Troubleshooting problems when moulding MDF

A detailed discussion of challenges in getting a smooth finish when putting profiles on MDF material. February 13, 2001

I am looking for advice on running MDF on a moulder. We have a six-head IIDA and use two heads to profile with, three knives per head. We are currently using carbide but are trying some treated high-speed steel. We use DML heads and are jointing and sanding.

I have noticed that the second time the head is ground, it seems to joint easier. The knife marks are evenly spaced, but often still have a problem at only 100 FPM. I have tried just going up to 140 FPM and hoping they would burn in, but often they don't.

Forum Responses
I am a technician that has worked with a large number of MDF moulding companies. In the past, the use of carbide or diamond has been the most common tool of choice. The type of moulder that is used will determine the ability to joint the tool.

If you are running a single knife machine (a non-jointed machine), you may still get a multi-knife finish if the MDF and tool material match well. I recently had this happen on a Weinig Profimat 22N. The company was able to get two knives cutting in the same cutting circle. This allowed them to double their feed rate. Prior to this run, this company was using a two-piece carbide system. When they achieved the multi-knife cut, they were using DGK-2000 knife steel. This knife steel is ground with a low cost ceramic grinding wheel and can be jointed (when needed) with a low cost stone. The DGK-2000 is a high-speed steel base with a special heat-reflective coating. This is what helps it to hold up against the heat produced when cutting MDF.

To date, the DGK-2000 has been used nationwide with excellent results in MDF, glued material and many hardwood applications.

Whatever the tool material is that is used, there are a few essentials:

1) All knives must be ground accurately to each other. This means within a .0005" tolerance.

2) Grind the knives to longer than 5/16" at the smallest cutting circle. This means that the tool to head body clearance is not too large. The further the knife sticks out of the head, the more problems will be seen when running both MDF and solid wood.

3) Make sure that the belts are properly tightened.

4) Balance the tools. One gram of imbalance equals 22 pounds of rotational force.

5) Work with an experienced company when deciding which knife material works with which MDF board. If you are using a particular brand of MDF, the type of carbide or steel can be adjusted to provide the best results.

6) Use the correct grinding wheel and process to prevent damage to the tool.

7) Run the correct feed rate. This will vary depending upon the type of tool and the type of MDF board. We have some machines running at 45 FPM with carbide, while the same machine can run 85 FPM with a different type of tool. On high-speed machines this range can be between 90 FPM up to 350 FPM.

8) Control the work piece the entire time it is entering the machine and going through the machine.

Dave Rankin, forum technical advisor

From the original questioner:
I have only been working with MDF for about 8 months. My first experience was on a Hydromat 29 and I had only a few problems, but could at least get up to 180 FPM, sometimes up to 280 with only three knives.

The runout on my top head spindle is .0002. I am making sure my knives are ground to at least .0005 and that everything in the head is balanced. I keep my hold downs and chip breakers as close and as tight as I can. I try to joint sparingly. I am using Weinig's blue grey stone. We are using WKW Bak-Pak at 23 degrees and the carbide is ground at 18. The MDF we are using is from Willamette. As far as vibration goes, I see and feel nothing inconsistent. The knife marks are very consistent.

You are running an IIDA profile grinder and an IIDA moulder. On your air table grinder, be sure that the air is not set too high. Normally less than 70 PSI will work well.

Overpressure can cause some inaccuracy in the grind. Grind as close as possible. When grinding the carbide, be sure to clean the wheel often, when it is slowing down.

When grinding the coated steel, you have to use a ceramic wheel and run it between 2200 and 2500 RPM. This will grind the best for you. Grind both types of tool material with flood coolant.

When setting the moulder, be sure to close the gaps around all the cutters to provide maximum control.

Dave Rankin, forum technical advisor

There is a large variation in machinability between MDF manufacturers. I feel Willamette is one of the hardest for controlling knife marks. I found that the boards with a strong smell of pine during the milling process milled the best. There was also a difference in weight. It could be a mix of pulp and bond material.

As for tooling, we started out with WKW Bak-Pak carbide. I had a problem jointing the knives. The knives had a perfect joint line but they were not running jointed. Centrifugal force would throw the knives out to the max and when they were being jointed the jointer would push them back in to the head. The same thing would happen when the tool made contact with the material. I switched to brazed inlay carbide and it solved the problem. I also had success with the Leitz carbide system.

Keeping the tooling sharp with minimal joint will help out tremendously. I rough out the carbide, then finish grind with a 220 grit diamond wheel, with great results. If you could find some coated steel that would work on MDF, I am sure you would have better finish results.

We have had good luck jointing WKW Bak-Pak with coarse grit aluminum oxide stones. They are easy to form and joint the carbide up quickly. Tapering the stone you're using to 45 degrees or more may clean up the joint. A lighter joint is better.

The Willamette material is pretty consistent. You may want to check your core and skin consistency. We have found Trupan, Ranger board, Willamette and Plum Creek to be consistent. Working with Masonite MDF can be more difficult.

Are you grinding the carbide and backer separately or together? A grit of 150 to 220 seems to work well, with a finer grit, around 400, to finish for profile wrapping. Carbide is especially prone to heat buildup during grinding. We normally recommend grinding carbide back clearance at 12-15 degrees. More than that won't allow enough backing material to provide adequate knife support.

