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Home » Technical Reference

Product Design


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Product Design-Threads
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Product Design-Threads
Molded-in threads can be designed into parts made of engineering thermoplastic resins. Threads always should have radiused roots and should not have feather edges — to avoid stress concentrations. The Recommended Design for Molded-in Thread illustration shows examples of good design for molded-in external and internal threads. For additional information, see molded-in threads in Fasteners. Threads also form undercuts and should be treated as such when the part is being removed from the mold i.e., by provision of unscrewing mechanisms, collapsible cores, etc. Every effort should be made to locate external threads on the parting line of the mold where economics and mold reliability are most favorable.

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Product Design – Radii
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Product Design Radii
It is best not to design parts with sharp corners. Sharp corners act as notches, which concentrate stress and reduce the part's impact strength. A corner radius, as shown in the Suggested Design for Corner Radius illustration, will increase the strength of the corner and improve mold filling. The radius should be in the range of 25% to 75% of wall thickness; 50% is suggested. The Stress Concentration as a Function of Wall Thickness and Corner Radius illustration shows stress concentration as a function of the ratio of corner radius to wall thickness, R/T.

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Product Design-Ribs and Gussets
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Product Design-Ribs and Gussets
When designing ribs and gussets, it is important to follow the proportional thickness guidelines shown in the Example of Rib Design illustration and the Example of Gusset Design illustration. If the rib or gusset is too thick in relationship to the part wall, sinks, voids, warpage, weld lines (all resulting in high amounts of molded-in stress), longer cycle times can be expected.

The location of ribs and gussets also can affect mold design for the part. Keep gate location in mind when designing ribs or gussets. For more information, see Gates. Ribs well-positioned in the line of flow, as well as gussets, can improve part filling by acting as internal runners. Poorly placed or ill-designed ribs and gussets can cause poor filling of the mold and can result in burn marks on the finished part. These problems generally occur in isolated ribs or gussets where entrapment of air becomes a venting problem.

Note: It is further recommended that the rib thickness at the intersection of the nominal wall not exceed one-half of the nominal wall in HIGHLY COSMETIC areas. For example, in the Example of Rib Design illustration, the dimension of the rib at the intersection of the nominal wall should not exceed one-half of the nominal wall.
Experience shows that violation of this rule significantly increases the risk of rib read-through (localized gloss gradient difference).

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Product Design-Bosses
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Product Design-Bosses
Bosses are used in parts that will be assembled with inserts, self-tapping screws, drive pins, expansion inserts, cut threads, and plug or force-fits. Avoid stand-alone bosses whenever possible. Instead, connect the boss to a wall or rib, with a connecting rib as shown in the Recommended Design of a Boss Near a Wall (with Ribs and Gussets) illustration. If the boss is so far away from a wall that a connecting rib is impractical, design the boss with gussets as shown in the Recommended Design of a Boss Away From a Wall (with Gussets) illustration.

The Recommended Dimensions for a Boss Near a Wall (with Rib and Gussets) illustration and the Recommended Dimensions for a Boss Away From a Wall (with Gusset) illustration give the recommended dimensional proportions for designing bosses at or away from a wall. Note that these bosses are cored all the way to the bottom of the boss.

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Draft Angle
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Draft Angle
So that parts can be easily ejected from the mold, walls should be designed with a slight draft angle, as shown in the Exaggerated Draft Angle illustration. A draft angle of 1/2° draft per side is the extreme minimum to provide satisfactory results.

1° draft per side is considered standard practice. The smaller draft angles cause problems in removing completed parts from the mold. However, any draft is better than no draft at all.

Parts with a molded-in deep texture, such as leather-graining, as part of their design require additional draft. Generally, an additional 1° of draft should be provided for every 0.025 mm depth of texture.

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Nominal Wall Thickness
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Nominal Wall Thickness
For parts made from most thermoplastics, nominal wall thickness should not exceed 4.0 mm. Walls thicker than 4.0 mm will result in increased cycle times (due to the longer time required for cooling), will increase the likelihood of voids and significantly decrease the physical properties of the part. If a design requires wall thicknesses greater than the suggested limit of 4.0 mm, structural foam resins should be considered, even though additional processing technology would be required.

In general, a uniform wall thickness should be maintained throughout the part. If variations are necessary, avoid abrupt changes in thickness by the use of transition zones, as shown in the Suggested Design for Wall Thickness Transition Zone illustration. Transition zones will eliminate stress concentrations that can significantly reduce the impact strength of the part. Also, transition zones reduce the occurrence of sinks, voids, and warping in the molded parts.

