Forming Standards

Thermoforming is an attractive process for prototype and development of a three dimensional plastic part due to it's quick turn around and low cost. As a result of it's inexpensive tooling and readily changeable fabrication, many projects are initiated from conceptual ideas, simple sketches or handmade models. We often find that our customers have a thorough knowledge of injection molding, metal stamping and other process techniques, but lack an understanding of vacuum and pressure forming variations. To aid in providing an insight into the variables of the process and to document the dimensional tolerance characteristics of this process, the following is provided for reference.

Variables Tolerances Draft Radii Dimensions

Variables

MACHINED TOOL - Our in-house CNC tools are produced to ± 0.002"

PATTERN - Normally a prototype starts from a hardwood pattern constructed by craftsmen in a woodworking shop. Expect ± .030" variation in the pattern.

CASTING - Normally an aluminum foundry sand casting. Aluminum shrinkage is .012 INCH/INCH ± .0015 INCH/INCH.

MATERIAL- Many factors integrate to cause variations in sheet materials. Extrusion thickness will vary .003" or 5% whichever is greater. Shrinkage will vary due to extrusion orientation, color additives and chemical composition with additional variations occurring between resin suppliers. Some common examples are:

¹ Polyethylene exhibits the most inconsistent shrink characteristics of all thermoformable plastics and should not be specified whenever dimensional stability is an important consideration.

TRIMMING - Jig and fixture designs for sawing, routing, drilling, bonding and assembly are subject to hand craftsmanship and influenced by the material variations.

Tolerances

FROM SMI CNC MACHINED ALUMINUM TOOL

1. Formed details at ±.020" up to 12" plus .001 INCH/INCH above 12"
2. Drilled hole centers at ±.015" up to 12" plus .001 INCH/INCH above 12".
3. Drilled hole diameters at ±.005".
4. Trim dimensions at ±.015" up to 12" plus .001 INCH/INCH above 12".

INDUSTRIAL THERMOFORMING FROM CAST ALUMINUM TOOL

1. Formed details at ±.030" up to 12" plus .001 INCH/INCH above 12"
2. Drilled hole centers at ±.015" up to 12" plus .001 INCH/INCH above 12".
3. Drilled hole diameters at ±.005" up to .375" diameter plus .001" per .063" above .375" diameter.
4. Trim dimensions at ±.015" up to 12" plus .001 INCH/INCH above 12" using CNC 5-axis equipment.
5. Joining with mechanical fasteners (rivets, bolts, etc.), all holes oversized by .015" to prevent stressing.

PACKAGING THERMOFORMING (less than .060" THICKNESS)

1. Formed details at ±.010" up to 6" plus .001 INCH/INCH above 6".
2. Punched hole diameters at ±.005" up to .375" diameter plus .001" per .063" above .375" diameter.
3. Punched hole centers at ±010" up to 6" plus .001 INCH/INCH above 6".
4. Trim dimensions at ±.02" up to 6" plus .001 INCH/INCH above 6".

Draft

VACUUM FORMING - 2° minimum to 6" high and 5° above 6" optimum for male tooling. Female cavities 0° draft.

PRESSURE FORMING - 0° minimum on polished surfaces and 2° minimum on textured surfaces. Note - 0° draft and undercuts can be obtained in many instances.

Radii

VACUUM FORMING - Outside radii at least one material thickness + .03" minimum to .06" ideal.

PRESSURE FORMING - Outside radii down to .03" (appearing as "0" radius).

Dimensions

All dimensions to be considered from mold side of material. Centerline dimensioning, when applicable, is preferred. XX ± .03 and XXX ± .02 unless otherwise estimated.

These standards are established by SMI to reflect what is normally bid when detailed dimensions are not specified or when drawings have been dimensioned to reflect the accuracies of other processes.

However, it is important to recognize that the key to cost effective production is to stay within these established process limits and then to loosen them whenever possible

Copyright © 2003-2005 by Specialty Manufacturing Inc.
Page last modified on 12 February, 2005 09:52 PM