Both WKW's and Leitz's systems have pros and cons. The newer Bak-Pak design has a tighter micro-corrugation fit to reduce slipping between the backer and the carbide. The thicker carbide seems to reduce breakage compared to other carbide knife systems. We have customers successfully using both systems. Carbide is tricky enough to work with. Adding MDF into the mix only complicates matters more.

When using carbide on MDF, the back clearance angle is important. While many people recommend a back angle of 12-15 degrees, I have found that in the field, the use of 18 degrees allows for a better jointed knife and thus longer production runs.

When grinding carbide you should never use a diamond wheel or any super-abrasive wheel more than 3 mm or 1/8" wide. If you use wider wheels, the heat build up normally damages the carbide.

When jointing any type of knife, including carbide, the correct stone is necessary. There are many types of jointing stones available and widely used. I recommend the use of the softer style of stones for machines running 6,000 RPM spindles or faster. The machine you are using is a 6,000 RPM IIDA. In the soft stone category, you can find a wide selection, in a range of 400 to 600 grit. I normally use a 500 grit stone, which is rated as a non-sparking stone. In some cases, a harder stone is needed. This is when I would use a 150 grit medium stone.

As for styles of carbide, there are many. We are now testing a new design of a two-piece system. It locks from the edges, thus allowing 100% contact on a flat surface between the carbide and the steel backer. This has shown to reduce breakage. Part of the test includes the carbide being treated with a heat reflective coating. So far, on the carbide and similar powderized materials, the tool life and usability has been excellent. We do these tests through the Grinderman's Association.

Dave Rankin, forum technical advisor

Can I put four knives in a six-pocket head? Would there be a problem with balance? This would allow us to recycle a lot of tools.

Cutter heads are designed to run a certain way. A four-knife head can run either two or four knives only. A six-wing head can either run three or six wings. If you try to use four knives in a six-wing head, you will run the head in an out-of-balance condition that would be unsafe. Four knives in a six-knife head will run in an egg shape. The only possible way to eliminate this out-of-balance situation is with an expensive dynamic balancer.

In addition, running four knives in a six-wing head will damage your bearings, produce a bad finish and in some cases injure the operator. At the Grinderman's Association Training Center, we would never think about doing this because of the risk of injury and damage to our machines.

I have contacted several of the major cutter head manufacturers and every one said the same thing. An experienced operator could explain how much the machine vibrates and how many bearings would have to be replaced.

Run three knives in this head and fill the other pockets with filler stock. This will allow for a true rotation if all of the knives and fillers are balanced.

Dave Rankin, forum technical advisor

From the original questioner:
We tried some coated HSS and it is not working so well. Three knives ground at 18 degrees. The best we got was 7000 ft at an average speed of 96 FPM. We spent one hour and 45 minutes on them. The finish at the end of the run was marginal at best. One the last try, we ran un-jointed. We paid attention to not honing the steel.

I have used un-coated steel on MDF before and actually got a better finish. The regular steel held up better (I did not have to grind so much back). There were a couple of differences, though. I was using heads with a 20 degree hook and a grinding angle of 22 on the regular HSS. Now we are using 15 degree heads and an 18 degree grinding angle. I was okay with it lasting as long as it did, but could not get up to the feed speed I needed (at least 150 FPM) to making it worth using.

I am going to try using six knives instead of three, using a soft jointing stone, and running at a much higher speed. I am hoping the knife stays cooler.

When it comes to running MDF with coated knives, the quality of the MDF board is critical. With coated knives I have seen a range of 5,000 lineal feet up to over 30,000 lineal feet in different types of MDF.

For the board that you run, the use of six knives should work better. The accuracy of the grind must be almost perfect. Most coated products are not designed to run MDF at all and there is now one that will work well for short runs.

Dave Rankin, forum technical advisor

To the original quesitoner:
How many knife marks per inch (CMI) in MDF are you shooting for? What are you getting? How do you measure them?

From the original questioner
I measure knife marks using carbon paper and a ruler. What I am shooting for is 40,000 lin ft. per seven hours, with no visible knife or heal mark after it is primed.

So you're getting nine cutter marks per inch, right?

Use a four-knife head jointed instead of the 3-knife for 12 cpi, which is just beneath the product's capabilities (usually around 14 cpi max on a sharp day). Run the feed speed down a bit (10%) as you get dull.

What is your knife's ground angle and knife or cutterhead pocket angle?

From the original questioner:
All of our heads are six-knife. We run close to 100 profiles and they are all in three-knife sets. So going to four-knife heads would not only mean buying all new heads but we'd have to scrap a lot of steel. I have experimented with angles and am back to 20 and 15. I have started slowly going to six-knife sets.

Jointed would be attempting to put more knife marks in an inch of MDF than it could take. You'll get skip, burn and shorter knife life, unless you increase the feed speed significantly.

Knives skip, bounce, scrape, scuff, burn, or all of the above when not enough wood is yet present to make a chip. This indication of too slow a feed speed shows up as irregular knife or tooth marks.

Strange, one would think slowing the feed speed would improve the smoothness of the finished surface. Often not.