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Disclaimer. The information contained in this document has been prepared by Valley Plastics Manufacturing Inc. Valley Plastics assumes no responsibility for the misappropriated use of any of this information. It is intended to be used as a Reference Guide Only . Engineers are advised that, although much of the information that follows is "common knowledge" throughout the Fabrication Industry, some of the wording and more specifically the "Class Rating Chart" utilize Valley Plastics terms. These terms and references may not be recognized by others in the industry.

Look for this ($) icon to find great tips on how to reduce cost and avoid costly errors.


Cost Effective Uses of Fabrication Technique.

Topics included in this Document:

Although most of the characteristics of various materials are available on "Material Specification Sheets" there have been a few changes in the industry over the years that spec sheets won't tell you. Here are a few tips that will keep your part costs down and get them to you quicker.

About Acrylic Colors

Acrylics that used to be manufactured in a wide range of colors, as "stock", are now only available by special order, and usually with fairly large minimum quantity requirements. Suppliers occasional have small amounts of non-stock items available but that doesn't mean they'll be there next time.

Tip: If you're unsure about the color being "stock" and you think you'll be making this part in the future, check with your Supplier to see if what you're looking for is a stock item.

About Brand Names

In the old days, certain manufacturers that pioneered the Acrylic markets established "Trade Names" that have stuck in the minds of most engineers. Quite often, these trade names appear on drawings in place of the generic word "Acrylic". In today's market place however, many competitive brands of acrylic are now available and some with much higher quality standards. Being in the industry, we have an opportunity to observe "quality trends" between manufacturers and we can purchase accordingly. We will select from those brands that are currently providing the most consistent performance.

Tip: Don't limit us to a specific Brand Name unless absolutely necessary. It's better to say: Material: Brand X or equivalent.


Materials, in general, are produced to standard "Manufacturers Tolerances" with regards to thickness. (Usually, + or - 10%. ) Fabricators must manufacture your parts in spite of these variances. Often times, engineers will design parts with bonded or bent sides, and do not consider the amount of thickness variations that the Fabricator must deal with. Your costs are effected drastically when we are required to maintain dimensions that result from a compounding of material variations.

Tip: Care should be taken to consider stock thickness variations and allow wider tolerances in situations that allow it.

Additionally, Many materials are now being manufactured, and are available, only in "Metric" sizes. We still see drawings calling out (.125 ) when the material is only made to 3 mm (.118)

Tip: Engineers should have current "Spec Sheets" available to assist them with the dimensioning of their parts.

We should note here that; Material can be special ordered to "Mil Spec" thickness, or, we can "Pre-Select" material and use only those pieces that fall within specific tolerance ranges. Either of these processes are available to you but will increase the overall cost of your product.

Tip: Keep in mind that; We will quote and produce your parts exactly to what you ask for on the drawing. Cost effective engineering requires that you consider your needs as well as our ability to manufacture the part efficiently.

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Acrylics, Polycarbonate. and A.B.S.

The Bonding of plastics is more of an Art than a Science. One could write page after page with regards to Bonding Technique and Quality. But, for this document, we can avoid discussing to much detail about "Technique", and concentrate instead on "Results", since this is more applicable to the engineering process.


There are different levels of quality with regards to Acrylic bonding We have attempted to establish a Quality Rating System that can be used to standardize the requirement so that both Engineer and Manufacturer are talking "apples to apples".

Following is a brief description of how each bond is achieved and the associated result.

Class 1. This is a "Bubble- free bond". Used in highly visible "appearance" applications. Sometimes referred to as Museum Quality. Bubble-free bonding requires more work. The mating pieces must have a very good surface finish prior to assembly. Additionally, assemblies require more time to dry, resulting in longer production times. It also takes a Fabricator with more experience to produce Bubble-free results consistently. Therefore: Bubble Free bonding is the most expensive.

Class 2. This is the most "typical" bond. With Class 2 bonding, the assembly is made with routed and cleaned surfaces, using a mixture of Methylene and Ethylene Chlorides and some directed pressure. The result is a Bond that is "Nice" but "Less than perfect". This is usually considered a high production bond, and is intended to provide the "Biggest Bang for the Buck", by delivering an attractive product at a reasonable price. Minor appearance flaws are to be expected. With thoughtful design, these imperfections can be hidden by the use of colored or tinted materials. The majority of our production is done with this technique and is quite acceptable for most applications.