A wall thickness variation of ±25% is acceptable in a part made with a thermoplastic having a shrinkage rate of less than 0.01 mm/mm. If the shrinkage rate exceeds 0.01 mm/mm, then a thickness variation of ±15% is permissible.

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Product Design-Undercuts
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Because of the rigidity of most engineering thermoplastic resins, undercuts in a part are not recommended. However, should a design require an undercut, make certain the undercut will be relieved by a cam, core puller, or some other device when the mold is opened.

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Mold Design


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Sprue Bushings
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Sprue Bushings
Sprue bushings connect the nozzle of the injection molding machine to the runner system of the mold. Ideally, the sprue should be as short as possible to minimize material usage and cycle time. To ensure clean separation of the sprue and the bushing, the bushing should have a smooth, tapered internal finish that has been polished in the direction of draw (draw polished.) Also, the use of a positive sprue puller is recommended. The Three Common Sprue Pullers illustration shows three common sprue puller designs.

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Mold Design-Runners
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Mold Design-Runners
Runner Geometry of Conventional Mold
Runner systems convey the molten material from the sprue to the gate. The section of the runner should have maximal cross-sectional area and minimal perimeter. Runners should have a high volume-to-surface area ratio. Such a section will minimize heat loss, premature solidification of the molten resin in the runner system, and pressure drop.

The ideal cross-sectional profile for a runner is circular. This is known as a full-round runner, as shown. While the full-round runner is the most efficient type, it also is more expensive to provide, because the runner must be cut into both halves of the mold.

A less expensive yet adequately efficient section is the trapezoid. The trapezoidal runner should be designed with a taper of 2 to 5° per side, with the depth of the trapezoid equal to its base width, as shown. This configuration ensures a good volume-to-surface area ratio.

Half-round runners are not recommended because of their low volume-to-surface area ratio. The Three Conventional Runner Profile illustration shows the problem. If the inscribed circles are imagined to be the flow channels of the polymer through the runners, the poor perimeter-to-area ratio of the half-round runner design is apparent in comparison to the trapezoidal design.

Runner Diameter Size
Ideally, the size of the runner diameter will take many factors into account — part volume, part flow length, runner length, machine capacity gate size, and cycle time. Generally, runners should have diameters equal to the maximum part thickness, but within the 4 mm to 10 mm diameter range to avoid early freeze-off or excessive cycle time. The runner should be large enough to minimize pressure loss, yet small enough to maintain satisfactory cycle time. Smaller runner diameters have been successfully used as a result of computer flow analysis where the smaller runner diameter increases material shear heat, thereby assisting in maintaining melt temperature and enhancing the polymer flow. Large runners are not economical because of the amount of energy that goes into forming, and then regrinding the material that solidifies within them.

Runner Layout
Similar multicavity part molds should use a balanced "H" runner system, as shown in the Runner System Layouts illustration. Balancing the runner system ensures that all mold cavities fill at the same rate and pressure. Of course, not all molds are multicavity, nor do they all have similar part geometry. As a service to customers, Dow Plastics offers computer-aided mold filling analysis to ensure better-balanced filling of whatever mold your part design requires. Utilizing mold filling simulation programs enables you to design molds with:
  • Minimum size runners that deliver melt at the proper temperature, reduce regrind, reduce barrel temperature and pressure, and save energy while minimizing the possibility of material degradation.
  • Artificially balanced runner systems that fill family tool cavities at the same time and pressure, eliminating overpacking of more easily filled cavities.
Cold Slug Wells
At all runner intersections, the primary runner should overrun the secondary runner by a minimum distance equal to one diameter, as shown in the Recommended Design of a Cold Slug Well illustration. This produces a feature known as a melt trap or cold slug well.

Cold slug wells improve the flow of the polymer
by atching the colder, higher-viscosity polymer moving at the forefront of the molten mass and allowing the following, hot, lower-viscosity polymer to flow more readily into the mold-cavity. The cold slug well thus prevents a mass of cold material from entering the cavity and adversely affecting the final properties of the finished part.

Runnerless Molds
Runnerless molds differ from the conventional cold runner mold (see Conventional Cold Runner Mold illustration) by extending the molding machine's melt chamber and acting as an extension of the machine nozzle. A runnerless system maintains all, or a portion, of the polymer melt at approximately the same temperature and viscosity as the polymer in the plasticating barrel. There are two general types of runnerless molds: the insulated system, and the hot (heated) runner system.

