Plastics, Forming, and Molding

Injection molding shops are challenging to schedule for several reasons.  In this article I’ll layout a few of the common challenges that we see and get into the details of how they can be handled in follow-up articles.

A typical injection molding operation goes something like this:

1. Raw material (plastic pellets) are melted in a heating unit and then injected into the cavities of a mold which has been mounted on a press.
2. The material fills the mold to create the shape of the product and then cools for a short period of time.
3. The mold is opened and the parts are ejected or removed and then advanced to subsequent finishing and packaging operations.

Sounds easy enough, right? 

Well, let’s look at some of the scheduling challenges that this presents:

Parallel Resources:  The operation requires a press and a mold (and maybe an operator) simultaneously.  To create a realistic schedule then, it’s necessary to schedule each of these resources and be sure that all needed resources are available for the duration of the operation.

Sequence-dependent Setup Time:  The time it takes to mount the mold into the press can be several hours.  Sometimes mold pre-heaters can be used to reduce the time, but some amount of time to setup the mold is going to be required whenever there is a change in the shape or material of the part to be made.  This setup time for a production run depends upon what was being made before it hence the “sequence-dependent” description.  The scheduling process needs to accurately account for and attempt to minimize time lost to setup (thus getting more productive part-producing time out of the shop).

Family Molds:  To further complicate matters, some shops use “family molds”.  These are less common than they used to be but are still used sometimes, especially when there are multiple parts that are usually made together — for example a “left” and “right” side of something that goes together.  If a family mold is in use then for each cycle of the mold there are multiple products produced– x of product 1 and y of product 2.  Most ERP (Enterprise Resource Planning) systems will create two production orders — one for product 1 and one for proudct 2.  Then when these orders are scheduled they need to be scheduled at the exact same time (”batched together”) since they are actually made concurrently.  There are also “built-up” family molds that can be configured to make various quantities of various products depending upon demand ratios of the products.  Of course, this gets even tougher to handle from a scheduling standpoint because now it’s not just a question of which molds to run and for how long but also how to setup the molds before running them.

The challenges described above are some of the major special problems that injection molders face every day.  This is in addition to the “normal scheduling challenges” such as: sequential operations, limited material availability, machine problems, resource eligibility, etc.  So you can start to get the idea of why injection molding scheduling is so difficult!

We’ll be covering solutions to these problems in the weeks ahead. 

Do you have other injection molding scheduling issues that you or one of your customers is wrestling with?  Let us know and we’ll see if we can help you out.  If not, at least we’ll feel your pain. =)