Thermoforming: The Most Commonly Thermoformed Plastics
To manufacture quality plastic parts consistently, competitively and profitably requires the right thermoforming machine, the right mold and the right plastic. This article and the March/April 2003 article in the Thermoforming Series will focus on how to select the right plastic for your application.
All thermoformed parts are formed from thermoplastic sheet. Thermoplastics are plastics that, when heated, will become soft and pliable. This soft and pliable plastic can be forced to the shape of a mold and when the plastic is cooled will remain in the shape of the mold. Thermoplastics are made of long chains of high molecular weight organic molecules. They do not normally undergo any chemical change during the forming process. The most commonly used thermoplastics are Acrylic (poly-methyl methacrylate or PMMA), ABS (Acrylonitrile-buta-diene- styrene), Polyesters (PETG/APET), PVC (Polyvinyl chloride), Polystyrene (PS), Polycarbonate (PC), Polyethylene (PE or HDPE) and Polyproplylene (PP.
Thermoplastics are generally classified as amorphous or crystalline. Amorphous thermoplastics molecules are arranged more randomly than in crystalline thermoplastics. Plastics have a critical temperature range where thermoforming takes place. Amorphous thermoplastics have a wider critical temperature range than crystalline. In general, amorphous thermoplastics are easier to thermoform than crystalline. Each thermoplastic has its own unique set of properties. In addition, the properties of a thermoplastic can be modified during the extrusion process. For example, polystyrene in a pure state has low impact strength and products formed from this sheet will crack easily, but the material can be modified during extrusion to give the plastic greater impact strength through the addition of plasticizers.
Most plastic sheet used for thermoforming is produced by the process of extrusion. Polymer resin pellets are heated to melting point and the melted plastic is forced through a die to form a sheet. Stabilizers, plasticizers and fillers are added to the melted plastic to change the characteristics of the sheet. Dyes and pigments are used to add color. Stabilizers protect the plastic from the effects of ultraviolet light and the heat of forming. Plasticizers can be added to reduce brittleness and fillers can be used to increase stiffness. The thermoformer must work closely with the extruder to specify the correct additives to produce the forming and operation characteristics needed from the plastic sheet.
The extruder can also commonly produce the sheet in the exact size and thickness required to form the product. Using the exact sheet size and thickness will reduce the cost of the finished part. Using plastic sheet thicker than required to produce the part not only increases the cost of material but also increases forming time and decreases productivity. Extruders can produce plastic sheet in a wide range of colors. Colors can also be custom blended to meet customers requirements. The colors added during the production of the sheet are uniformly distributed through the sheet. Custom colors usually require a minimum order size of 1,000 to 2,000 pounds.
Coextrusion is the process of making multi-layered plastic sheet. A plastic sheet can be produced from multiple layers of similar polymers. Coextrusion can be used to add a special property of one polymer to the character of another polymer. For example, a thin film of a more expensive ultraviolet-resistant plastic can be coextruded over another material that is adversely affected by UV, producing a new material that has improved outdoor uses. Coextrusion can also be used to create plastic sheet that has one color on one side and another color on the opposite side or a high gloss, special color can be coextruded over a thicker plastic of low cost regrind.
Cell Cast acrylic is generally considered to have the best optical clarity of all the materials. It comes in many colors and thicknesses and is easier to create custom colors and special effects. However, it also has the down side of having more thickness variation, which may present difficulties in applications where thickness tolerance is very important like thermoforming. Cell Cast acrylic can be thermoformed but will not produce fine detail, small radius curves or deep draws. Continuous Cast is the next best thing for clarity, and it has the benefit of maintaining uniform thickness as sheets and in thermoformed parts. It is the material of choice for most skylight manufacturers. During thermoforming, it does not exhibit the differences in the amount of shrinkage that extruded materials can. It has the down side of not being as hard as Cell Cast materials which means that it may show scratches more readily. Additionally, it is also not offered in very many colors or thicknesses, making it somewhat limiting in design options when compared to cell cast.
Thermoforming - Regrind
Thermoformed part production usually produces a relatively high amount of plastic scrap. This scrap can be recycled to help keep part costs competitive. Plastic scrap can be used by the plastic sheet extruder to produce new plastic sheet. The scrap used is this manner is called regrind. The thermoformer can use a granulator to grind the scrap material into pellets that are sold back to the extruder. Granulating the scrap will not only reduce costs but reduce space required to house scrap.
Using regrind can reduce the cost of plastic sheet. Virgin sheet is produced only from pellets of resin from the resin manufacturer. When utilizing regrind, the per-cent can be specified to the sheet extruder. The percentage of regrind can range from very small to sheet that is all regrind. The percent of regrind is determined by the intended use of the finished part. Parts that require special colors usually require virgin material. Parts used in the food and medical industries also usually require virgin material. But parts such as general containers, guards, etc. can be formed using various percentages of regrind.
Plastic sheet can also be extruded in a wide assortment of surface textures. The texture is placed into the plastic during the production of the sheet. The texture can range from very smooth to grain patterns that simulate natural textures. Your sheet supplier can provide samples of the different textures they offer. The cost to add texture to a sheet is usually small.
Thermoforming - Orientation
The process of extrusion imparts mechanical stress into the plastic sheet called orientation. The orientation can be either in the length or width and can affect the thermoforming process. Excessive orientation can produce stress in the sheet that is great enough to physically pull the sheet from the sheet clamps when heated. The direction of orientation should be specified when ordering the sheet. In some plastics, biaxial orientation can increase the tensile strength or increase impact strength.
Plastics that absorb moisture are called hygroscopic. Hygroscopic plastic can cause problems when thermo-formed. The intense heat of the thermoforming process drives out moisture trapped within the sheet. The escaping water can pit mark the surface. Plastics that are hygroscopic should be shipped from the extruder dry and wrapped in plastic film that will keep moisture out. If the sheet is used quickly and kept wrapped, many times hygroscopic plastic can be thermoformed without any problems. In some cases pre-drying the sheet is necessary. Plastics that are not hygroscopic do not require pre-dying when forming, special wrapping or moisture control.
Thermoforming - Sheet Size
Commercially available 4’ x 8’ sheet can be used for prototype and short run thermoforming. For production forming, custom extruded or cast sheet sizes can be used to lower part cost.