Vacuum forming, commonly known as vacuforming, is a simplified version of thermoforming, whereby a sheet of plastic is heated to a forming temperature, stretched onto or into a single-surface mold, and held against the mold by applying vacuum between the mold surface and the sheet.
The vacuum forming process can be used to make most product packaging, speaker casings and even car dashboards.
Normally, draft angles must be present in the design on the mold (a recommended minimum of 3°), otherwise release of the formed plastic and the mold is very difficult.
Vacuum forming is usually – but not always – restricted to forming plastic parts that are rather shallow in depth. A thin sheet is formed into rigid cavities for unit doses of pharmaceuticals and for loose objects that are carded or presented as point-of-purchase items. Thick sheet is formed into permanent objects such as turnpike signs and protective covers.
Relatively deep parts can be formed if the form-able sheet is mechanically or pneumatically stretched prior to bringing it in contact with the mold surface and before vacuum is applied.
Suitable materials for use in vacuum forming are conventionally thermoplastics, the most common and easiest being High Impact Polystyrene Sheeting (HIPS).
This is molded around a wood, structural foam or cast/machined aluminum mold and can form to almost any shape. Vacuum forming is also appropriate for transparent materials such as acrylic which are widely used in applications for aerospace such as passenger cabin window canopies for military fixed wing aircraft and “bubbles” for rotary wing aircraft.
Common problems encountered with vacuum forming
Moisture absorption: absorbed moisture can expand forming bubbles within the plastic’s inner layers. This may be solved by drying the plastic for an extended period at high but sub-melting temperature.
Webs may form around the mold, which is due to overheating the plastic and so must be carefully monitored. Webbing can also occur when a mold is too large or parts of the mold are too close together.
Vacuum forming heats a plastic sheet until it becomes pliable, stretches the material onto a mold, and then uses vacuum pressure to obtain the desired part geometry. The simplest type of plastic thermoforming, vacuum forming uses wooden patterns and epoxy tools in addition to temperature-controlled aluminum tooling.
Types of Vacuum Molds
Vacuum forming uses either male or female molds. With male molds, the thermoplastic sheet is placed over the mold. With female molds, the thermoplastic sheet is placed inside the mold. Male molds are used when the inside dimensions of the plastic part are a priority. Female molds are used when outer dimensions are more important than inner dimensions.
The Process: How Vacuum Forming Works
Vacuum forming clamps a thermoplastic sheet in a frame and applies heat until the sheet begins to sag. The frame seals against the mold and vacuum pressure is applied. This causes the plastic sheet to pull against the mold and form the final shape. The bottom of the sagging portion contacts the mold first and tends to retain its thickness. Material in the corners and at the top edges stretches the most and, therefore, thins the most. After cooling, the plastic part is removed from the frame and excess material is trimmed away.
Billow Forming and Plug Assists
Because the final piece is non-uniform in thickness, straight vacuum forming can be supplemented with additional techniques. Billow forming and plug assists are designed to yield more uniform wall thicknesses in the finished part. Plug assists can be used with billow forming, or with straight vacuum forming.
With billow forming, the softened sheet is blown into a bubble with approximately the same surface area as the finished part. The pre-stretched sheet is then pulled against the mold walls by vacuum pressure to produce a part with less pronounced variation in wall thickness. If the depth of draw is especially deep, a plug assist may be used to facilitate an even distribution of material.
Vacuum forming is one of the oldest and cheapest methods for plastic molding and is widely used in our everyday life, from smaller objects to huge industrial machinery. The vacuum forming process is being used at a large scale due to its low cost, efficiency, speed of imitation, and ease of use for shaping smaller objects molds. Vacuum forming is a process in which a layer of plastic is placed on the mold, and then a suction force is applied to shape the plastic according to the desired shape. Vacuum forming is also known as the simplest form of plastic thermoforming as only a mold is required, and the plastic is placed over it.
There are two types of molds that are used in vacuum forming that are: Male or positive mold
Female or negative mold
The male or positive mold is a convex-shaped one. The plastic is placed on the outside layer of the mold, which helps contour the inner dimensions of the plastic, while the female or negative mold is concave shaped. The plastic is placed inside the mold to contour the outside dimensions of the plastic accurately.
Male or Positive Mold / Female or Negative Mold
Vacuum forming, as discussed earlier, is the simplest of all forms. Still, now advanced technology is being introduced, such as heat, hydraulic and pneumatic controls to produce more precise and desired products at a reasonable production speed. Many products are made from vacuum forming, such as bath and shower trays, vehicle parts, refrigerator liners, plastic storage boxes, etc.
Difference between Thermoforming, Vacuum Forming and Pressure Forming
A process of heating a plastic sheet to make it flexible and then contouring in desired molds, trimming the final product is called thermoforming. Thermoforming is then categorized into two types:
Vacuum formin / Pressure forming
The main difference in these is the number of molds used in their product manufacturing.
Vacuum forming is done using a single mold and a vacuum pump. The heated sheet is placed into the mold, and a vacuum is applied to place it properly into the mold of the desired shape. It is mainly used in the contoured packaging of food and electronics etc. At the same time, pressure forming is done with the help of two molds. The sheet is placed within one mold and then pressed by placing the other mold on it rather than using suction from the vacuum pump. This process enables precise and aesthetically good-looking molds such as appliances casing etc. Furthermore, pressure forming is very suitable for manufacturing the plastic parts that are needed to be shaped evenly and that go deeper into a mold.
Thermoforming is one of the oldest methods of processing plastic materials. Vacuum formed products are all around us and play a major part in our daily lives. The process involves heating a plastic sheet until soft and then draping it over a mould. A vacuum is applied sucking the sheet into the mould. The sheet is then ejected from the mould.