Articles
Pressure Forming: It Makes More Sense Than Ever - From Job Shop Technology

In the last 5-7 years there has been more innovation and progress in Thermoforming than in the prior 60. This means it could now be the best process for your product.

At a recent SPE Thermoforming conference a member of the plastic press corps, was heard to remark "This stuff really looks injection molded." Yes pressure forming has really come a long way in the last 15 years. Many designers who formerly weren't confident enough in the process to take advantage of its tremendous savings over injection molding costs are now making it the process of choice in many circumstances.

Pressure forming has been a real asset to designers who want plastic parts with a high price look but don't have the volume to justify expensive injection molding tooling. Pressure formings low cost tooling and its increasing capability to supply sharp detail, is becoming more appreciated each year. Louvers, label recesses, logos, and undercuts for mounting hardware or for hidden mating surfaces and numerous other features can be molded in. Many Pressure formers now use sophisticated CNC mills and routers for trimming. This allows many details to be trimmed precisely at lower cost and many features not available before can be added. This makes it possible for designers to improve the way products look and feel. Additionally many structural features can be added without sending costs through the roof.

Look and feel are as important as performance in the customer's mind. Design features often are what set products apart in the market place. To get the tight look injection molding is generally the process of choice. Quite often there is uncertainty about whether a products volume will be enough to justify injection molding's expensive tooling and long lead times. This is especially true with larger complex parts or medical equipment with high sales prices but lower volumes. Pressure forming can give you the look and feel you need for your product to sell. Then if sales volume increases enough you can move to Injection molding and your product will look, feel and perform the same. If sales volume doesn't increase to those levels you still have a good looking product in the marketplace.

How The Process Works

Pressure Forming is a natural extension of the vacuum forming process, which has been around since the 1930s.

Vacuum forming simply requires the heating of a sheet of plastic until it reaches a forming temperature then sucking it into or around a mold. It has been used widely for products from camper shells to cold drink cups.

Pressure Forming uses air pressure as a forming aid to increase the detail on the mold side.Features that could not be achieved by vacuum alone can be molded with pressure forming. The mold can be textured or the part painted to get the desired surface finish. The result is the customer achieves the look and feel of an injection or structural foam molded part at a price close to a vacuum formed one.

Design Considerations

When designing for pressure forming you should consider the following things:

Select a vendor early in the design process to insure your design is compatible with the process.You can also get their suggestions to lower costs on second operations by molding in as many features as possible. If you work with them closely they often can incorporate new forming and trimming techniques that can get you far more part for your processing dollar than in years past.

It's best to dimension everything from the molded side of your part. This process can only tightly control the one side of the part so all data should fit into that. Bosses and other machined features should reference a datum from the molded side of the part. The remaining dimensions should be called off the machined feature. This will give you close tolerance capabilities for locating hole patterns, and other key post molding features.

The following guidelines should help determine what shapes you can incorporate in your design.

Draw ratios in the past have generally been 3:1, i.e. parts 3 times wider than they are tall. However exceptions to this rule are becoming more commonplace if properly designed and placed de-bossed sections and louvers can have draw ratios up to 1:2 and sometimes more.

Undercuts are fairly simple to 1/2 inch but can be larger under the right circumstances. This will depend primarily on the depth of draw and material thickness. Parts with 1-2" undercuts can be made but tooling and piece cost will increase due to more rigid tooling requirements and the need for thicker starting material.

Under cuts allow you to hide attachment hardware for clean part lines.

Louvers and bezels can be obtained at the lowest cost when molded in and only the back needs to be trimmed off. Blind venting is achieved by molding in the louver and machining off one side from the back. Which side depends on how your product faces the customer. These can wrap around the sides of the part if enough draft is allowed for them to release from the tool. Molded in fines and grooves can serve as stiffening ribs to strengthen the part, ensure flatness and give the part a more interesting appearance. Ribs can also be molded horizontally into the sides of parts using moving cores in the mold. Not all vendors have this capability.

If you want molded in texture you will generally need to allow for 3 degrees or more of draft on side walls. The basic rule of thumb is 2 degrees draft plus a 1 degree for every .001" of texture depth. If this rule is ignored your texture often will stay behind in the tool, being scraped off as the part ejects. This can vary depending on the texture you choose. Texture with no draft is possible but requires fairly complex tooling and will limit the number of vendors who can make your product.

Be sure you specify materials clearly. There is a big difference between general purpose ABS and UL fire rated 94VO/V5 ABS. It can change your cost as much as 20%. Most commercial products require these ratings. If not specified from the start you could be in pre production with your budgets set and be asked "Oh by the way did you need fire rated material for this?" This is often where that great price from the marginal vendor ends up very close to the vendor with certified QA who got passed over for that great price on the initial quote.