Insulated Runners
The insulated runner system (see Insulated Runner Mold illustration) allows the molten polymer to flow into the runner, and then cool to form an insulating layer of solid plastic along the walls of the runner. The insulating layer reduces the diameter of the runner and helps maintain the temperature of the molten portion of the melt as it awaits the next shot.

The insulated runner system should be designed so that, while the runner volume does not exceed the cavity volume, all of the molten polymer in the runners is injected into the mold during each shot. This full consumption is necessary to prevent excess build-up of the insulating skin and to minimize any drop in melt temperature.

The many advantages of insulated runner systems, compared with conventional runner systems, include:
  • Less sensitivity to the requirements for balanced runners.
  • Reduction in material shear.
  • More consistent volume of polymer per part.
  • Faster molding cycles.
  • Elimination of runner scrap — less regrind.
  • Improved part finish.
  • Decreased tool wear.
However, the insulated runner system also has disadvantages. The increased level of technology required to manufacture and operate the mold results in:
  • Generally more complicated mold design.
  • Generally higher mold costs.
  • More difficult start-up procedures until running correctly.
  • Possible thermal degradation of the polymer melt.
  • More difficult color changes.
  • Higher maintenance costs.
Hot Runners
The more commonly used runnerless mold design is the hot runner system, shown in the Hot Runner Mold illustration. This system allows greater control over melt temperatures and other processing conditions, as well as a greater freedom in mold design — especially for large, multicavity molds.

Hot runner molds retain the advantages of the insulated runner over the conventional cold runner, and eliminate some of the disadvantages. For example, start-up procedures are not as difficult. The major disadvantages of a hot runner mold, compared with a cold runner mold, are:
  • More complex mold design, manufacture, and operation.
  • Substantially higher costs.
These disadvantages stem from the need to install a heated manifold, balance the heat provided by the manifold, and minimize polymer hang-ups.

The heated manifold acts as an extension of the machine nozzle by maintaining a totally molten polymer from the nozzle to the mold gate. To accomplish this, the manifold is equipped with heating elements and controls for keeping the melt at the desired temperature. Installing and controlling the heating elements is difficult. It is also difficult to insulate the rest of the mold from the heat of the manifold so the required cyclic cooling of the cavity is not affected.

Another concern is the thermal expansion of the mold components. This is a significant detail of mold design, requiring attention to ensure the maintenance of proper alignment between the manifold and the cavity gates. (For more information on thermal expansion, see the information on thermal stress analysis in Thermal Properties.)

Currently there are many suppliers and many available types of runnerless mold systems. In most cases, selection of such a system is based primarily on cost and design limitations — be careful in evaluating and selecting a system for a particular application.

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Moldflow Analysis


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Moldflow Analysis Sample-1
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Moldflow Analysis Sample-1

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Mold design
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Mold design and construction requires special attention for optimal product quality and reliable molding. A detailed specification is required in advance:
  • product shape and tolerances
  • mold in relation to molding equipment
  • parting lines; venting
  • number of cavities
  • runner lay-out and gating system
  • ejection system
  • cooling system lay-out -type of tool steel
  • surface finish.

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Plastic Product


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Abbreviations for the Plastics
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Injection molding is a manufacturing process where plastic is forced into a mold cavity under pressure. A mold cavity is essentially a negative of the part being produced. The cavity is filled with plastic, and the plastic changes phase to a solid, resulting in a positive. Typically injection pressures range from 5000 to 20,000 psi. Because of the high pressures involved, the mold must be clamped shut during injection and cooling. Clamping forces are measured in tons.

