Thermoforming or vacuum forming is a common method of processing plastic materials, and vacuum formed products are ubiquitous in daily life. The process involves heating a plastic sheet until soft and then placing it over a mold. A vacuum then draws the soft sheet into the mold. The sheet is allowed to cool and is then removed from the mold.
What is Vacuum Forming?
Vacuum forming is a thermoforming process. The processing principle of vacuum forming is to heat and soften the thermoplastic plastic sheet, then stick the sheet on the mold, and vacuum the side of the coating, so that the sheet can be more closely attached to the mold. After the sheet has cooled, it is set to the shape of the mold.
In simple terms, vacuum forming is a manufacturing method used to shape plastic material by heating a sheet of plastic and then pulling it around a mold using suction.
Processing of Vacuum Forming:
While the vacuum forming principle itself is fairly basic, vacuum forming processes often utilizes sophisticated pneumatic, hydraulic, and thermal controls to enable higher production speeds and a wider range of precision vacuum forming applications.
Applications of Vacuum Forming:
Vacuum forming can be used for a wide range of manufacturing applications, from small custom parts produced on benchtop equipment to large parts made on automated industrial machinery. It is often used in the production of blister packaging, handmade boxes, closures, workboxes, fruit and food boxes, and other products. Almost all thermoplastics can be manufactured by vacuum forming, including PVC, PTE, PS, PP, etc.
Production characteristics of Vacuum Forming:
- Vacuum forming offers several processing advantages over other forming processes. Relatively low-cost tooling can be achieved using low forming pressures. Since the vacuum forming process uses low pressure, the requirements for the mold material are not high, and the mold manufacturing time is relatively short. Therefore, it is a relatively economical production method to produce prototypes or to custom make small quantities of large parts. It can be used for continuous automated production of high-volume items such as disposable cups.
- Unlike other thermoplastic forming processes, vacuum forming uses extruded plastic sheets. For vacuum forming, secondary processing may be required to trim the formed sheet to complete the finished part. The trimmed waste can then be reground and recycled.
How does vacuum forming work?
- Clamps: Place a piece of plastic in the open frame and clamp it in place.
- Heat: Use a heat source to soften the plastic sheet until it reaches the proper molding temperature and becomes pliable.
- Vacuum: The heated flexible plastic sheet and frame are lowered over the mold and pulled into place by the vacuum on the inner side of the mold. The mold has small holes drilled in it so that the vacuum can pass through the holes to draw the thermoplastic sheet onto the mold.
- Cooling: Once the plastic is drawn around the mold, it is allowed to cool. For larger workpieces, fans and cooling mist are sometimes used to speed up this step in the production cycle.
- Demolding: After the plastic has cooled, it can be removed from the mold frame.
- Trimming: Excess material needs to be cut from the finished part, and the edges may need to be trimmed, sanded, or smoothed.
The heating and vacuuming steps are fast and usually only take a few minutes. Depending on the size and complexity of the part being manufactured, cooling, trimming, and making the mold can take longer.
What are the Methods of Vacuum Forming?
Vacuum forming methods mainly include concave mold vacuum forming, convex vacuum forming, concave and convex mold vacuum forming successively, blown vacuum forming, auxiliary convex vacuum forming and vacuum forming with an air buffer device.
Concave vacuum forming:
Concave vacuum forming is the most common vacuum forming method. After the plastic sheet has been softened and placed over the mold, the air under the sheet is evacuated to form a vacuum. The plastic sheet is formed against the mold under atmospheric pressure. The plastic parts are cooled and compressed air is introduced into the lower air holes to blow out the formed products. Concave vacuum forming is suitable for products with a small depth. If the depth of the product is too large, the plastic sheet will be stretched too much, which will cause the bottom to be too thin.
Convex vacuum forming:
Convex vacuum forming is usually used where the inner surface of the molded plastic part requires a high dimensional accuracy. The softened plastic is drawn over the mold, so that the mold forms the inside of the item. This method is used for thin-walled plastic parts with convex shapes.
The concave and convex molds are vacuumed successively:
After softening, the heater is removed, and compressed air is blown through the mold to make the plastic sheet bulge. The mold is then inserted downward into the bulging plastic sheet and vacuumed into the mold. At the same time, compressed air is passed into the female mold, so that the plastic sheet is pushed against the outer surface of the mold and formed. With this molding method, since the softened plastic sheet is blown and stretched before forming, the sheet thickness is relatively uniform, making it useful for forming plastic parts with deep cavities.
Blown vacuum forming:
Some blister parts that require a more uniform wall thickness can be vacuum formed by first stretching the plastic sheet by blowing it up like a balloon inside an airtight box. First the sheet is heated and compressed air is blown into the airtight box to blow the sheet outward. The mold is then raised up under the sheet and the air is evacuated from the box. Outside atmospheric pressure will then form the sheet around the mold. With this molding method, the thickness of the plastic sheet is reduced in advance so that the thickness will remain uniform throughout the final molding.
Convex vacuum forming methods:
Convex vacuum forming is divided into downward vacuum forming and upward vacuum forming.
For the upward vacuum forming process, the plastic sheet is heated over the mold in an airtight container. The mold is them pushed up so that the air inside the container pushes and stretches the plastic sheet up. Air is then let out of the container so that the stretched sheet can be drawn back down over the mold using regular vacuum forming.
The advantage of upward vacuum forming is that the plastic sheet does not contact the mold as it is being pushed up and expanding. If downward vacuum forming is used, the sheet is not initially stretched, so when it first contacts the mold, the thickness till be less uniform and sturdy.
Vacuum forming with air buffer:
After heating the plastic sheet, a combination of a plunger and compressed air is gently pressed against the sheet. Compressed air is then blown into the cavity around the mold to blow the heated plastic sheet onto the mold. At this time the sheet is between two air buffer layers, and the mold gradually descends. The compressed air is then evacuated so that the plastic sheet forms around the mold. The wall thickness of the plastic parts formed by this method is relatively uniform, and deeper plastic parts can be formed.
What is the Difference Between Vacuum Forming, Thermoforming, and Pressure Forming?
- Thermoforming is a manufacturing process in which a piece of plastic is heated to make it pliable and then shaped or contoured using a mold and trimmed to make a final part or product. Both vacuum forming and pressure forming are different types of thermoforming processes. The main difference between pressure forming and vacuum forming is the number of molds used.
- Vacuum forming is a relatively simple type of plastic thermoforming that uses mold and vacuum pressure to achieve the desired part geometry.
- Thermoforming includes two basic mold types, male and female. For male molds, plastic sheets are placed on the mold to outline the internal dimensions of the plastic part. For female molds, thermoplastic sheets are placed within the mold to precisely form the outer dimensions of the part.
- In compression molding, a heated plastic sheet is squeezed between two dies. Compression forming is ideal for making plastic parts that require precise shaping or require deeper drawing.