The shell will be moulded in approximately 5mm wall sections and needs to be very strong as there are cantilevered arms. No gas-assist is involved in the moulding.
Plastic Parts Company is a leader among mould makers. Our main focus is creating the highest quality products. Plastic Parts Company is an america injection mould company. Our company concentrates on mould design and mould manufacture. The main products are Plastic Injection Mould, Injection Moulding and Custom Moulds.
We are in the middle of an IML project and have found that the supplier of the "injection moulding" is as critical, if not more, than the tooling/moulding source. Our supplier had suggested a higher cost, longer lead time Japanese supplier for the custom moulds - however time pressures pushed us to a faster Korean supplier - the end result, we are now 2 months behind in production and having to retool at the first injection moulding supplier.
I spent a few years as an IMD specialist for a Tekra and Bayer films. The most helpful advice I can offer is to get all parties involved at the very beginning of the design phase- tooling, moulding, mould maker, mould printer, mould former/trimmed (if not being done in house). Both the part and all the related tooling need to be designed as a team for a good chance of success- retrofitting for IMD after the fact is almost always a recipe for failure. If at all possible, make sure all partners in the project have IMD/IML background.
I and others made other points relevant to the question, even though the original questioner never acknowledged the contributions! Incidentally, Romano, it's not clear from your brief question whether you want to stiffen an existing PP chair, in which case material changes are the only options without tool modifications, or whether you're looking for suggestions to help with designing and developing a new one, in which case geometry and material can & should be considered together. The use of gas/water-assisted moulding and glass-filled PP has achieved fantastic results in more than one chair in the last few years.
Underling problem is we all can make a damn good chair. The Wall-marts of the world will not allow it as it would be a chair that 1) last 2) durable 3) recyclable friendly and last 4) will make a chair that will last 15 years. Cost would be $20 per chair vs. $5.99. With our current retail market / industry / society and wanting something cheep - forces us all to make a finished product to last minimum time, forcing society to buy again and again. Good for overall economy - well - yes. I went to a golf driving range this past Saturday. Was letting my kids hit some balls. I was sitting in a "PP injection moulded chair". It was brand new - I leaned back - and the entire bottom seat and back legs buckled, I weight 205. Why - this is the highest quality the retail market will allow. I was born in 1961. My Mom was given an iron from her Mom. When I left the house in 1980, my Mom still was using the iron - yes cord was wrapped in electrical tape - but still good iron. We buy a new piece of shit iron every year that cost 500 times than a iron made in 1960. Different world. Now can we make resin for automotive, food grade, pharmaceutical better today than ever? Yes - different market.
On a related tangent, one of the most interesting plastic chairs I have seen is the Navy 111 chair. I saw it at the NPE show in Orlando this past April, and it won first prize in the plastics design competition. The basic shape it copied from the design of an aluminum chair that was produced for the US Navy in the 1940's. It is made of 111 recycled PET bottles using gas assist injection moulding technology.