What Else to Consider

What pressure can they form at? Most commercial pressure forming equipment can form at pressures of 50-150 psi. Remember in this process air pressure and tool quality are what give you the detail. Don't be afraid to ask "will my part be vacuum formed or pressure formed?" Air driven equipment will need some method for clamping the platens together or pressure cannot be applied to the part Most pressure forming machines have either hydraulic clamps or heavy-duty motor driven platens to ensure a tight seal and good mold pressure.

What kind of tooling are they using? Your part will be no better than your tool so this is one of the most important considerations. The best tools are machined from solid aluminum. Many vendors still use sand castings. The quality of these can vary greatly. Many castings are quite porous so any machining degrades the surface affecting part quality and appearance. Also when texturing your tool the etching can expose the porosity spoiling your finish. So caution is in order when trying to use cast tools for close tolerance high appearance parts.

If your tolerances are extremely tight and high detail is important a CNC machined tool is probably best for the job. The most advanced thermoformers can take your CAD files and make your tools directly from them. With 3D CAM Software, compound angles, complex curves, and other formerly machinable features can be easily accommodated. Your design integrity is assured because your tools are made right off your CAD database. This insures the highest quality tooling possible.

Although it's not as common as it used to be, if a vendor suggests wood or epoxy tools for pressure forming, you're at the wrong place. Your tool will not be around long and you will be unhappy with what parts you do get.

After the Forming...

Since forming is only half the process, the second most important consideration is how will they finish trim my part? Capabilities can range from full CAD/CAM & CNC machining centers and 5 axis routers, to plywood fixtures and pistol drills. Many shops can pressure form a part but can't perform the complex trim operations needed to finish it Remember the object is to emulate injection molded parts. It can't be done with bad tools. Often complex machining operations are needed to finish the job. These are easily performed on CNC equipment but can be quite difficult by hand. So the amount, complexity and quantity of secondary operations can be the determining factor in choosing a vendor. Shops with CNC capabilities can almost always supply higher quality product than those without it.

The ability to hold tolerances can vary between vendors. Good vendors can hold .002 per inch on molded parts. Tighter tolerances can be held but again this is a factor of mold quality and vendor capability. On post mold trimming vendors should be able to hold ±.015" or better. If the vendor you are considering has problems holding these tolerances look elsewhere. At Freetech we do our post molding trimming on CNC mills that can hold tolerances of ±.0001" at time of machining. We admit this is over kill for Thermoforming, but it's good to know your vendor is not working to the edge of the tolerance all the time.

Quality

Does the vendor have a quality control department or is he just going to ship it out hoping you'll need it bad enough to take it? Do they have in- process quality control or will they just final inspect our product hoping the crew got it right? A quality shop can provide an inspection report with each shipment. With JIT ship to stock programs there can't be any question whether your parts are right or not. If the vendor's eyes glaze over when you mention ISO 9000, Sigma or other quality standards you might want to seek product elsewhere. Most quality oriented vendors are working toward ISO 9000 compliance if not certification. If you are shipping product overseas this could be quite important to you.

Do they ship on time? Find out what a vendor's quality and delivery ratings are with several customers. If three vendors quote 8 weeks, and one says four weeks, caution is in order. Good tooling requires a certain amount of time. Many a four week delivery promise has found its way to our door 10 to 12 weeks late with no product shipped and a useless tool.

Insist on seeing samples. Look at their samples carefully. Is the finish work what you want? Are vents and louvers straight, detail crisp and clean or is ever somewhat muted. Ask to see unpainted samples. This will really show you what a vendor can do. If parts are rough and uneven you may end up spending more for paint and finishing than you should. A good thermoformer should be able to give you finishes of injection molded quality on the mold side of the part Most molders' samples are the best parts they have. If the samples you are shown don't meet your standards, neither will the product they ship you.

If you follow these guidelines along with other good standards for supplier selection you should be able to find the tight vendor for you. They will be able to make your product for a fair price and the quality level you need. As with any process you want to beware of the vendor with a super low price. This could signal someone new in the process or poor workmanship.

This guide while establishing some basic guidelines is not the last word. If you have a design that bends the rules, make sure your vendor can too. A vendor who's staff are Members of the Society of Plastics Engineers Thermoforming Division are more likely to be able to handle those kinds of products. This is due to participating in continuing education programs that keep them abreast of the latest innovations in the plastics forming industry.

So when considering processes for your next Plastic part take a close look at Thermoforming. It could be the best process for the job.