold shoes and leader pins Use tapers, wedges, or pins for cavities and cores Step 12: Review Work thus far Is it best layout?Consider other design possibilities. What materials will be used for parts? Are mold dimensions reasonable for thickness, length, taper,etc Establish minimum outside dimensions

Mold Layout Rules Mold Shoe Design Step 13: Draw Machine Platen Layout Shoe all size and location of tie bars, mounting holes, ejector holes and edges of platens Decide whether have toe clamps or through bolt pattern Check the min daylight on platen with the height of mold Step 14: Complete Mold Elevation (Cross Section) Show all necessary plates to the right and left of stack. Step 15: Show Shell (Stack Outline) Show all of the outside dimensions of the stack in 3-Directions Step 16: Show Both Plan Views Show cavity and core shells. Views include plan of cavity, plan of core, and cross sections.

Mold Layout Rules Mold Shoe Design Step 17: Symmetry of Layout Position all stacks (shells) in a symmetrical layout about the vertical and horizontal center lines of the mold for balance runners and balanced clamping forces. Show ejector pins and moving core pins to size ejector plate in relation to cooling lines and air return lines. Step 18: Design Review Meeting Discuss the work to date and discuss fits, clearances, operation, timing, roadblocks, costs, etc… Step 19: Cooling Lines Show cooling lines and the cooling circuit including external manifolds or tubing for the cavity and the core. Show how the cooling lines will be drilled.

Mold Layout Rules Mold Shoe Design Step 20: Alignment and Other Mold Features Select location of leader pins. If possible place leader pins near corner of mold. Make sure leader pins do not interfere with cooling lines or ejector plate. Step 21: Mold Supports Select locations of support pillars, especially near ejector pins Step 22: Mold Mounting Show mounting of mold to the machine platens Preferred is with screws either from the rear through the holes in the platens or from the front using threaded holes or T-slots. Mold clamps are not as efficient and can move during operation Note: All stack features may still require relocation due to changes

Mold Layout Rules Mold Shoe Design Step 23: Check Machine Specs w/ Current Mold Layout Is shut height acceptable? If too large then reduce thickness of plates. If too small then additional plates may be required. Step 24: Finalize the Mold Shoe Outside Plan Dimension Select a size that allows the use of existing stock plate sizes or commercial standard plate sizes without wasting material If layout is not standard size, see if the components can be rearranged to use standard components and sizes. Step 25: Screws Show placement of all screws for mold mounting. Use standard hole pattern for the mounting plate so that many molds will mount the same in the platen.

Mold Layout Rules Mold Shoe Design Step 26: Complete Assembly Drawings Detail and tolerance all views (minimum of 3 views, top front, and isometric) Include specifications, e.g., screw torque, adjustments, installation dimensions for leader pins, cooling lines, ejector locations, injection location. Step 27: Review Design and Mfg Process Final design review with design team before tool build Step 28: Cross Hat Sections Show cross hat sections only where it can clarify tough areas. Step 29: Identify (Balloon) the Components Identify each mold item with letters and numbers and list on the Bill of Material (BOM)

Mold Layout Rules Mold Shoe Design Step 30: Fill in the Title Block and Finish Drawing Show all information on drawing including Scale, materials, heat treatment, can finishes Pertinent notes from design changes Job number and mold number Designer name and checker name Date of completion for drawing and of checking drawing Date of release to shop

Mold Layout Rules Balancing of Lateral Forces In molds the forces F inside the cavity are at right angles to the mold opening motion balance each other. (Fig 6.7) If the projected area is not symmetric and one side has a flange and the other side does not, then the forces can be unbalanced. (Fig 6.8) A wedge insert can be used to balance the force (Fig 6.9) P/L P/L F F F Balanced Unbalanced P/L F F F Wedge Insert

Mold Layout Rules Balancing of Lateral Forces Hot runners and cold runners can be balanced with the use of reorientation of the cavities to have equal right angles on both sides of the mold. If side cores are required, the side cores must be wedged (Fig 6.12) and then locked in position by individual wedges. P/L F Unbalanced Hot Runner Mold Unbalanced Cold Runner Mold P/L F F F P/L F F F Core Backing Plate or Wedge Balanced Hot Runner Mold Balanced Cold Runner Mold

Mold Layout Rules Balancing of Runners Balanced flow depends upon The amount of redirection of the plastic at the point of change Accuracy of the machining and finish Temperature difference within runner and mold Machining tolerances of the gates How sensitive the plastic is to flow changes and shear rates Venting uniformity Quality difference in molding surface

Mold Layout Rules Balancing of Runners Balanced flow into the cavities is a prerequisite for a quality part. This can be achieved by changing the runner size and length. Whenever possible, a naturally balanced runner system should be used to balance the flow of material into the cavities. If a naturally balanced runner is not possible, then the runner system should be artificially balanced, as shown below.

Mold Layout Rules Balancing of Runners Encourage flow to the cavities farthest from the sprue by reducing the diameter of runners feeding the other cavities. Note that decreasing the runner diameter too much may cause it to freeze prematurely, causing a short shot. On the other hand, increased frictional shear heating may actually reduce the resin's viscosity, and thus, resistance to flow and fill the cavity even faster.

