tc 29d rear hood question

[HeavyToy]

I think that I detect a tone . . . but before you get all sguirrelly please recognize that I was responding to the posting at the same time you were and not intending to refute your guestimation. Though generic "fiberglass" and SMC both use glass as reinforcement, they are not the same, which is why you can not use fiberglass to repair. I was saying that it was not generic "fiberglass". However, SMC is the process used for the hood but most likeley not for the grill/light nose or the instrument cowling. Note that on your hood the two are bonded together. It is not SMC, because it is in sheet form and does not flow as well to form tall ribs and bosses in conpression molding. BMC (Bulk Molding Compound), which would be like a pelletized SMC, is what would be able to be injection molded to get the ribs, bosses and details. My assasment was that it is injection molded and I was thinking that if the cowl was structural foam or glass reinforced structural foam, (widely used in the equipment industry) the part would be able to be painted directly out of the mold and could be a repairable thermoplastic.

Since I was not aware that we were bumping chests and thought that we were trying to figure out a way to repair the part, I was just advising that structural foam will be difficult to repair but a solvent will probably be the best bet as the plastic may be a thermoplastic rather than thermoset. If it has glass fill as well, it will be even more difficult as there will be a lesser percentage of plastic to bond in the crossection of the crack.

 

[HeavyToy]

According to your codes ABS-PC, you might want to try methylene chloride acrylic. This is a solvent and not an adhesive. If the part is ABS, you will be able to inject this solvent into the crack, using a syringe, and press the parts back into intimate contact with each other (as long as there are no pieces missing). The solvent will cause the plastic to be bonded together rather than adhered together. You can get this solvent at a plastics or hobby store and maybe an Auto supply. You may then want to grind the backside surface of the part and add the plastic weld that the shop recommended. I would use a glass tape to press into the weld compound for reinforcement, as the glass strands along the crack are no longer fused to the plastic.

No matter how you repair the part I would recommend using the glass tape to reinforce the repair since the stresses from the bolts will be translated through the repair. If you do not have glass tape, you can use a piece of metal window screen (or fiberglass) but not nylon screen. Press the reinforcement material into the weld such that the weld compound flows through the openings and surrounds the strands.

Check that any build-up of material on the backside does not interfere with the fit of the parts.

Just a thought.

 

[daTeacha]

It sounds like you may have some plastics engineering background.

In the event that I somehow crack a fender or hood on my DX29, would I be able to effect a functional if not show quality repair with fiberglass cloth or perhaps wall joint tape covered with fiberglass resin and hardener mixed and applied to the underside?

I'm just wondering if the materials in the 'glas resin mix will eat the materials in the hood and fender. If not, will they adhere to it?

 

[HeavyToy]

I am not sure that all of the parts, though reinforced with glass, are of the same plastic. This issue with the instrument cowl is with respect to the ABS-PC and I beleive that the engine hood is a different plastic, but not sure just sitting here.

The joint tape would be perfect as the reinforcement but I do not beleive that the polyester resin will bond to the ABS or PC. If you notice that the engine hood of the tractor and the grill/light part are bonded together. The bonding material is probably the plastic weld compond that dqdave1 was told to use for repairs. Notice that this is done with a lap joint so the structure is carried primarily within the joint and not the bonding material.

If you perform all of the repair efforts from the inside of the part, you will keep the mess out of sight. However, it is inevitable that the paint finish will chip around the crack revealing the plastic color, and this will need to be addressed from the outside for a showroom look.

That said, some repairs such as the to the step area will not be sufficiently repaired from a structural standpoint. Once the part breaks, the cohesion of the glass to plastic is violated along the fracture and the structural integrety is compromised. The bonding of the plastic back together, even with reinforcment, will never have the same integrity as the original part and may require replacement or measures such as rivoting metal straps across the fracture or bonding a perforated metal plate to the underside.

 

[daTeacha]

I have had a fair degree of success repairing some small plastic parts by the simple expedient of heating small finishing nails red hot and then imbedding them across the joint. I realize some plastics can't be repaired by melting and allowing to cool. Are the tractor parts of this variety or could a guy "weld" them together with careful heating?

 

[HeavyToy]

You sound quite clever and resourceful.

Thermoplastic plastics have an amorphic molecular structure which allows them to flow again when the temperature is elevated or ultrasonics excite the molecules, such as ABS. Thermoset plastics have a crystaline molecular structure and will not flow again, such as polyester resin in "Fiberglass".

Welding by virtue of heating the two parts so that they flow together does not work as in metals because plastic is an insulator and does not want to readily conduct the heat through the thickness of the material. Thus the plastic when transitioning to liquid tends to draw away from the seam.

In answer to your question, some tractor parts yes and some no.

 

[Doc_Bob]

I asked a body shop about it and they told me to use a Permatex product called Plastic Weld item # 84115 to repair it. Grind a "vee" along crack and fill.

Sounds like we could all learn, take photos!
Bob