When specifying the material for various mold components, the mold designer should also consider the uniformness of the stock material as well as its ability to be finished and treated. Most metals are cast and subsequently rolled/formed/slit to their supplied shape. The resulting grain structure and properties are a complex function of not only the constitutive alloying elements but also the thermal and structural history during processing. Mold designers, mold makers, and end-users should be aware that there are many issues such as porosity (voids), contaminants, inhomogeneity, and residual stress that may impact the quality of the machined mold.
For these reasons, it is recommended that the mold components be machined from annealed or normalized steel with minimal residual stress and uniform properties. These treatments can provide for lower hardness and faster machining. After the mold has been machined and finished, the finished component may again be annealed to verify dimensional stability and then carburized (also known as case hardening) to improve surface hardness by increasing the carbon content at the surface. Carburizing is performed at 900–950°C, and can cause dimensional changes in machined components. There are many alternative surface treatments
that are available including nitriding, boriding, plating, vapor deposition, anodiz- ing, and others. Table 4.2 provides a comparison of several surface treatments that may be selected for specific application-specific purposes including increased hardness, reduced coefficient of friction (COF), hardness, improved corrosion re- sistance, and others [7, 8].
Table 4.2 Common Surface Treatments
Treatment Properties Purpose & notes
Diamond chrome plating
at ~60°C 50 µm thick; 85 RC; 0.15 COF Super hard low-friction coating but requires anode Hard chrome plating
at ~70°C Up to 0.5 mm thick; 72 RC; 0.2 COF Hard coatings; can be thickened for repair of surfaces but requires anode Nickel-boron nitriding at
~600°C 10 µm thick; 67 RC; 0.05 COF Thin layer for low friction and excellent abrasion resistance Electroless nickel coating
at ~80°C 50 µm thick; 62 RC; 0.4 COF Lower-cost alternative to hard chrome for corrosion and abrasion resistance Nickel-PTFE coating 45 RC;
0.10 COF Excellent release for deep ribs, cores with low draft, textured surfaces, and tacky polymers Carburizing at ~900°C 60 RC surface Used with low carbon steels to “case harden”
surfaces to depth of 6 mm Physical vapor deposition
(PVD) of titanium nitride 5 µm thick; 70 RC; 0.1 COF Very thin but hard coatings applied at ~500°C in reduced pressure nitrogen atmosphere Thermoreactive diffusion
(TRD) of silicon carbide 5 µm thick; 70 RC; 0.1 COF Very thin but hard coatings applied by molten salts at high temperatures Aluminum anodizing 50 µm thick;
65 RC Provides hard, corrosion and wear resistant surface for aluminum
In most applications, mold makers will outsource the mold components to be treated by service providers that specialize in surface treatments. These treat- ments will increase the initial purchase cost of the mold but can greatly increase the longevity and reduce associated maintenance costs, especially when process- ing abrasive resins or long production runs. Molders also often rely on surface treatments to resolve issues such as improving lubricity of mold surfaces to ease part ejection, reduce wear between sliding components, repair scratches, or improve the surface of welded sections. Diffusion processes (e. g., carburizing, nitriding) do not grow the thickness of the mold surface as opposed to coatings, which may add substantial mass. Mold designers and mold makers should take note of the thickness and consistency of the applied coating, so that cavity wall thicknesses are designed to provide an allowance for its thickness as appropriate. Fortunately, many coatings may be stripped and reapplied if repair or alterations in surface properties are needed.
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4.5 Chapter Review
4.5Chapter Review
The mold layout design process includes the examination of the part geometry to be molded to identify the parting line, parting plane, and shut-offs. The core and cavity inserts are then sized and located relative to each other. Afterwards, a suitable mold base is chosen or designed that can efficiently hold and support the core and cavity inserts. The mold layout process finishes with the selection of the materials used for the mold base as well as the core and cavity inserts. In many mold-making companies, these materials are immediately ordered concurrently with the detailed analysis and design of the mold subsystems.
After reading this chapter, you should understand:
How to identify the mold opening direction(s) and parting line(s) for a molded
part,
How to design a parting plane and shut-offs to separate the core insert from the
cavity insert,
How to size the length, width, and height dimensions for the core and cavity inserts,
The advantages and disadvantages of different cavity layouts,
How to lay out a given number of mold cavities, How to size a mold base for a given mold cavity layout,
How to verify that a mold is appropriate for a molding machine, and
Properties and selection of mold materials and surface treatments.
Figure 4.28 provides a mold design checklist for high performance, standard, and basic molds; the details are presented throughout the book. The next chapter examines the mold cavity filling process, which is required to 1) verify that the part design can be produced at available melt pressures, and 2) estimate the load- ing that will be placed on the mold components. Afterwards, the analysis and de- sign of the feed system will be addressed.
