Low pressure injection moulding enables production of large parts

Paul Boughton

This is the strategy that has been successfully pursued by AVK Plastics, based in the small Dutch town of Balk. A subsidiary of the Danish AVK company, it is the only plastics component producer within a group of companies that specialises in producing valves for the wastewater industry.
AVK Plastics was founded in 1990 and after a few years started to switch from conventional moulding to injection-moulding. Initially it produced parts for smaller valves and plastic alternatives for heavy steel wastewater outlets that the parent company made. Traditionally a lot of these products were made with low-pressure injection moulding on Hettinga machines. Looking for expansion, in 1999 AVK plastics took over a company that produced plastic pallets.
Although completely different to its original area of expertise, plastic pallet production became a growth sector and of importance for the future of AVK Plastics. Soon the company found rising demand from the medical and food industry and, last but not least, the Chinese market that otherwise only accepts specially treated wooden pallets.
In these cases, the costs of a plastic pallet, which can be up to twice the price of a wooden pallet, are irrelevant because of the better properties that plastic offers. Starting off with smaller size pallets, AVK Plastics now produces Euro size pallets and even larger models. As a result, its position is growing within the European market.

Low pressure moulding

A good example of a company product is the 100x120cm pallet. It is produced for Q-Pall, a distribution centre.
AVK Plastics consulted Foma Engineering and asked the company to look at utilising an older machine with a large clamping force and back plate that could inject the 32kg of foamed regrind polypropylene. To achieve this kind of shot weight with high-pressure injection moulding would require an enormous machine.
AVK Plastics therefore choose the path of low-pressure injection moulding. Foma then calculated that a clamping force of 1300tonnes was needed with an injection pressure of 800-900bar into a 40°C mould. The injection time was estimated at six seconds.
The machine that was needed now was within reasonable size, but the only issue was the shot weight. To prepare and inject sufficient melt flow within an acceptable cycle time Foma designed a double barrel. The upper barrel prepares the necessary melt flow, and then pushes it into the lower barrel that injects it into the mould.
The pallet required an intelligent and compact mould design, otherwise it would not fit in the machine that had been chosen for it. Neither AVK nor Foma had much experience with hot runners so additional expertise was needed to complete the project.
Only a limited number of suppliers are able to meet the requirements for a hot runner system of this size. AVK Plastics contacted Synventive Moulding Solutions, whose representatives immediately understood the high level of customisation that would be required for the success of this project. After discussing the application Synventive teamed up with Foma to bring the project to a successful conclusion.
Because of the high amount of foamed material that had to be injected, a hot runner system with large flow bores was needed. Large flow bores also minimise the amount of pressure loss, essential for low-pressure moulding. Valve gates were needed to shut off the needles, preventing the expanding material from flowing out of the nozzles when opening the mould and to control the melt flow during the injection phase. AVK Plastics had a filling analysis that indicated that nine nozzles would result in the optimum flow pattern.

Synventive advised on the specific layout of the system, the special connection with the machine and the type of insert that would ensure optimal processing. The nine nozzles are divided over multiple zones which are operated by the machine instead of a separate controller.
At start up, the results were even better then calculated. The required clamping force ended up as just 800tonnes, the injection pressure was 400bar maximum into a 22°C mould. The total injection time is around 18 seconds.
AVK Plastics will now produce 70000–80000 pallets a year for Q-Pall and gradually switch to hot runners for the production of other pallets that are currently still made in the conventional way.