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H&S Products was established in 1981 as a full service supplier of custom plastic products. We specialize in vacuum and pressure forming with the capability of.
H& S PRODUCTS Division of Holman Plastics, Inc.

An Inside Look at Vacuum & Pressure Forming

INTRODUCTION H&S Products was established in 1981 as a full service supplier of custom plastic products. We specialize in vacuum and pressure forming with the capability of forming parts from 6” x 6” to 84” x 84” x 48” deep or 48” x 96” x 36” deep. At H&S Products we are dedicated to serving our customers to the best of our abilities and that is the primary reason we do all of our own in-house tooling. This way we can stand by each product with the knowledge that the tool forming the part is of the highest possible quality. The primary advantages of pressure forming over injection molding are its low-cost tooling and shorter lead time. Design engineers and purchasing agents with medium to large production volume requirements find pressure forming an effective method of manufacturing. The ability to produce parts with sharp definitions gives the look of injection molding from the exterior of the part. Undercuts, zero or minus drafts, hidden vents, and reveal lines are easily achieved. A varied range of mold texturing is sometimes used when painting is not permissible. To get maximum detail, molds should be machined aluminum rather than cast. A machined mold will usually last the life of the part. By piecing the mold together, future modifications are cost effective. The need to add vacuum holes is eliminated by shimming inserted pieces, making paint preparation less time consuming. Vacuum and pressure formed parts are lightweight and have excellent structural integrity with high-impact strength. Also, a fairly uniform wall thickness can be achieved using different plug assists. With the addition of bosses and threaded inserts, internal parts such as PC boards and touch screens can be easily inserted. It is difficult to explain vacuum and pressure forming in its entirety due to the wide range of equipment and different types of materials used. However, this guide will give you a basic illustration of vacuum and pressure forming techniques. When designing a package for your equipment, whether it’s for the medical, electronic or recreational industries, we recommend you consider this process.

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Even though vacuum forming has been around for years, the techniques in producing high quality parts have changed: larger, deeper, and more detailed parts can be produced with advanced machine design.

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Conventional vacuum forming is accomplished by evenly heating the plastic material and placing it over a male or female mold which makes a seal and evacuates the air. The part is then cooled over the mold until it is cool enough to remove. By pre-drawing the materials into a hemispherical shape before forming over a male mold, deep parts can be produced without cold flow or excessive thickness variation. An alternative process called billow forming can also be used. Billow forming is a technique that blows a bubble into the material. The mold is then forced into the bubble and the vacuum snaps the material over the mold. This process is sometimes called vacuum snap-back. The drawings illustrate the various techniques of vacuum forming when inside dimensions are critical. When outside dimensions are required, female molds are used. When sharp details on the outside are required, we recommend pressure forming.

ILLUSTRATIVE LOOK AT THE PROCESS

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1. EMI-RFI Shielding

FCC approved coatings shield and protect components. Coatings can be applied to all inside surfaces as required.

2. Textures & Finishes

A wide range of coats and surface textures can be either molded in or painted depending on customer’s corporate color schemes.

3. Vents & Louvers

Vents can be formed vertically or horizontally on any surface. Formed vents are tight, clean and sharply defined.

4. Logo Depressions

Areas for logos or label plates add a custom personalized look. Logos can be either embossed or debossed. Other methods include hot-stamping, silkscreening and painting.

5. Mating Surfaces

Formed mating surfaces such as undercuts and relief lines allow a smooth, clean fit that is strong and durable.

6 6. Cut-Outs

Formed cut-outs allow for A/C plugs, cables and connectors. Counter-sunk holes are for mounting additional hardware.

7. Zero (0º) Draft

Zero draft or negative draft is easily achieved.

8. Mounting Bosses & Flanges

Projecting collars or rims known as bosses or flanges are used to hold parts together and give them strength. Bosses, available with or without threaded inserts, are accurately located within plus or minus .010 of an inch.

9. Trimmed Openings

Openings allow for easy access to hardware. Components such as keyboards, LED displays and switches can protrude through these openings.

PRESSURE FORMING Pressure forming is a refined extension of vacuum forming and is by far the more advanced method of thermoforming. Pressure formed parts are produced using a female mold. From the outside, pressure formed parts are virtually indistinguishable from injection molded parts.

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In pressure forming, as in Vacuum Forming, the plastic sheet is first heated. A pressure vessel is then lowered over the sheet onto the top of the mold and vacuum chamber. The two platens on the machine lock together for a positive seal. Next, vacuum pressure is applied to the mold below the plastic sheet and air pressure is simultaneously forced on top of the sheet. The air pressure can be as high as 150 pounds per square inch and is constant and steady. After the part is formed, the platens separate and fans cool it while the vacuum acts as a clamp.

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The same techniques used in vacuum forming, such as vacuum snap-back and pre-stretching the material, can also be used in pressure forming when necessary.

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MATERIALS Most thermoplastic sheets can be processed using vacuum or pressure forming. Listed below are the most popular materials used. Heavier gauge materials, ranging from .060” to .375” thick, give you a wide range of products. Acrylonitrile Butadiene Styrene (ABS), used for cases, computer and monitor housings, side panels, and other applications, is the most common material in pressure forming. ABS is durable, forms well, and is moderately priced. It also comes in a variety of opaque colors. Flame retardant material meeting UL-94V0 rating is also available. Polycarbonate is used when high-impact, flame retardance, and relatively high heat resistance are required. Acrylic is commonly used in vacuum forming when clarity is required. Pressure forming is not recommended for this reason. Acrylic has medium impact strength and is resistant to ultraviolet light. Polypropylene, a member of the Polyolefin family of thermoplastics, exhibits many properties that make it an excellent choice for corrosion resistant applications. High-Impact Polystyrene has a high dielectric strength and is widely used in packaging applications where cost is an issue. Polyvinyl Chloride (PVC) is the most widely used member of the vinyl family. Its major use is for corrosion resistance applications. Unlike other olefins, PVC can be cemented when bonding is required. PVC resins are normally mixed with other additives such as impact modifiers and stabilizers, providing hundreds of PVC based materials with a variety of engineering properties. Copolyester (PETG) sheet material features excellent clarity and delivers impact strength that acrylic cannot withstand. It may be used in deep draw situations. Mold detail reproduction is outstanding when PETG is used.

H& S PRODUCTS

Division of Holman Plastics, Inc.

48368 Milmont Drive, Fremont, CA 94538

P (510) 659-1550

F (510) 659-0777

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