Plastic and Resin: Two Names, One Thing

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Introduction

This article came into being as a response to a client’s questions, and I have decided to leave it in that form although I have extended it to provide a more thorough reference. Some of the terms I use are necessarily technical in nature; most of these terms that I do not explain here are linked to entries in my Glossopedia or to other technical references.

Name vs Essence

By way of introduction, here are a “plastic” pen (a Waterman Kultur) and a “resin” pen (a Montblanc Dostoevsky):

Fountain pen Magnifying glass
Fountain pen Magnifying glass

(If there is a magnifying-glass symbol (Magnifying glass) next to an image, click the magnifying glass to view a zoomed version for more detail.)

Q:  I notice that less expensive/economy pens have plastic barrels and caps. Yet all the high end, more expensive pen makers advertise that their pens are made of resin. Isn’t resin just a form of plastic?

It’s actually the other way around: plastic is a form of resin.

Natural resins are thick, sticky organic liquids that are insoluble in water. The sap of pine and other coniferous trees, often called pitch, is a resin. The materials we call plastics (polystyrene, ABS, acrylics, polyethylene, etc.) are synthetic resins in the form of long-chain polymers derived from (usually) petroleum. Even Celluloid qualifies as a plastic; it’s made by using nitric and sulfuric acids to first convert cellulose, the material that forms the basic building block of plant cell walls, into cellulose nitrate (a violently explosive substance known as guncotton). The process is completed by plasticizing the cellulose nitrate (converting it into a long-chain polymer) by mixing it with camphor. This pen was made c. 1924 using a duPont celluloid that Sheaffer called Radite:

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Before the manufacture of celluloid pens began c. 1920, celluloid was used for objects such as billiard balls, flatware handles, jewelry, and toiletry items such as hand mirrors, hairbrushes, sewing implements, and the lady’s dresser-top hair receiver shown here. This DuBarry-made piece is 51/32" (12.8 cm) in diameter, and it is made of ivory-toned Pyralin (celluloid).

Edwardian hair receiver

Before there were plastics as we know them today, the word “plastic” was an adjective, not a noun. It’s still used as an adjective, and it means “able to be shaped or molded.” Modeling clay is plastic because you can squoosh it into pretty much any shape you want, but it’s not a resin; hence, it’s not a plastic as we think of the term.

The Basic Types of Plastics/Resins

Like celluloid (and its less explosive cousin cellulose acetate), today’s plastics are mostly of the type referred to as thermoplastic resins: heating a thermoplastic resin softens it so that it can be molded (usually by injection molding, in which the softened resin is forced into the cavities of a mold). When the resin cools, it hardens again, retaining the shape into which it was molded.

Different thermoplastics have different properties. Polystyrene is popular for the manufacture of low-priced pens because it is inexpensive to produce. It is hard, it can be made in virtually any color, and it can be molded easily into intricate and very finely detailed shapes with smooth, shiny surfaces. But it softens at too low a temperature to be good for machining in a production environment, and it is more brittle than its cousin ABS (Acrylonitrile Butadiene Styrene) — which is somewhat more machinable but doesn’t work as well as polystyrene for very fine molded detail. Polyethylene is relatively soft and flexible; the moldability of a given formulation is related to the material’s hardness. Acrylics are extremely hard and respond well to machining and polishing, but they are less appropriate for injection molding. And so on.

There is another type of plastic, called thermosetting resin. This kind of material usually begins as a liquid at room temperature; when heated, it hardens permanently. Bakelite is a transparent thermosetting resin made from formaldehyde and phenol; it is described technically as a phenolic resin. To make an object of Bakelite, the manufacturer pours the liquid resin into a mold and then heats the mold to set the resin. Bakelite works well for machining because it does not melt when it is warmed — but it is brittle and becomes more so with age. The 1920s Dunn-Pen shown here has a ruby Bakelite barrel.

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To make Bakelite that is not brittle, the manufacturer mixes in a fibrous filler. The filler renders the material opaque, however, and there are other opaque materials that are better for use in pens; during the early part of the 20th century, the better material was hard rubber, and today it is principally plastics.

Which Name, and Why?

In the manufacture of pens, the use of the term “resin” usually implies a material of higher quality than the more plebeian polystyrene or ABS that low-priced pens are made from. Low-priced pens are made of thermoplastic resins that are injection molded to shape. Higher-line pens are made of acrylic resins (still thermoplastic in nature) that are supplied as cast or compression-molded rod stock or as cast sheet stock and are machined to shape.

Casting and compression molding produce different results.

Clear and solid-colored acrylics are made as cast rods or sheets. The pen shown here, a New Postal Reservoir Pen, illustrates sheet-cast acrylics in solid opaque black and transparent ruby:

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Patterned cast resin is often pearlescent; sheet stock often has the appearance of having been poured into a pan in layers, and pens made from sheet generally display differing degrees of brilliance and/or translucency when viewed from different angles. Shown here is a Bexley Submariner Grande made of Blue Pearl sheet-cast acrylic.

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Compression-molded resin is usually made of “chips and chunks” as illustrated by this Sheaffer Balance II.

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The different manufacturing techniques yield products with different mechanical qualities. Cast resin is free of internal stresses, while compression-molded material usually retains significant internal stresses and is much more prone to crack. One of the most notorious compression-molded resins of recent years is the very attractive Aspen (shown above) that Sheaffer used in its Balance II pens. This material is highly prone to cracking.

The advantage of machining rod or sheet stock is that you can achieve luxurious finishes that can’t be produced by injection molding. Unlike that of the Bexley Submariner Grande above, the patterned appearance of the injection-molded Waterman Philéas, for example, has no depth or translucency. It is a photolithograph that’s applied to the surface of the molded part, not a pattern that’s molded in. The pattern on every red second-generation Philéas (shown below) is identical to the pattern on every other red second-generation Philéas, and similarly for the first-generation version.

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The disadvantage of using machined acrylics is that it can be difficult (or in some cases impossible) to achieve certain complicated shapes that are easy to make by injection molding, e.g., the shapes of the section and body of the Lamy Safari (transparent Vista version shown here).

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A Rose by Any Other Name…

Q:  Further, I notice that some pen makers claim to use fine Italian custom resin. Is this better than plain resin?

Custom resins are available in patterns and colors that “ordinary” resins don’t come in. For example, the acrylic resins that the Bexley Pen Company uses to make its pens are primarily cast sheet stock, but they’re “standard” colors that come from a catalog. If Bexley wanted to make pens in a color that nobody else had, it could order a sheet made to its own specs. That’s all a custom resin is; the “fine Italian” part is a little bit of marketing hype based on the fact that Italian acrylic manufacturers are known for the quality and beauty of their products.

The Giant-Killer Question

Q:  And finally, is resin more durable?

Acrylic resins (“resin”) are much more durable than celluloid or styrene-based resins (“plastic”). They’re harder and more resistant to heat and the corrosive action of chemicals such as the oils and weak acids that skin produces.

In the end, the choice of material for a given pen model depends on factors such as the intended price, the target audience, and how much hand work the manufacturer can or will do. The more hand work that is required, the higher the pen’s price; in the past, labor was often less costly than materials, but today that relationship is largely reversed. When the maker plans to produce thousands of a particular model, more money can be invested in automated tooling; this reduces cost and improves consistency at the expense of the luxury imparted by the human touch.


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