Metal is considered a heat-conducting surface. When placed on the surface pile of carpet, heat from the seam tape is drawn towards the metal seam weight causing moisture to form. The moisture and heat can lead to discoloration from the seam weight especially if there is any rust on the bottom of the metal plating, and potential twist loss of carpet fibers. Carpet fibers are “heat set” meaning that heat sets the twist in place and re-heating can lead to twist loss of fibers.
Steel retains heat and does not allow heat to dissipate very quickly. So why is it used? I believe that the steel seam weights originated with the area rug fabricators. Rug fabricators still use steel seam weights when they “back tape” or seam from the backside of the carpet. Many rug fabricators use either a tape without a paper backing or a fiberglass mesh with a hot glue system to bond the seams together, to cool the seam, a steel weight is used; this does a couple of things. The steel weight maintains flatness of the seam and also allows for the seam weight to be removed and then moved along the seam as the steel does not bond to the hot melt tape or mesh and hot melt system. Photos 1, 2 and 3 show the procedure for back seaming. Tape is placed over the seam area and the hot melt iron is placed onto the backside of the tape. The seam tape shown in the photo has a releasable paper. Once the seam has cooled, the paper is removed (Photos 4 and 5).
The difference between back tape and seaming from the face of the carpet is that one seam is constructed from the back and one from the top. When the seam is constructed from the back, the heat is pulled away into the heat conducting weight. When constructing a seam from the top, the metal weight is placed on the fibers, the heat draws from the tape up through the fibers and does not dissipate, which leads to distortion to the face yarns and discoloration at the seams. Heat will always flow from the hotter material to the colder material.
Wood and plastic are non-heat conducting materials. Several manufacturers sell seam weights with either wood or plastic, here are a few.
Manufacturer seam weights
(Photos 6 and 7) A metal tool tray with a grooved plastic surface on the bottom of the tray to dissipate the heat (Orcon)
(Photo 8) A steel weight with a wood plate that has a spacer between the wood and the steel. The wood also has cut outs enabling the heat to diffuse quicker. This seam weight also has an area on the top deck, which holds the seam iron tray so that the seam weight and seam iron can be transported together. (Carder Industries)
(Photo 9) A steel weight with a cooling fan and plastic bottom (Bullet Tools)
(Photo 10) Not really a seam weight but a vacuum system that creates a flow of air that draws the heat from the tape. The heat from the thermoplastic is sucked up through the vents in the bottom of the tool, which cools the thermoplastic hot melt much quicker than a seam weight and keeps the carpet fibers cool. (Seamer Down Now)
Homemade seam weights
(Photos 11 and 12) A steel seam weight with plastic mounted on the bottom side; note the width. The installer has the right idea with the non-heat conducting surface but the width needs to be a minimum of 8 inches to maintain a wider area of flatness.
(Photo13) Steel seam weight with a wood base. This installer’s seam weight can have more or less weight added by adding or removing the barbell weights.
(Photo 14) a wooden box covered with carpet and used as a seam weight; the installer told me that he added approximately 15 pounds of weight inside the box.
(Photo15) A steel straightedge and barbell seam weight. This is all heat conducting material that has the potential to cause issues.
(Photo 16) A 30-pound barbell that has been adhered to a wood base. This seam weight can be used for seams as well as arm curls while the seam cools making for one strong installer!!
(Photo 17) A manufacturer’s seam weight and a toilet tank lid. Both are non-heat conducting surfaces but one of these does not look professional on the job site. Unfortunately, toilet tank lids are used far too often.
(Photo 18) This installer prefers two seam weights, both with wood as the base.
(Photos 19 and 20) A wood seam weight, 2 inches thick by 9 inches wide by 19 inches long weighing 4 pounds. You will notice that this seam weight does not weigh very much. If a seam is properly sealed, a seam board is placed underneath the seam tape, proper heat setting on the iron, use of a seam roller; this is all that is needed to keep the seam flat.
(Photo 21) A seam board that is used under the seam tape; this creates a hard, flat surface. When a proper seam roller with a hard flat surface is used, pressure can be applied to the roller without distorting the backing, hot melt can be transferred up into the backing of the carpet for a stronger seam.
Remember, SEAMS ARE NOT INVISIBLE, but with the proper tools and techniques, a quality seam can be constructed.
For quality hot melt seams:
Seal ALL seams unless manufacturer states otherwise.
Construct seams on a flat surface.
Make sure hot melt iron is not overheating seam tape.
Use a smooth seam roller for cut pile products, a star roller is acceptable for loop pile products. The use of a seam roller adds considerable strength to the overall seam.
Use a non-heat conducting seam weight at least 8 inches wide. While some weight is beneficial, a heavy weight is not necessary.