Diamond Systems Tritan Guia de Resolução de Problemas descarregar pdf (Página 22)

Family molds

Family molds contain two or more cavities that mold different

parts. Eastman

™

polymers are being used successfully in family

molds. Like any other polymer, their ﬂow into the individual

parts must be balanced. All parts should ﬁll evenly and equally.

Otherwise, uneven packing will occur; some parts will be

overpacked and highly stressed, leading to warpage, and other

parts will be underpacked or not completely ﬁlled.

Note: Family molds should be avoided if possible because

balanced ﬂow is difﬁcult to achieve. If family molds are required,

ﬂow should be balanced by varying runner diameters, not

gate size.

Mold polishing and texturing

Mold polishing

Eastman

™

polymers provide excellent gloss and pick up mold

ﬁnish very well. Keep in mind that surfaces polished smoother

than required for ejection only add to mold cost. In most cases,

highly polished surfaces can hinder ejection if there is a vacuum

drawn in low or no draft areas. Where no vacuum is drawn,

polished surfaces generally eject better.

The following guidelines are suggested:

• Specify SPI mold ﬁnish standards.

• Specify surfaces smooth enough to minimize ejection force.

• Specify ﬁnal polish in the direction of draw to minimize scufﬁng.

• Add a light 320 dry grit blasting (SPI B3 ﬁnish) to drafted walls

to reduce the possibility of a vacuum forming during ejection.

Texturing mold surfaces

Texturing is useful in hiding weld lines, ﬂow marks, gate blush,

sink marks, and scufﬁng. There are hundreds of standard

patterns available. Basically, anything that can be drawn in

black and white can be used as the basis for a texture pattern.

It is important to decide on a texture pattern early in the design

process so that the proper draft angles and contours can be

incorporated into the part.

Typical texture depth is 0.06–0.08 mm (0.0025–0.0030 in.). To

aid ejection, 1°–1.5° draft should be added for each 0.025 mm

(0.001 in.) of texture depth.

Texturing methods

There are many different methods for applying textured surfaces:

• Mold polishing—User controls degree of gloss by varying

polishing grit diameter.

• Photoetching—Most common texturing procedure

• Electric Discharge Machining (EDM) or spark erosion—Makes

economic sense if the cavity of the tool is eroded by EDM also

• Sandblasting—Suitable only for mostly ﬂat surfaces

• Matte chromium platin—Produces a matte, wear-resistant

texture

Preventing surface defects

Careful consideration should be given to the part design if a

surface ﬁnish is expected on the ﬁnal product. Abrupt changes

in wall thickness, relatively thick sections, or heavy ribs can

cause variations in surface gloss. Uniform mold temperatures

are needed to ensure that the texture is even throughout the

pattern.

To achieve a higher matte ﬁnish, double or triple the texture

etching.

If weld lines are visible on the ﬁnal product, one alternative is

to move the gate positions so that the weld line is formed in a

less visible place. If this is not possible, apply a rounded texture

pattern to help hide the blemish.

If the material you are molding is expected to have poor scratch

resistance, use a rounded texture pattern to hide potential marks.

Draft angle guidelines

In most cases, 1° draft per side is suggested to aid ejection.

However, 1⁄

° per side can be used to obtain reasonable dimensions

in ribs, bosses, and other design features. Attention to the

thickness at the top of ribs or bosses is needed to ensure

structural strength.

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