Injection Molding Tool: What Design Mistakes to Avoid

Designing a mold tool is all about designing it to produce the part required as efficiently as possible with maximum output and minimal tool down time. Unfortunately, few mold makers in China apply best practices, and few importers are knowledgeable enough to catch design issues.

A few common errors to avoid are shown below. They can be categorized in two groups: tooling design and construction, and part design.

1. Tooling Design Points

1.1 Incorrect material selection – The coefficient of thermal expansion for all steel, hardened steels, copper & aluminum, must be taken into consideration when designing and building tools. Large differences can cause damage and uneven wear to all components.

1.2 Incorrect back plates – Most molds have 2 back plates, one on the fixed side and one on the moving side. Back plates have 3 main functions:

  • To hold the mold in the molding machine using clamps
  • To form part of the runner system
  • To support the entire mold against excessive platen deflection

If the back plates are too thin, the resultant repetitive deflection during each cycle eventually causes the following part quality problems:

  • Flashing
  • Short shots
  • Weight variation
  • Voids
  • Sink marks
  • Balancing issues in the runner system

These quality problems occur because the plates do not provide enough support to stabilize the mold against cavity injection pressure and clamp tonnage.

1.3 Incorrect runner design – A runner system that is not balanced will cause filling issues and inefficiency in running the tool. Some parts may be over-packed and other under-filled; both scenarios could cause quality issues with the part.

1.4 Incorrect gate design – Selecting the incorrect gate has a similar effect with filling issues and inefficiency in running the tool. The gate is the area where the polymer transits from the runner to the part, and if this is wrong the gate could cause restrictions, incorrect polymer flow, turbulence, or other defective filling issues.

2. Part Design Points

2.1 Part designed with incorrect wall thickness – The part should be designed with the thinnest wall section the part can tolerate (taking into account strength, functionality and other critical aspects of the design). The two risks as:

  • Too thin – if the wall section is too thin it may not fill correctly or it may break off in the tool.
  • Too thick – wall sections that are too thick take longer to cool and solidify before ejecting and may result in sink marks, warping and even cracking.

2.2 Sharp internal corners – Having a sharp corner at an intersection will cause stress within the part and could result in part failure. Best practice is to design corners with a radius.

2.3 Thick solid sections – Designing a part with thick solid section because you need the strength or rigidity is not the answer. Having a thick solid section will only result in sink marks and defective molding due to the cooling effect of the polymer after injection. Best practice is to core out the section and add ribs to strengthen that area (this will allow the molten polymer to fill the part and for cooling to be even).

2.4 Deep pocket with parallel sides – In order for a part to be ejected off of the core without sticking, there has to be draft on the walls. A parallel-sided part will inevitably cause molding issues and high reject rates. Adding draft will overcome these problems.

3. A few real-life examples we saw in China

Some of the issues our team has experienced, and that have caused issues with the tool running, are listed below:

  • Not standard parts used in the construction of the tool, springs cut to length and not ground flat resulting in uneven forces being applied to ejector plates. This caused ejector system to snag and pins to get stuck.
  • Core/cavity finished to the wrong dimensions so the toolmaker ‘welded the tool’ to add metal then reworked that area to the correct dimension. This ultimately led to inconsistent parts. Remedy was to machine out that section, add an insert of the correct steel specification and rework the shape and dimensions.
  • Poor workmanship with incorrect tools being used for the job, steel hammers on the tool, screwdrivers to upon the tool on the bench, tools not protected in storage resulting in rust and contamination.


What other design issues did you encounter?

Selection of Sourcing Countries Beyond China

Sourcing New SuppliersI am reading The Rise Of The New East: Business Strategies For An Increasingly Complex World by Ben Simpfendorfer, a consultant and economist based in Hong Kong.

This book explores the region’s growing commercial opportunities and complexities. It deals with “the East”, a wide area starting in Egypt and ending in China.

I thought it might be interesting to reproduce some comments by Richard Thomas, head of sourcing for the Far East region at Marks & Spencer. He explains what drives his company to select new sourcing countries:

Thomas emphasizes that cost only partly explains the changes in the industry. “Sure, it plays a role. But it’s also about balancing your portfolio. You need to source in countries with duty-free access to Europe or even local markets here in the region. You need to re-risk from countries with labor shortages. You need to dual-source some products,” he says. “It’s all these pressures that force change and so while we source one-third of our goods from China, we’ve also gone into Vietnam, Cambodia, and Bangladesh, and we’re looking at new opportunities in Myanmar and Pakistan.”

Factory migration also has its limits. “We are also increasingly focused on efficiencies in the markets we are already in,” he emphasizes. “It is critical we maintain leading standards of ethics and quality. We are absolutely committed to the company’s sustainability program, Plan A, especially when entering new markets.”

Marks & Spencer purchases mostly textile products from those countries. They are known for taking smart decisions, and for helping the key manufacturers they work with in improving their quality and efficiency.

If you have an interest in reading about the latest trends, I encourage you to read Ben’s book. It is well written, full of examples and anecdotes, and very easy to read.

Why Does Making a Mold In China Take a Month?

Last week I wrote about the cost of making a mold. This week I am addressing another common question from clients: why does it generally take about one month?

The length of time it takes to produce a mold tool depends on a number of factors. Primarily, the more complex the part, the longer it takes to manufacture the mold. Other factors to take into account are the size of the part to be molding, and the number of cavities within the tool.