Abbreviations for the Plastics Industry
Plastics, Preproducts and Rubbers

AAS Methacrylate-acrylic-styren PEEK Polyaryletherketone
ABS Acrylonitrile-butadiene-styrene PEI Polyetherimide
ACM Acrylic acid ester rubber PEO, PEOX Polyethylene oxide
ADC Allyl diglycol carbonate PEPA Polyester block amides
AES Acrylonitrile-ethylene- propylene-styrene PEP Polyethylene-propylene
AMMA Acrylonitrile-methyl methacrylate PES Polyester sulphone
ANM Acrylic acid ester rubber PET, PETP Polyethylene terephthalate
APP Atactic polypropylene PETG Polyethylene terephthalate, glycol comonomer
ASA Acrylonitrile-styrene-acrylic ester PF Phenol formaldehyde
AXS Acrylonitrile-styrene- terpolymers PFA Perfluoro alkoxy alkaline
BR Cis-1, 4-polybutadiene rubber PFEP Polytetrafluorethylene- perfluoro-propylene
BS Butadiene-styrene rubber PFF Phenol-furfural
CA Cellulose acetate PI Polyimide
CAB Cellulose acetate-butyrate PIB Polyisobutylene
CAP Celluose acetate propionate PIBI Butyl rubber
CF Cresol formaldehyde PIR Polyisocyanate
CHR Epichlorhydrine PMCA Polymethyl - chloroacrylate
CMC Carboxymethyl cellulose PMI Polymethacryloimide
CN Cellulose nitrate PMP Poly- 4 -methylpentene-1
CP Cellulose propionate POM Polyoxymethylene, polyacetal
CPE Chlorinated polyethylene (correctly:PEC) PP Polypropylene
CPVC Chlorinated polyvinylchloride (correctly:PVCC) PPC Chlorinated polypropylene
CR Chloroprene rubber PPMS Polyparamethylstyrene
CS Casein PPO(S) Polyphenylene oxide (styrene)
CSM Chlorosulfonated polyethylene PPOX Polypropylene oxide
CTA Cellulose triacetate PPS Polyphenylene sulfide
DAP Diallyl phthalate PPSU Polyphenylene sulfone
EC Ethyl cellulose PS Polystyrene
ECB Ethylene-cop-bitumen PSB Styrene butadiene rubber
ECTFE Ethylene-chlorotrifluoro- ethylene PSU Polysulfone
EEA Ethylene-ethylacrylate PTFE Polytetrafluorethylene
EMA Ethylene-methacrylic acid PTP Polyterephthalates
EP Epoxy epoxide PUR Polyurethane
EPDM Ethylene-propylene teropolymer rubber PVAC Polyvinyl acetate
EPM Ethylene-propylene rubber PVAL Polyvinyl alcohol
EPS Expanded polystyrene PVB Polyvinyl butyral
ETFE Ethylene-tetrafluroethylene PVC Polyvinyl chloride
EVA, EVAC Ethylene-vinyl acetate PVCA Polyvinyl chloride-acetate
FEP Perfluoro ethylene-propylene PVCC Chlorinated polyvinyl chloride
FF Furan formaldehyde PVDC Polyvinylidene chloride
GR-I Butyl rubber PVDF Polyvinylidene fluoride
GR-N Nitrile rubber PVFM Polyvinyl formal
GR-S Styrene-butadiene rubber PVK Polyvinyl carbazole
IIR Butyl rubber PVP Polyvinyl pyrrolidone
IPDI Isophorone diisocyanate RF Resorcin formaldehyde
IR Cis-1, 4-polyisoprene rubber SAN Styrene-acylonitrile
MBS Methylmethacrylate- butadiene- styrene SB Styrene-butadiene
MC Methyl cellulose SBR Styrene-butadiene rubber
MDI Diphenylmethane diisocyanate SI Silicone plastics
MF Melamine formaldehyde Si Silicone rubber
MMA Methylmethacrylate SMA Styrene-maleic anhydride
MPF Melamine-phenol- formaldehyde SMS Styrene - methylstyrene
NBR Nitrile rubber SRP Styrene-rubber-plastics
NC Cellulose nitrate TAC Triallylcyanurate
NR Natural rubber TFA Fluor-alkoxy-terpolymer
PA Polyamide (nylon) TDI Toluyl diisocyanate
PAA Polyacrylic acid TMDI Trimethyl-hexamethylene diisocyanate
PAI Poly-amideimide TPU Thermoplastic polyurethane
PAK Polyester alkyd TPX Polymethylpentene
PAN Polyacrylonitrile UF Urea formaldehyde
PB Polybutene-l UP Unsaturated polyester
PBAN Polybutadiene-acrylonitrile VAC Vinyl acetate
PBS Polybutadiene-styrene VC Vinyl chloride
PBTP Polybutylene therephthalate VCE Vinyl chloride-ethylene
PC Polycarbonate VCEVA Vinyl chloride-ethylene- vinyl acetate
PCD Polycarbodiimide VCOA Vinyl chloride-octylacrylate
PCTFE Polymonochlorotri fluoroethylene VCVAC Vinyl chloride-vinyl acetate
PDAP Polydiallyl phthalate VCVDL Vinyl chloride-vinylidene chloride
PE Polyethylene VF Vulcanized fibre
PEC Chlorinated polyethylene    

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Exploded View of Moulds
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Exploded View of Moulds
Exploded View of Moulds
Description
The modular spacer molds provide the exploded view of head component mold defining a first opening, a head connector positioned within the first opening of the head component mold, a stem component mold defining a second opening, and a stem connector to fit within the second opening of the stem component mold to mateably engage the head connector. The complete molding can be provided with optional accessories and related kit as per the requirements of the customers.

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