What is Vacuum Forming?
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What is Vacuum Forming?

Thermoforming or vacuum forming is one of the common methods of processing plastic materials. Vacuum forming products are ubiquitous and are used to produce all kinds of plastic objects in daily life.
Published: May 04, 2022
What is Vacuum Forming?

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:

The process of vacuum forming involves heating a plastic sheet until soft and laying it over a mold. The sheet is drawn into the mold by vacuum and then ejected from the mold. The vacuum forming process utilizes a sophisticated pneumatic, hydraulic, and thermal controls to enable higher production speeds and a wider range of precision vacuum forming applications.

Applications of Vacuum Forming:

Suitable materials for vacuum forming include PVC, PTE, PS, PP, etc. It is often used in the production of a blister, handmade boxes, closures, workboxes, fruit and food boxes, and other products. Almost all thermoplastics can be manufactured by vacuum forming.

A wide range of manufacturing applications, from small custom parts produced on benchtop equipment to large parts made on automated industrial machinery, can be processed by vacuum forming.

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 custom small quantities of large parts.
  • 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 reach the finished part. The trimmed waste can then be reground and recycled.

How does vacuum forming work?

  1. Clamps: Place a piece of plastic in the open frame and clamp it in place.
  2. Heat: Use a heat source to soften the plastic sheet until it reaches the proper molding temperature and becomes pliable.
  3. Vacuum: The heated flexible plastic sheet and frame are lowered over the mold and pulled into place by the vacuum on the other side of the mold. The female (or punch) mold needs to drill small holes in the gap so that the vacuum can effectively draw the thermoplastic sheet into the proper shape.
  4. Cooling: Once the plastic is around/molded around the mold, it needs to be cooled. For larger workpieces, fans and cool mist are sometimes used to speed up this step in the production cycle.
  5. Demolding: After the plastic has cooled, it can be removed from the mold and demolded from the frame.
  6. Trimming: The finished part needs to be cut from the excess material and the edges may need to be trimmed, sanded, or smoothed.

During vacuum forming, 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, punch vacuum forming, concave and convex mold vacuum forming successively, foam blowing vacuum forming, auxiliary punch vacuum forming and vacuum forming with a gas buffer device.

  1. Die vacuum forming:
    Die vacuum forming is the most common vacuum forming method. The board is fixed and sealed above the mold cavity. The heater heats the board to soften, and then the air in the cavity is evacuated to form a vacuum. The board is formed against the mold cavity under atmospheric pressure. After the plastic parts are cooled and shaped, compressed air is introduced into the lower air holes to blow out the formed products. Die 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.
  2. Punch vacuum forming:
    The plastic sheet is clamped above the punch by the clamping frame and heated to soften. Then the clamping frame is moved down, and the softened plastic sheet is covered on the punch like a tent, that is, it is cooled and loses its thinning ability. Then, the air between the sheet and the punch is evacuated to form a vacuum, and the edge and the periphery of the plastic sheet are closely attached to the punch to be thinned and formed. The products formed by the punch vacuum forming method have high internal dimensional accuracy, and the bottom is thicker and not thinner. Punch vacuum forming is mostly used for thin-walled plastic parts with convex shapes, and the inner surface of the molded plastic parts has high dimensional accuracy.
  3. The concave and convex molds are vacuumed successively:
    The plastic plate is fastened to the die and heated. After softening, the heater is removed, and compressed air is blown through the punch, while the punch is evacuated to make the plastic sheet bulge. Finally, the punch is inserted downward into the bulging plastic plate and vacuumed, and at the same time, compressed air is passed into the female mold, so that the plastic plate is attached to the outer surface of the punch and formed. In this molding method, since the softened plastic plate is blown, the plate is extended and then formed, so the wall thickness is relatively uniform, and it can be used for forming deep cavity plastic parts.
  4. Blown vacuum forming:
    Some blister parts that require a roughly uniform wall thickness can also be vacuum formed by blowing bubbles. Formed with punches placed in an airtight box. First, heat the sheet, then send compressed air into the airtight box, blow the sheet outward, and then lift the punch to form a sealed state with the sheet. Finally, it is evacuated from the air hole on the punch, and it is formed by the outside atmospheric pressure. This molding method is to reduce the thickness of each part of the sheet at the same time in advance so that the thickness of the plastic part is roughly the same in the final molding.
  5. Auxiliary punch vacuum forming:
    Auxiliary punch vacuum forming is divided into downward vacuum forming and upward vacuum forming.
    For the downward vacuum forming process, first, heat the plastic plate with the die set to a softened state. Then remove the heater, use the auxiliary punch to push the plastic plate down, so that the air in the die is compressed, the softened plastic plate is extended due to the thrust of the auxiliary punch and the movement of the closed air in the cavity, and then the punch is pulled out. Vacuum forming.
    The advantage of upward vacuum forming is that the material does not contact the die first, and the material expands and hangs after heating. At this time, the auxiliary punch rises upward, and the sheet is drawn down into the initial shape without the die so that the thickness of the sheet can be changed more uniformly. If downward vacuum forming is used, the sheet first contacts the die, the contact is cooled, the thickness does not change, and the forming effect is poor.
  6. Vacuum forming with gas buffer:
    A combination of a plunger and compressed air. After heating the plastic plate and the frame, gently press it against the die, and then blow compressed air into the cavity of the die to blow the heated plastic plate. The heated air is blown out through the holes of the auxiliary punch on the plate, at this time the plate is between the two air buffer layers, and the auxiliary punch gradually descends. Finally, the blowing of compressed air into the auxiliary punch is stopped, and the cavity is evacuated so that the plastic plate is attached to the cavity of the cavity for forming, and the auxiliary punch is raised at the same time. 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.
Published: May 04, 2022 Source :bpf, Source :formlabs, Source :newton

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