Recommended Runner Design Full-Round Runner The best in terms of a maximum volume-to-surface ratio, which minimizes pressure drop and heat loss. Tooling cost is generally higher because both halves of the mold must be machined so that the two semi-circular sections are aligned when the mold is closed. Trapezoidal Runner Works well and permits the runner to be designed and cut on one side of the mold. Commonly used in three-plate molds and at the parting line in molds, where the full-round runner interferes with mold sliding action.

Sizing Runners Hydraulic diameter and flow resistance To compare runners of different shapes, you can use the hydraulic diameter, which is an index of flow resistance. The higher the hydraulic diameter, the lower the flow resistance. Hydraulic diameter can be defined as: Equivalent hydraulic diameters

Design Rules Runner size Runner cross section Cold runner diameter The cross-sectional area of a runner should not be smaller than that of the sprue, to permit rapid, unaltered flow of the material to the gating area. Cold runner diameter The selection of a cold runner diameter should be based on standard machine tool cutter sizes. The minimum recommended runner diameter for most materials is 1.5 mm (0.06 inches). Typical runner diameters for various unfilled generic materials are shown in Typical runner diameters for unfilled generic materials. Formula Following is the formula for runner dimension design: where D = runner diameter (mm) W = part weight (g) L = runner length (mm)

Design Rules Runner size Pick diameter based upon weight or mass of part and the average or nominal thickness. Draw a horizontal line from the mass until it intersects the thickness line and then read the value on x-axis

Design Rules Branched Runners Each time a runner is branched, the diameter of the branch runners should be smaller than the main runner Because less material flows through the branches and it is economically desirable to use minimum material in the runners. Where N is the number of branches, the relationship between the main runner diameter (dmain) and the branch runner diameter (dbranch) is

Design Rules Runner Intersections All runner intersections should have a cold solid slug well to help the flow of material through the runner system and into the cavity. Diagram below shows that the length of the well is usually equal to the runner diameter. Cold slug well is typically an extension of the runner at an intersection with another runner.

Mold Layout Rules Balancing of Runners Factors affect runner balancing Speed of injection In cold runners, the influence of mold cooling gradually narrows and restricts the cross section of the channels toward the gates, especially at slow injection speeds when the skin is large compared to the core To achieve a balance runner Use Moldflow balance runner program Try and keep cross sections similar when going from big diameter to smaller ones Runners should be as short as possible and as balanced as practical

Mold Layout Rules Two-Plate Molds (Cold Runners) Runner length Short as possible Runner cross section Small as possible. Circular section is most common. Trapezoidal is easier to machine and best. Flat runner will cool too quickly. Cavity Spacing Spaced closely (Section 6.1) Cavity Supports Need supports for ejector pins. Gate location Follow guidelines from before

Mold Layout Rules Two-Plate Molds (Cold Runners) Distance and time required for free fall Important for high speed molding Clearance between tie bars Molecular alignment of plastic and skin/core effects Center gating on edge is rarely used because it can cause a weak spot in the plastic. Better if end gated on same end, but clamp forces are unbalanced Best if gated on opposite ends

Mold Layout Rules Cavity Layout Single Cavity Edge gating in single cavity mold with open center (Fig 6.15) Can use 4 or 8 or 1 gate depending on weld line acceptance Used in simple parts, e.g., frames, rings, U-shaped products, etc. Can leave a gate mark Center gating in single-cavity mold with closed center (Fig 6.16) Sprue gate (Cold or hot) can be used. Can leave a unsightly gate mark. Used for pails, cups, trays, and many large products. If not acceptable then it must be edge gated (Fig 6.17) The cavity will be off centered, causing unbalanced loads. Useful to add another cavity to balance load and flow.

Mold Layout Rules Cavity Layout Dual Cavity (Fig 6.18) 2 basic layout for two cavities. Both are practical and used For Greater than 2 cavities many number of layouts are possible, including a circular layout or pattern (Fig 6.19) A combination of circular and rectangular layout can be used Circular pattern Has a shorter runner, but has a limited number of cavities layout in a circle. Rectangular pattern is easier to produce since designed in rectangular versus radial coordinates.

Mold Layout Rules Cavity Layout Dual Cavity Comparison of circular and rectangular layouts Shape of the product Small elongated parts are best with a circular (Spoons) Size of mold Circular layout uses a smaller mold. Balancing of runners Circular layout has a naturally balanced runner Point of gating The gate will be the narrowest with a circular layout Layout depends upon shape and size of the product, location of gate, runner, ejectors, cooling General Rules for Layout design Balanced runners, Symmetric layout, and drop height of product

Mold Layout Rules Three-Cavity Molds Four-Cavity Molds and greater Difference in drop height is important for high speed molding (Fig 6.20) Cavities are spaced 120° and are balanced Four-Cavity Molds and greater 4-cavity Fig 6.21 shows circular layout. H is larger than rectangular Fig 6.22 shows rectangular layout. Typical H & X type runners Rectangular Circular H H H H

Mold Layout Rules 5 cavities 6 cavities 7 cavities 8 cavities Design with Circular layout. Sometimes used 6 cavities Design with Circular layout or Rectangular (Fig 6.23) with an H style runner is preferred or 2 Y runners 7 cavities Design with Circular layout is possible though not recommended. Rectangular is preferred. 8 cavities Design with Rectangular layout. Fig 6.24 with X or Y branched runners. Variations with elongated product