Class 3. Bonding directly to a sawn edge will show the saw cuts. This is the least expensive method to choose. It is intended for structural performance only. If you are designing a guard for your machine shop, or some other in-house use where cosmetics aren't important, you should note this on your drawing and it will help keep your costs down.

Tip: Choose your bond requirement according to the desired end result. Help keep your costs down by specifying the Class level expected.

Bonding Polycarbonate.

The process of bonding Polycarbonate is very similar to Acrylic except that the results are quite different. Esthetically, Acrylic is by far superior. Polycarbonate has a tendency to turn "milky" when bonded. Additionally, bubbles are sometimes a problem. Strength, Impact Resistance and a higher Burn Rating are the main points for choosing Polycarbonate. If cosmetics are important, try to avoid bonds altogether, and consider designing your part to accommodate "Strip Bending" instead. ( see Strip Bending )

Tip: Specify cosmetic requirements and give the Fabricator the option to Bend instead of Bond.


A.B.S. is a General Purpose material. It comes in a variety of colors, with or without texture, and, is considerably less expensive than Acrylic or Polycarbonate. Bonding is relatively simple and good quality can be achieved easily. A.B.S. bonds, provide a high degree of structural integrity as well as excellent Cosmetic qualities.

Engineers should note; When using textured materials, the texture is on the surface only. Every bond will therefore have an area, (the edge and thickness of the material), without texture. For cosmetic purposes, this should be incorporated into the design.

Tip: Consider Vacuum Forming your ABS product and do away with the bonding altogether.

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Process Overview.

Strip bending is a process by which material is heated within controlled parameters in order to create the desired result. By varying the width of the "Heat line" the fabricator can create specific internal or external radii. Once heated, the now pliable material can be placed into a fixture or set-up and allowed to cool and thereby maintain this shape. Although it may sound simple enough, due to the many differences in materials, it is actually quite involved. There are "bend allowances" that must be calculated into the flat state dimensions, and, these vary with thickness, radii and the many different materials. The Fabricator must be creative in his approach to fixturing while utilizing a variety of tooling, jigs, and fixtures.

From this large group of variables come a few conditions that are fairly constant. Conditions that are predictable results of the strip bending process. We believe that an understanding of this information is crucial if one intends to design strip bent parts effectively.

Material thickness and Tolerances

Just as with bonding, there are thickness tolerances to contend with. In general, try to not to design parts that require tighter that + or - .010 across bent dimensions.

Tip: Allow Standard tolerance for any strip bent details of + or - .020.

Bow as a result of bending.

There is a certain amount of "bow" that occurs when material is bent. As a rule, "the longer and narrower a part is, the greater the bow." This can be minimized by the use of a shallow "kerf". A kerf is a small groove at the place where the bend occurs. The result is a straighter part and increased consistency with regard to dimensions. Keep in mind however, the use of a kerf will eliminate any inside radius. There is also some sacrifice to structural integrity because the kerf acts as a scribe line on the inside surface. Pulling on the part, as if to bend it flat again, could snap it apart.

Tip: Use a kerf to reduce Bow. A kerf will reduce cost by reducing the heat time required to bend your part.

Edge Flare. There is a certain amount of "flare" that occurs at both ends of a bend. It is a result of the fact that, the outside surface of the material has to stretch farther than the inside surface causing it to pull back while the inside bulges out. In a situation that calls for the bend to be bonded to an end piece, the part will have to be made long and the flare will need to be cut off. On many simple U shaped parts, where bonding is not required, engineers should make it clear on their drawings by noting whether or not this flare is permissible. Removing it will cost a bit more, and, our first assumption is that you want it removed.

Tip: Specify, "Flare Permissible" to reduce costs where applicable.

Stress from forming.

As with all types of thermoforming, Strip bending creates stress in the material and future contact with isopropyl alcohol may cause a condition known as "Crazing. (small cracks along the heated area) Avoid all cleaners with Isopropyl Alcohol in them.

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Process Overview.

Vacuum forming is a process that requires selected materials to be heated and pulled down over a mold. Molds can be produced from many different materials with consideration being given to, design confidence, life expectancy, and cost. In many cases, V-forming can drastically reduce the part cost compared to fabrication.

Engineers must not forget to consider, "draft angles" and "material thinning" in the design and dimensioning of their parts. The amount to which a material will thin is a result of part design and starting thickness of the material. Certain steps in the V-form process can be taken to reduce the amount of thinning that will occur, but, there are limits. A complete understanding of the process is recommended before deciding to utilize the V-form process.