The basic parts of a mold tool are:

  • Bolster or support block, this allows the tool to be fitted to the injection molding machine.
  • The core and cavity, this is where the shape of the part to be molded is machined — the time to generate this part of the tool depends on how complex the part is, as mentioned above.
  • Injection system — includes the locating ring, sprue, sprue bush, and the runner system which is machined into the mold to allow the part to be filled correctly.
  • Cooling system – the tool needs to be maintained at an optimum temperature for the injection process to function at its best, however once the molten plastic has been injected into the tool, it needs to be cooled down before being ejected.
  • Ejector system — this comprises of the ejector housing, ejector plate, ejector pins, ejector return pins, and the spur puller pin. All ejector pins need to be positioned so that the part is pushed out of the tool cleanly.

From a manufacturing point of view, a number of different machines and processes are used. And an element of hand finishing goes into every tool. If we focus on just the core and cavity, the steps would include:

  • Initial rough shape would be created from CNC machining centre or CNC mill
  • Electrical Discharge Machining commonly known as EDM is then normally used to produce the final shape and dimensions of the part required. The basic EDM principle is where an electrical spark is created between an electrode that has been machined to a specific shape, and the work piece. The electrical spark is so hot (up to 12,000 Deg C) that is melts virtually everything with the idea being that the work piece takes on the shape of the electrode. The key thing is to the control the spark so that it only affects a very small amount of the work piece at a time. Accuracy of EDM can be controlled to 0.0025 mm.
  • Once the shape has been generated to the correct dimensions, the next stage is the surface finish; this could include final grinding, texturing, sand blasting, shot peening, or hand polishing.

One of our engineers tells me of a time during a factory visit (inspecting a set of tools for a client) where he witnessed a worker sitting on a wooden stool inside a tool cavity, hand polishing the surface. When our engineer asked how long the worker had been polishing that tool, he was told this would require 1 week to complete — just hand polishing (bearing in mind the size of the tool for him to be sitting inside it!)

Beyond the core and cavity, there are many other items that go into making a mold tool. And machining, finishing, polishing and assembly of a tool just takes time.

Supplier Management and Sourcing Issues: Speaking Engagements

I will take part in several conferences in October and November. I listed them below.

China Sourcing Fairs (Global Sources)

Topic: The 5 Biggest Challenges in Importing from China and How to Manage Them

Dates and times: October 19 (Sunday), 1:00pm – 2:00pm, and October 27 (Monday), 2:30pm – 3:30pm.

Venue: Level 2, AsiaWorld Expo, Hong Kong.

It will take place at the same time as the trade shows organized by Global Sources in Hong Kong.

TPM Asia conference (organized by the Journal of Commerce)

Topic: I will be part of a panel discussion on the topic of sourcing in Asia.

Date and time: October 16

Venue: InterContinental Shenzhen Hotel

We’ll try to respond to questions such as:

  • What are the main drivers of sourcing decisions today?
  • What countries are benefiting, and which are losing competitiveness?

Profitable Sourcing (European Chamber of Commerce in China)

Topic: Best practices for managing suppliers – To deliver solid performance in a difficult environment, buyers need to improve their existing supplier base. In many cases, it means helping manufacturers stabilize their quality, gain in efficiency, fine-tune their planning and organization, and improve worker safety and morale.

Date and time: November 11 in the afternoon

Venue: Sofitel Hotel Guangzhou (not far from the East train station)

Registration is already open here.

There will probably be 5 speakers. Details will be announced a little later.

I hope to see you in one of these conferences!

What Determines the Cost of a Plastic Injection Mold in China?

Many clients have asked me “what determines the cost of a mold?” or “how to get plastic injection molds made for low prices in China?” So I asked one of our engineers for an explanation.

The short answer is, “it all depends”. Mold tools can cost anywhere from a few thousand dollars to hundreds of thousands of dollars and the main factors are the complexity of the part, size and the number of cavities required in the tool.

If there are undercuts or screw sections that need molding, this requires additional moving sections to be added to the tool, which will add cost to the tool.

Let’s take a dashboard in a car that is produced by injection molding and look at some of the elements required to mold this type of product.

To start with, this is a relatively large part and would require a large tool in order to mold such an item. Along with the physical size, a dashboard has a complex shape with surfaces at all different angles as well as apertures and different wall thicknesses all over it. This requires a complex tool.

Every feature that cannot be produced from the mold in the straight open and shut direction will require a moving part added to the tool. Some of these moving sections are referred to as sliders, collapsible cores, and unscrewing molds. Each of these moving parts makes the tool bigger, requires more work and ultimately increases the price.

The surface finish of the part can also affect the price of the tool slightly. If the surface finish is not critical then a machined finish may be acceptable — this does not require secondary finishing thus reducing the time and cost to produce the tool. Hand polishing takes time and effort and has to be built into the cost of the tool.

Another factor when it comes to cost is the material selection. Simplified, this can be broken down into the following options:

  • Rapid tooling – Aluminium or soft steel – produce between 100’s and a few 1000 shots
  • Low volume parts – P20 or 718S steel – produce around 300,000 shots
  • Medium volume parts – 718HH or 1.2728H – produce 600,000 shots
  • High volume parts – H13 or 1.2343 – produce 1,000,000 minimum shots

(Note: volumes will depend upon polymer used.)

The basic rule of thumb is ‘a higher volume of parts will require a superior material, which costs more money’.

Many businesses choose to have their tooling manufactured in China where you can typically see a cost saving of upwards of 40% compared with a typical tool price from Europe or the USA.

Getting a tool made in China can save a lot of money on the capital expenditure, however you need to take into consideration the expenses of travelling to the factory a number of times to ensure the tool is built and runs correctly. The cost of this can run into the thousands of dollars.

Alternatively you can use a third party company to carry out process audits and tooling inspections which will provide the correct feedback at a much reduced cost.