Practical techniques of injection molding.

The changes in the rubber industry over the last 30 years have been dramatic. Most notable perhaps has been the rapid increase in the industry's acceptance of injection molding for the production of many parts categories, most of which had been produced by compression/transfer molding processes.

At the same time injection molding was becoming the standard as a production technique. Engineers developed a number of press controls and electronic data interfaces which enabled a machine to literally monitor its own production and productivity. This article will begin with a review of the press types available, the press features which any good production machine must include and some of the auxiliary press equipment now available. The requirements of running a successful molding operation in the immediate future will also be addressed.

Press types

The basic types of machines most utilized are vertical single station, vertical two station commonly referred to as a shuttle, horizontal single station and multi-station. Each machine has its use in the rubber factory. However, the most suitable and versatile is a vertical single station. About 85% of all machines sold are of the vertical single station type, while the other three types make up the remaining 15%.

The most versatile and practical single station has the injection unit on the top of the machine with the closing platen approaching from the bottom. The advantages of a top injection machine include the following:

* Better access to the injection unit for general operation and routine maintenance;

* better operator safety;

* complete access to all sides of the machine for demolding and automated systems;

* minimal chance for contaminants to enter the injection unit;

* shorter distance from injection cylinder to center of mold, which means less unwanted auto-heating and less pressure drop;

* easier feeding of raw material eliminating the experience of feed rolls and the labor to load them;

* cavity placement not restricted as on a parting line inject machine;

* injection compression and injection transfer far superior since the compound is not required to lift the mobile platen, which creates auto-heating during injection;

* top injection machines allow gravity to open the mold and do not require hydraulic mechanisms to lift the mold open, which increases set-up time and decreases versatility.

The industrially accepted method of injecting rubber is the screw ram injection system.

Technical details which are often represented as significant, are in fact often the difference between success and failure.

Years of experience with this system have proven details such as:

* Proper feed port geometry for feeding silicone and low green strength materials;

* proper "L" over "D" ratio;

* proper flute pitch for optimum plasticizing;

* proper compression ratio;

* non-restrictive check valve, free of dead spots;

* amply dimensioned passages to minimize unwanted auto-heating;

* amply dimensioned passages that have a minimum length to guarantee maximum cavity pressure;

* self contained temperature regulation as close to the control source as possible to ensure fast and accurate temperature regulation;

* hot oil regulation for optimum heat transfers and maintenance-free operation;

* volumetric shot control with repeatability within grams;

* adjustable feed back-pressure for optimum compound viscosity;

* volumetric bumps to insure repeatability of bump position;

* programmable injection pressures and programmable injection speeds;

* variable injection unit contact pressure for use with cold runners.

More than 40 years of experience with the screw ram system indicate this method will produce the shortest cures and the widest range of processing possibilities. Rep believes an injection machine should be equipped with the future in mind. An injection machine should have the basic categories of features included:

* Parameter adjustments for any mold and compound;

* parameter adjustments for the integration of automatic

systems;

* safety control systems to guarantee proper machine operation;

* safety control systems for operator and personnel safety;

* on-board computerization to network all data on every cycle;

* control allowing easy integration of future evolutions.

These categories must be evaluated prior to the purchase of a machine or the machine will lack versatility in performance. Invariably, the features that are called bells and whistles by some will indeed pay off handsomely in the near future. Our advice is to evaluate any quotation carefully, as most injection press manufacturers quote a base price and don't mention the options unless specifically requested to do so.

Standard features needed

Some of the features that should be considered standard that are commonly found optional on injection machines are:

* Electronically programmable machine functions;

* closed loop control of all operating parameters;

* auto tuning of all temperature controls;

* modular construction;

* use of lubrication free bushings;

* safety-sensing devices on hydraulics;

* adjustable machine parallelism;

* electrically interlocked safety system, with high speed operation;

* zero-based hydraulics for minimum power consumption;

* microswitch and pressure sensitive mold safety.

