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March 26, 1999

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Thixomolding design standards

TABLE I. THIXOMOLDING TECHNICAL DESIGN SPECIFICATIONS


1. LINEAR TOLERANCES


Dimension, mm

<25

25-50

50-100

100-150

200-300

Linear tolerance, mm

±.03
.06

±.100
.150

±.150
.250

±.250
.350

±.350
.600


2. CALLOUTS


 CALLOUT

 REFERENCE SURFACE

TOLERANCE mm/mm

Flatness

X

.002

Parallelism

X+Y

.003

Perpendicularity

Z+Y

.002

Concentricity

Z+Y

.003

Roundness

Z

.003


3. FILLETS AND EDGES


Sharp corners lead to stress concentrations

Radius in the range of .4 to .8 mm

Sharp edges not recommended

Radius in the range of .2 to .4mm


4. DRAFT ANGLE


<25mm

.25°

25 to 50 mm

.50°

50 to 80 mm

.75°

80 to 120 mm


5. WALL THICKNESS


Minimum

.4 mm

Maximum

20 mm

Elsewhere in this issue is a report on a new internationaljoint venture scheduled to start production next year of net-shapedmagnesium (Mg) parts using the relatively new Thixomolding process.Thixomolding offers tremendous business potential to molders,moldmakers, and end users serving a number of markets. This isbecause it is a means of manufacturing reliable, high-performanceproducts in high volumes and yields, and at a low cost (the materialcosts about 10 cents/cu inch).

Design standards for Thixomolding have begun to emerge. D. DunstanH. Peiris, director of technology, and his company, Advanced MaterialsTechnologies (AMT, Singapore), have contributed to the developmentof such standards to help familiarize parties more familiar withconventional processes like plastics injection molding or metaldiecasting with the Thixomolding materials and process. (AMT isan equity partner in the new international joint venture, calledSSIM Technologies, along with Amptech Corp. of Calgary, Alberta.)

TABLE II. POSITION OF THIXOMOLDING MATERIALS


 Material

Process

Density,
g/cu cm

Ultimate
tensile strength,
MPa

Yield
Strength,
MPa

Elongation,
%

Thermal
conductivity,
W/mK

AZ91D

Thixomolded

1.80

259

157

6

72

AM60

Thixomolded

1.80

282 

159

5

72

AZ91D

Diecast

1.80

220

130

8

62

Al 380

Diecast

2.70

250

.50

3

100

Steel

Cast

7.86

517

400

22

42

ABS

Injection molded

1.21

38

30

18

.2

Acetal, 25% PTFE

Injection molded

1.51

46

40

8

.26

98% Alumina ceramic

Pressed and sintered

3.81

284*

.35*

?

25

 

 

 

 (*Flexural strength)

(*Modulus of elasticity )

 

 


In addition to the table of specifications and the tablepositioning Thixomolding materials performance in relation tocompetitive materials in competitive processes, Peiris offersthe following 10 basic design guidelines.

  • Allow space for optimum gate location. Avoid gating at cosmetic surfaces.

  • Concentrate on uniform wall thickness and both progressive and free-flowing cavity filling. Choose the shortest metal travel path possible.

  • Reduce material by designing in cores and holes.

  • Design for functional surfaces rather than parting lines or trim surfaces.

  • Include reasonable draft or taper. Make use of the low draft capability of the process; even zero draft has been used.

  • Design a comfortable location for ejector pins.

  • Match bosses and studs to wall thickness.

  • Make reasonable use of the thin-wall capabilities of the process. Designing extremely thin walls results in increased costs. Design lettering to protrude. Place external undercuts on the parting line.

  • Think of rigidity and strength in lighter, thinner, shorter, and smaller product designs.

  • Design parts to minimize any secondary operations. For example, external threads can be molded in. Zero-degree draft eliminates boring holes. Most parts are produced net shape where no machining is required. Holes as small as 1 mm diameter and 1 inch deep can be molded.

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