Plastic Headlight Materials and Bonding Methods

this thread can lay the future groundwork for a sticky if we get enough usefull info in here.

i am working on fixing a retro that jwhite did for me. as a whole its not all that bad, but the craftsmanship is bad and the fit-and-finish is very sloppy.

You're reading: Plastic Headlight Materials and Bonding Methods

being the perfectionist that i am, i simply won’t bolt these on my car as-is, except for testing and aiming.

with that said, lets get to the point of this post. in my application (+61404532026 accord) with tsx retro, the back of the headlamp housing has to be cut open to fit the projector. once cut open, i feel there needs to be a way to cover up the hole, and i’m not cool with just aiming the projector, and then filling the hole up with some kind of goop.

jwhite cut open the back of my housing up with a soldering iron, or hot knife of some sort. not only does it look terrible, but there is a large gap above the top of the projector, where the factory gasket does not cover. i do not want water/moisuture/sand entering my expensive modded headlights. i have seen the effects of water/sand in a headlight. after months of sloshing around, the chrome is completely sanded away and you are left with dull plastic where there used to be chrome.

on my headlights, and i am assuming just about every other headlight out there, there are castings in the back of the headlight that tell you usefull information about the headlight. they tell you what type of bulb goes into the headlight, and more importantly, for the purpose of this thread, they tell you what they are made of.

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my +61404532026 accord sir headlight housings are made of 3 different kinds of plastics. molded into the back of my headlights is the following:
hsg. : pp
lens : pc
and the corner light has abs casted into the back of it.

the clear lenses on the outside of the lights are made from pc, which stands for polycarbonate. polycarbonate, like many plastics, can be glued together with solvents. the glueing process actually melts the surfaces of the parts being glued together and forms whats called a “chemical weld” you are probably already familair with this, its the same exact thing as using pvc glue for pvc plumbing. this makes a water tight and air tight seal if done properly.

the grey housing of the headlamp itself is made from pp, which stands for polypropylene. polyproplyene belongs to a family of plastics which are known for their superior chemical resistance. this means they can not be easily be glued together. when attaching pieces of pp, the most efective way is to actually “weld” the plastic together using a plastic welder. like welding metal, the pp is heated to a melting point, and then melted pp rod is flowed into the joint.

my corner light (i.e. parking light) is made from abs plastic, which stands for acrylonitrile butadiene styrene. abs and polycarbonate can be chemically welded together and the clear pc lens is simply solvent-glued directly to the abs housing. there is no need for headlamp sealant here because the two plastics both melt with the same glue. the corner light can not be taken apart using the oven method. because there is no sealant between the lens and the housing to soften up from the heat. i used a utility knife and a hammer to get them apart. i gently tapped all the way around between the pc lens and the abs housing. this cracked the chemical weld apart without harming either the lens or the housing. i got the idea after watchin an episode of man vs. wild where bear grills used a knife, and a stick as a hammer, to cut through thick-ass tree branches. haha. worked great.

the reason you can take your oem headlamps apart in the oven, is because polycarbonate lens can not be glued to polypropylene and they have to use a headlamp sealant that is very sticky. the sealant keeps moisture out and the clips and tabs that go around the headlight are used to hold the two plastics together. the oven heats the sealant and causes it to “flow”, you then pry back the clips and tabs, and you can get the headlamps apart.

so for my retrofit, i took apart my corner lenses. i removed the clear plastic chip from inside the corner lens. i sprayed the back side of the clear chip with vans jdm amber lens paint. then i popped the now orange chip back into the corner light. i will be chemically glueing the pc lens back to the abs corner. everything looks factory and the clear lens now appears to be made from clear orange plastic. a great glue for pc is a product called ips weld-on #16. its a clear, thick, syrupy cement, that fills gaps well, and is widely used for chemically welding acrylic. it can also be used for bonding styrene, butyrate, pvc and polycarbonate, as well as other plastics and porous surfaces to themselves and to eachother.

now on to the back of my headlamp housing. its made from pp. pp can’t be glued. i want to cut the back of them out cleaner than jwhite did. i want to have a nice round hole back there instead of a melted mess. i want to have the back of the headlamp enclosed with a removable cover, just like factory tsx headlights. so i am ordering a plastic welder and some pp welding rods. i am going to source a pp drinking cup for my larger diameter hole. i will weld the cup to the back of the headlight and have it stick out far enough to enclose the entire rear of the projector and the ignitor.

we should start a sticky thread about types of plastics used in different headlamps and methods for working with these plastics. i also realize that there are expoxies and urethanes and glues that can provide a mechanical bond if the plastics are sanded with coarse grit sand paper and then glued together. from what i have been reading though, hardly anything wants to stick to pp.

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to summarize:

ips weld-on #16
writings great for: acrylic / polycarbonate / abs / styrene / butyrate / pvc. these can be glued to eachother and to themselves

polypropylene, polyethylene can not be glued with solvents and must be attached to unlike plastics using mechanical methods. they can be welded to themselves using heat.

most headlamps for most cars will be made from the following materials:

-acrylic or polycarbonate for lenses

-abs, pvc, pp, pe, *hdpe, *hdpp for housings (*hd=high density)

please add usefull info.

Category: Headlights