In many new presses, a microcomputer controls the injection machine. This microcomputer allows exact repeatability of press parameters cycle after cycle. It also provides a self-diagnostic system that will free the user of any problem that might exist on the machine. This type of machine may seem too complicated but is in fact engineering to the point of simplicity. It is the future of the rubber industry, available to molders today.

Production techniques

Simply stated "production technique" is close consideration of the entire makeup of the machine cycle...all machine movements, mold movements, systems movements and operator movements. The correct number of cavities is usually less than the maximum possible. Since most rubber parts must be mechanically or manually removed from the mold, the demolding actually becomes the most critical portion of this cycle and, unfortunately it is usually given the least consideration. This, of course, leads to long open times and longer cycle times. The difference between a 120 second and a 150 second cycle is 20% in productivity. This 20% difference makes a tremendous difference in manufacturing costs. You won't get very many jobs if your cost is 20% higher than your competitor.

A current popular method for minimizing open time is a system developed and industrialized by REP. It is a simple setup where cores, core plates or cavity plates are on a rotating system which allows demolding and mold servicing while another cycle is in progress. Whenever possible, the mold is designed so the runner exits with the plate or core bar thus allowing other cycle savings.

When the proper consideration is given to the production technique, these systems will double the output and, for all practical purposes, eliminate mold damage usually associated with double core setups.

Another system is the automatic ejector. These systems can be placed on the front, rear or sides of the machine and will automatically demold parts and clean plate surfaces of any residual flash. Typical open time with such a system is 20 to 30 seconds. Done manually, this operation would take 120 seconds to 150 seconds.

Recently perfected is the top and bottom hydraulic ejector system for faster demolding where less flash and trim are involved or space is limited. The Rep Quick Latch system allows the molder to take molds in and out of the press in a much faster and efficient manner. Mold changeover is often the leading cause of non-productive downtime.

A silicone stuffer has been developed to help process this particularly difficult material. Under the careful control of the microprocessor, the silicone stuffer preps and literally stuffs silicone stock pig into the extruder port causing better material flow as the result.

The basic molding methods commonly utilized are: Classic injection, injection transfer and injection compression.

Piece part design will usually dictate the selection of the method. Generally parts of 2" in diameter and over would be best suited for classic injection, while parts of 2" and under would be best suited for injection transfer. Injection compression has limited application. Generally the parts must be shallow such as an "O" ring or a diaphragm and must be located within a 9" or 10" cavity area in order to obtain proper pinchoff. Metal contaminants in the rubber compounds present some mold damage problems with this technique. The introduction of injection transfer had a significant impact on injection molding, allowing a large number of mold cavities, a sound method to fill them, and also the possibility of flashless injection molding.

Before building any injection molds, be certain you investigate the possibility of a cold runner or a wasteless system. Generally, these systems, if applied properly will reduce cycle time, make automation realistic and save substantial amounts of material. With material costs as they are now, return timetables on this additional investment are usually very appealing. Within the last 10 years all of these systems and techniques described above have become commonplace in the rubber industry.

Table 1--the 10 commandments of press buying

1. Know thy business - The biggest mistakes often are made not in choosing an injection machine that's bad, but in choosing one that's bad for your company,

2. Do not hold false hopes - An injection machine won't solve all your problems. An injection machine will find better ways to do what your company already does correctly.

3. Thy patience shall be rewarded - Don't buy an injection machine with the idea that everything will work out immediately. It's going to take at least six months of on stream production.

4. A bargain beckons only fools -If you're basing your buying decision solely on price you'd best forget the whole thing.

5. Weigh thy options - If you end up with the wrong piece of equipment you've no one to blame but yourself. Satisfaction comes to those who explore all possibilities.

6. Sustain thy injection machine - A vital step to successful injection molding is organizing sufficient personnel to manage and operate the system.

7. Embrace the future - The future is now when it comes to injection machines, A good machine is one that adapts to change.

8. Thou shaft make thine own blunders - Repeat this three times: injection machines do not make mistakes. People do.

9. Good service is its own reward - Buy a machine from a company which stands behind, in front of, and along side its equipment

10. To thine own bottom line, be true - Profitability is directly proportionate to productivity, Get the best machine you can, for you must live with your decision every day.