Stag e Highest Volume/Qualit y Standard Volume/Qualit y Basic/Prototypin g
Inspecon Item ( means important, means oponal)
Number of cavies & type of mold carefully selected Mold specificaon/purchase agreement completed Part design reviewed: thickness, gang, dra, undercuts, etc. Shrinkage guidance provided by molder or end-user Mold design standards (DIN, JIS, inch) specified Supply chain preferences specified
Target producon quanty/rate specified P20 (DME#2, DIN 1.2311) or stainless mold base H13 or similarly hard cavity & core insert materials Hot runner system without sub-runners Electrical connectors (male) at top of mold Limit switches on slides/pulls for confirming posion Mulple knock-out locaons spanning ejector plate Early return ejectors (posive return)
Wear plates on four sides of mold
Parng line interlocks with cavity/core inserts Cavity pressure/temperature transducers
Thermal insulaon to minimize heat transfer to mold/platens Springs on slides to prevent accidental movement Greased ways with fings on inaccessible slides Parts/mold designed for robot interface Mold cycle counter
Mold designed for fully automac operaon Uniform cooling provided to all core & cavity inserts Cooling "O" rings/seals recessed to avoid damage Ejector plates guided by pins/bushings
Leader pins/bushings engage prior to other components Venng provided at end of flow and in blind pockets Dra used to assist ejecon, esp. deep cores & textures Mulple ejectors to push on sff secons of parts Ejector & core pins through-hardened Parng line interlocks
Dowel pins used for mang with locaon fits Horn pins less than 20 degrees to avoid wear
Mold guide pins/bushings to engage prior to mold closure All bolts, pins, plates, etc. standard stock parts when possible Recessed water connectors at bo¡om of mold
Date/shi/logo inserts for run idenficaon Mold-actuated lis/slides used instead of core pulls Mold inserts for runners or other high-wear areas Mold cavity surfaces hardened & heat treated Hardened stainless used for corrosive resins
Chrome or nitrided surface treatements used for abrasive resins Mulple support pillars with preload
Early return ejectors (springs)
AISI4130 (DME #2 , DIN1.1312) mold base P20 or similarly hard cavity & core materials
Hot sprue bushing or hot runner system with sub-runners Runner shut-offs for changing flow pa¡erns
Sucker pin on cold runners & near gates Steel/aluminum/no mold base (or master unit die) Cavity & core from 6160 or other prototyping materials
Start of Desig n Performance Desig n Standard Desig n Basi c
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4.6 References
4.6References
[1] Sachs, E., et al., Production of injection molding tooling with conformal cooling channels using the three dimensional printing process, Polym. Eng. Sci. (2000) 40(5): pp. 1232–1247
[2] Kovács, J. G., et al., Thermal simulations and measurements for rapid tool inserts in injection molding applications, Appl. Therm. Eng. (2015) 85: pp. 44–51
[3] Sachs, E., et al., Three dimensional printing: rapid tooling and prototypes directly from a CAD model,
J. Manuf. Sci. Eng. (1992) 114(4): pp. 481–488
[4] Roberts-Austen, W., On the Hardening and Tempering of Steel, Sci. Am. (1889) 28: pp. 11520–11522
[5] Pavlina, E. and C. Van Tyne, Correlation of yield strength and tensile strength with hardness for steels,
J. Mater. Eng. Perform. (2008) 17(6): pp. 888–893
[6] Oberg, E., et al., Machinery’s Handbook, Green, R. E., (Ed.), 24th ed. (1992)
[7] Surface Treatments Applicable for Molds, Kata Gijutsu (1989) 10
5
For an acceptable molded part to be produced, the polymer melt must completely fill the mold cavity. Accordingly, the wall thickness of the molded part and the gat- ing locations must be specified such that the melt is able to traverse from the gates to the edge of the cavity. Mold filling analysis is used to ensure that the melt can not only fill the mold at achievable molding pressures, but fill the mold as intended to achieve the desired quality.
5.1Overview
Cavity filling analysis may be performed for a variety of purposes. On the most basic level, mold filling analysis is useful to ensure that the mold cavity can be filled with the plastic melt given the melt pressure that can be delivered by the molding machine. Typically, the melt pressure required to fill the cavity is less than 100 MPa (about 15,000 psi) even though most modern machines can supply twice this amount. This safety margin between the required and available melt pressures provides an allowance for the pressure drop in the feed system, and also ensures that the mold can be filled given possible variances in the material proper- ties or molding process.
Cavity filling analysis is also performed to ensure that the filling pressures are not too low, since very low melt pressures are indicative of a poor molded part design or improper processing conditions. Excessively thick wall sections will result in low pressures, excessive material costs, and extended cycle times. In such cases, the nominal wall thickness should be decreased and ribs or other features used to provide the necessary strength and stiffness. In some cases, very low melt pres- sures can indicate improper filling time, mold temperature, or melt temperature. These processing conditions should be adjusted to reduce the processing time and cost at the expense of higher melt pressures.