Thermoforming 101

Thermoforming 101

Dr. James at jthrone@tampabay.rr.com.
Website: www.foamandform.com

 

Alphabet Soup »
It seems that plastics people never tire of their alphabet soup – ABS, PTFE, PVC, PUR, and on and on. This lesson defines common acronyms of thermoforming.

Basic Heat Transfer
There are three modes of heat transfer which are important during the heating and cooling of the plastic.
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Collecting Thin Gauge Parts
Thin-gauge forming operations generate many, many parts per hour. And these parts need to be rapidly and accurately collected or collated.
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Common Sheet Materials and Suppliers
Reference sheet of common sheet materials and suppliers.
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Comparing Concept to Reality
If we are serious about fabricating the customer’s concept, we need to understand the methodology in reducing a concept to reality.
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Cooling The Formed Part
This part considers how the sheet cools. As discussed earlier, the sheet stretches differentially against the mold surface.
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Corners »
Most plastic parts have corners. And most corners are radiused. Designers often seek sharp corners or more properly, corners with very small radii. Aesthetics is often cited as the reason for this.

The Cutting Edge
In this lesson, we focus on the edge or periphery of the part. The first thing we need to realize is that the part we’ve just thermoformed is still attached to the plastic that held it in the clamp frame while it was being formed.
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Down Gauging – It’s a Good Thing
How many of us quote jobs and specify the starting gauge? I would suggest that the majority of custom thermoformers are accustomed to quoting this way.
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Draft Angles
If the mold is male or positive, or if even a portion of the mold is male or positive, the plastic will shrink onto the mold surface. And if the mold is not properly designed, we will have a devil of a time getting the part off it. Thus we face the subject of draft angles.
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The Forming Temperature
“Is the sheet ready to be formed?” This is the most difficult question in all of thermoforming. Part of the difficulty lies in the broad spectrum of polymers and part designs.
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The Impossible Draw Ratio
For this issue I have chosen to talk about a subject that my predecessor Jim Throne wrote about in 2000 and 2001 – pre-stretching the sheet. But this time I want to discuss what type of pre-stretching should be used in a very difficult application.
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Heaters
There are three primary energy sources for heating plastic sheet in thermoforming. Electric heat is used more than gas heat or hot fluid heat. Some common heating sources include hot air, hot water or steam, sun lamps, nichrome spiral wire or toaster wire, steel rod heaters, steel or nichrome tape, tungsten and halogen…
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How to Interpret Technical Articles
So, in this tutorial, we pause to examine perhaps the thorniest issues confronting even the smartest thermoformer. First, why on earth do we need these abstruse technical articles, anyway? And second, is there really something important among all the graphs and equations?
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In The Beginning
The thermoforming cognoscenti among you know that this series has focused on some of the general concepts in thermoforming. We began with brief descriptions of polymers, then discussed heat transfer, mold materials, heaters, oven design.
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Mold Materials
Most commercial thermoforming molds are made from aluminum. Aluminum is used because it is light, it is easily worked, is relatively inexpensive and has a very high thermal conductivity. It is also used because the forming forces against the finished mold are low when compared.
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Oven Design
The focus of this article is on non-contact heating. The oven serves several purposes. It holds the sheet while it is being heated. For the most part, it isolates the heating sheet from the environment outside the oven. It provides a rigid structure for the heaters.
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Polymers Properties »
Thermoforming involves stretching of rubbery solid plastic sheet. When force is applied to any material, it stretches or elongates. The amount that it stretches depends on the amount of force per unit area, or ‘stress,’ applied to the sheet, the nature of the material and its temperature.

Process – Cycle Time
Foreign competition has forced us to maximize efficiency and become more competitive. So let us review the basic factors that determine cycle time.
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Recrystallization – What Does That Mean?
In the last TF101, I mentioned recrystallization. In this tutorial, I will explain what it is and why it is important in thermoforming. Amorphous and Crystalline Plastics – A Brief Review.
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The Rim
This lesson focuses on some of the important issues dealing with the rim. In the next lesson, we’ll look at the characteristics of the trimmed edge itself.
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Stretching the Sheet
This is a three-part tutorial in sheet behavior during heating and forming. This part focuses on sheet behavior while it is still in the oven.
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Square 1 Observe Your Sheet as it Heats
Here we consider how we can observe the effects of orientation or frozen-in stretching as the sheet is being heated.
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Square 1 Polymer Selection
In the next set of lessons, we consider additional characteristics that are needed to produce quality thermoformed products.
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Square 1 Polymer Selection Orientation »
We began by summarizing the extrusion process, then focused on polymer characteristics that influence the extrusion process. In this lesson, we continue our investigation of the extrusion process.

Thermoformable Polymers
Although we generally consider the words ‘plastics’ and ‘polymers’ interchangeable, the term ‘plastics’ refers to the product delivered as resin pellets or sheet. Nearly all plastics contain ‘polymers,’ the pure long-chain hydrocarbons, but they also contain shopping lists of additives such as thermal stabilizers…
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Trimming 1 General Comments
The polymer material that is not a portion of the formed part(s) is known in the industry as trim, web, or skeleton. It is not known as scrap, since this material is destined to be reground and reprocessed into sheet or used in non-thermoforming applications. This discussion is part of three parts on trimming.
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Trimming 2 Thin Gauge
What is Used to Trim Thin Sheet?
The steel rule die is the most common method of trimming thin-gauge sheet. The steel rule die is basically a special-grade steel strip that has been sharpened on one edge. The strip is bent to the contours of the…
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Trimming 3 Thick Gauge
As defined earlier, heavy-gauge or thick-gauge forming refers to parts formed from sheet having thickness greater than abut 3mm, 120 mils or 1/8-inch. Typically, heavy gauge parts are formed from cut sheet around it being removed from the machine clamp frame to an off-line trimming station.
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The Ubiquitous Draw Ratio
This lesson focuses on the concept of draw ratio.
Is There More Than One Definition?
Unfortunately, yes. There are at least three definitions. Let’s define the common ones.
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Understanding How a Sheet Stretches
Forming into a Mold v. Forming onto a Mold
In the not-so-politically- correct jargon of the day, if we form into a mold cavity, the mold is called a “female cavity.” A better PC2 phrase is “negative mold.” If we form onto a mold, the mold is called a “male mold.”
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What Part of Regrind Don’t You Understand
Thermoforming is burdened by two serious economic albatrosses. First, thermoforming is considered a secondary process. That is it is a process that takes place after the primary process of extrusion, used to produce the sheet that represents our incoming material.
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Why Part Design is Important
..it is time to review at least some of the generic aspects about thermoformed part design. We try to do this in the next series of lessons. And we begin by considering some of the limitations to the thermoforming process.
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