Tip: Consider making multiple molds that will increase yield and reduce run time to maximize efficiency in the process. This will keep costs down.

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The quality of an Edge or Surface finish will depend on the method used to produce it. In Fabrication, the most common edge is produced with a "Router Bit". A routed edge is typically a #63 finish, which is also a typical machined surface. It is possible however, to receive parts with a "Saw Cut" edge. A Saw Cut will not meet #63 requirements and "blade marks" are clearly visible. If you do not specify a #63 surface finish, a Fabricator is well within his rights to "Saw" material to size.

Be aware! You may not like the results.

Here are a few different surface and edge finishes commonly available.

Sawn Edges. As mentioned above, "Saw cut", which comes straight from the Saw. Be careful to watch for "chipping" that may occur along edges and at corners.

Routed edges are the most common. Router marks are visible, but, because of their consistency, don't usually harm the appearance.

A Sanded surface. Router marks can be removed by progressively working your way down to a fine grit sandpaper. Starting with 220 grit and finishing with 600. This is pretty time consuming and will drive the cost of production way up. It is a prerequisite for polished parts that require "Clear Surfaces" with very little distortion.

Buffed or Flame Polished Edges can be buffed to a high gloss or they can be Flame Polished. Buffing and Flame Polishing are good ways to improve the esthetics of a part. Most of the time, Flame polishing is used right over a Routed surface. Do not allow a buffed or Flame Polished surface to be cleaned with Isopropyl Alcohol.

Polycarbonate can not be Flame Polished. It has a tendency to turn black. The best way to polish Polycarbonate is to "Solvent Polish". Using great care, the Fabricator will apply a solvent mixture of Methylene and / or Ethylene Chloride, either as a liquid or vapor, to the area to be polished. The result is a highly polished surface.

Tip: Keep in mind that, the quality of the polished surface is relevant to the starting condition of that surface. Therefore: A polished Saw Cut edge will still show the blade marks. A polished routed edge will still show the router marks.

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There are a number of different ways to remove sharp edges or burrs from plastics. With most parts, a variety of hand tools are used, to which, the quality of the work depends on the skill of the operator.

Small parts are often "Tumbled" in a parts tumbler (very similar to a Rock Tumbler) with different sized media. They are tumbled along with some sort of liquid, such as soapy water or coolant. You must be specific if residues from particular solutions will contaminate your parts. Tumbling may also leave a uniform, frosty, surface over the entire part because the media comes in contact with more than just the edges.

Flame polishing will also remove burrs from Acrylic.

Tip: In most cases a note to "Break all sharp edges" is adequate, but, if you're concerned about finish or contamination, specify, "Do Not Tumble" on your drawing.

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Once a part is manufactured, it goes into "Final Packaging". Here the part is stripped of its original protective paper and inspected for cosmetic flaws. Upon passing this inspection it must be protected once again. After cleaning we apply a layer of transparent film called "Register Check". Many times Register Check can stay on to protect your parts right through assembly. You may even wish to ship them that way.

Parts may also be wrapped in tissue or simply placed into a Poly Bag. All Packaging requirements must be clearly stated either on the print or in your Company's "Packaging Standards"

Tip: Clearly state packaging requirements. Do not assume them.

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Part Marking may be required. In these instances, it is important to provide instructions as to the location and size of characters desired. You may also wish to have us "bag and Tag" when parts are too small or odd shaped.

When requesting "Part Marking", keep these things in mind:

  • What is the purpose for which part marking is required?
  • Is it for Incoming Inspection or Field Service?
  • Inventory Control or Assembly?

A clear understanding of it's purpose will enable you to choose the most "cost effective" method to suit your needs. It is much cheaper to have us Bag and Tag in lots of 25, than to individually bag and tag. For example, you wouldn't want to pay to have us individually tag 1000 washers!

You can also request "anti-static packaging" for static sensitive areas.

Tip: Be specific about your part marking needs.

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Parts should be cleaned using a soft cotton cloth with mild soap and water or a qualified plastic cleaner. Your Fabricator should be able to make a recommendation that suits your particular needs.

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Also see our material care section for details regarding care for polycarbonates.



Valley Plastics Mfg Inc., 968 Piner Rd. Santa Rosa, CA 95404
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